Damage of stability of a ship and how its change .pptx
1. CANADIAN UNIVERSITY OF BANGLADESH
Course: Fault Detection and Damage Prevention
Code: SME 4455
L-2: Damage Stability
Ehamadul Haque Ani
Lecturer
Shipping and Maritime Science
2. Stability
Ship stability can be defined as its
characteristics or tendency to return to its
original state or upright state, when an external
force is applied on or removed from the ship.
In general terms, stability can be divided into
two parts:
A. Intact stability
B. Damage stability
3. Damage stability
The study of damaged stability of a surface ship
comes of use when the ship’s watertight hull is
affected in a way that allows water to flood any
compartment within the ship’s hull. Since this changes
the stability parameters of the ship, the extent of
which depends on the extent of damage and
flooding.
4. Damage stability
Before going into the methods that are used to
evaluate the damaged stability of a ship, we will
discuss the effects of flooding on a ship:
Change of draft
Change of trim
Heeling
Change in stability
Change in freeboard
Loss of ship
5. Change of draft
Flooding results in entry of water into the ship’s
damaged compartment. This will cause a change in
draft to the point where the displacement of the
undamaged (intact) part of the ship will be equal to
the displacement of the weight of the water that
entered the ship after flooding. To express it:
6. Change of trim
Ingress of water in a compartment can be
considered as an addition of weight to any point
along the length of the ship. This causes a change in
the trim of the ship.
7. Heeling
The ship will heel if the damaged compartment is
unsymmetrical positioned about the centerline of the
ship. If the meta centric height of the ship in the
flooded condition is negative, the damaged ship is
rendered unstable. In such a case, the ship would
capsize if the flooded compartment is unsymmetrical
about the centerline, and is prone to capsize by
action of any external heeling moment if the
compartment is symmetrical about the centerline.
8. Change in stability
The metacentric height of the ship changes due to flooding. This
can be explained by the general expression of metacentric
height:
GM= KB+BM-KG
The explanation of reduction in stability can be done in two
ways, which is exactly what we will study in this article, but at a
later stage. For now, it should suffice to understand that when a
ship is flooded, it results in a change of KB and BM values. The
KB rises due to flooding, and it may rise further due to change
in the ship’s trim. However, there is a significant reduction in BM,
which is a function of the area moment of the ship’s water plane.
9. Change in freeboard
The increase in draft of the flooded ship results in
reduction of freeboard, which poses a great threat
to the residual buoyancy of the ship. Even though
the metacentric height may be positive after
flooding, reduction in freeboard to a point where
the deck is immersed, decreases the ship’s range of
stability. This means that the ship could now capsize
due to external forces of wind or waves.
10. Loss of the ship
One of the most common terms that is used in
damaged condition of a ship, is Margin Line. The
Margin Line of a ship is an imaginary waterline
considered 75 mm below the uppermost continuous
watertight deck. If a ship is damaged, it is
considered to be safe only if the margin line is not
immersed. Once the waterline reaches the margin
line at any point along the length of the ship, the
ship is considered to be unsafe, and evacuation
becomes mandatory.
