Answer
Damage Stability
– calculations are required in order to achieve a min degree of safety after flooding.
- In order to assess the behavior of the vessel after damage 2 approaches have been developed
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1.Deterministic 2. Probablstic
1.DETERMINISTIC APPROACH
Subdivision is calculated based on theoretical principle
It is based on standard dimension of damage extending anywhere along the ship’s length or between transverse bulkheads depending on the relevant requirements.
a. The consequence of such standards of damage is the creation of a group of damage cases, the no. of which and as well as no. of compartments involved in each case, depends on ship’s dimensions & internal subdivisions.
b. For each loadn condition, each damage case is to be considered & all applicable criteria are to be complied with.
c. Different deterministic methods in damage stability have been developed depending on ship’s type, on freeboard reduction and the kind of cargo carried.
d. The determnstc method to be applied for passenger ships, oil tankers, chemical tankers, g as carriers and special purpose ships.
e. The determinstcs method to be applied in cases of freeboard reduction.
2.PROBABILISTIC APPROACH
This method applies to cargo ships of length more than 80 m and for which no detrmnstc method applies.
a.This method was devised in year 1973. This showed a pattern in accidents which could be used in improving the design of the ships. Most damage were sustained in the forward part of the ship hence it seemed logical, to improve the standards of forward subdivisions rather than those of stern.
b.It is based on statistical evidence conserning what actually happens when ships collide in terms of sea state and weather conditions, extent and location of damage, speed & course of ship & whether the ship survived or sank.
c.This concept is believed to be more realistic than the determnstc method in which ship’s
subdivision is based in theoretical principles.
d.It is based on 3 probabilities related to subdivision & damage stability requirements
I. Probability that the ship may be damaged
II. Probability as to the location of damage & extent of flooding i.e. deends on the arrangement of watertight sub-divisions which have a direct influence on the location & extent of hull damage.
III. Probability to assess the ability of the ship to survive in flooded situation i.e. depends on the
buoyancy & stability in flooded condition which will further depend on following –
• Location & extent of damage
• Permeability of flooded space
• Draft & stabily before floodin
• Applied forces & moments
e.This method takes the probability of survival after collision as a measure of ship safety in damaged condition referred to as – Attained Sub-Division Indx –A & Req. Sub-Division Index-R which is defined in terms of persons & size of vessel.
A>R where
A=0.4As + 0.4Ap + 0.2 A1; As & Ap should not be less than 0.9R for Passenger ships & 0.5R for Cargo ships.
R = 1-{(128/Ls)+152}; for Cargo ships > 100m in length (Ls) which is fixed for ship as per the size & no. of persons