guide to offshore structures design for engineers

DESIGN OF OFFSHORE STRUCTURES
For Practising Engineers and Students
Dr.S.Nallayarasu
Associate Professor
Department of Ocean Engineering
Indian Institute of Technology Madras,
Chennai - 600036, India

Contents
1 INTRODUCTION 17
1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
1.2 Types of Offshore Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.3 Fixed Platforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.3.1 Steel template Structures . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.3.2 Concrete Gravity Platforms . . . . . . . . . . . . . . . . . . . . . . . 19
1.4 Compliant Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.4.1 Compliant Tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.4.2 Guyed Tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1.4.3 Tension Leg Platforms . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1.4.4 Articulated Tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
1.5 Floating Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
1.5.1 Floating Production System . . . . . . . . . . . . . . . . . . . . . . . 26
1.5.2 Floating Production, Storage and offloading System . . . . . . . . . . 27
1.6 Subsea System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
1.7 Fixed Platform Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
1.7.1 Functional Classification . . . . . . . . . . . . . . . . . . . . . . . . . 28
1.7.2 Geometrical Classification . . . . . . . . . . . . . . . . . . . . . . . . 28
3

Contents Contents
1.7.3 Foundation Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2 LOADS 31
2.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.2 Types of Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.3 Gravity Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3.1 Dead Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3.2 Facility Dead Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3.3 Fluid Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3.4 Live Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.3.5 Drilling Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.4 Environmental Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.4.1 Wind Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.4.2 Wave and Current Loads . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.4.3 Current Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.4.4 Marine Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.4.5 Morison Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.4.6 Wave-Current Interaction . . . . . . . . . . . . . . . . . . . . . . . . 39
2.4.7 Selection of Wave Theory . . . . . . . . . . . . . . . . . . . . . . . . 40
2.4.8 Wave Load on a Member . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.4.9 Maximum Global Loads . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.4.10 Buoyancy Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
2.4.11 Ice Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
2.4.12 Mud Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
2.5 Load Estimation and Distribution . . . . . . . . . . . . . . . . . . . . . . . . 51
4

Contents Contents
2.6 Load Combinations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3 MATERIALS AND CORROSION 57
3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.2 Structural Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.2.1 Structural Member Classification . . . . . . . . . . . . . . . . . . . . 58
3.2.2 Steel Grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.2.3 Manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.2.4 Chemical Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.2.5 Carbon Equivalent (CE) . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.2.6 Mechanical Requirements . . . . . . . . . . . . . . . . . . . . . . . . 61
3.2.7 Notch Toughness Requirements . . . . . . . . . . . . . . . . . . . . . 61
3.2.8 Supplementary Requirements . . . . . . . . . . . . . . . . . . . . . . 61
3.3 Structural Steel Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.4 Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.4.1 Seawater Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.4.2 Factors influencing corrosion . . . . . . . . . . . . . . . . . . . . . . . 62
3.4.3 types of corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.4.4 Rates of corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3.5 Corrosion Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.6 Cathodic Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.6.1 Sacrificial Anode System . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.6.2 Impressed Current System . . . . . . . . . . . . . . . . . . . . . . . . 69
4 GLOBAL STRUCTURAL ANALYSES 73
4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5

Contents Contents
4.2 Structure Geometry selection . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4.3 Geometry Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.3.1 Axis System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.3.2 Joints or Nodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.3.3 Members and properties . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.3.4 Offsets and Eccentricities . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.3.5 Wish-bone Connections . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.3.6 Dummy Structure Models . . . . . . . . . . . . . . . . . . . . . . . . 80
4.4 Foundation Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.4.1 Pile Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.4.2 Soil Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4.4.3 Pile Group Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4.5 Load Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.5.1 Dead Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.5.2 Equipment Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.5.3 Fluid Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.5.4 Drilling Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.5.5 Live Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.5.6 Wind, Wave and Current . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.6 In-place Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4.6.1 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4.6.2 Geometry Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
4.6.3 Load Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4.6.4 Topsides Load COG Shift Envelope . . . . . . . . . . . . . . . . . . . 92
4.6.5 Minimum and Maximum Water Depth . . . . . . . . . . . . . . . . . 92
6

Contents Contents
4.6.6 Maximum Wave Loads . . . . . . . . . . . . . . . . . . . . . . . . . . 92
4.6.7 Allowable Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
4.7 Dynamic Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
4.7.1 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
4.7.2 Geometry Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
4.7.3 Load Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
4.7.4 Foundation Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . 95
4.7.5 Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
4.8 Seismic Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
4.9 Fatigue Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
4.9.1 Deterministic Fatigue Analysis . . . . . . . . . . . . . . . . . . . . . . 96
4.9.2 Wave Scatter Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
4.9.3 Spectral Fatigue Analysis . . . . . . . . . . . . . . . . . . . . . . . . 99
4.9.4 Fatigue Life Calculation . . . . . . . . . . . . . . . . . . . . . . . . . 101
4.9.5 S-N Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
4.9.6 Stress Concentration Factors . . . . . . . . . . . . . . . . . . . . . . . 104
4.9.7 Foundation Linearisation . . . . . . . . . . . . . . . . . . . . . . . . . 105
4.10 Boat Impact Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
4.10.1 Impact Vessels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
4.10.2 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
4.10.3 Method of Computing Impact Energy Dissipation . . . . . . . . . . . 107
4.10.4 Energy dissipation by member . . . . . . . . . . . . . . . . . . . . . . 108
4.10.5 Energy Dissipation by Boat . . . . . . . . . . . . . . . . . . . . . . . 110
4.10.6 Energy Dissipation by Overall jacket deflection . . . . . . . . . . . . . 110
4.10.7 Structural Strength During Impact . . . . . . . . . . . . . . . . . . . 110
7

Contents Contents
4.10.8 Post Impact Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
4.10.9 On-Bottom Stability Analysis . . . . . . . . . . . . . . . . . . . . . . 110
5 DESIGN METHODOLOGY 115
5.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
5.2 Design Stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
5.2.1 FEED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
5.2.2 Basic Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
5.2.3 Detailed Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
5.3 Design of Topsides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
5.3.1 Platform Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
5.3.2 Deck Arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
5.3.3 Deck Elevations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
5.3.4 Deck Leg Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
5.3.5 Beam/Girder Selection . . . . . . . . . . . . . . . . . . . . . . . . . . 122
5.3.6 Deck Construction method . . . . . . . . . . . . . . . . . . . . . . . . 122
5.3.7 Deck Appurtenances . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
5.4 Design of jacket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
5.4.1 Jacket Framing Concepts . . . . . . . . . . . . . . . . . . . . . . . . . 124
5.4.2 Lift versus Launch . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
5.4.3 Main pile versus skirt pile . . . . . . . . . . . . . . . . . . . . . . . . 124
5.4.4 Mudmat arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
5.4.5 Jacket Appurtenances . . . . . . . . . . . . . . . . . . . . . . . . . . 124
6 DESIGN OF TUBULAR MEMBERS 125
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
8

Contents Contents
6.2 Factors affecting strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
6.2.1 Tubular fabrication methods . . . . . . . . . . . . . . . . . . . . . . . 126
6.2.2 Material properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
6.2.3 Imperfections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
6.3 Ultimate Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
6.3.1 Section properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
6.3.2 Boundary conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
6.3.3 Buckling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
6.4 Allowable Stress Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
6.4.1 Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
6.4.2 Applied Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
6.4.3 Allowable Stress in Tension . . . . . . . . . . . . . . . . . . . . . . . 128
6.4.4 Allowable Stress in Compression . . . . . . . . . . . . . . . . . . . . . 128
6.4.5 Allowable Stress in Bending . . . . . . . . . . . . . . . . . . . . . . . 130
6.4.6 Combined axial compression and bending interaction . . . . . . . . . 130
6.4.7 Combined axial tension and bending interaction . . . . . . . . . . . . 131
6.4.8 Hydostatic Collapse . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
6.4.9 Hydrostatic Collapse Rings . . . . . . . . . . . . . . . . . . . . . . . . 132
6.4.10 Axial Load and Hydrostatic Pressure . . . . . . . . . . . . . . . . . . 133
6.4.11 Optimum Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
6.5 Load and resistance Factor Design . . . . . . . . . . . . . . . . . . . . . . . . 135
6.5.1 Code Provisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
6.5.2 Optimum Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
7 DESIGN OF TUBULAR JOINTS FOR STATIC LOADS 137
9

Contents Contents
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
7.2 Tubular Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
7.2.1 Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
7.2.2 Load path dependency . . . . . . . . . . . . . . . . . . . . . . . . . . 142
7.3 Joint geometrical Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 142
7.4 Static Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
7.4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
7.4.2 Failure Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
7.5 Design methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
7.6 Parametric equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
7.6.1 API RP2A (WSD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
7.6.2 Design Practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
7.6.3 API RP2A (LRFD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7.6.4 AWS D1.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7.7 Design method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7.7.1 Axial load capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7.7.2 Moment capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7.7.3 Interaction equation . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7.7.4 Effect of thickened can . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7.7.5 Design practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7.7.6 Strength check (50%) . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7.8 Material Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7.9 Special Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7.9.1 Grouted pipe-in-pipe joints . . . . . . . . . . . . . . . . . . . . . . . . 151
7.9.2 Ring stiffened joints . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
10

Contents Contents
7.9.3 Overlapped joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
8 DESIGN OF TUBULAR JOINTS FOR CYCLIC LOADS 153
8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.2 Hot spot and stress concentration . . . . . . . . . . . . . . . . . . . . . . . . 154
8.3 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.4 Fatigue assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.4.1 Fracture mechanics method . . . . . . . . . . . . . . . . . . . . . . . 154
8.4.2 Fatigue damage method . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.5 S-N curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.6 SCF-Parametric equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.7 Deterministic Fatigue Assessment . . . . . . . . . . . . . . . . . . . . . . . . 154
8.7.1 Wave scatter data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.7.2 Hot spot stress range . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.7.3 Fatigue damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.7.4 Factor safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.8 Spectral fatigue assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.8.1 Sea state definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.8.2 Spectral response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.8.3 Fatigue damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.9 Grouted pipe-in-pipe joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.10 Ring stiffened joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.11 Overlapping joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
9 FOUNDATION DESIGN 155
9.1 Main Pile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
11

Contents Contents
9.2 Skirt or Cluster Piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
9.3 Pile Axial Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
9.4 Design Factors of Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
9.5 Pile-Soil Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
9.5.1 P-Y Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
9.5.2 T-Z Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
9.5.3 Q-Z Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
9.5.4 Pile Group Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
9.6 Pile Driveability analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
9.7 Pile Design for Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
10 FABRICATION 157
10.1 Fabrication Yard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
10.1.1 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
10.1.2 Quay side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
10.1.3 Skidways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
10.1.4 Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
10.2 Fabrication of tubulars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
10.2.1 Fabrication of Tubulars . . . . . . . . . . . . . . . . . . . . . . . . . . 158
10.2.2 Tubulars Braces and Cans . . . . . . . . . . . . . . . . . . . . . . . . 158
10.2.3 Tubular Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
10.2.4 Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
10.2.5 NDT Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
10.2.6 PWHT Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
10.3 Jacket Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
12

Contents Contents
10.3.1 Jacket Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
10.3.2 Roll up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
10.3.3 fittup and welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
11 DESIGN FOR INSTALLATION FORCES 159
11.1 Loadout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.1.1 Skidded Loadout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.1.2 Trailer Loadout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.1.3 Lifted Loadout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.1.4 Jacket Loadout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.1.5 Deck Loadout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.1.6 Ballast Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.1.7 Loadout arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.1.8 Loadout procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.1.9 Design Loads for Loadout . . . . . . . . . . . . . . . . . . . . . . . . 160
11.1.10Analysis methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.1.11Support settlement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2 Sea Transportation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.1 Transportation route . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.2 Weather forecast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.3 Design seastate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.4 Barge selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.5 Cargo Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.6 Transportation methods . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.7 Classification of barges . . . . . . . . . . . . . . . . . . . . . . . . . . 160
13

Contents Contents
11.2.8 Barge Motion Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.9 Barge Motion Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.10Barge Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.11Longitudinal Strength . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.12Barge Flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.13Inertia Loads during transportation . . . . . . . . . . . . . . . . . . . 160
11.2.14Analysis Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.15Basic Loads and combinations . . . . . . . . . . . . . . . . . . . . . . 160
11.2.16Allowable stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.17Sea fastening design . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.18grillage design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.2.19Transportation Fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.3 Lifting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.3.1 Lifting Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.3.2 Jacket Lift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.3.3 Module and Deck Lifts . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.3.4 Single and Doible Hook lifts . . . . . . . . . . . . . . . . . . . . . . . 160
11.3.5 Lift methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.3.6 Dynamics of lift system . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.3.7 Weight control procedure . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.3.8 Lift load factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.3.9 Code Provisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.3.10Lifting Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.3.11Lift point design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.3.12Spreader bars and Frames . . . . . . . . . . . . . . . . . . . . . . . . 160
14

Contents Contents
11.4 Launching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.5 Upending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.6 Topsides Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.6.1 Deck Module Installation . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.6.2 Floatover installation . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.7 Pile Driving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.7.1 Piling scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.7.2 Main and skirt piles . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.7.3 Pile guides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.7.4 Pile segments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.7.5 Wave equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.7.6 Analysis Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.7.7 Soil Resistance to Driving . . . . . . . . . . . . . . . . . . . . . . . . 160
11.7.8 Pile refusal criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.7.9 Pile driving fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.7.10Pile remedial schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.8 Jacket Leveling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.8.1 Grouting system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.9 Riser Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.10Bridge Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.11Intallation Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
15

Chapter 1
INTRODUCTION
1.1 General
One of the greatest discovery of 20th century was oil and it has so many applications that
it cannot be separated from mankind. The oil exploration has started as early as —– and
the oil exploration initially was concentrated on on land. As the need for oil expands in an
explosive rate, need for find new discoveries was eminent. During the middle of 20th century,
oil discovery started in near shore and medium range of water depth.
The need for qualified offshore structural personnel are rapidly increasing as the oil industry
moves into deeper water in the search for additional supplies of oil and gas, new technology
is emerging at a rapid peace for the development of new concepts for offshore platforms.
This book gives brief introduction to offshore engineering with basic concepts of various
types of offshore structures and provide insight into various design issues and requirements,
fabrication and installation techniques.
Chapter 2 gives introduction in to types of offshore platforms based on water depth require-
ments, geometry and installation concepts.
Chapter 3 gives basic loads applied on offshore structures and techniques of calculations of
such loading.
Chapter 4 gives introduction to material requirement for offshore structures including cor-
rosion.
Chapter 5 describes various structural analysis techniques required for the offshore structures
before it can be designed.
Chapter 6 gives introduction to design methodology of offshore structures, various design
17

1.2. Types of Offshore Structures Chapter 1. INTRODUCTION
methods such as Working Stress Design (WSD) and Load and Resistance factor Design
(LRFD) and code provisions.
Chapter 7 gives basic principles involved in the design of a tubular member in the offshore
structure including code provisions and optimum member design.
Chapter 8 gives tubular joint design including various types and configurations. This chapter
provides some insight into the design of tubular joints for fatigue loading including Stress
Concentration Factors (SCF) of stiffened and un-stiffened joints.
Chapter 9 gives some insight in to different type of foundations for offshore structures and
its design method
Chapter 10 give some basic principles of fabrication and installation of offshore platform
components.
1.2 Types of Offshore Structures
The offshore structures built in the ocean to explore oil and gas are located in depths from
very shallow water to the deep ocean. Depending on the water depth and environmental
conditions, the structural arrangement and need for new ideas required. Based on geometry
and behaviour, the offshore structures for oil and gas development has been divided into
following categories.
1. Fixed Platforms
• Steel template Structures
• Concrete Gravity Structures
2. Compliant tower
• Compliant Tower
• Guyed Tower
• Articulated Tower
• Tension Leg Platform
3. Floating Structures
• Floating Production System
• Floating Production, Storage and Offloading System
18

1.3. Fixed Platforms Chapter 1. INTRODUCTION
1.3 Fixed Platforms
The fixed type of platform shall exhibit a low natural period and deflection again environ-
mental loads.
1.3.1 Steel template Structures
The steel template type structure consists of a tall vertical section made of tubular steel
members supported by piles driven into the sea be with a deck placed on top, providing space
for crew quarters, a drilling rig, and production facilities. The fixed platform is economically
feasible for installation in water depths up to 500m.
These template type structures will be fixed to seabed by means of tubular piles either driven
through legs of the jacket (main piles) or through skirt sleeves attached to the bottom of the
jacket.
The principle behind the fixed platform design is to minimize the natural period of the
structure below 4 seconds to avoid resonant behaviour with the waves (period in the order
of 4 to 25 seconds. The structural and foundation configuration shall be selected to achieve
this concept.
1.3.2 Concrete Gravity Platforms
Concrete gravity platforms are mostly used in the areas where feasibility of pile installation is
remote. These platforms are very common in areas with strong seabed geological conditions
either with rock outcrop or sandy formation.
Some part of north sea oil fields and Australian coast, these kind of platforms are located.
The concrete gravity platform by its name derive its horizontal stability against environmen-
tal forces by means of its weight. These structures are basically concrete shells assembled in
circular array with stem columns projecting to above water to support the deck and facilities.
Concrete gravity platforms have been constructed in water depths as much as 350m.
19

Figure 1.1: Different types of Offshore Structures
20

Figure 1.2: Fixed Template type platform
21

Figure 1.3: Concrete Gravity Platform
22

1.4. Compliant Structures Chapter 1. INTRODUCTION
Figure 1.4: Complaint Tower Platform
1.4 Compliant Structures
In addition to the developing technologies for exploration and production of oil and natural
gas, new concepts in deepwater systems and facilities have emerged to make ultra-deepwater
projects a reality. With wells being drilled in water depths of 3000m, the traditional fixed
offshore platform is being replaced by state-of-the-art deepwater production facilities. Com-
pliant Towers, Tension Leg Platforms, Spars, Subsea Systems, Floating Production Systems,
and Floating Production, Storage and Offloading Systems are now being used in water depths
exceeding 500m. All of these systems are proven technology, and in use in offshore production
worldwide.
1.4.1 Compliant Tower
Compliant Tower (CT) consists of a narrow, flexible tower and a piled foundation that
can support a conventional deck for drilling and production operations. Unlike the fixed
platform, the compliant tower withstands large lateral forces by sustaining significant lateral
deflections, and is usually used in water depths between 300m and 600m.
23

Figure 1.5: Tension Leg Platforms
1.4.2 Guyed Tower
Guyed tower is an extension of complaint tower with guy wires tied to the seabed by means of
anchors or piles. This guy ropes minimises the lateral displacement of the platform topsides.
This further changes the dynamic characteristics of the system.
1.4.3 Tension Leg Platforms
A Tension-leg platform is a vertically moored floating structure normally used for the offshore
production of oil or gas, and is particularly suited for water depths around 1000m to 1200
metres (about 4000 ft). The platform is permanently moored by means of tethers or tendons
grouped at each of the structure’s corners. A group of tethers is called a tension leg. A
feature of the design of the tethers is that they have relatively high axial stiffness (low
elasticity), such that virtually all vertical motion of the platform is eliminated. This allows
the platform to have the production wellheads on deck (connected directly to the subsea
wells by rigid risers), instead of on the seafloor. This makes for a cheaper well completion
and gives better control over the production from the oil or gas reservoir.
Tension Leg Platform (TLP) consists of a floating structure held in place by vertical, ten-
sioned tendons connected to the sea floor by pile-secured templates. Tensioned tendons
24

Figure 1.6: SeaStar
provide for the use of a TLP in a broad water depth range with limited vertical motion. The
larger TLP’s have been successfully deployed in water depths approaching 1250m.
Mini-Tension Leg Platform (Mini-TLP) is a floating mini-tension leg platform of relatively
low cost developed for production of smaller deepwater reserves which would be uneconomic
to produce using more conventional deepwater production systems. It can also be used as a
utility, satellite, or early production platform for larger deepwater discoveries. The world’s
first Mini-TLP was installed in the Gulf of Mexico in 1998.
SPAR Platform (SPAR) consists of a large diameter single vertical cylinder supporting a
deck. It has a typical fixed platform topside (surface deck with drilling and production
equipment), three types of risers (production, drilling, and export), and a hull which is
moored using a taut catenary system of six to twenty lines anchored into the seafloor. SPAR’s
are presently used in water depths up to 1000m, although existing technology can extend its
use to water depths as great as 2500m.
1.4.4 Articulated Tower
Articulated tower is an extension of tension leg platform. The tension cables are replaced
by one single buoyant shell with sufficient buoyancy and required restoring moment against
25

1.5. Floating Structures Chapter 1. INTRODUCTION
lateral loads.
The main part of the configuration is the universal joint which connects the shell with the
foundation system. The foundation system usually consists of gravity based concrete block
or some times with driven piles.
The articulated tower concept is well suited for intermediate water depths ranging from 150m
to 500m.
Figure 1.7: Articulated Tower Platforms
1.5 Floating Structures
1.5.1 Floating Production System
Floating Production System (FPS) consists of a semi-submersible unit which is equipped
with drilling and production equipment. It is anchored in place with wire rope and chain,
or can be dynamically positioned using rotating thrusters. Production from subsea wells is
26

1.6. Subsea System Chapter 1. INTRODUCTION
Figure 1.8: Floating Production storage and offloading system (FPSO)
transported to the surface deck through production risers designed to accommodate platform
motion. The FPS can be used in a range of water depths from 600m to 2500m feet.
1.5.2 Floating Production, Storage and offloading System
Floating Production, Storage and Offloading System (FPSO) consists of a large tanker type
vessel moored to the seafloor. An FPSO is designed to process and stow production from
nearby subsea wells and to periodically offload the stored oil to a smaller shuttle tanker.
The shuttle tanker then transports the oil to an onshore facility for further processing. An
FPSO may be suited for marginally economic fields located in remote deepwater areas where
a pipeline infrastructure does not exist. Currently, there are no FPSO’s approved for use in
the Gulf of Mexico. However, there are over 70 of these systems being used elsewhere in the
world.
1.6 Subsea System
Subsea System (SS) ranges from single subsea wells producing to a nearby platform, FPS,
or TLP to multiple wells producing through a manifold and pipeline system to a distant
production facility. These systems are presently used in water depths greater than 1500m.
27

1.7. Fixed Platform Concepts Chapter 1. INTRODUCTION
1.7 Fixed Platform Concepts
For the last few decades, the fixed platform concept has been utilized extensively over 300m
depth with various configurations.
1.7.1 Functional Classification
The offshore platforms for oil and gas exploration purpose can be classified based on func-
tionality and purpose of installation.
• Wellhead platform - primarily meant for drilling and supporting wellhead equipment.
It supports very few equipment such as wellhead control panel and piping. Occasionally
it also supports helicopter landing structure for emergency evacuation.
• Process Platform - primary meant for production facilities (oil or gas) and it may
support in addition to equipment for production, such as power generation, utilities
and living quarters.
• Riser Platform - This is another kind of structure specially built to support all the
incoming and outgoing risers on a planned complex. This will also be connected to the
main platform by bridge.
• Living Quarters Platform - Some times due to safety requirements, the living quar-
ters will be supported on a separate structure away from the wellhead and process
platforms. This types of platform will be located atleas 50m away from the neighbor-
ing process platforms and will be connected by a bridge.
• Flare Support Platform- The flare boom structure to flare the excess gas from well
reservoirs may be supported on a separate structure either a tripod or four legged
jacket for safety reasons. This is to avoid excessive heat on wellhead and process
equipment on the neighboring platforms. Usually this will located away by a distance
to be calculated based on the heat output during flaring.
1.7.2 Geometrical Classification
The structural configuration of fixed template type structures vary extensively from location
to location depending on the requirement and environmental conditions such as water depth,
wave and current loads etc. Based on geometry, jackets can be classified in to following
categories.
28

Figure 1.9: Platform Complex
• Tripod - basically to support minimum facility such as few wellhead and riser or to
support a bridge between two major platforms or to support a flare boom
• 4 Legged- typically for wellhead platforms
• 6 or 8 Legged - mainly for process complex
1.7.3 Foundation Concepts
The offshore platforms shall be fixed to the seabed by means of piles either driven through the
main legs of the jacket or through skirt sleeves attached to the jacket legs or the combinations
of both main and skirt piles. This kind of arrangement is shown in the following pictures.
29

30

Chapter 2
LOADS
2.1 General
2.2 Types of Loads
Loads on offshore structures are gravity loads and environmental loads. Gravity loads are
arising from dead weight of structure and facilities either permanent or temporary. Seismic
loads are arising from gravity loads and is a derived type.
Environmental loads play a major role governing the design of offshore structures. Before
starting the design of any structure, prediction of environmental loads accurately is impor-
tant. Various environmental loads acting on the offshore platform is listed below.
• Gravity Loads
– Structural Dead Loads
– Facility Dead Loads
– Fluid Loads
– Live Loads
– Drilling Loads
• EnvironmentalLoads
– Wind Loads
– Wave Loads
– Current Loads
– Buoyancy Loads
31

2.3. Gravity Loads Chapter 2. LOADS
– Ice Loads
– Mud Loads
• Seismic Loads
2.3 Gravity Loads
2.3.1 Dead Loads
Dead loads includes the all the fixed items in the platform deck, jacket, bridge and flare
structures. It includes all primary steel structural members, secondary structural items such
as boat landing, padeyes, stiffeners, handrails, deck plating, small access platforms etc.
The primary structural steel members will be calculated based on the structural information
in the model automatically when a computer program is used to analyse the structure. But
the weight of secondary structural steel items shall be calculated applied to the structural
model at appropriate locations.
2.3.2 Facility Dead Loads
The structure built either for drilling or wellhead type platform or for process type platform
supports various equipment and facilities. These are fixed type items and not structural
components. they do not have any stiffness to offer in the global integrity of the structure
and shall not be modelled. The weight of such items shall be calculated and applied at the
appropriate locations according the plan of the structure. These items include
• Mechanical equipment
• Electrical equipment
• Piping connecting each equipment
• Electrical Cable trays
• Instrumentation items
2.3.3 Fluid Loads
The fluid loads are weight of fluid on the platform during operation. This may include all the
fluid in the equipment and piping. The weight of these items shall be calculated accurately
and applied to the correct locations.
32

2.4. Environmental Loads Chapter 2. LOADS
2.3.4 Live Loads
Live loads are defined as movable loads and will be temporary in nature. Live loads will only
be applied on areas designated for the purpose of storage either temporary or long term.
Further, the areas designed for laydown during boat transfer of materials from boat shall
also be considered as live loads.
Other live load include open areas such as walkways, access platforms, galley areas in the
living quarters, helicopter loads in the helipad, etc. These loads shall be applied in accordance
with the requirement from the operator of the platform. This load vary in nature from owner
to owner but a general guideline on the magnitude of the loads is given Table 1.1.
Sl. No Location Load (kN/m2
)
1 Storage / laydown 10
2 Walkway 5
3 Access Platform 5
4 Galley 10
Table 2.1: Live Loads
2.3.5 Drilling Loads
Drilling loads are due to drill rigs placed on top of the platform for drilling purposes. These
are large equipment assembled together and placed on top. Normally, drilling rigs are as
heavy as 500 Tonens to 1000 Tonnes. These will deliver reaction forces on the deck and the
stiffness of the drilling rigs are not considered in teh structural analysis. Hence the weight of
the structure shall be applied as load on the structure. Further, during drilling, additional
loads will be developed due to drill string and pulling operations. these loads also shall be
considered in the analysis.
2.4 Environmental Loads
2.4.1 Wind Loads
The wind speed at 10m above LAT (Lowest Astronomical Tide) is normally provided (Vo).
This wind speed shall be extrapolated to the height above for the calculation of wind speed.
The extrapolation shall be calculated as below.
V = Vo
( y
10
)1
8
(2.1)
33

where y is the elevation of point in consideration in m above LAT and V is the velocity at
that point. Wind loads shall be calculated as per API RP2A guidelines.
Sustained wind speeds(10min mean) shall be used to compute global platform wind loads
and gust wind (3 second) shall be used to compute the wind loads to design individual
members.
The wind pressure can be calculated as
fw =
ρg
2
V 2
(2.2)
where F is the wind pressure per unit area, ρ (0.01255 kN/m3
) is the density of air, g is
the gravitational acceleration (9.81 m/sec2
) and V is the wind speed in m/sec. the above
equation can be simplified by substituting the values and can be expressed as
fw = 0.6V 2
kN/m2
(2.3)
The total wind load on the platform can be calculated using the wind blockage area and the
pressure calculated as above. The shape coefficient (Cs) shall be selected as per AP RP2A
guidelines. But for the calculation of global wind load (for jacket and deck global analysis)
shape coefficient can be 1.0.
The total force on the platform can be calculated as
Fx = fwAxCs (2.4)
Fy = fwAyCs (2.5)
The exposed areas (Ax and Ay) shall be calculated as lengthxheight or widthxheight de-
pending on the axis system followed.
Wind load on oblique directions can be calculated using following relationship.
Fθ = Fxcos(θ) + Fysin(θ) (2.6)
In practical design, it is often only Fx and FY will be calculated and applied in the structural
analysis as basic laods and the wind load effect due to non-orthogonal directions are simulated
using factors in terms of Fx and Fy in the load combinations. The factors can be calculated
as
34

Y
X
θ
Fθ
Ay
Ay
A1
A2
Fy
Fx
The projected areas can be calculated as A1 = Ax cos(θ) and A2 = Ay sin(θ)
Fθ = fw (A1 + A2)) (2.7)
Fθ = fw (Axcos(θ) + Aysin(θ)) (2.8)
Fθx = fw (Axcos(θ) + Aysin(θ)) cos(θ) (2.9)
Fθy = fw (Axcos(θ) + Aysin(θ)) sin(θ) (2.10)
where Fθx and Fθy are the components of Fθ in x and y directions respectively. Ratio between
Fθx and Fx can be expressed as
Fθx
Fx
=
fw (Axcos(θ) + Aysin(θ)) cos(θ)
fwAx
(2.11)
Fθx
Fx
= cos2
(θ) + (Ay/Ax)sin(θ)cos(θ) (2.12)
Similarly, ratio between Fθy and Fy can be expressed as
Fθy
Fy
=
fw (Axcos(θ) + Aysin(θ)) sin(θ)
fwAy
(2.13)
35

Fθy
Fy
= sin2
(θ) + (Ax/Ay)sin(θ)cos(θ) (2.14)
2.4.2 Wave and Current Loads
Methodology
In applying design waves load onto the offshore structures, there are two ways of applying
it.
• Design Wave method
• Spectral Method
In design wave method, a discrete set of design waves (maximum) and associated periods
will be selected to generate loads on the structure. These loads will be used to compute the
response of the structure.
In the spectral method, a energy spectrum of the seastate for the location will be taken and
a transfer function for the response will be generated. These transfer function will be used
to compute the stesses in the structural members.
Design Wave method
The forces exerted by waves are most dominant in governing the jacket structures design
especially the foundation piles. The wave loads exerted on the jacket is applied laterally on
all members and it generates overturning moment on the structure.
Period of wind generated waves in the open sea can be in the order of 2 to 20 seconds. Theses
waves are called gravity waves and contain most part of wave energy.
Maximum wave shall be used for the design of offshore structures. The relationship between
the significant wave height(Hs) and the maximum wave height (Hmax) is
Hmax = 1.86Hs (2.15)
The above equation correspond to a computation based on 1000 waves in a record.
36

Region 1 year 100 year
Bay of Bengal 8 18
Gulf of Mexico 12 24
South China Sea 11 24
Arabian Sea 8 18
Gulf of Thailand 6 12
Persian Gulf 5 12
North Sea 14 22
Table 2.2: Maximum design waves in various regions
The design wave height for various regions is tabulated below.
API RP2A requires both 1 year and 100 year recurrence wave shall be used for the design
of jacket and piles. Appropriate combination of loads with these waves shall be used in the
design. A one-third increase in permissible stress is allowed for 100 year storm conditions.
Spectral Method
Instead of simulating the design wave environment by discrete maximum wave, a design
seastate described by energy spectrum of for the given site will be used in the load simulation.
A directional spectrum can also be used to simulate the changes design wave seastate.
Various seastate energy spectra are available for use, some of which are listed below.
• PM Spectra
• Jonswap Spectra
• ISSC Spectra
2.4.3 Current Profile
Oceans currents induce drag loading on offshore structures. These currents together with
the action of waves generate dynamic loads.
Ocean currents are classified into few types based on their nature viz, tidal current, wind
driven current and current genertaed due to ocean circulation. Wind driven currents are
small in nature and it varies linearly with depth where as tidal currents vary nonlinearly
with depth. Similarly, the currents generated due to ocean circulation will vary nonlinear
with depth and can be as much as 5 m /sec.
37

Surface
Seabed
Wind Driven Current Tidal Current
X
Y
Figure 2.1: Current Profiles
The current variation with depth is shown in Figures and can be expressed as below.
VT = VoT
(y
h
)1
7
(2.16)
where VT is the tidal current at any height from sea bed, VoT is the tidal current at the
surface, y is the distance measure in m from seabed nd h is the water depth.
VW = VoW
y
h
(2.17)
where VW is the wind driven current at any height from sea bed, VoW is the wind driven
current at the surface, y is the distance measure in m from seabed nd h is the water depth.
2.4.4 Marine Growth
Marine growth is an important part in increasing the loads on offshore structures. The
growth of marine algae increases the diameter and roughness of members which in turn
cause the wave or current loading to increase. Detailed discussion on the member roughness
and its relationship with hydrodynamic coefficients can be found in API RP2A.
The thickness of marine growth generally decrease with depth from the mean sea level and
it is maximum in the splash zone. The thickness of marine growth in the splash zone can
be as much as 20cm and will reduce below to 5cm. In deeper zones, the thickness may be
negligible.
Splash Zone is a region where the water levels fluctuate between low to high. The actual
elevation of the bottom and top of these vary from location to location due to different tidal
conditions. In general terms, the splash zone will vary from -3m to +5m.
38

In structural analysis, the increased diameter of the member (D = d + tm) shall be included
so that the wave and current loads can be calculated correctly. D and d are the diameter
of increased member and original member respectively and tm is the thickness of marine
growth.
The roughness of the marine growth is an important parameter in determining the drag and
inertia coefficients. reference shall be made relevant API RP2A clauses for more details.
2.4.5 Morison Equation
Wave and current loading can be calculated by Morison equation.
Morison equation can be written as :
FT =
1
2
CD ρw D V |V | +
πD2
4
CM ρw a (2.18)
where FT is the total force, ρw is the density of water, CD and CM are the drag and inertia
coefficients respectively, D is the diameter of the member including marine growth, V is the
velocity and a is the acceleration.
The first term in the equation is drag component (FD)and the second term is the inertia
component (FI). This can be expressed as
FT = FD + FI (2.19)
Most of the time, current exist in the same direction of the wave propagation and hence
the current shall be taken into consideration in the load calculation. However, algebraic
sum of wave and current loads is different from calculation of load by adding the horizontal
water particle velocity with the current velocity and computing the loads. This is because
of nonlinear term in the drag equation.
Current velocity shall be added vectorially with the water particle velocity before computa-
tion of drag force, i.e. V = Vw + Vc where V is the total velocity, Vw is the Velocity due to
waves and Vc is the velocity of current. This is required since there is a square term in the
drag force equation.
2.4.6 Wave-Current Interaction
The wave current interaction is an important phenomenon since the waves propagate on
the current. Both current modifies the wave and wave modifies the current exist. But the
39

former takes most priority in the calculations of wave loads. This interaction modifies the
wave parameters and modifies the wave field. Depending on the direction of current in
respect of wave direction, it either stretches the wave longer or shortens it.
If the current travels in the same direction as the wave, then the wave period becomes longer
and it is called apparent wave period (Tapp). Recommendation of API RP2A shall be used
to estimate the apparent wave period. Following simultaneous equations shall be solved to
obtain the apparent wave period.
L
T
=
L
Tapp
+ VI (2.20)
Tapp =
2πL
g tanh kh
(2.21)
VI =
4π/L
sinh kh
∫ 0
−h
Uc(z) cosh 2k(z + h)dz (2.22)
Refer to API clause 2.3.1.b (1) for more details on how to compute this apparent wave period.
2.4.7 Selection of Wave Theory
The computation of wave kinematics such as velocity and acceleration involves the equations
from wave theory. There are various kinds of solutions available depending on the accuracy
required, and parameters involved in the computation. The various wave theories are listed
below.
• Linear / Airy Wave Theory
• Stokes Wave Theory (up to 5th order approximations)
• Stream Function Wave Theory (up to 22nd order approximations)
• Cnoidal Wave Theory
Depending on the location such as deep water or shallow water and associated wave param-
eters, a suitable wave theory shall be selected for use. API RP 2A recommends to use a
chart for such selection based on d/gT2
and H/gT2
as the X and Y axis. Refer to Figure.
The wave theories discussed above are for non-breaking waves. For H/h ¿ 0.78, these theories
are not applicable as the waves tend to break. In such situation, empirical equations shall
be used to calculate the breaking wave forces on the structures.
40

Figure 2.2: Selection of wave theory for load calculation
2.4.8 Wave Load on a Member
The various steps involved in calculating loads on a member can be shown graphically. The
extract from API RP2A is shown in Figure 6.1
Morison equation is a general form and can not be applied to all members in the offshore
structure. It was developed specifically for a surface piercing cylinder like pile of a structure.
But in reality, the members of the offshore structure may be horizontal or inclined in space
and can not used without modification.
41

Figure 2.3: Procedure for Calculation Wave Plus Current Loads
Water Wave Kinematics
Airy wave theory is considered in the calculation of wave kinematics. Consider a progressive
wave with water surface elevation depicted by cosine curve,
ζ =
H
2
cos(kx − ωt) (2.23)
and the corresponding velocity potential is given by
ϕ = −
H
2
ω
k
cosh k(h + z)
sinh kh
sin(kx − ωt) (2.24)
The horizontal and vertical velocity and acceleration of water particle can be calculated
using the following equations.
Vh = −
∂ϕ
∂x
=
H
2
ω
cosh k(h + z)
sinh kh
cos(kx − ωt) (2.25)
Vv = −
∂ϕ
∂z
=
H
2
ω
sinh k(h + z)
sinh kh
sin(kx − ωt) (2.26)
42

ah =
∂Vh
∂t
=
H
2
ω2 cosh k(h + z)
sinh kh
sin(kx − ωt) (2.27)
av =
∂Vv
∂t
= −
H
2
ω2 sinh k(h + z)
sinh kh
cos(kx − ωt) (2.28)
where k is the wave number defined by 2π/L, ω is the wave circular frequency defined by
2π/T, L is the wave length, and x is the distance of the point in consideration from origin.
Mean Sea Level (M.S.L)
Highest Astronomical Tide (HAT)
Lowest Astronomical Tide (LAT)
h
x
y
Direction of Wave Propagation
Figure 2.4: Wave Loads on Jacket Structure
Maximum Load on a vertical member
Consider a case of a surface piercing cylinder such as pile of a structure or a leg of a jacket,
the combined drag and inertia force (total force) varies with time and will be maximum only
43

at one occasion. In order find the maximum force, phase angle at which the maximum force
occurs shall be found first.
Let us express the total force on the pile by substituting the velocity and acceleration com-
ponents and integrating between the limits (from surface to seabed, i.e., 0 to -h),
FT =
1
2
CD ρ D
π2
H2
T2
cos θ| cos θ|
sinh2
kh
[
sinh(2kh)
4k
+
h
2
]
−CM ρ
πD2
4
2π2
H
T2
sin θ
k
(2.29)
The total force will be maximum when,
∂FT
∂θ
= 0 (2.30)
Substituting the values of velocity and acceleration components in to the drag and inertia
force equation and differentiating with respect to θ and rearranging the terms, we get
θmax = cos−1
[
−
πD
H
CM
CD
2 sinh2
kh
(sinh 2kh + 2kh)
]
(2.31)
Maximum Load on a horizontal member
Consider a case of a horizontal cylinder such as brace of a jacket, the combined drag and
inertia force (total force) varies with time and will be maximum only at one occasion. In
order find the maximum force, phase angle at which the maximum force occurs shall be
found first.
Let us express the total force on the pile by substituting the velocity and acceleration,
FT =
1
2
CD ρ D
H2
ω2
4
cos θ| cos θ|
[
cosh2
k(z + h)
sinh kh
]
−CM ρ
πD2
4
Hω2
2
sin θ
[
cosh2
k(z + h)
sinh kh
]
(2.32)
44

The total force will be maximum when,
∂FT
∂θ
= 0 (2.33)
Substituting the values of velocity and acceleration components in to the drag and inertia
force equation and differentiating with respect to θ and rearranging the terms, we get
θmax = sin−1
[
−
πD
2H
CM
CD
sinh kh
cosh k(h + z)
]
(2.34)
Maximum Load on a inclined member
The resultant force on a arbitrarily oriented circular cylinder in water waves can be calculated
using vector analysis combined with Morison equation.
The resultant force on a cylinder in general has component normal to the cylinder axis Fn
and a component along the axis of the cylinder (a tangential component) Ft. Thus, the total
force per unit length of the cylinder can be written as
⃗
F = ⃗
Fn + ⃗
Ft (2.35)
Each of these components can be expressed as functions of the fluid particle motions by
using Morison’s equation. The force in normal direction can be expressed as
⃗
Fn = ⃗
Fn
D + ⃗
Fn
I (2.36)
where Fn
D and Fn
I are the drag and inertia forces respectively. These forces can be expressed
as
⃗
Fn
D =
1
2
Cn
DDρ ⃗
Vn| ⃗
Vn| (2.37)
⃗
Fn
I =
1
4
πCn
M ID2
ρ ⃗
an (2.38)
where
45

Cn
D = Drag coefficient for flow normal to the cylinder
Cn
M = Inertia coefficient for flow normal to the cylinder
D = Diameter of cylinder
ρ = Density of seawater
⃗
Vn = Velocity of fluid particle normal to the cylinder axis
⃗
an = Acceleration of fluid particle normal to the cylinder axis
In the tangential direction, only a skin friction drag term exists since inertial component
along the member axis does not exist unless a axial inertia coefficient is specified. Hence the
equation for tangential force can be written as
⃗
Ft = ⃗
Ft
D (2.39)
⃗
Ft
D =
1
2
Ct
DDρ⃗
Vt|⃗
Vt| (2.40)
where
Cn
D = Drag coefficient for flow tangential to the cylinder
⃗
Vt = Velocity of fluid particle tangential to the cylinder axis
These forces can be summed and expressed in terms of cylinder local axis as below.
⃗
Fx =
1
2
Ct
DDρ⃗
Vt|⃗
Vt| (2.41)
⃗
Fy =
1
2
Cn
DDρ ⃗
Vn| ⃗
Vy| +
1
4
πCn
M ID2
ρ⃗
ay (2.42)
⃗
Fz =
1
2
Cn
DDρ ⃗
Vn| ⃗
Vz| +
1
4
πCn
M ID2
ρ⃗
az (2.43)
The maximum forces can only be found numerically by calculating the forces for one wave
cycle.
2.4.9 Maximum Global Loads
Maximum global loads on a platform can be calculated using two principles.
• Maximum Base Shear Method
46

• Maximum Overturning Moment Method
It is important that the wave loads on the structure be checked for both conditions. The
maximum overturning moment method will give more pile loads than the other. Similarly,
the maximum base shear method may govern the design of some jacket leg members near
seabed due to high shear.
Maximum Base Shear
Maximum base shear or maximum total force on a structure has to be determined for the
global analysis of structures. As the wave propagates across structure wave force on each
member is different and all the locations will not be attaining the maximum forces. To find
the maximum total force a structure, following steps need to be considered.
• Position the wave crest at the origin of the structure as shown in Figure.
• Divide one wave cycle into number of segments either in terms of θ or in terms of
length.
• Compute the wave forces on all members at that instant of time using water wave
velocities and accelerations computed.
• Sum up the forces in horizontal direction for all the members.
• Repeat the calculation in step 4 for all the points for one wave cycle.
• The maximum of all the total forces computed in step 5 is the maximum base shear
or total force.
Maximum Overturning moment
Maximum overturning moment on a structure can be determined following the procedure for
the maximum base shear case. In this case, the loads on the members shall be multiplied by
the lever arm from mud-line. This shall be summed up and the procedure shall be repeated
for all the steps in the wave.
2.4.10 Buoyancy Loads
The offshore structural members mostly made buoyant by air tight sealing of the welds
to avoid water entry. This is purposely planned so that the overall structure will have
adequate buoyancy during installation. Typical example is the jacket structure. This kind
47

of structure requires at least a reserve buoyancy of 10% to 15%. The reserve buoyancy is
defined as buoyancy in excess of its weight. To obtain this buoyancy, structural tubular
members are carefully selected such that their buoyancy / weight ratio is greater than 1.0.
This means that the member will float in water. On other hand, if the member is part of a
structure supported at its two ends and forced to be submerged by weight of other members,
this member will experience a upward force equal to the displaced volume of water. This is
called buoyancy force. The buoyancy force can be calculated by two methods.
• Marine Method
• Rational Method
Water Surface
Buoyancy by
Rational Method
Selfweight including
buoyancy (Marine Method) Self Weight alone
Figure 2.5: Buoyancy Calculation methods
The marine method assumes that the member in consideration considered to have rigid body
motion. This means that the weight of the member is calculated using submerged density
of steel and applied to the member vertically down as an uniformly distributed load. This
buoyant weight WB of the member per unit length can be calculated as
WB =
1
4
π(D2
− (D − t)2
)(ρs − 1.025) (2.44)
where ρs is the density of steel
Unlike gravity which is a true body force acting on every particle of a body, buoyancy is the
resultant of fluid pressure acting on the surface of the body. These pressures can only act
normal to the surface.
48

The rational method takes in to account this pressure distribution on the structure, results
in a system of loads consisting of distributed loads along the members and concentrated
loads at the joints. The loads on the members are perpendicular to the member axis and in
the vertical plane containing the member. The magnitude of this distributed member load
can be expressed as
BB =
1
4
πD2
ρw cos α (2.45)
where α is the angle between the member and its projection on a horizontal plane
The joint loads consists of forces acting in the directions of all of the members meeting at the
joint. These joint forces act in a direction that would compress the corresponding members
if they acted directly on them, and have magnitude of :
PB = ρwAh (2.46)
where
A = ”displaced” area i.e. the material area for flooded members,
the enclosed area for non-flooded members
h = water depth at the end of the member being considered
2.4.11 Ice Loads
For structures located in polar regions and cold countries, ice loading shall be considered in
the design. In this regions, the ice sheets of varying thicknesses can move from one location
to other due to tide and under water current. These ices sheets when come closer and hit
the offshore structures, large impact force is experienced by the structure.
This kind of force cannot be calculated by means of analytical tools. However, based on
experimental studies, an empirical equation is available and can be used to estimate the
force Fice.
Fice = C f A (2.47)
where
fice = Crushing strength of ice vary between 1.5 MPa to 3.5 MPa
Cice = Ice force coefficient vary between 0.3 to 0.7
A = Area struck by ice (Diameter of member x ice sheet thickness)
49

F
Figure 2.6: Ice Loading on a structure
2.4.12 Mud Loads
Platforms located in the vicinity of the river mouth (shallow water platforms) may experience
the mud flow loads. The river flow brings sediment transport and nearby mud towards the
platform and may slide through the location.
Sometimes over a long period of time sediment settlement at the location of the platform
may have sloping surface and mud slides can also generate mud loads.
These loads can be calculated using
Fmud = Cmud τ D (2.48)
where
Cmud = Force Coefficient vary from 7 to 9
τ = Shear strength of soil 5 KPa to 10 kPa
D = Diameter of pile or or member)
50

2.5. Load Estimation and Distribution Chapter 2. LOADS
Stiff and Stable Soil
Soft or Unstable soil
Mud Slide
Figure 2.7: Mud Loading on a structure
2.5 Load Estimation and Distribution
The gravity loads on the topside shall be estimated with care, especially at the initial stage
of the project. During initial stage, no reliable information may be available. based on past
experience weight of various items shall be assumed. Hence the possibility of weight overrun
during the project can occur.
In order to reduce the risk of such situation and further consequences of re-design, a planned
method of estimation and control shall be introduced called ”Weight Control procedure”.
In the weight control procedure, the weight elements of an topside shall be divided into the
following categories.
• Primary Structure
• Secondary Structure
• Mechanical Equipment
• Piping & Bulks
51

2.5. Load Estimation and Distribution Chapter 2. LOADS
• Electrical Equipment
• Instrumentation
In a typical wellhead platform, the distribution or break down of total weight or payload is
shown in Table.
No Category %
1 Primary Structure 20
2 Secondary Structure 15
3 Mechanical Equipment 25
4 Piping & Bulks 25
5 Electrical Equipment 10
6 Instrumentation 5
Total 100
Table 2.3: Typical Break down of total topside loads
The weight estimation of various components will start as early as FEED stage. Structural
analysis of concepts selected will require reasonable data for loading the structure with
and subsequent analysis work. However, the equipment and other information may not be
available at so early in the project. Hence weight estimation with suitable ”contingency”
shall be used in the design. Further, during the progress of the project, design requirement
may change due to fundamental change in process techniques, equipment selection etc. This
contingency is used to allow for such unforeseen growth in the topside load. This contingency
is normally kept at higher during the initial stage and can be reduced once the information
on each item is available.
No Category FEED Basic Detail Fabrication Weighing
Design Design Stage Stage
1 Primary Structure 25 20 15 10 5
2 Secondary Structure 35 30 25 15 10
3 Mechanical Equipment 45 30 25 15 10
4 Piping & Bulks 35 30 25 15 10
5 Electrical Equipment 40 30 15 10
6 Instrumentation 35 30 25 15 10
Table 2.4: Variation of contingency through project duration
Further, the variation in weight due to mill tolerances, fabrication tolerance, welding etc
shall also be accounted for in the design. This allowances can be normally estimated and
included. The total allowance for this shall be at least 5%.
There is another allowance normally kept aside during the design is the allowance for future
expansion. This is normally decided by owner or developer of the field and identified future
expansion. This is called Future expansion allowance.
52

2.6. Load Combinations Chapter 2. LOADS
10
20
30
40
50
FEED BASIC DESIGN DETAIL DESIGN FABRICATION
DESIGN / FABRICATION STAGES
Upper bound
Lower bound
Figure 2.8: Variation of Contingency through project duration
2.6 Load Combinations
The load combinations used for adequacy checking of any offshore structure can be divided
into following four categories.
• Normal Operating Case - Maximum gravity loads arising from normal operation of
the platform with 1 year return period wave, current and wind. This case is used to
check the structure against loads during the normal operation of the platform.
• Hydro-test case - This is a case where hydro-testing and commissioning of equipment
and piping is carried out offshore. In this case, the equipment and piping will be fully
filled with sea water and maximum gravity loads will be developed. It should be noted
that not all equipment will be hydro-tested simultaneously. It will be done one by one.
This case is used to check the local strength of the deck structure.
• Extreme Storm Case - Maximum gravity loads arising from extreme case with 100
year return period storm wave, current and wind. This case is used to check the
structure due to loads during 100 year return period storm together with platform
gravity loads.
– Some platforms are designed to be unmanned and will be operation during the
storm with remote control. Typically, a wellhead platform with only very mini-
mum personnel onboard will be fully functional except that the personnel will be
evacuated during the storm.
53

– Second type of platforms are process platforms with attached living quarters where
large number of people staying on board, platform will be shutdown, and the
people will be evacuated during a severe storm.
• Pull out case - Minimum gravity loads arising from extreme case with 100 year return
period storm wave, current and wind. This case is used to check the maximum tension
loads on the piles and structural members.
• Seismic case - Maximum gravity loads with seismic loads from either strength level
earthquake or ductility level earthquake as per actual design requirement. This case
is used to check the seismic condition if the platform is located in seismically active
region.
Depending on type of platform operation such as process, welhead / drilling or living quar-
ters, the required load combinations during a normal operation or extreme storm may change.
This is illustrated in the Tables.
Normal Hydro-test Extreme Tension
No Load Category Operating case Storm Pullout Seismic
case case case case case
1 Gravity Loads
Structural Dead Loads 1.00 1.00 1.00 0.90 1.00
Mechanical Equipment 1.00 1.00 1.00 0.90 1.00
Piping & Bulks 1.00 1.00 1.00 0.90 1.00
Electrical Equipment 1.00 1.00 1.00 0.90 1.00
Instrumentation 1.00 1.00 1.00 0.90 1.00
Operating Fluids 1.00 - 1.00 - 1.00
Hydro-test Fluids 1.00 1.00 - - -
Live Loads 1.00 0.50 - - 0.50
2 Drilling Loads
Drilling Equipment 1.00 1.00 1.00 0.90 1.00
Supplies 1.00 - - - 0.50
Hook Loads 1.00 - - -
Rotary Loads - - 1.00 -
3 Environmental Loads
Operating Wind 1.00 1.00 - -
Extreme Wind - - 1.00 1.00
Operating Wave+current 1.00 1.00 - - -
Extreme Wave+Current - 1.00 1.00 -
4 Seismic Loads - - - - 1.00
Table 2.5: Load Combination for an offshore platform design based WSD
The above table is based on the Working stress method (WSD) and does not include any
contingencies. Suitable contingency shall be added to the load combination.
54

Normal Hydro-test Extreme Tension
No Load Category Operating case Storm Pullout Seismic
case case case case case
1 Gravity Loads
Structural Dead Loads 1.00 1.00 1.00 0.90 1.00
Mechanical Equipment 1.00 1.00 1.00 0.90 1.00
Piping & Bulks 1.00 1.00 1.00 0.90 1.00
Electrical Equipment 1.00 1.00 1.00 0.90 1.00
Instrumentation 1.00 1.00 1.00 0.90 1.00
Operating Fluids 1.00 - 1.00 - 1.00
Hydro-test Fluids 1.00 1.00 - - -
Live Loads 1.00 0.50 - - 0.50
2 Drilling Loads
Drilling Equipment 1.00 1.00 1.00 0.90 1.00
Supplies 1.00 - - - 0.50
Hook Loads 1.00 - - -
Rotary Loads - - 1.00 -
3 Environmental Loads
Operating Wind 1.00 1.00 - -
Extreme Wind - - 1.00 1.00
Operating Wave+current 1.00 1.00 - - -
Extreme Wave+Current - 1.00 1.00 -
4 Seismic Loads - - - - 1.00
Table 2.6: Load Combination for an offshore platform design based LRFD
The above table is based on the Load and resistance Factor method (LRFD) and does not
include any contingencies. Suitable contingency shall be added to the load combination.
55

56

Chapter 3
MATERIALS AND CORROSION
3.1 General
Selection of suitable material for the offshore structure is a very important aspect. Materials
for offshore applications include the following.
• Structural Materials
– Structural Steel - Low carbon steel used for ll kinds of structural members in the
platforms
– Structural concrete - Marine grade concrete used for construction of columns and
bases of gravity type platforms
– Cement grout - used for grouting annulus between the pile and jacket leg or skirt
• Piping Materials
– Carbon Steel used for normal process piping
– Stainless steel
– Steel Alloys such duplex steel, super duplex steel etc for transporting corrosive
fluids such crude oil etc
– Fibre Reinforced Plastics - used for sea water service such as fire water system
• Miscellaneous Materials
– Fibre Reinforced Plastics - used for splash zone protection of piles and members,
marine growth preventer etc
– Fibre Glass - used for grating on offshore platforms
– Timber - used for miscellaneous applications such as launch skid, temporary
works, mudmat, etc.
57

3.2. Structural Steel Chapter 3. MATERIALS AND CORROSION
3.2 Structural Steel
Offshore structures are generally constructed of structural steel. But these structures are
mostly either fixed template type structures and deep water complaint structures. Some
times the concrete gravity type structure are also constructed. In this chapter, the type of
steel materials and its behaviour under water is discussed.
3.2.1 Structural Member Classification
Structural members in the offshore structures can be classified into following categories based
on the criticality.
• Primary Structural Member - Primary Structural members shall include members
and components essential to the overall integrity and safety of the primary structure.
• Secondary Structural Member - Secondary Structural Steel shall include members
and components essential to the local integrity of the structure where failure of these
members will not affect the overall integrity and safety of the primary structure.
• Special Members - Special members are parts of primary members located in, or at
the vicinity of, critical load transmission areas and of stress concentration locations.
• Ancillary Members - Ancillary’ members are minor members and attachments,
which do not fall in the above categories.
Typical examples in each category is given in Table 3.1.
Category Deck Jacket
Primary Legs, main truss members, Legs, piles, braces
plate girders
Secondary stringers, floor plates Minor braces, caissons,
conductor supports
Special Nodes, lifting padeyes Nodes, padeyes
Ancillary handrails, platforms, supports walkways, bumpers and guides
Table 3.1: Typical member types in Deck and Jacket
3.2.2 Steel Grades
The four (4) grades of steel, which may be used to fabricate plate, rolled shapes, built-up
girders and beams, and tubulars, are
58

• Mild Steel (MS)
• Special Mild Steel (SMS)
• High Strength Steel (HS)
• Special High Strength Steel (SHS)
Mild Steel (MS)
a) Mild Steel (MS) plate, shapes, and plate for tubulars, shall conform to ASTM A 36,
”Specification for Structural Steel”.
b) Structural grade seamless line pipe may be used for tubular less than 508 mm O.D., for
handrails, grating supports, walkways, or as shown on the Fabrication Drawings. Line pipe
shall meet the requirements of API Specification 5L Grade B or ASTM A 53 Grade B.
c) All mild steel shall have a minimum of 250 MPa yield strength.
Special Mild Steel (SMS)
Special Mild Steel (SMS) plate shall conform to ASTM A 36, except that this steel shall be
manufactured to a fully killed, fine grain practice, grain size six (6) and finer as determined
by ASTM E 112.
High Strength Steel (HS)
a) High Strength Steel (HS) plate shall conform to the requirements of API 2H Grade 50 or
ASTM A 633 Grade C.
b) High Strength Steel (HS) for rolled shapes shall conform to the requirements of ASTM A
633 Grade D.
c) These grades of steel shall be fully killed, fine grain practice, grain size six (6) or finer as
determined by ASTM E 112.
d) The carbon equivalent index shall be for Grade 50 steels as defined by API Specification
2H.
e) High Strength Steel for tubular less than 508 mm O.D. may conform to seamless API
Specification 5L.
59

Special High Strength Steel (SHS)
• Two (2) special high strength steels may be used in the structure(s):
• ”SHS-50” shall conform to the requirements of API 2H Grade 50 or ASTM A 633 Grade
C, Normalized High Strength Low Alloy Structural Steel with API 2H supplements S-1,
S-4 and S5 applicable to either designation.
b) These materials shall be fully killed and normalized, grain size six (6) or finer as
determined by ASTM E 112.
• The steels shall meet the requirements of ASTM A 578, ”Straight Beam Ultrasonic
Examination of Plain and Clad Steel Plates for Special Applications” and ASTM A770.
For ultrasonic examination acceptance level II shall be used and the entire plate shall
be scanned. Supplementary requirements S1 and S4 shall apply.
• The maximum carbon equivalent shall be as defined by API Specification 2H for Grade
50 steels.
3.2.3 Manufacturing
Materials shall be produced by a manufacturing process which includes ladle vacuum de-
gassing, calcium argon stirring, or other suitable techniques which shall result in removal or
shape control of the complex oxy-sulfides. The supplier shall provide a guarantee and sub-
stantial proof that the required properties and weldability can be achieved with the proposed
process. Melting by the open hearth process shall not be allowed.
3.2.4 Chemical Requirements
The maximum allowable sulfur content shall be 0.006 percent. Rare earth metal (REM)
additions shall not be permitted. This approval shall be contingent upon a review of the
details concerning the type and amount of REM additions, how and at what point in the
melting - ladle degassing process are the REM added, the sulfur level and the extent of
degassing prior to the REM addition, and the method used for analysis and control.
3.2.5 Carbon Equivalent (CE)
CE = C +
Mn
6
+
Cr + Mo + V
5
+
Ni + Cu
15
(3.1)
60

3.3. Structural Steel Specifications Chapter 3. MATERIALS AND CORROSION
3.2.6 Mechanical Requirements
For grade SHS-50, the minimum yield strength shall be 345 MPa and the minimum tensile
strength shall be 483 MPa for plates through 100mm in thickness.
3.2.7 Notch Toughness Requirements
The Charpy V-notch impact test shall be used for all high strength ASTM materials for
notch toughness requirements. The test shall be conducted and acceptance criteria shall be
in accordance with API Specification 2H.
3.2.8 Supplementary Requirements
3.3 Structural Steel Specifications
3.4 Corrosion
Corrosion of metals is a electro-chemical process of loss of material from one location. This
may get deposited in the same metal in another area. Corrosion in a wet environment attacks
the metal by removing the atoms on the metal surface. The metal atoms at the surface lose
electrons and become actively charged ions that leave the metal and enter the wet electrolyte.
The metal ions join with/to oppositely charged ions from another chemical and form a new,
stable compound.
3.4.1 Seawater Corrosion
Corrosion in a wet environment attacks the metal by removing the atoms on the metal
surface. The metal atoms at the surface lose electrons and become actively charged ions
that leave the metal and enter the wet electrolyte. The metal ions join with/to oppositely
charged ions from another chemical and form a new, stable compound.
Corrosion requires energy. During corrosion the reacting components go from a higher to
a lower energy state and release the energy needed for the reaction. In the dry corrosion
process of Figure 1 the metal and the oxygen combine to produce the oxide on the surface
because the reaction leads to a compound (the oxide) at a lower energy level.
The oxide layer shields the metal from the oxygen and forms a barrier. The oxide will not
react with the oxygen in the air or the metal. The barrier makes it difficult for oxygen in
61

3.4. Corrosion Chapter 3. MATERIALS AND CORROSION
the air to contact the metal and it eventually grows so thick that the movement of electrons
and ions across it stop. Provided the oxide layer does not crack, or is not removed, the
metal is protected from further corrosion. But if the bare metal is exposed to the oxygen,
it will again react to form the oxide. In this case the presence of oxygen benefits the metals
protection. Removal of the oxygen removes the metals ability to create its own protective
corrosion barrier.
Corrosion of steel in marine environmental is basically a electro-chemical reaction. This is
caused by flow of electrons from one location to another location results in recombined metal
or rust deposited in the receiving location called Cathode. The location from where the
metal is being taken away is called Anode.
The essential requirement of such metal transfer to occur is closed circuit between the two
locations. This is provided by the surrounding seawater which is a best electrolyte. The
presence of chloride, accelerates the process of corrosion.
This process is schematically showing in Figure 3.7.
In the above process the electrons from the corroding anode metal move to the connected
cathode where they recombine with the atoms of oxygen and water in the electrolyte to make
a new hydroxyl ion (OH-). This new negatively charged ion then reacts to make a stable
compound with the positively charged metal ions (F2
+) that originally lost the electrons.
In this case, the electrons have a continuous pathway to escape the parent metal and the
parent metal, which cannot develop a protective barrier, disassociates or falls apart. Once
corrosion starts it continues until the ingredients are all used up.
3.4.2 Factors influencing corrosion
The factors influencing or accelerating the sea water corrosion process are listed below.
• PH value of sea water- The electrolyte in wet corrosion can be neutral, acidic or
alkaline. For corrosion in near neutral solutions (pH 6 8) under oxygenated conditions
the predominant cathodic reaction is the oxygen absorption reaction (O2 + 2H2O +
4e- = 4OH-) shown in Figure 1. If instead the bimetallic cell has no oxygen present
in the electrolyte the hydrogen evolution reaction (H+ + e- = H followed by H +
H = H2 gas) becomes the cathodic process and the anode continues to corrode. This
reaction is a much slower reaction (the H+ ion has a very low concentration in solution)
than the oxygen absorbing reaction. In acidic solutions (pH 0 - 6) the hydrogen ion
concentration is higher and the hydrogen evolution reaction is the predominant one.
Corrosion rates become extreme as the pH drops (acid gets stronger).
• Salt Content - The presence of chloride in sea water increase the chemical reaction
that takes place during the corrosion process.
62

Cathode
Cathode
Anode
Electrolyte - Sea Water
Corroding
Metal
-
Fe
2e−
2e−
Presence of Oxygen
H2O
O2
2H2O + O2 + 4e−
→ 4OH−
Fe2+
+ OH−
→ Fe(OH)2
Fe → Fe2+
+ 2e−
→ Rust
Figure 3.1: Schematic showing the sea water corrosion process
• Temperature of water - The effect of temperature on corrosion rate is very impor-
tant. The rate of corrosion increases as the electrochemical reaction gets faster in high
temperature.
• Velocity of flow - The rate of corrosion is higher in water circulating or flowing
through than in the stagnant water. During corrosion, ions build up immediately
around the anode and cathode saturating their respective regions. The corrosion rate
begins to fall due to the concentration of stagnant ions blocking the creation of more
ions in the electrolyte. If the ions are removed or more voltage is provided the corrosion
rate again picks up. If you want fast corrosion then agitate the electrolyte and add
oxygen.
• Presence of Oxygen - Presence of oxygen is main cause of corrosion process to
start. The corrosion reaction requires oxygen and where oxygen is present the metal is
cathodic and where oxygen is depleted the metal is anodic and corrodes. The parts of
63

the metal in contact with the highest oxygen concentration become cathodic and are
protected, and the areas where oxygen concentration is low will corrode. Steel posts
dug into the ground will rust just below the surface because of this effect.
3.4.3 types of corrosion
• General / Uniform Corrosion - this is basically a global corrosion occurring uniform
over the exposed surfaces of the metal. Typical example of uniform corrosion is the
corrosion of offshore structures in sea water.
• Localised Corrosion - Some of the localised corrosion effects are listed with brief
description of hot it happens.
– Pitting Corrosion - This type of corrosion happen due to presence of some kind
of opening in the metal surface such as fatigue cracks. In pitting corrosion the
metal at the top of the pit has access to the oxygen in the air and becomes the
cathode. At the bottom of the pit oxygen is depleted and the metal becomes the
anode. The deeper the pit is the less the oxygen available at the bottom and the
corrosion rate increases.
– Crevice Corrosion - A crevice is created whenever two objects are brought to-
gether. Unless they are perfectly flat a crevice is present and oxygen cannot easily
enter the gap but is plentiful outside. Corrosion starts in the crevice because of
differential aeration.
– Stress Corrosion - Metal under tensile stresses can corrode at higher rates than
normally expected. The stressed areas have changed electrical potentials to the
neighbouring metal and are also more likely to develop microscopic surface cracks.
Both situations promote increased corrosion rates.
– Galvanic Corrosion -
Galvanic corrosion needs to be watched. Dissimilar metals of different potentials
joined together by an electrolyte, like process water or rainwater, will cause the
more anodic metal to corrode. Running copper water pipe to a galvanised tank
will cause the tank to corrode very quickly. Joining copper to steel is nearly as
bad. In the galvanic series listed in Table 1 only join metals that are near each
other.
Some protection from galvanic corrosion can be achieved if the electrolyte is not
present. Without the availability of water molecules the corrosion reaction stops
because the electrons cannot find a host to complete the chemical reaction. Where
dissimilar metals must be used, for example aluminum fins on the copper coils of
a refrigeration chiller condenser, protect them from contact with water. If water
must be used in contact with dissimilar metals insure it is deionised and oxygen
free.
64

Air
High Oxygen
Low Oxygen
Sea Water
Anode
Cathode
Rust
Steel
FE2+
Steel
Air
High Oxygen
Low Oxygen
Sea Water
Anode
Cathode
Rust
Steel
FE2+
Steel
Figure 3.2: Schematic showing the pitting and crevice corrosion
A metal can corrode without being in contact with another metal. In this case different areas
of the metal take on different electrical potentials. This can occur because of variations in
the metal metallurgical properties or because of variations in the surface oxide layer, such
as a break, thinning, inclusion like mill scale, contaminant like dirt, etc.
In pitting corrosion the metal at the top of the pit has access to the oxygen in the air and
becomes the cathode. At the bottom of the pit oxygen is depleted and the metal becomes the
anode. The deeper the pit is the less the oxygen available at the bottom and the corrosion
rate increases. Figure 2 shows the mechanism of pitting corrosion.
3.4.4 Rates of corrosion
Variation of corrosion along the depth of water depends on many parameters discussed in the
previous section. However, a general trend is that the corrosion rate decrease as the depth
increase. This is due to lack of presence of oxygen and decrease in temperature. However, the
salt content increases with the depth. Hence the rates of corrosion is a complex phenomenon
changes from location to location. The corrosion rate of 0.1mm to 0.5mm per year in the
splash zone and 0.05mm to 2mm below the splash zone has been given in the literature.
In case of jacket type structures, generally the corrosion allowance in the splash zone vary
from 6mm to 12mm and for other regions below, normally no corrosion allowance is provided
since sacrificial anodes are provided to protect the steel. The longer the design life of the
structure, the corrosion allowance in the splash shall be higher. But some times, a non-
corrosive splash zone protection in the form wrapping using FRP materials are also provided
inplace of corrosion allowance.
65

3.5. Corrosion Protection Chapter 3. MATERIALS AND CORROSION
Zone 1 : Atmospheric
Zone 2 : Splash Zone
Zone 3: Tidal
Zone 4: Continuosly
Zone 5: Sub-soil
Submerged
0 0.10 0.20 0.40 0.50 0.60
Corrosion Rate mm/year
Figure 3.3: Rate of corrosion along depth
3.5 Corrosion Protection
There many ways of protecting the structures against corrosion. The possible methods are
listed below.
• Select base materials such that they have corrosion resistant property inherently.
• Providing protective coating or other means to stop the environment from attacking
the steel surface.
• Cathodic Protection by means of sacrificial anodes or impressed current system.
• Providing corrosion inhibitors to stop the corrosion process
• Providing sacrificial corrosion allowance in areas where protection by other method
does not work.
3.6 Cathodic Protection
Cathodic protection has applications in most of the offshore structures and facilities.
66

3.6. Cathodic Protection Chapter 3. MATERIALS AND CORROSION
• Offshore Platforms
• Jack-up Rigs
• Semi-Submersible Platforms
• Jetties and Wharves
• Submarine Pipelines
• FPSOs
3.6.1 Sacrificial Anode System
A schematic showing the sacrificial anode system is shown in Figure 3.4. The metal surfaces
to be protected shall be connected to a sacrificial anode. In this method, the protective
current that flow from the sacrificial anode prevents the flow of corrosive current flowing
from anode to cathode. This way the metal corrosion is stopped. Instead, the sacrificial
anode corrodes during the process.
Anode
Sacrificial
Cathode
Cathode
Anode
Protective
current
Corroding
Metal
-
Fe
Fe2+
2e−
2e−
Presence of Oxygen
H2O
O2
2H2O + O2 + 4e−
→ 4OH−
Fe2+
+ OH−
→ Fe(OH)2
Fe → Fe2+
+ 2e−
Anode
Vathode
Result → Rust
Figure 3.4: Schematic showing the sacrificial anode system
For a freely corroding mild steel in sea water has a potential difference of -0.50 to -0.60
Volts when compared with Silver/Silver Chloride reference electrode. This negative potential
67

increases further when cathodic protection is applied. This negative volts increases more than
-0.80 Volts and this indicates that the cathodic protection system is in working condition.
This method is very suitable for offshore fixed type platforms. This method does not require
any maintenance and no external resources for operation. A typical fixed offshore platform
as shown in Figure 3.5 is provided with many number of anodes distributed from mudline
to LAT. It is to be noted that the cathodic protection by means of sacrificial anodes does
not work in the splash zone due to intermittent exposure. Hence the anodes need not be
provided in the splash zone.
The amount of sacrificial anodes required to protect the structure depends on the following
parameters and shall be carefully studied.
• Seawater Resistivity, Salinity, temperature and flow velocity
• Total Surface area to be protected
• Type of Anode Material and its composition, size and shape
Among the various types of anodes used in the industry, the slender stand-off type anode
commonly selected due to its simplicity.
Anode Calculations
The calculation of number of anodes required for a jacket can be divided into three stages.
• Initial Condition
• Mean Condition
• Final Condition
Initial Condition
At initial condition, the current demand is high. The current requirement can be calculated
as
Iinitial = βi (AbDbi + AaDai) (3.2)
Rinitial =
ρ
2πl
[
ln
(
4l
ri
)
− 1
]
(3.3)
68

LAT
HAT
Splash Zone
Anode
Seabed
Conductors
Piles
Soil
Figure 3.5: Offshore platform protected with sacrificial anodes
3.6.2 Impressed Current System
69

Anode Material - Aluminum Alloy
Anode Core - Steel Tube
L
l
h
d
b
B
Cross Section
Figure 3.6: Typical details of slender stand-off sacrificial anode
Anode
Current
Cathode
Cathode
Anode
Protective
current
Corroding
Metal
-
Fe
Fe2+
2e−
2e−
Presence of Oxygen
H2O
O2
2H2O + O2 + 4e−
→ 4OH−
Fe2+
+ OH−
→ Fe(OH)2
Fe → Fe2+
+ 2e−
Anode
Vathode
Result → Rust
Impressued
DC Power Source
Figure 3.7: Schematic showing the impressed current system
70

Transformer Rectifier
Anode
Figure 3.8: Offshore platform protected with sacrificial anodes
71

72

Chapter 4
GLOBAL STRUCTURAL
ANALYSES
4.1 General
With the advancement in computer and software technology and availability of computers,
the structural analysis of structures has been made easy and fast. There are a number of
commercial computer programs available specifically coded to carry out three dimensional
structural analysis for offshore structures. Few programs are listed below.
• SACS - Structural Analysis Computer System - from Engineering Dynamics Inc. USA
• Strucad - Also from Engineering Dynamics INC. USA
• SESAM - from Det Norske Veritas, Norway
The modern day offshore development project schedules does not permit designers to carry
out hand calculations due to faster requirement of design and drawings for fabrication.
Usually, the first discipline to produce documents and drawings is structural so that the
materials can be ordered to mill for production. hence the designers are under very high
pressure from fabricators to produce the structural material take off for order placement.
The use of structural analysis programs with fast computers has made possible some of the
largest structures to be designed in 6 to 8 months of period.
The global structural analysis of offshore structure under either WSD or LRFD is still based
linear elastic frame analysis.
Following preparatory activities are required before analysis and design can be carried out.
73

4.2. Structure Geometry selection Chapter 4. GLOBAL STRUCTURAL ANALYSES
• Structure Geometry selection
• Geometry Simulation
• Foundation Simulation
• Load Simulation
4.2 Structure Geometry selection
Structure geometry shall be selected based on various requirement such as layout, water
depth, environmental condition, installation methodology and topside loads etc. A detailed
method of selection of deck and jacket geometry is discussed in detail in Chapter 5.
Figure 4.1: Computer Model of a Wellhead Jacket and Deck
74

4.3. Geometry Simulation Chapter 4. GLOBAL STRUCTURAL ANALYSES
4.3 Geometry Simulation
A geometric model of a structure contains a database of following information.
• Joints or Nodes
• Members and Properties
• Foundation
• Loads
Each of the above information can be entered in a planned and systematic way so that the
post processing and correlating the design drawings with analysis results becomes easier and
faster.
4.3.1 Axis System
Any computer model of structure require origin and coordinate system so that the structure
information such as nodes and loads can be modeled. Normally, orthogonal coordinate
system with X, Y and Z will be used. The origin of such system shall be at the geometric
center of the platform in plan at the work point and at Mean Sea Level (M.S.L) in elevation
as shown in Figure.
The axis origin at the center of the jacket helps in modeling and transformation of geometry
for further installation analyses.
4.3.2 Joints or Nodes
Joints or nodes are defined as three dimensional coordinates of junctions of members in the
space frame. Joints are work points (W.P.) of ends of members and the member shall be
formed along the centreline of the member. Unique numbering system shall be followed so
that identification of location of joint becomes easy. For example following joint numbering
can be used.
Some software programs has the ability to generate automatic joint numbering based on
initial number given by the user. Each panel or plane framing can be given with unique
numbering. This numbering schemes will help a lot during post processing of results and
identification of members or joints that require redesign.
For example, in the deck joint numbering scheme, we can split the numbering system in
to blocks and use them in a systematic way. For cellar deck, D1 to D399, and for main
75

X
Z
Y
Joint
Member
L
E
G
L
E
G
BRC
X
B
R
Figure 4.2: Computer Model of a Jacket with Axis system
No Item Description Joint Numbering
1 Jacket J1 to J999
2 Deck D1 to D999
3 Flare Boom F1 to F999
4 Bridge B1 to B999
5 Living Quarters Q1 to Q999
Table 4.1: Joint Numbering scheme
deck D400 to D799 and for upper deck D800 to D999 and so on. This way, we can easily
identify which node is lying on which deck etc. This method of modeling can only come by
experience and can be improved to produce a more user friendly model. This will definitely
help during quick processing of results and compiling the information from various analyses
for a jacket such as inplace, fatigue, seismic, loadout, transportation and launching etc.
4.3.3 Members and properties
A member in structural model is a an element (usually a beam element in terms of Finite
Element Terminology) connected between two nodes or joints. This member represents the
presence of a structural member in the analysis. The member can be defined by two joints
76

(one at each end of member) and a member group name (under which the member property
such as diameter and wall thickness can be assigned).
Similarly, the member properties can also be given in systematic way. Same member group
shall not be given to too many segments or members.
No Item Description Member group
1 Jacket
Legs Prefix with either L or J
Horizontal Braces Prefix with H
Elevation braces Prefix with V
Launch Truss Prefix with T
Boat Landing Prefix with BL
2 Deck
Cellar Deck Prefix with C
Main Deck Prefix with M
Upper Deck Prefix with U
Crane Pedestal Prefix with P
3 Flare Boom Prefix with F
4 Bridge Prefix with B
5 Living Quarters Prefix with Q
Table 4.2: Member Group scheme
the above is list in an indication of how systematic a structural model can be made and it
is upto the designer to build it the way he wanted.
4.3.4 Offsets and Eccentricities
The members connecting at one common joint can be joined to one single joint if the offset
between the work points (W.P.) is less than the API RP2A allowable distance of D/4 where D
is the diameter of the chord member at the joint. This is based some studies on tubular joints
with eccentricities showed that the additional moment generated by these eccentricities have
consequences on the member and joint adequacy. The shear capacities of tubular members
are generally high and may not be serious issue is the eccentricity is within D/4.
However, this rule shall not be applied to deck truss. The column, brace and plate girder
junction shall be modelled with additional joint even if the eccentricities are less than D/4.
This is required to model the shear transfer between the brace and plate girder through the
web of the girder. If a separate joint is not modelled, the vertical component of the braces
forces will directly taken by the column and the shear capacity of the plate girder at the
joint may be checked correctly.
77

Figure 4.3: Computer Model of a vertical elevation of jacket
4.3.5 Wish-bone Connections
The conductors are tubes inserted through series of guides in the jacket at various levels and
driven in to the seabed for at least 30 to 40m. These tubes are later used to pass through
the drill safely without damaging the jacket members. Further, these tubes prevent the
drill pipes (normally very small diameter, typically 13 to 16 inch) from failure due to large
displacement and vortex induced vibration. These conductors are not structurally connected
to jacket members but just touching the conductor guides at different elevations. Total
number of such conductors may vary from platform to platform, in some cases, there may as
much as 30 to 40 conductors of 30 inch diameters. Wave and current loading on conductors
need to be included in the structural analysis and hence they need to to modelled.
The conductors and the structural elements at the conductor guide level will be modelled
through a ”stiff element” called wish bone with only axial stiffness so that it will transfer
the lateral loads from wave and current on conductors. The length of the wish bones will be
78

Brace
B
r
a
c
e
Brac
e
Chord
e1 e2
For e1 and e2 less than D/4
For e1 and e2 greater than D/4
Common Joint
for all the braces
Separate Joint
for each brace
e1 e2
Figure 4.4: Joint offset or eccentricities in jacket tubular Joint
Brace
B
r
a
c
e
Colu
mn
Plate Girder
e1 e2
Should be modelled even if e1 and e2 are less than D/4
Separate Joint
for each brace
e1 e2
Increased web thickness
Increased Flange Thickness
Figure 4.5: Joint offset or eccentricities in deck truss Joint
very small. This is done by creating a joint with almost same coordinate of the structural
joint.
79

Conductor
Support Braces
Conductor Guide
Anulus Gap
Wish bone
Figure 4.6: Wish bone modelling of conductor / guide interface
4.3.6 Dummy Structure Models
Jacket structure has many appurtenances attached to it. The wave and current loading on
them shall be taken into account but the stiffness need not considered in the global analysis
of jacket. This is to simplify the load path and to understand the behaviour clearly. Further
the materials of such appurtenances may not be in line with the main structure, it is obvious
to ignore them in the global structural stiffness. Such structures are listed below.
• Boat Landing
• Barge Bumber
• Launch Runner
• Mudmat
• Anodes
• Conductors
• Risers
• J-tube
Similarly, the deck structure has many appurtenances as listed below.
• Walkways
• Stairways
80

4.4. Foundation Simulation Chapter 4. GLOBAL STRUCTURAL ANALYSES
• Platforms
• Equipment Supports
• Monorails
But these items on the deck does not have wave or current loading but may have wind loading
on them. Since the wind loading on deck is normally considered as block area, this may not
be the case. But these appurtenances need to be modelled to simulate the vertical COG.
This is required in the sea transportation analysis and seismic analysis. Refer to relevant
section for more details.
The dummy structure can be modelled in two ways. The first method is to model as a
structural member and give very high Young modulus so that the stiffness is very low. This
method will lead to large displacements and may not be correct.
The other method is to remove such members after the loads are calculated and summed
to the nearest joint. Some of the software programs have this ability to simulate dummy
structure based on this method.
4.4 Foundation Simulation
4.4.1 Pile Modelling
In an offshore structure, the piles hold them on to the sea bed. This needs to be simulated
in the structural analysis involving their inplace strength and stability. There are type of
pile system that can be used in the offshore structures.
• Main Pile
• Skirt Pile
As it can be seen from the figure that the skirt pile is always grouted with the skirt sleeve
of the jacket. But in the case of main pile, the annulus between the pile and the jacket leg
may be grouted or not grouted depending on the design water depth. Like other structural
elements of the jacket structure, pile is also a structural member and shall be modelled
according to the diameter, wall thickness and material properties. It is the load transfer
mechanism between the jacket leg and pile that requires special care in simulation of actual
load transfer.
For the case of grouted skirt piles and main piles, the model becomes much easier by simply
specifying the cross section as a ”Composite Section” containing jacket leg, pile and the
81

Jacket Brace
Jacket Leg
Pile
Wish-Bone
Structure Node
Pile Node
Pile to Deck Connection
Pile To Jacket Connection
Pile Head Node
Figure 4.7: Pile Simulation for an Offshore jacket
82

annulus filled with cement grout. The equivalent axial area, shear area and bending stiffness
can be calculated using the equivalent section concept and used in the analysis.
But for the case of main pile, this cannot be done. The pile and jacket are two parallel
members physically connected at the top of jacket by means welded connections and else
where no welding but spacers are placed inside the jacket leg to provide contact points for
load transfer. These spacers are specially located at the horizontal framing such that the
lateral loads from the wave and current can be easily transferred to the piles.
4.4.2 Soil Simulation
Piles below seabed shall be modelled in the structural analysis to reflect the vertical and
lateral behaviour of pile soil system. This is very essential to simulate the jacket and deck
deflections and pile stresses. This can be done in three ways.
• Equivalent Pile Stub -
• P-Y, T-Z and Q-Z Curves -
• Linearised Pile Stiffness Matrix -
4.4.3 Pile Group Effect
The skirt piles for very large jackets normally arranged in cluster at each corner to resist the
forces from gravity and environmental loads.
These pile clusters can be arranged in various ways but due to construction limitations
usually they will arranged in closed manner as shown in the Figures 4.10 and ??. The
distance between the jacket leg and the farthest pile shall be kept to a minimum possible for
fabrication to avoid unnecessary bending on jacket legs as well on the pile sleeves.
It is a good practice to space the centre to centre of adjacent piles at a distance of 3D
where D is the diameter of the pile. This will prove a clear distance between the pile face of
2D.Even with this separation, the effect of load on one pile will affect the behaviour of the
adjacent pile. The issues to be looked into are two categories as listed below.
• Effect Axial Capacity
• Effect on P-Y, T-Z and Q-Z behaviour
83

Jacket Legs
Brace
Skirt Sleeve
Main Pile
Soil Lateral
Resistance
Figure 4.8: Pile Simulation for an Offshore jacket
84

Figure 4.9: Pile Group arrangements for 4 legged platform
Figure 4.10: Pile Group arrangements for 8 legged platform
85

4.5. Load Simulation Chapter 4. GLOBAL STRUCTURAL ANALYSES
4.5 Load Simulation
4.5.1 Dead Loads
The dead loads of primary structural members such as deck beams, braces, jacket legs and
braces, piles etc shall be calculated by the program automatically based their dimensions
and unit weight of material supplied. Hence in the simulation of geometry, the structural
members shall be modelled as close as possible to the actual arrangement including member
offsets. Otherwise, the weight of the member may not be calculated accurately. Especially
for jacket, if the offsets at the joints are not modelled, then the buoyancy will be either over
estimated or under estimated depending on the member weight to buoyancy ratio. Typical
K joint of a jacket is shown in Figure.
4.5.2 Equipment Loads
Generally, the equipment weight are manually entered based on VENDOR supplied infor-
mation. The weight of the equipment shall be distributed to the deck beams or plating
depending the load transfer method adopted for the design of the equipment skid. Equip-
ment skids may have been designed for four corner support or continuous supports. Provision
of continuous supports shall be carefully examined as it may be very difficult to achieve.
Hence these equipment loads shall be distributed to the deck beams at appropriate locations
based on equipment Center of Gravity (COG) as point loads.
Only very small equipment may be placed directly on the deck plating. This can be applied
as patch loads on the deck plating.
4.5.3 Fluid Loads
The fluid loads are based on equipment operating weight. This can also be obtained from
the equipment manufacturer. The contents of the equipment can be calculated as below.
Wfluid = WOper − Wdry (4.1)
where Woper and Wdry are the weight of equipment in operating and dry conditions. Similarly,
the contents of the piping during operation can be estimated based on length of pipes,
diameters etc. This can only be estimated very approximately and large contingency shall
be applied on this item.
86

4.5. Load Simulation Chapter 4. GLOBAL STRUCTURAL ANALYSES
These loads shall be applied as member load on the deck beams. The distribution of these
loads shall be as per the estimated overall piping COG at each deck levels.
4.5.4 Drilling Loads
Drilling equipment include rig, drill strings, mud tanks, etc. These equipment are also similar
to the other equipment described above except that the drilling rig is not an fixed equipment.
Usually, the drilling rig is designed to be used for drilling more than one well. Each platform
may have a matrix array of wells either 3x3 or 4x4 depending on the field development plan.
Hence these drilling rings will be mounted on skid beams (part of deck with raised T sections
as rails) so that the drilling rig can be moved longitudinally. Across the other direction, the
drill dig will have a skid arrangement to move. This arrangement will produce different
reaction on to the deck structure at each drilling location.
Normally, these shall be applied as point loads on the skid beams. There may be several load
cases to cover all the well positions. The complication is due to application of wind loads on
these drilling rig structure. Each time the drilling rig changes position, the wind load also
shall be applied to the corresponding load point. This will lead to several combinations in
the global inplace analysis.
4.5.5 Live Loads
The live loads shall be applied on open areas not occupied by equipment or facilities. This
can be applied as member loads,
4.5.6 Wind, Wave and Current
Wind
Wind loads are normally calculated manually and applied to deck edge usually on nodes at
the periphery. Diagonal or non-orthogonal wind load cases can be generated from loads from
orthogonal cases. For example, the loads in +X, +Y, -X and -Y will be applied manually.
The load case for 45 degree can be generated suitably using loads in +X and +Y. The load
combination factors are discussed in the earlier Chapter ”Loads”.
87

4.6. In-place Analyses Chapter 4. GLOBAL STRUCTURAL ANALYSES
Wave and Current
The wave and current shall be simulated using software contained modules. Manual cal-
culation and application of these loads will lead large errors and approximations since the
number of members are very high. However, following points shall be noted in selecting
various parameter for calculation.
• Wave and Current Directions - Normally for a 4 or eight legged jacket, at least
8 directions shall be considered. For tripod structures, at least 12 directions shall
be considered. Diagonal directions shall be selected to maximize the pile loads. For
example, if the platform geometry in plan is a square, 45 degree angle will produce
maximum pile load. But if the geometry is a rectangle, then the angle shall be the
diagonal angle joining the diagonal pile groups as shown in Figure 4.11
• Selection of suitable Wave Theory
• Hydrodynamic Coefficients
– CD and CM
– Current Blockage Factor
– Conductor Shielding
– Kinematics Factor
– Apparent Wave Period
• Simulation of Non-structural items
– Boat Landing
– Barge bumper
– Walkways
– Anodes
– launch Runner
– Skirt Sleeve Connections
– Risers and J-tubes
– Mudmat
4.6 In-place Analyses
4.6.1 Principle
In-place analysis can be carried out in two ways.
88

X
Y
L
L
Single Pile or Group
0o
180o
90o
270o
225o
315o
45o
135o
X
Y
B
L
θ2
θ1
Single Pile or Group
0o
180o
90o
270o
180 + θ1
180 + θ1 + θ2
360 − θ1 + θ2
360 − θ1
θ1
θ1 + θ2 180 − θ1 − θ2
180 − θ1
θ1 = tan B
L θ2 = 180 − 2θ1
Figure 4.11: Wave attack angles for platform with square and rectangular base
• Pseudo Static Analysis
• Wave Response Analysis
The pseudo static in-place analysis can be carried out as below.
• Geometry Simulation - Simulate the geometry of the jacket as per the drawings and
Basis of Design (B.O.D)
• Load Simulation - Simulate the gravity and environmental loads as per weight control
report and meta-ocean report.
• Modal Analysis - Evaluate the dynamic characteristics of the deck/jacket system
and determine the dynamic amplification factor (DAF).
• Foundation Simulation - Simulate the foundation pile and soil as per the geo-technical
report
• Pile/Soil Interaction - Carry out static analysis with pile/soil interaction
• Post processing of results. - Post process the results to obtain member forces, joint
deflections, pile loads, etc.
The wave response in-place analysis can be carried out as per the steps below.
• Geometry Simulation - Simulate the geometry of the jacket as per the drawings and
Basis of Design (B.O.D)
• Load Simulation - Simulate the gravity loads as per the weight control report and
generate results of the analysis.
89

• Foundation Simulation - Simulate the foundation pile and soil as per the geo-technical
report
• Modal Analysis - Establish the dynamic characteristics of the deck/jacket system
and generate modal masses, modal frequencies and mode shapes
• Wave Response Analysis -Carry out time series wave response analysis together
with the pile/soil interaction.
• Combine Gravity and Wave Response - Combine the results of the gravity static
analysis due to gravity loads and wave response analysis with appropriate load combi-
nation factors.
• Post processing of results. - Post process the results to obtain member forces, joint
deflections, pile loads, etc.
The analysis will be performed on a 3D space-frame computer model representing the jacket.
All primary structural members, caissons and appurtenances will be modelled.
4.6.2 Geometry Simulation
A 3-dimensional rigid spaced frame structural computer model with all members contributing
to its stiffness will be generated. This consist of:
• Legs - Jacket legs normally consists of normal sections and can section. Member
between two adjacent horizontal frames shall be defined using member segments rather
than additional joints. Care shall be taken to provide correct length of the CAN at
the joints.
• Braces - Brace members shall be modelled with offsets from the jacket legs. Any
eccentricities more than D/4 shall be modelled by adding additional joints.
•
•
•
Simplified Topside Model
The primary frame members within the integrated deck North (Module M1 and South,
Module M2) will be included in the sub-structure models in order to correctly represent the
90

stiffness of the deck and the load transfer from the topsides to the jacket. The other struc-
tures, living quarters, drilling rig, drilling support module and vent boom will be explicitly
modelled. The loads on these modules and structures will be applied accurately within the
structure itself to reflect the actual COG of the loaded structure. Stiff members to appropri-
ate locations within the integrated deck model will connect the individual structural units
for the representation of load transfer and stiffness simulation.
4.6.3 Load Simulation
Dead Load
Program SACS using element areas and densities computes the dead weight of all jacket
and topside structural elements. The weight of non-modelled components, such as leg di-
aphragms, pile sleeve guides and appurtenance steel will be input as additional member
or joint loads, at appropriate points of application on the structure. The non-structural
appurtenance dead weights will be applied as point loads at their points of attachment to
the jacket. Where it is impractical to input these as individual point load, to reflect the
best current estimate of the structure dead weight, the member densities will be factored
up to include the weight of those items. These include miscellaneous pipes, joint and ring
stiffeners, conductor casing program, etc. Upper and lower values of jacket weight will be
evaluated for use in the load combinations. The lower value of jacket weight will be the base
weight. This will be used to check for maximum tension uplift force in the piles. A factor of
1.00 shall be used. The upper value of the jacket weight will include all design allowances.
Wave and Current Loading
Drag and inertia forces on individual members will be calculated using Morison’s Equation.
Shielding or interaction effects within the structure will be considered.
The water particle velocities and accelerations for the design waves will be computed using
Stream Function Theory in accordance with API RP 2A. For any given position along the
wave profile, the specified current velocity profile will be from seabed to the free water
surface. Current and wave directions will always be assumed parallel and of the same sense;
resultant particle velocities being the vector sum of these components.
For the initial stage of the analysis, the structure will be modelled using a combination of
tubular structural and wave force elements. Structural elements are those elements that
attract wave forces and contribute to the stiffness of the structure. Wave force elements
attract wave force but are considered to possess zero stiffness. Wave and current forces will
be computed using Morison’s equation.
Pile sleeve bottle sections will be modelled as tubular members with effective section prop-
91

erties equal to the pile pitch circle diameter plus sleeve diameter. Pile stick-up of 9.0 m will
be allowed on all skirt sleeves. Elements with attachments will have wave loading calculated
based on the nominal member section with modified Cm and Cd values.
Similarly, where two or more members are combined into one for the purposes of the wave
load analysis, the hydrodynamic coefficients will be modified to give the correct total drag
and inertia response.
Drag and inertia coefficients for non-tubular and/or complex geometry will be calculated
using an equivalent diameter. The equivalent diameter will be based on the circumscribing
circle.
In the calculation of all effective drag and inertia coefficients, the increase in diameter due to
marine growth of both the true structural members and the equivalent wave force members
will be taken into account where appropriate.
4.6.4 Topsides Load COG Shift Envelope
The topsides operating weight and COG, including all allowances and factors will be taken
from the latest topside model available at the time of the analysis and used in the jacket
in-place analysis. This, by implication implies, that the values used in the detailed design
analyses may well differ later on when topside detail engineering becomes more advanced and
information more accurate. The load contingency percentages are included to take care of
possible weight growth. The load to be used in the Detailed Design analysis will be applied
as described below.
An envelope within which the topsides centre of gravity is shifted will be included in the
in-place analysis. A load case shifting the COG to each of the four extreme corners of the
envelope as well as in between the extreme corners will be included in order to maximise leg
loads/frame loads. A COG shift of 1.0 m between north and south, 1.5 m east to west will
be envisaged (rectangle box).
4.6.5 Minimum and Maximum Water Depth
4.6.6 Maximum Wave Loads
• Maximum Base Shear
• Maximum Overturning Moment
92

4.7. Dynamic Analysis Chapter 4. GLOBAL STRUCTURAL ANALYSES
4.6.7 Allowable Stresses
4.7 Dynamic Analysis
4.7.1 Principle
Basically, the dynamic analysis is carried out to determine the natural periods, mode shapes
etc for further use in the seismic analysis using Response Spectra Method. Further, these
mode shapes and and periods will be used for the calulation of Dynamic Amplification Factor
(DAF) for both inplace storm analysis and fatigue analysis.
4.7.2 Geometry Simulation
The stiffness model for the jacket will be developed from the in-place model with the specific
modifications detailed below:
The model incorporates plan framing and vertical trusses, providing overall stiffness simula-
tion.
Additional joints required for mass modelling are provided at the centre of gravity positions
of the drilling, drilling support midule, LQ and other deck equipment for mass modelling.
These joints will be connected by triangulated (pyramid form) stiff links to the deck model.
Stiff links will be created by applying large section properties to the link members, however
densities should be set to zero to ensure that there are no effects due to mass of these
members.
” The jacket model used in the natural frequency analysis is similar to the one used in
the in-place analyses. Modifications will comprise alteration to the Foundation Model as
described below, preparation of a complete mass model and any alterations to appurtenance
simulation necessary to minimise the number of potential spurious local vibration modes.
” Stiffness matrices developed for extreme storm condition, corresponding to the 100-year
wave height will be used to generate the linearized foundation stiffness.
4.7.3 Load Simulation
The mass model comprises the structural mass, water added mass, contained mass and
marine growth. Added mass is the mass of water assumed to move in unison with the
structural member as it deflects. For tubular, a value of mass numerically equal to the
mass of water displaced by the submerged member is used including marine growth where
93

applicable. Contained mass is the water contained or enclosed by the submerged members.
In this analysis the water depth is taken at mean sea level to be the fatigue design water
depth (maximum still water depth). All members below this depth will therefore have an
added mass value. The four corner legs will be flooded to top of leg, while all other members
will be considered non-flooded.
Pile sleeves, their added masses and contained grout mass will be computed manually and
lumped proportionately at the nodes of the members defining the elements.
Computer Generated Mass
The SACS Program will internally compute the structural mass, water added mass, flooded
mass and mass of marine growth for all active members (only) of the structural model.
Members will be temporarily designated active or inactive depending on whether computer
mass generation or direct input mass is required.
For steel mass calculation, active members of the jacket will consist of elements in the corner
legs, plan levels and frames. Additional jacket mass due to the nodes will be generated by
SACS based on variable sectional properties input for each member so that the total ”stick
mass” and node mass add up to the overall steel mass of the jacket. SACS will also calculate
the water added mass of each active structural member and the mass of marine growth on
it. Each mass will be calculated for a structural member and assigned equally to the joints
bounding the member.
Direct Input Mass
The masses that are not generated by program will be calculated by hand and allocated
to their appropriate centre of gravity positions at each level. Masses will be based on the
weight reports for jackets and topsides and will include the following:
• Deck Structure
• Caissons and Supports
• Anodes
• M&E piping
• Walkways
• Pile sleeves, pile and grout
• Entrained water and marine growth masses not generated by the program.
94

4.7.4 Foundation Simulation
The dynamic analysis cannot be performed together with the iterative nature of soil-pile
interaction.
The non-linear soil-pile system of the jacket foundation is replaced by linear foundation
model obtained through Pile Soil Interaction (PSI) analysis of the SACS Program. The
foundation model will comprise a 6 x 6 stiffness matrix, representing each pile to provide a
linear elastic approximation to the soil stiffness.
4.7.5 Solution
The soultion to the following equation will give the eigen values and eigen vectors.
[M]X + [K]X = 0 (4.2)
An eigen value analysis will be performed for the jacket model using computer program.
The program uses the standard Householder-Guyans extraction technique in solving for the
eigen values and eigen vectors of the reduced sets of equations for the model.
The consistent mass approach is adopted to generate the mass matrix. The program first
assembles the overall stiffness and mass matrices corresponding to all degrees of freedom
of the model. Subsequently, both the stiffness and mass matrices are reduced to have only
master or retained degrees of freedom using Guyan reduction procedure.
Matrix decomposition and solution yields eigen values and eigen vectors from which the
natural periods are extracted and the mode shapes of the structure plotted. At least twenty-
five modes will be considered in this analysis.
Dynamic Amplification factor
From the structural periods derived, the ratio of structural period to wave period will be
computed for the relevant waves.
The dynamic amplification factors (DAF’S) will then be derived using the relationship for a
single degree-of-freedom system, i.e.
DAF =
1
√[(
1 − T2
n
T2
)2
+
(
2ζT2
n
T2
)2
] (4.3)
95

4.8. Seismic Analysis Chapter 4. GLOBAL STRUCTURAL ANALYSES
where
T = Wave Period
Tn = Natural period of structure (first mode)
ζ = Damping factor (5% for steel structures in water)
These DAF’s will be used in the fatigue analyses and in-place analyses if necessary, when
the natural period exceeds three seconds.
4.8 Seismic Analysis
4.9 Fatigue Analysis
Fatigue analysis can be carried out using the following two methods.
• Deterministic Method - In the deterministic method, the seastate energy is is sim-
ulated using discrete frequencies and wave heights with corresponding number of oc-
currences. Structural responses and hot spot stresses are generated for each of these
discrete waves. The summation of fatigue damages due to these discrete wave load
cases are then summed up to obtain the total damage during the life of the structure.
• Spectral Method- Spectral method uses the seastate energy spectra us used to gen-
erate the transfer function for the structural response. This transfer function is then
used to generate the hot spot stresses in the joints.
4.9.1 Deterministic Fatigue Analysis
Deterministic Fatigue analysis of jacket involves the following steps.
• Establish a wave scatter diagram for the field location including ranges of wave height,
wave period and occurrences
• Simulation of jacket structure stiffness and deck stiffness accurately and make the
model simple enough to understand the behaviour.
• Simulate deck and jacket mass accurately including vertical COG to determine the
dynamic characteristics of the platform
• Establish Dynamic amplification factors (DAF) either using discrete approximate method
or using wave response analysis
96

4.9. Fatigue Analysis Chapter 4. GLOBAL STRUCTURAL ANALYSES
• Select appropriate wave theory and generate wave loading on all the members
• Compute stress range using appropriate method
• Determine Stress Concentration factors for all the joints using suitable empirical equa-
tions
• Establish Hot Spot stresses for the tubular joints
• Select suitable S-N curve for the joint configuration
• Determine Fatigue damage due to all the wave load cases and sum up to obtain the
total fatigue damage
4.9.2 Wave Scatter Data
The wave scatter data contains the sea state information such as wave height, wave period
and their distribution over a certain period of time. This information may be available for a
1 year period based on measurements at the site or may be based on numerical simulation
validated with bench mark measurements. These data will give a indication of each set of
waves with (definitive height and period) will occur how many times in a given period of
time. Further, this will also define the direction of approach to the platform. Normally, at
least 8 direction sectors (45 degrees each) shall be used for fatigue analysis.
A typical wave scatter data for the middle east (Persian Gulf)is given in Table 4.3.
97

31536000
N
NE
E
SE
S
SW
W
NW
Total
T
assoc
H
s
H
max
DAF
Number
of
225
180
135
90
45
0
315
270
sec
m
m
Occurances
(Nc)
10.4
5.9
6.4
7.6
9.6
8.5
33.6
18
100
%
%
%
%
%
%
%
%
%
2.53
0.25
0.47
1.000
69
89363
50697
54993
65304
82489
73037
288713
154668
859264
2.92
0.50
0.93
1.000
39
43790
24842
26948
32000
40421
35790
141475
75790
421056
19.3%
3.75
0.20
0.37
1.000
79
69158
39234
42559
50538
63838
56523
223433
119696
664979
3.75
0.70
1.30
1.000
97
84915
48173
52256
62053
78383
69402
274342
146969
816493
3.75
1.20
2.23
1.000
52
45522
25825
28013
33266
42020
37205
147070
78787
437708
28.9%
5.02
0.20
0.37
1.000
64
41843
23738
25749
30577
38624
34198
135184
72420
402333
5.02
0.70
1.30
1.000
100
65379
37090
40233
47777
60350
53435
211225
113156
628645
5.02
1.20
2.23
1.000
64
41843
23738
25749
30577
38624
34198
135184
72420
402333
5.02
1.70
3.16
1.000
27
17652
10014
10863
12900
16294
14427
57031
30552
169734
5.02
2.20
4.09
1.000
9
5884
3338
3621
4300
5431
4809
19010
10184
56578
25.0%
6.29
0.20
0.37
1.000
20
10434
5919
6421
7625
9632
8528
33711
18059
100329
6.29
0.70
1.30
1.000
62
32346
18350
19905
23638
29858
26437
104503
55984
311021
6.29
1.20
2.23
1.000
65
33911
19238
20868
24781
31303
27716
109560
58693
326070
6.29
1.70
3.16
1.000
36
18782
10655
11558
13725
17337
15350
60679
32507
180593
6.29
2.20
4.09
1.000
23
11999
6807
7384
8769
11076
9807
38767
20768
115379
15.6%
7.56
0.20
0.37
1.000
9
3906
2216
2404
2855
3606
3193
12620
6761
37560
7.56
0.70
1.30
1.000
18
7813
4432
4808
5709
7212
6385
25240
13522
75120
7.56
1.20
2.23
1.000
23
9983
5663
6143
7295
9215
8159
32252
17278
95987
7.56
1.70
3.16
1.000
27
11719
6648
7212
8564
10817
9578
37861
20283
112681
7.56
2.20
4.09
1.000
18
7813
4432
4808
5709
7212
6385
25240
13522
75120
7.56
2.70
5.02
1.000
14
6076
3447
3739
4440
5609
4966
19631
10517
58427
6.9%
8.83
0.20
0.37
1.000
4
1486
843
915
1086
1372
1215
4802
2572
14292
8.83
0.70
1.30
1.000
9
3344
1897
2058
2444
3087
2733
10804
5788
32156
8.83
1.20
2.23
1.000
9
3344
1897
2058
2444
3087
2733
10804
5788
32156
8.83
1.70
3.16
1.000
9
3344
1897
2058
2444
3087
2733
10804
5788
32156
8.83
2.20
4.09
1.000
8
2973
1686
1829
2172
2744
2430
9604
5145
28583
8.83
2.70
5.02
1.000
12
4459
2530
2744
3258
4116
3644
14406
7717
42875
2.7%
10.10
0.20
0.37
1.000
2
650
369
400
475
600
531
2099
1124
6247
10.10
0.70
1.30
1.000
6
1949
1106
1199
1424
1799
1593
6297
3373
18741
10.10
1.20
2.23
1.000
7
2274
1290
1399
1662
2099
1858
7346
3936
21864
10.10
1.70
3.16
1.000
6
1949
1106
1199
1424
1799
1593
6297
3373
18741
10.10
2.20
4.09
1.000
5
1624
921
1000
1187
1499
1327
5247
2811
15617
10.10
2.70
5.02
1.000
8
2599
1474
1599
1899
2399
2124
8396
4498
24988
1.6%
Total
1000
690126
391514
424693
504323
637039
564045
2229638
1194449
6635826
100.0%
7
Tz
H
max
=
1.86
Hs
Nos.
of
Cycles
=
T
T
*N
c
*P
d
/
T
assoc
Percent
MIDDLE
EAST
-
OFFSHORE
FATIGUE
WAVE
DATA
Direction
Distribution
Percentage
(P
d
)
IN
SECONDS
=
(365*24*60*60)
T
T
=
98

It should be noted that sometime such information may be only available with significant
wave height and zero crossing periods. This shall be converted into individual maximum
wave heights and peak period using appropriate factors as shown below the table.
4.9.3 Spectral Fatigue Analysis
The spectral fatigue analysis will consider the dynamic response of the structure to a range of
wave heights covering a frequency range that includes all sea-states contributing significantly
in terms of fatigue damage. The response transfer functions and the sea-state spectra will be
used to generate stress spectra according to the well- known power spectral density approach.
Assuming that the joint stress histories conform to a Rayleigh distribution, the fatigue
damage will be calculated using Miner’s Rule.
Wave Spectra
Methods of fatigue analysis recognise the probabilistic nature of sea states. The random
sea surface can be represented in a deterministic domain by superimposing a finite number
of sinusoidal components of amplitude and phase. An energy spectral density function is
constructed from the mean square of the amplitude components for each narrow band of
frequency, f, and plotted against the appropriate wave frequency.
The two-parameter ISSC modified Pierson-Moskowitz wave spectrum will be used to repre-
sent sea surface elevation and is given by:
Sh(f) =
H2
s Tz
4π(fTz)5
exp
[
−1
π
(fTz)−4
]
(4.4)
where
Sh(f) = Single sided spectral density of wave amplitude (m2
unit frequency)
Hs = Significant wave height (m)
Tz = Zero up-crossing period (sec)
f = Wave frequency (Hz)
Wave Theory Selection
Water particle velocities and accelerations will be calculated using stream function.
In determining the water depth to use in the deterministic fatigue analysis, care will be
taken to recognise that higher water particle velocities, and hence higher local member wave
99

loads, result from shallower water depths. To this end, the Mean Sea Level (MSL) will be
used which reflects the long-term average of water depth. Current will be excluded from the
analysis.
Wave Steepness
In a fatigue analysis, the forces are assumed to be proportional to wave height. If this
assumption were always true, the selection of wave height (wave steepness) would be imma-
terial to the analysis. However, non-linear effects are present in wave force calculation due
to drag influences.
Wave steepness is defined by the ratio of wave height to wavelength. The wavelength is
obtained from the appropriate wave theory. For small amplitude deepwater waves, the
wavelength in metres is given by:
wavesteepness =
Hd
L
where L = 1.56T2
(4.5)
where T is the wave period and Hd is the design wave height.
From experience a wave steepness of 1:16 will be used for generation of wave forces.
Transfer Functions
In order to perform a fatigue analysis, the wave height spectra must be transformed into
a series of member-end hot-spot stress-amplitude response spectra at each of 8 equally-
spaced circumferential points around each end of each member for each sea state. This
transformation is accomplished by multiplying the appropriate frequency ordinate of the
spectrum for the sea state under consideration, by the square of the corresponding ordinate
of the hot spot transfer function. The transfer function ordinates are defined as the ratio of
hot spot stress amplitude at a particular member-end circumferential point divided by the
wave amplitude that generates the stress amplitude at that point as a function of frequency.
Consequently, units of the transfer function will be, for example, MP a/m of wave amplitude.
Selection of Frequencies
The inaccuracy introduced when a transfer function is used to convert a wave spectrum is
dependent on the number of frequencies for which transfer function ordinates are available.
If an infinite number of ordinates were available, the response spectrum would be ”exact”.
This, however, is obviously not practical. All that is really required is a sufficient number of
100

ordinates to describe adequately the variation of transfer function over the frequency range
of interest. For this to be done, the frequencies at which transfer function peaks and troughs
can be expected must first be anticipated. Ordinates are then computed at these points and
at other intermediate points. A minimum of 8 ordinates will be considered.
To generate a transfer function ordinate for a particular fatigue wave direction, selective
waves of various heights but constant steepness are used to load the structure. These waves
need not necessarily be the waves from the fatigue environment, but waves chosen based on
the following criteria:
Dynamics Criterion : The waves with frequencies corresponding to first three modal
frequencies will be considered.
Leg Spacing Criterion: Certain frequencies are chosen such that the corresponding wave-
lengths are approximately integral (n) or half integral (n/2) multiples of the distance between
legs. This criterion is used to locate the crests and troughs in the transfer function.
Minimum and Maximum Wave Height Criterion: Waves with a minimum height of
0.25 m and a maximum wave height obtained among the fatigue wave climate will also be
considered for generating transfer function.
Stress Amplitudes
As the wave profile should be a pure sinusoid for the response to be interpreted as a transfer
function, the Stream Function theory does result in wave profiles that are pure sinusoids.
The stress is calculated at various wave positions and the difference between the maximum
and minimum stress called the stress range is determined for each wave. Dividing these stress
ranges by one-half of the corresponding wave height produces stress ranges for waves of unit
amplitude (for sinusoidal waves, wave height equals twice the wave amplitude). The rela-
tionship between the stress ranges of unit amplitude and the corresponding wave frequency
for all waves considered is the transfer function.
4.9.4 Fatigue Life Calculation
Calculation of Stress Ranges
If for the direction being considered, the wave height spectral density Sh(f) of a particular
sea-state is known and the transfer function Hi(f) for the point can be calculated, then
the statistical cyclic stress range (RMS cyclic stress range) at that point for this particular
101

sea-state is given as:
σRMSi =
√∫ ∞
0
H2
i (f)Sh(f)df (4.6)
Cumulative Damage calculation
For every RMS stress, there exist an average time, TZ, between zero crossings with a positive
slope for a stationary Gaussian process with zero mean. This period called the zero crossing
period is given by:
Tz =
σRMSi
√∫ ∞
0
f2H2(f)Sh(f)df
(4.7)
For a narrow band process, this is the average period or the reciprocal of the average fre-
quency of the process. The expected number of cycles associated with this sea-state during
the design life of the structure is:
n(s) =
mL
Tz
(4.8)
where, L is the design life of the structure and m is the fraction of the design life that this
sea-state prevails.
For a given stress range s, the number of cycles to failure, N(s), can be found from the S-N
curve used. Then, the expected damage from the given sea-state is given as:
D =
n(s)
σ2
RMSi
∫ ∞
0
s
N(s)
exp
(
s2
σ2
RMSi
)
ds (4.9)
The total expected damage for all sea-states during the life of the structure is the sum of
the damage for each individual sea-state. The expected fatigue life is equal to the design life
divided by the expected damage.
4.9.5 S-N Curves
For a particular stress range s, there exists a theoretical number of cycles N(s) at which
fatigue failure may occur. The relationship between this number of allowable cycles and the
stress range is usually expressed as an S-N curve. The API RP 2A-WSD curve X applicable
for normal welding and curve X′
applicable for profile control welding.
102

Figure 4.12: S-N Curve [API RP2A 2000]
103

4.9.6 Stress Concentration Factors
In most tubular joints, local stresses on both the stub and chord side of the stub-to-chord
weld will be considerably higher than the nominal stub and chord stresses resulting from a
global frame analysis of the structure. The magnitudes of these stress concentrations are
dependent upon the weld profile and the joint geometry. Weld profile stress concentrations
result from notches in the weld and are difficult to quantify explicitly using full-scale test
results or finite element techniques. Consequently, they are incorporated into the empirically
derived S-N curves.
Tubular Joints
Several researchers for various unstiffened joint geometry have developed geometric stress
concentrations, but those developed by Efthymiou is preferred.
Hot spot stresses on the chord and brace sides of the chord-to-brace weld are determined by
multiplying the nominal stub stress by chord and stub stress concentration factors (SCFs)
respectively. The manner in which each SCF is computed is dependent on the details of the
geometry of the joint under consideration. The following joint configurations exist.
Unstiffened Non-overlapping Joints
SCFs will be derived from the formulae of Efthymiou for all joints. These formulae include
parameter ranges within which they are said to be valid. In many cases there is no alternative
but to use the formulae outside these ranges, but in these cases the joint will be the subject
of special consideration. Normally, SCFs will be computed for the actual parameter values,
and again using values on the limit of exceeded ranges. The maximum of those two values
will be used.
Unstiffened Overlapping Joints
SCFs will be derived from the Efthymiou’s formulae. However, overlapping joints shall be
avoided wherever possible.
iv) Internally Stiffened Joints
The SCF’s for internally stiffened joints will be calculated based on the Lloyd’s Register
formulae.
104

Cone Transitions
The geometrical SCF’s for conical transitions will be determined as below:
SCFgeom = (fa + fb + fb1)/( fa + fb)
where:
fa = Axial stress fb = Bending stress fb1 = Secondary bending stress as defined in API
RP2A Section 2.5.
Fabrications Misalignments / Thickness Transitions
The Fabrication Specification will require that all plate misalignments in excess of 10% of
the minimum thickness of the plates being joined must be profiled to a slope of 1:4 and
that smaller misalignments will be joined with a tapering weld. Thickness transitions will
be effected with a 1:4 taper on the thicker plate.
Minimum Stress Concentration Factors
In general, for tubular node joints, the minimum axial and bending SCF for all stubs and
all chords is taken as 2.0. SCFs less than 2.0 may be used only for fully backed-up leg nodes
when justified by finite element or acrylic model test results. Similarly, the minimum SCF
for stiffened joints is taken as 2.5 unless otherwise justified.
4.9.7 Foundation Linearisation
The fatigue analysis involves series of structural analysis for each direction and each wave
set. It will be very time consuming if every analysis has to be carried out with pile/soil
interaction. Further, the modal analysis cannot be carried out with pile/soil interaction.
Hence the nonlinear foundation shall be linearised and a stiffness matrix that corresponds to
six degrees of freedom at the pile head shall be used for the analyses. But to generate this
spring stiffness, a representative wave height and direction shall be selected that represents
the fatigue seastate.
The foundation spring stiffness will be derived using loads corresponding to an equivalent
wave height, He. From the wave scatter diagram, it is possible to derive a fatigue damage
scatter diagram on the basis of the following:
Di = Hbm
si
Pi
Tzi
(4.10)
105

4.10. Boat Impact Analysis Chapter 4. GLOBAL STRUCTURAL ANALYSES
where
Di = Damage of the ith sea-state
m = Slope of the S-N curve
b = Slope of log-linear wave height versus stress(Assumed = 1.80)
Hsi = Significant wave height of the ith sea-state
Tzi = Zero up-crossing period of the ith sea-state
Pi = Probability of occurrence of the ith sea-state
The above assumes a log-linear wave height versus stress relationship.
After constructing the fatigue damage scatter diagram using the above formula, its centroid
will be determined using the following equations to identify the sea-state causing the most
damage:
H̄s =
∑
DiHsi
Di
(4.11)
¯
Tz =
∑
DiTsi
Di
(4.12)
The deterministic design wave (Hd, Td) which represents the seastate at the centre of the
damage scatter diagram is usually taken to be:
Hd = 1.86H̄s (4.13)
Td = 1.27 ¯
Tz (4.14)
4.10 Boat Impact Analysis
4.10.1 Impact Vessels
In am offshore development, often service boats and supply vessels have to serve the offshore
operation. During their trips, due to harsh weather conditions, it may some time drift and
hit the the jacket legs or braces. These vessels during their normal approach to the platform
may arrive in with normal operating speed or may arrive at accidental speed depending on
the weather conditions at the time of arrival. API RP2A specifies a operating a speed of 0.5
m/sec and accidental speed of 2 m/sec.
The jacket legs and braces in the splash zone shall be designed of such loads to avoid
premature failure and collapse of the platform. Where such impacts are not allowed, a
106

properly designed boat impact guard (sacrificial) shall be provided. For example, the risers
located outside the jacket perimeter shall be protected with riser guard or riser protector
and this kind of riser guard shall be located sufficiently away (at least a 1m) so that during
vessel impact, risers does not experience large deflection.
The purpose of the boat impact analysis is:
• Normal Impact - To ensure the adequacy of the jacket leg and brace members in the
splash zone such that they can absorb the energy imparted by a design vessel traveling
at normal operating velocity.
• Accidental Impact To ensure the adequacy of the jacket leg and brace members in
splash zone such that they can absorb the energy imparted vessel traveling at accidental
velocity.
• Post Impact Strength - To ensure the compliance of the damaged platform for
operating (1-year wave) design requirements after the boat impact.
4.10.2 Principle
The general methodology for ship impact analysis involves the following three primary steps:
• Impact LocalImpact analysis to estimate the damage to the members which are
directly hit by the ship and the impact force.
• Impact Global Analysis - Static analysis to ensure that the jacket will be able to
withstand the impact force safely. In this analysis the member and joint stresses are
allowed up to yield limit as this force is temporary.
• Post Impact Analysis Static analysis to verify if the damaged platform can sustain
the 1 year operating environmental loads.
4.10.3 Method of Computing Impact Energy Dissipation
As the boat hits any member, equal and opposite forces are applied to the member and to
the boat. These forces cause the following effects:
• Local denting of the member under the point of impact, which causes a local re-
duction in the effective cross-sectional area and section modulus of the member. The
relationship between dent depth and lateral forces causing the dent is given by the
DNV curve (Furnes and Amdahl) for knife-edge contact.
107

• Bending of the member, initially elastic and subsequently, elasto-plastic until a
plastic mechanism form.
• Denting of the boat - The force/indentation curves contained in DnV TN A202
document is used for this purpose.
• Global deformation - Elastic deformation of the rest of the structure.
4.10.4 Energy dissipation by member
The total energy dissipation is computed in four stages as described below by iterating on
the dent depth.
• Stage 1 : Elastic Beam Bending - Stage 1 of the energy absorption process covers
the period from the moment of impact until the section starts yielding due to the axial
load and moments at the dented section. The impact is conservatively assumed to
occur at midspan of the member. For a given dent depth, X, the lateral force, Pd ,
causing the dent depth is calculated in accordance with the DNV curve for B/D = 0,
where, B is the central length of the dent and D is the mean diameter of the member.
The equation given below approximates the DNV curve for B/D = 0:
Pd = 15mp(D/t)(X/R) (4.15)
where
mp = is the plastic moment capacity of tube wall equal toFyt24
D = is the mean diameter of the tubular member
R = is the mean radius of the tubular member
t = is the thickness of the tubular member
X = is the dent depth
The energy absorbed, Ed , by a dent of given depth, X, is found by integration of the
force over the dent and is given as,
Ed = 14.14mp
X3/2
t
(4.16)
The lateral force, Pd, causes a change in the bending moment distribution of the
member. The moments at the ends and at the middle depend on the original moments
and the rotational stiffness of the ends of the member. The energy absorbed by the
bending deformation is given by,
Eb = 0.5Pdδ (4.17)
108

Where, δ is the lateral displacement of the member centre line. The other energy ab-
sorbing components are energy due to the longitudinal strain, global structural transi-
tional energy and ship distortion energy. Stage 1 of the energy absorption mechanism
ends when the sum of the axial and bending stresses at the centre of the tube is equal
to the plate yields stress. This point is found by iterating on the dent depth.
• Stage 2 : Elasto-Plastic Beam Bending -
The behaviour of the member in Stage 2 is similar to that in Stage 1. Stage 2 ends
at the formation of full plasticity at the dented section. By iterating on the depth of
dent until the formation of first plastic hinge, the end of Stage 2 is found. The energy
associated with the formation of the first plastic hinge is calculated in the same way
as in Stage 1.
• Stage 3 : After Formation of First Hinge
This phase of energy absorption process continues until a 3-hinge mechanism is formed.
For a given dent depth, the lateral force is found. The deflection at the centre of the
beam is calculated assuming that the dent force is carried by the two cantilevers. If
the energy absorbed at the end of Stage 3 is less than the impact energy of the ship,
then the absorption mechanism enters Stage 4.
• Stage 4 : After Formation of Three-Hinge Mechanism -
At this stage, it is assumed that no further dent growth occurs and the dent has
absorbed all the energy it is capable of. The energy absorption at this stage is due to
the triangulation effect of longitudinal strain, structure distortion and ship deformation
energy. Rupture of the brace being impacted and consequently penetration of the
boat within the jacket shall be prevented by limited the tension strain to 10%. The
displacement of the braces on gridline 1 shall be limited to prevent damage of the
conductees.
All the computations involved in the above four stages of energy absorption process are
incorporated in the IMPACT program. Starting from Stage 1, the energy computations
are continued over the four stages till the full ship impact energy is absorbed.
109

4.10.5 Energy Dissipation by Boat
4.10.6 Energy Dissipation by Overall jacket deflection
4.10.7 Structural Strength During Impact
4.10.8 Post Impact Analysis
4.10.9 On-Bottom Stability Analysis
General
Once the jacket is either launched from the barge or lifted and lowered in to the sea, it will
be made upright by either controlled upending or self upending method. Once this activity
is completed, the jacket will be resting on the soft seafloor and should not settle excessively.
Further the jacket should have sufficient sliding and over turning stability due to its self
weight, buoyancy, and wave/current loads.
Normally, piling operations can not be carried out in very harsh weather conditions. If the
weather conditions are good immediately after making the jacket up right, still it may require
considerable time (around 6 to 10 hours to set up the hammer, upend the pile etc) before
any piling operation can complete. During this period of time, the jacket on its own weight
shall resist the design installation wave forces without sinking in to the soft seabed.
In order to limit the vertical settlement of jacket to within allowable limits (usaually within
few 100 mm as there is no physical requirement or code requirement rather than just to
avoid too much sinking) a system of flat supports provided at the bottom of jacket is called
”Mudmat”. Excessive sinking of jacket may require additional leg piece (green piece) to be
welded before the installation of the deck to keep the deck level same as designed.
Figure 4.13 and 4.14 shows shows various kinds of mudmat system adoptable to different
kind of jacket geometry.
Design Wave Height
The design wave height for on-bottom stability shall be carefully selected based on seastate
conditions at the time of installation. Normally the weather forecast for at least two weeks of
the installation time shall be considered for selection of wave and current data. But during
the design stage (normally design will be done 10 to 12 months before the installation time).
The general criteria for installation wave height shall be 1 year return wave, current and
wind shall be selected for the design purposes.
110

X
Y
Mudmat Y
X
Mudmat
Figure 4.13: Mudmat arrangement for 4 legged jackets
Y
X
Mudmat
Mudmat
X
Y
Figure 4.14: Mudmat arrangment for 8 legged jackets
In some cases, reduced wave height may be permitted, if a longer weather window is per-
missible by the installation contractor.
In general, a design significant wave height of 3m and with a wave period of 6 sec is normally
used in the design. The associated current may be taken as 1 m/sec at the surface.
Factors of Safety
The factor of safety against sliding, overturning and bearing shall be as stated in Table below
as per API RP2A.
111

Load Bearing Sliding Overturning
Selfweight & Buoyancy 2.0 1.5 2.0
Welfweight & Buoyancy + Wave & Current 1.5 1.5 2
Table 4.4: Factor of Safety - On Bottom Stability
Analysis
A static analysis with the following parameters can be carried out to obtain the selfweight,
wave loads and their reactions on the mudmat for further calculation of factor of safety
against stability and mudmat design.
• The water depth for the analysis shall be selected using Mean sea level with 1 year
return storm surge.
• No Marine growth shall be used as the new installed jacket will not have any marine
growth.
• No topside loads shall be applied.
• Boundary conditions shall be applied to the jacket legs to restraint the model.
• Wave and current shall be applied to at least 8 directions around the jacket.
A jacket subjected to gravity and environmental forces is shown in Figure 4.15
Vertical Load
Overturning
Sliding
Wave
Figure 4.15: Onbottom Stability schematic diagram
112

h1
b1
H
B
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
R
R
X
X
Y
Y
Mudmat
x
z
y
Figure 4.16: Mudmat arrangement
Unpiled Stability Checks
Mudmat Design
Moment of inertia of the mudmat system at the seabed level can be written as
Ix =
b1h3
1
12
+ b1h1
(
H − h1
2
)
(4.18)
Iy =
h1b3
1
12
+ b1h1
(
B − b1
2
)
(4.19)
σj,i =
Fzi
A
−
Mxiyj
Ix
+
Myixj
Iy
(4.20)
113

114

Chapter 5
DESIGN METHODOLOGY
5.1 General
The design of offshore structure is not an single step design process. The structural config-
uration, arrangement, member sizes and its specification requirements can be arrived after
few design cycles. In order to achieve a optimum design suitable for the installation method
proposed and satisfy the final operating requirements, a design procedure suitable for the
project shall be developed.
In an offshore project, the design of structural elements cannot be initiated unless the basic
understanding of the needs are identified. The basic needs are
• What is the type of platform ? Oil or Gas, Process or Wellhead or Quarters etc
• What is floor area of topsides required ?
• Expected maximum weight of facilities ?
• What is basic water depth and environmental parameters such as wave and current ?
• Where is it located ?. Earthquake prone ?.
• What is type of installation ? Lift installed or Launch installed ?
• Any CAPEX constraints ?
The answer to the above questions will give some indication of type of jacket and topsides
required.
115

5.2. Design Stages Chapter 5. DESIGN METHODOLOGY
5.2 Design Stages
The various design stages in an offshore project is listed below.
• Front End Engineering Design (FEED) or Concept Selection
• Basic Design
• Detailed Design
5.2.1 FEED
The first step in initiating an offshore project is a FEED or concept selection. This stage of
project will involve following steps in all disciplines such as Process, Mechanical, Electrical
and Instrumentation in addition to Structural Engineering.
• Collection Process Data and identifying process needs and equipment
• Preliminary equipment sizing and area requirements
• Weight estimation based on past projects
• Identification of Structural configurations
• Preliminary estimation of structural weight
• Identification of installation methods
• Estimation of CAPEX within ±40%.
The above activities will define the project to a basic understanding and will provide enough
insight into carrying out further engineering activities.
5.2.2 Basic Design
At this stage of the project, the data collected during the FEED stage will be further verified
to make sure the authenticity and reliability of such data for further use. A detailed weight
estimates of all items involved in the project will be carried out. The process and mechanical
requirements will be further defined and identified. A Design Basis (DB) will be developed
for the proposed facility containing following information.
116

• Process information containing type of well fluid (oil or gas) and its characteristics,
safety requirements and kind of process technology to be adopted.
• Mechanical requirement such as type of facility and basic equipment required for the
process, and material handling and safety
• Electrical requirement such power generation equipment, lighting and switch gears
etc
• Instrumentation requirement such as basic control system, feedback requirement etc.
• Piping information such as pressures, pipe sizes required etc.
• Meta-Ocean information such as water depth, wave, current, wind and tidal infor-
mation at the site.
• Structural requirement such as materials proposed or available for use in the country,
design method to be adopted, codes and specifications to be used etc.
• Installation information such as type of barge, lifting crane, loadout-method, piling
hammer etc.
At the basic design stage, the deck area required, number deck levels, etc will be defined.
This will lead to identification of number of legs required to support the deck. Normally the
spacing between deck legs for a typical platform can vary from 10m to 20m beyond which it
may become uneconomical to design a braced deck truss structure.
Basic weight estimates for various disciplines such as structural, mechanical, electrical, in-
strumentation and piping will be carried out. Based on the weight of total deck, it may then
be decided to fabricate the deck in one piece or in various modules. This kind decision can
only be taken together with the viable installation options such ”Availability of Heavy Lift
vessels in the region” or use of float-over technique. In case such methods are not possible,
then the total topsides shall be divided in to various functional modules such as compres-
sion module, process module module, utility module, quarters module, etc. These modules
are self contained units with structure, piping, equipment etc fabricated and transported to
the site. These modules are then installed on top of the ”module Support Frame”, which
transfers the loads to the jacket. Some times this module support frame may not needed, if
the modules are organised properly over the legs. This kind of basic ideas shall be made at
the basic design stage.
5.2.3 Detailed Design
Detailed design of offshore platform will be initiated once the basic design confirms the
economic viability and technical feasibility.
117

Figure 5.1: Deck Installation Concepts - Integrated Deck
118

Utility Module
Living
Quarters
Wellhead Module Process Module
Module Support Frame
Drill Rig
Compression Module
Living Quarters
Process Module Utility Module
Offshore Splice
Figure 5.2: Deck Installation Concepts - Modular Deck
119

In the Detailed design or engineering of an offshore platform following items of the jacket
and deck will be developed in detail.
Deck Jacket
Framing Plans Framing Plans
Framing Elevations Framing Elevations
Joint Details Joint Details
Plate Girder Details Pile make-up
Connection Details Launch Truss Details
Welding Details Welding Details
Deck plating & grating Launch Cradle details
Stairways and Walkways Walkways
Lifting Padeyes Lifting Padeyes
Transportation tie-down Transportation Tie-down
Monorails Details Caissons and supports
Equipment Support Details Conductor guides
Flare Boom Barge Bumbers
Bridge Boat Landing
Crane Pedestal Closure Plate details
Riser Clamps
Table 5.1: Detailed Design Items
All necessary analysis required to complete the design above items shall be carried out.
Deck Jacket
Inplace Inplace
Loadout Loadout
Sea Transportation Sea Transportation
Lifting or Floatover Lifting or Launch
Miscellaneous Calculations Upending
Dynamic
Fatigue (Deterministic or Spectral)
On bottom Stability
Pile Driveability
Cathodic Protection
Ship Impact
Table 5.2: Detailed Design Analysis
120

5.3. Design of Topsides Chapter 5. DESIGN METHODOLOGY
5.3 Design of Topsides
5.3.1 Platform Geometry
Figure 5.3: Platform Geometry
121

5.4. Design of jacket Chapter 5. DESIGN METHODOLOGY
5.3.2 Deck Arrangement
5.3.3 Deck Elevations
5.3.4 Deck Leg Sizing
5.3.5 Beam/Girder Selection
5.3.6 Deck Construction method
5.3.7 Deck Appurtenances
• Stringer Beam
• Deck Plate
• Monorails
• Stiffeners
• Leg to Plate Girder connections
• Crane Pedestal Fatigue
• Crane Pedestal Manway
• Lifting Padeyes
• Spreader Frames
122

Deck Foot Print layout
Number of Deck Levels
Deck Gravity Loading
Deck Leg Spacing
Deck leg Sizing
Deck / Jacket Interface
Jacket Concept
Jacket Framing
Foundation Concept
Met-Ocean Data
Geotechnical data
Installation Equipment Data
Seismic Data
Platform Owners Requirements
Deck and jacket Structural Model
Gravity Loading estimate and applicatiom
Wind, Wave and Current parameters
Determine Ciritical Directions
Installation Methods
Preliminary Pile Dia
Deck Bracing Concept
Estimation of Deck and Jacket Weight
Estimation of Deck and Jacket Appurtenances
Determine Hydrodynamic Parameters
RUN PRELIMINARY INPLACE ANALYSIS
Final In-place Analysis
Review D/t and KL/r
Finalise Member Sizes
Check Punching Shear of Joints
Maximum Pile Loads
Pile Sizing and Penetration
Dynamic Analysis
Review Natural Period
Estimate DAF
Check Mode Shapes
Fatigue Analysis
Spectral Or Deterministic ?
Include Dynamic Effects
Installation Anlyses
Loadout, Transportation, and Lifting
Upending and Onbottom Staibility
Pile Driveability
Seismic Anlyses
API RP2A requirements
Strength Level and Ductility Level
Ship Impact Anlyses
Establish Boat Sizes
Establish Impact Locations
Riser Protecors ?
Check SCF and Joint Configs
Finalise Can Thicknesses
Design Appurtenances
Launch Cradle Lifting Padeyes
Mudmat Upending Padeys
Anodes Boat Landings
Barge Bumpers
Walkways
Closure Plates
Skirt Sleeve Connections
Riser Clamps Condutcor Guides
Caissons and Supports
Pile handling
Figure 5.4: Flow Chart Showing design steps for a Jacket
123

5.4 Design of jacket
5.4.1 Jacket Framing Concepts
5.4.2 Lift versus Launch
5.4.3 Main pile versus skirt pile
5.4.4 Mudmat arrangement
5.4.5 Jacket Appurtenances
• Pile Sleeve Grouted Connection
• Barge Bumber
• Riser Protector
• Boat Landing
• Ring Stiffeners
• Closure Plates
• Closure diaphragms
• Pile Catcher
• Pile Grippers
124

Chapter 6
DESIGN OF TUBULAR MEMBERS
6.1 Introduction
Tubular members are invariantly used in the offshore structures. Deck legs, deck braces,
jacket legs, braces and piles are of made of structural steel tubular members. In addition,
the secondary structures such as flare booms, bridges etc also made of steel tubular members.
Design of such members will involve calculation of actual stresses induced in the member
during the life of the structure and compare it with the allowable stresses based on certain
code of practice.
The design of structural elements of any cross section either rectangular, wide flange rolled
shape, tubular, or even fabricated sections, many design methods can be followed.
In practice, following two methods are employed in many countries.
• Working Stress Design (WSD)
• Load and Resistance Factor Design (LRFD)
The API RP2A has both WSD and LRFD versions of the design and will be discussed in
the following sections.
125

6.2. Factors affecting strength Chapter 6. DESIGN OF TUBULAR MEMBERS
6.2 Factors affecting strength
6.2.1 Tubular fabrication methods
6.2.2 Material properties
6.2.3 Imperfections
6.3 Ultimate Strength
6.3.1 Section properties
6.3.2 Boundary conditions
6.3.3 Buckling
6.4 Allowable Stress Design
6.4.1 Principles
Working stress is method of design has been in use for onshore and offshore structures for
a long period of time since man made design of structures started many decades back. The
method is very simple and easy to adopt. Basic principles of design can be stated as below.
StressesduetoLoads ≤ AllowableStresses (6.1)
The stresses due to axial, bending, shear, torsion and other effects shall be calculated using
linear elastic theory (linear geometrical, material) of structures. Allowable stresses shall be
calculated from material ”Yield strength” based on ”Predefined Factor of Safety”
against each category of stresses as defined by codes of practice.
6.4.2 Applied Stresses
The linear elastic structural analysis of the 3D frame of the jacket will yield member forces
(member end forces). The forces include the following six components.
126

6.4. Allowable Stress Design Chapter 6. DESIGN OF TUBULAR MEMBERS
Y
Z
X
L
Fy
Fx
Fz
Mz
My
Mx
Fy
Fx
Fz
Mz
My
Figure 6.1: Member Local Coordinate System and Forces
• Axial Force (Fx)
• Vertical Shear (Fy)denoted also as Vv
• Horizontal Shear (Fz)denoted also as Vh
• Inplane Bending Moment (My)
• Out-off Plane bending Moment (Mz)
• Torsion (Mx)denoted also as Mt
These component forces shall be used to calculate the stresses in the members based on their
geometrical properties such as cross sectional area, moment of inertia etc.
The stresses in the member using simple beam bending theory is summarized below.
fa =
Fx
A
(6.2)
fv =
Vv
0.5A
fh =
Vh
0.5A
(6.3)
127

fvt =
Mt(D/2)
Ip
(6.4)
fby =
My(D/2)
Iy
(6.5)
fbz =
Mz(D/2)
Iz
(6.6)
where
D = Outside diameter of Tubular member
t = Thickness of wall
A = π
4
(D2
− (D − 2t)2
)
Iy = π
64
(D4
− (D − 2t)4
)
Iz = π
64
(D4
− (D − 2t)4
)
Ip = π
32
(D4
− (D − 2t)4
)
6.4.3 Allowable Stress in Tension
The allowable stress in axial tension is 0.6Fy where Fy is the yield strength of steel. This
gives a factor of safety in stress of about 1.67.
6.4.4 Allowable Stress in Compression
The allowable axial compressive stress depends on the following factors.
• Global buckling or Column Buckling
• Local Buckling
The column buckling is characterized by the member slenderness ratio (Kl/r). Column
buckling will happen when the slenderness of the member is very high. For typical jacket
128

primary members, the slenderness shall be kept below 120 and 200 for members subjected
to compression and tension respectively.
The local buckling to tubes will be a problem if the D/t ratio exceed 60. For primary
structural members of jacket such as legs and braces, the D/t shall be kept well within limit
to avoid the local buckling. Generally local buckling may happen due to external hydrostatic
pressure or due to presence of ovality during fabrication. API Spec 2B allows a fabrication
tolerance of out-off roundedness of about 0.2% in diameter. This is incorporated in the
calculation of critical buckling stress.
Global or Column Buckling Local Buckling
Figure 6.2: Global and Local Buckling
Following formula shall be used to calculate the allowable axial stress in compression when
D/t is less than 60.
Fa =
(
1 −
(Kl/r)2
2C2
c
)
Fy
5/3 +
3(Kl/r)
8Cc
−
(Kl/r)3
8C3
c
for Kl/r < Cc (6.7)
Fa =
12π2
E
23(Kl/r)2 for kL/r ≥ Cc (6.8)
where
Cc =
[
2π2
E
Fy
]1/2
(6.9)
129

E = Youngs Modulus of elaticity N/mm2
K = Effective length factor
l = Unbraced length in mm
r = Radius of gyration defined by
√
I
D/2
in mm
I = Moment of inertia of cross section in mm4
Is is to benoted that when D/t ratio exceeds 60, Fy in the above expressions shall be sub-
stituted by the critical local buckling stress (smaller of Fxe or Fxc).
6.4.5 Allowable Stress in Bending
The allowable bending stress shall be calculated as per the following expressions.
Fb = 0.75Fy for
D
t
≤
10340
Fy
(6.10)
Fb = (0.84 − 1.74
FyD
Et
)Fy for
10340
Fy
<
D
t
≤
20680
Fy
(6.11)
Fb = (0.72 − 0.58
FyD
Et
)Fy for
20680
Fy
<
D
t
≤ 300 (6.12)
It is to be noted that the above empirical expressions are based on SI units of measure, i.e.,
the stresses, yield strength, and Young modulus shall be in N/mm2
and the diameter and
wall thickness shall be in mm.
6.4.6 Combined axial compression and bending interaction
For members subjected to axial compression and bending loads, the members shall be pro-
portioned to satisfy the following interaction equations at all points along the length of the
member.
For fa/Fa > 0.15
fa
Fa
+
Cm
√
f2
bx + f2
by
(
1 − fa
F′
e
)
Fb
≤ 1.0 (6.13)
130

fa
0.6Fy
+
√
f2
bx + f2
by
Fb
≤ 1.0 (6.14)
For fa/Fa ≤ 0.15
fa
Fa
+
√
f2
bx + f2
by
Fb
≤ 1.0 (6.15)
fa
Fa
+
v
u
u
t
[
Cmxfbx
1 − fa
F′
ex
]2
+
[
Cmyfby
1 − fa
F′
ey
]2
Fb
≤ 1.0 (6.16)
where F′
e,F′
ex and F′
ey shall be calculated using the following expression.
Fe or Fex or Fey =
12π2
E
23(Kl/r)2 (6.17)
6.4.7 Combined axial tension and bending interaction
Members subjected to axial tension and bending shall be proportioned to satisfy the following
equation at all points along the length of the member.
fa
0.6Fy
+
√
f2
bx + f2
by
Fb
≤ 1.0 (6.18)
6.4.8 Hydostatic Collapse
The difference between land based structure and the offshore structure is presence of hy-
drostatic pressure all around the tubular members. This pressure generates hoop stress in
tubular members. This shall be included in the checking of members. The design hydrostatic
pressure can be calculated as
p = ρwHz (6.19)
131

D
L
Beff
Dc
h
Nuetral Axis
Internal RIng
External Ring
Figure 6.3: External and internal rings
where ρw is the seawater density and Hz is the design hydrostatic head. This shall include
the water surface elevation due to tide, wave etc. The hoop stress due to hydrostatic pressure
can be calculated using
fh =
pD
2t
(6.20)
fh ≤
Fhc
SFh
(6.21)
where Fhc is the critical hoop buckling stress and SFh is the factor of safety against hydro-
static collapse. A minimum factor of safety of 2.0 shall be applied.
6.4.9 Hydrostatic Collapse Rings
External or internal rings can be provided to avoid local buckling of tubular shells against
external hydrostatic pressure. This is normally needed when the D/t ratio exceeds 60 or
the external pressure is very high. Such situation may arise either during whole life of the
structure such as jacket legs and braces left permanently sealed or temporarily like buoyancy
tanks. Buoyancy tanks are normally designed to be temporary and will be flooded during
upending of the jacket and finally removed.
This kind of temporary structures may be designed with thin shell and provided with rings
since there may not considerable amount of other forces except during launch.
The moment of inertia of circumferential stiffening rings shall be calculated as below.
Icrequired =
tLD2
8E
(6.22)
132

where the variables are as shown in Figure xxx. D shall be taken as effective diameter to
the centroid of rings Dc for external rings. Beff shall be calculated as 1.1(Dt)1/2
. It is to
be noted that the yield strength of rings and tubular is same and size of stiffening rings shall
be proportioned to avoid buckling. Minimum size of stiffening rings shall be 10mm x 76mm
and 13mm x 102mm for internal and external rings respectively.
The moment of inertia of the T shaped stiffener / shell composite section can be calculated
as
IcProvided = (6.23)
6.4.10 Axial Load and Hydrostatic Pressure
The structural members of the jacket are designed and built to be buoyant and will be
subjected to external pressure during the life of the structure. This pressure in addition to
the loads from the topsides and environmental load will act together.
The interaction due to axial loads (either tension or compression) with hydrostatic pressure
shall be checked as per API RP2A clause 3.3.4 and 3.3.4. The factor of safety to be applied
against these combinations are given in clause 3.3.5.
6.4.11 Optimum Design
The design of offshore structural members is an iterative procedure since alteration of member
sizes would alter the loads substantially and in some cases may end up into serious design
changes. Hence a one step design is not feasible.
Initially, a member size shall be assumed based on previous experience, or from existing
similar design. Based on the first stage design results, a revised size shall be arrived. A
complete design cycle is explained in Chapter 5.
In any design process, many solutions to a particular design may be feasible. But a fit for
purpose design is called Optimum design. The optimum design in offshore environment shall
be characterized by the following requirements.
• Minimum diameter - minimizes the environmental load
• Minimum weight - reduces the cost
• Minimum weight / buoyancy ratio - increased buoyancy
• Minimum D/t ratio as it increases the allowable stresses and no local buckling
133

• Maximum Utilization - the calculated unity check as per combined axial and bending
shall be close to one.
As it can be seen that the selection of diameter and wall thickness for a given set of loads,
and intended purpose of the member is an important task.
Maximum Slenderness
The slenderness of a member is defined by its ratio of effective length (Kl) and radius of
gyration(r) where K is the effective length factor and l is the length of the member. This
is denoted as Kl/r. This is an important parameter determining the column buckling as
the length increases, the tendency of the column buckling increases and consequently the
allowable axial stresses decrease.
For jacket structural design, the slenderness of primary structural members shall be limited
to 120 and 180 for members in compression and tension respectively.
Maximum D/t Ratio
D/t ratio plays an important role in the selection of member size for the tubular structures.
The allowable stresses in bending is reduced as the D/t ratio increases. The local buckling
become the problem when D/t ratio exceeds 60.
On the other hand, when the tubular is fabricated from plates, the rolling method requires
D/t ratio to be greater than 20 to avoid excessive strain in the rolled tubular.
Hence for an optimum design, for a jacket primary structural member, the D/t ratio shall
be between 20 and 60.
In some cases, such as buoyancy tanks, it may economical to design a tank with higher D/t,
and with internal ring stiffeners to avoid local buckling instead of thicker shell. But the
fabrication costs and associated time delay has to be taken into consideration.
Typical Example
Consider a jacket brace with following parameters and applied loads.
Length L = 20m
Effective Length factor = 1.0
Applied Loads Fx = 1500 kN, Fy = 1500 kNm and Mz = 1000 kNm
134

6.5. Load and resistance Factor Design Chapter 6. DESIGN OF TUBULAR MEMBERS
Yield strength of steel E = 2 x 105
N/mm2
Yield strength of steel Fy = 345 N/mm2
Density of steel ρs = 7850 kg/m3
Density of sea water ρs = 1025 kg/m3
applied loads and designed with two different diameter and wall thickness. It can be seen
from the table, both design is acceptable.
6.5 Load and resistance Factor Design
6.5.1 Code Provisions
6.5.2 Optimum Design
135

6.5. Load and resistance Factor Design Chapter 6. DESIGN OF TUBULAR MEMBERS
No Parameter Design 1 Design 2
1 Diameter D (mm) 762 914
2 Wall Thickness t (mm)
3 Weight (W) kg/m
4 Buoyancy (B) kg/m
5 Buoyancy ratio
6 Slenderness ratio Kl/r
7 D/t ratio
8 Axial Stress fa
9 Bending Stress fbx
1 Diameter D (mm) 762 914
2 Wall Thickness t (mm)
3 Weight (W) kg/m
4 Buoyancy (B) kg/m
5 Buoyancy ratio
6 Slenderness ratio Kl/r
7 D/t ratio
8 Axial Stress fa
9 Bending Stress fbx
Table 6.1: Optimum Design Example
136

Chapter 7
DESIGN OF TUBULAR JOINTS
FOR STATIC LOADS
7.1 Introduction
The main structure of offshore platform (either topside or jacket) will consists of tubular
members. These members are joined together to form three dimensional frame both in
horizontal and vertical planes. The tubular joints are used extensively offshore, particularly
for jacket structures. Typical tubular frame used for the deck and jacket is shown in Figure
8.1.
These connections are formed by fusion welding (called full penetration welding) along the
surface of the tubular being connected. Two main calculations need to be performed in order
to adequately design a tubular joint. These are:
• Static strength considerations
• Fatigue behaviour considerations
7.2 Tubular Joints
In a three dimensional space frame, the type of connections that may be encountered are
many since members from various directions will join together at joints. Theses configura-
tions can be classified in to following simple types based on their geometry.
137

7.2. Tubular Joints Chapter 7. DESIGN OF TUBULAR JOINTS FOR STATIC LOADS
X Joint
J
a
c
k
e
t
L
e
g
B
r
a
c
e
TY Joint
TK Joint
J
a
c
k
e
t
C
a
n
Figure 7.1: Typical jacket frames showing various joint configurations
θ
Chord
Brac
e
T Joint
Chord
Y Joint
θ
B
r
a
c
e
θ2
K Joint
B
r
a
c
e
2
Brace1
Chord
θ1
L
D
d1
d2
Chord
Brace
θ2
θ1
X Joint
Figure 7.2: Various Joint Types
138

• T Joint
• Y Joint
• K Joint
• X Joint
These are uni-planner joints and are shown in Figure 2.
139

7.2.1 Classification
140

Figure 7.3: Load path dependant joint classification
141

7.3. Joint geometrical Parameters
Chapter 7. DESIGN OF TUBULAR JOINTS FOR STATIC LOADS
7.2.2 Load path dependency
7.3 Joint geometrical Parameters
The behaviour of tubular connection mainly depends on the geometry of the joint and loads
applied on the connection. The geometrical parameters included chord and brace diameters
and wall thicknesses and the connection angle between the chord and brace. These basic
parameters will be used to derive some configuration parameters to define the various joint
types and will be used in the formulation of the empirical equations for the joint design.
Typical joint showing all the geometrical parameters are shown in Figure.
Brace
Chord
Can Length L
Crown
Crown
Brace
Saddle
Saddle
d1
d
2
t1
D
t
2
g
θ2
θ1
T
Figure 7.4: Joint Geometrical Parameters
The basic geometrical parameters and derived parameters are defined below.
D = Chord diameter β = d
D
(0.4 to 0.7)
T = Chord wall thickness τ = t
T
0 ≤ τ ≤ 1
d = Brace diameter γ = D
2T
(24 to 60)
t = Brace wall thickness
θ = Brace angle (θ1, θ2 etc)
g = gap
7.4 Static Strength
7.4.1 General
The are no closed form solutions for the static strength of tubular joints for the practical
cases of tubular connections. For last few decades, empirical equations and results from
142

7.4. Static Strength Chapter 7. DESIGN OF TUBULAR JOINTS FOR STATIC LOADS
Finite Element Methods (FEM) have been used for practical design purposes. In some
cases, analytical shell theory have been used for extrapolation and modification to obtain
the empirical equations for the design.
There have been many research studies on the high stress at the interface between the chord
and brace. These stresses often exceed yield of the material and can be several times the
nominal stress in the brace member.
For all practical design purposes, empirical equations derived based on experimental inves-
tigations, FEM analyses have been historically used.
7.4.2 Failure Modes
The braces carrying loads deliver it on the surface of the chord in the form line loads along
the interface of the chord / brace. These line loads vary in shape and position depending on
the geometrical parameters of the connection. These line loads induce deformation on the
chord member locally and globally. Some of the failure modes identified by earlier research
(Marshall, Graw) are listed below.
• Unzipping of weld
• Local Failure of Chord
• General Collapse of Chord
• Material Problem
Each of the above failure modes is discussed in detail in the following sections.
Unzipping of weld
There are many causes for unzipping of the weld but two important items which can be
controlled during fabrication of the connection is discussed here.
The first is the non-uniform distribution of the stresses along the weld length and the local
hot-spot stresses can be 5 to 8 times of the nominal stress in the member or other locations
on the weld. This variation in the stress on the weld some times redistributed during the
loading process. However, if the peak stresses are beyond yield, the weld will start unzipping
before re-distribution. This will cause the weld to progressively open-up.
The second item is due to defects in the weld. The welding of tubular connection by mean of
full penetration welding is very important and requires to be checked for voids, incomplete
143

penetration, flaw etc. This is normally done by means of Non-Destructive Testing (NDT)
methods such as Ultrasonic testing (UT), X-ray, Magnetic Particle (MP) and Dye Penetrant
Test (DPT) etc. But due to large number of joints, some times not all the joints are inspected.
Despite the above inspection, some time the defects in the weld will lead to local unzipping of
the weld may happen and subsequent re-distribution of load will lead to progressive unzipping
and failure of the connection between the chord and brace.
Local failure of chord
This local failure of the chord may happen at the interface between the chord and brace if
the chord is comparatively stiff and non of the global general collapse does not happen. Take
a case of chord with low D/T ratio (between 15 to 20) with short chord length (distance
between supports or point of contra-flexure) and β less than 0.3, then the chord will behave
as a stiff support and will take the loads without appreciable deformation (in cross section
and longitudinal). In such situation, the local failure of chord in the vicinity of the foot print
of the brace connection will happen. This will be in the form local punching shear.
Brace
Chord
P
Local punching
Figure 7.5: Punching Stress Concept
General Collapse of chord
The general collapse of chord involves more widespread modes of failures as listed below.
• Ovalisation
144

• Beam bending
• Beam Shear
• Sidewall web buckling
• Longitudinal distress
The ovalisation is a very common mode of collapse of tubular joint as shown in Figure 8.6.
This is mainly due to the ovalisation of the chord due to un-balanced loads from the braces.
B
r
a
c
e
Brace
Brace
Chord
SECTION A-A
A A
Ovalisation Beam bending
P
Chord
Brace
Support
Figure 7.6: General Collapse of Chord
The other mode of general collapse is the beam bending. This is basically a simple beam
bending problem due to applied load on the brace and the chord supported between adjacent
joints. This mode of failure is not normally independently as the local capacity of the joint
is normally higher than the beam bending mechanism of the member. This mode normally
happens together with the other mode of collapse such as ovalisation. The exact contribution
of these modes of general collapse cannot be predicted accurately due to the complexity and
varied nature of the joints.
Material Problems
The failure of the connection may happen due to problems associated with the material and
welding as listed below.
145

7.5. Design methodology
• Fracture and fatigue
• Lamellar tearing
• Weldability and Heat affected Zone (HAZ)
Welded joints having crack at the tip of the joint may initiate a fracture of the weld. The
thickness of the chord at the joint is normally thick and in some cases may exceed 75mm.
In such cases, due to the presence grains across the thickness may lead to Lamellar tearing
as shown in Figure 8.10.
During the welding of the chord and brace, the heat generated will alter the properties of
both chord and brace parent metal. This is called Heat Affected Zone (HAZ). This zone is
weaker than the other part of the connection and may lead to failure.
7.5 Design methodology
It can be concluded that the stress distribution along the interface between the chord and
brace is very complex and cannot be determined using close forms solutions. Alternatively,
it can be represented by simple punching shear concept as shown in Figure 8.12.
The average punching shear stress vp acting at the perimeter of the brace to chord intersection
is defined as
Acting vp = τ sin θ(fa + fb) (7.1)
where fa and fb are the nominal axial and bending stresses in the brace. The acting punching
shear stress is calculated normal to the chord wall.
The acting punching shear stress Vp shall be compared with the allowable punching shear
stress (allowable shear stress is calculated with appropriate factor of safety). For stocky
chords (D/2T) less than 7), the material shear strength of Vp = 0.4 Fy will govern.
For other cases of mixed failure modes, the allowable punching shear stresses are obtained
from empirical equations derived based earlier experimental and theoretical studies. The
recommendations based on the above has been incorporated in codes of practice such Amer-
ican Petroleum Institute Recommended Practice 2A (API RP2A) and American Welding
Society - Welding Code D1.1 (AWS). These recommendations together with some example
problems will be discussed in subsequent sections.
146

7.6. Parametric equations
Vp
D
T
θ
Chord
Brace
P
d
t
Figure 7.7: Punching Stress Concept
7.6 Parametric equations
This section provides some insight into the code provisions from API RP2A and AWS D1.1.
7.6.1 API RP2A (WSD)
Acting Punching Shear Stresses
Punching shear stress (vp) due to each of the applied component from brace loads such as
axial (AX), inplane bending (IPB) and out-off plane bending (OPB) can be calculated as
below.
(vp)AX = τfAX sin θ (7.2)
(vp)IPB = τfIPB sin θ (7.3)
(vp)OPB = τfOPB sin θ (7.4)
where fAX, fIPB and fOPB are nominal brace stresses in axial, inplane and out-off plane
bending respectively.
147

Allowable Punching Shear Stresses
Allowable punching shear stress in the chord wall can be calculated using
(vpa)AX = (Qq)AX(Qf )AX
Fyc
0.6γ
(7.5)
(vpa)IPB = (Qq)IPB(Qf )IPB
Fyc
0.6γ
(7.6)
(vpa)OPB = (Qq)OPB(Qf )OPB
Fyc
0.6γ
(7.7)
where Qq and Qf are factors to account for loading and geometry and longitudinal stresses
in the chord respectively. The allowable stresses shall also be calculated for each component
separately. Qq can be calculated based on empirical equations given in Table 4.3.1-1 in API
RP 2A and reproduced here. Qf shall be calculated using the following expression.
Qf = 1.0 − λγA2
(7.8)
where
λ = 0.030 for brace axial stress
= 0.045 for brace in-plane bending stress
= 0.021 for brace out-of-plane stress
A =
√
¯
f2
AX + ¯
f2
IPB + ¯
f2
OPB
0.6Fye
(1/3 increase applicable to denominator)
where ¯
fAX, ¯
fIPB and ¯
fOPB are nominal stresses in axial, in plane bending and out-off plane
bending respectively in the chord. Qf shall be set to 1.0 for the case of tensile stresses in
the chord member.
148

Source : API RP2A
Combined effects of Axial and Bending
The combined effect of axial and bending stresses shall be evaluated based following equa-
tions.
(
vp
vpa
)2
IPB
+
(
vp
vpa
)2
OPB
≤ 1.0 (7.9)
(
vp
vpa
)
AX
+
2
π
arcsin
√(
vp
vpa
)2
IPB
+
(
vp
vpa
)2
OPB
≤ 1.0 (7.10)
7.6.2 Design Practice
Following considerations while configuring the tubular connections shall be adopted.
• Thicker wall sections required for the chord shall be extended at least by D/4 or 305mm
which ever is larger.
• The minimum gap between the two braces connected at a joint shall be 51mm.
• The thicker brace stub or special steel shall be extended to a distance of at least by
brace diameter d or 610mm which ever is larger.
149

• The joint offset between braces shall not exceed D/4.
• The brace angle shall not be less than 30 degrees.
• The ratio of D/d shall not be close to 1.0
B
r
a
c
e
Brace
Chord
Minimum Gap 51mm
Offset not to exceed ±D/4
Heavy wall section of chord
Minimum
D/4 or 305mm
D
d
Stub of heavy
or special steel in brace
M
inim
um
d
or
610m
m
Figure 7.8: Simple Tubular Joint Design Considerations
150

7.7. Design method Chapter 7. DESIGN OF TUBULAR JOINTS FOR STATIC LOADS
7.6.3 API RP2A (LRFD)
7.6.4 AWS D1.1
7.7 Design method
7.7.1 Axial load capacity
7.7.2 Moment capacity
7.7.3 Interaction equation
7.7.4 Effect of thickened can
7.7.5 Design practice
7.7.6 Strength check (50%)
7.8 Material Properties
7.9 Special Joints
7.9.1 Grouted pipe-in-pipe joints
7.9.2 Ring stiffened joints
7.9.3 Overlapped joints
151

7.9. Special Joints Chapter 7. DESIGN OF TUBULAR JOINTS FOR STATIC LOADS
152

8.1. Introduction Chapter 8. DESIGN OF TUBULAR JOINTS FOR CYCLIC LOADS
Chapter 8
DESIGN OF TUBULAR JOINTS
FOR CYCLIC LOADS
8.1 Introduction
8.2 Hot spot and stress concentration
8.3 Methodology
8.4 Fatigue assessment
8.4.1 Fracture mechanics method
8.4.2 Fatigue damage method
8.5 S-N curve
8.6 SCF-Parametric equations
8.7 Deterministic Fatigue Assessment
8.7.1 Wave scatter data
8.7.2 Hot spot stress range
8.7.3 Fatigue damage
154

9.1. Main Pile Chapter 9. FOUNDATION DESIGN
Chapter 9
FOUNDATION DESIGN
9.1 Main Pile
9.2 Skirt or Cluster Piles
9.3 Pile Axial Capacity
9.4 Design Factors of Safety
9.5 Pile-Soil Interaction
9.5.1 P-Y Simulation
9.5.2 T-Z Simulation
9.5.3 Q-Z Simulation
9.5.4 Pile Group Effect
9.6 Pile Driveability analysis
9.7 Pile Design for Installation
156

10.1. Fabrication Yard Chapter 10. FABRICATION
Chapter 10
FABRICATION
10.1 Fabrication Yard
10.1.1 Layout
10.1.2 Quay side
10.1.3 Skidways
10.1.4 Equipment
10.2 Fabrication of tubulars
10.2.1 Fabrication of Tubulars
10.2.2 Tubulars Braces and Cans
10.2.3 Tubular Joints
10.2.4 Welding
10.2.5 NDT Requirements
10.2.6 PWHT Requirements
10.3 Jacket Fabrication
158

11.1. Loadout Chapter 11. DESIGN FOR INSTALLATION FORCES
Chapter 11
DESIGN FOR INSTALLATION
FORCES
11.1 Loadout
11.1.1 Skidded Loadout
11.1.2 Trailer Loadout
11.1.3 Lifted Loadout
11.1.4 Jacket Loadout
11.1.5 Deck Loadout
11.1.6 Ballast Plan
11.1.7 Loadout arrangement
11.1.8 Loadout procedure
11.1.9 Design Loads for Loadout
11.1.10 Analysis methods
11.1.11 Support settlement
11.2 Sea Transportation
160

guide to offshore structures design for engineers

More Related Content

What's hot (20)

Similar to guide to offshore structures design for engineers (20)

Recently uploaded (20)

guide to offshore structures design for engineers