Trans RINA, Vol 153, Part A1, Intl J Maritime Eng, Jan-Mar 2011
has been formulated generally using the same approach as Resolution A.265(VIII).
2. PROBABILISTIC DAMAGE STABILITY 2.1 THE PROBABILISTIC METHOD
A probabilistic methodology has been accepted as the way forward in applying a unified approach to assessing the survivability standard for varying ship types. Certain vessel types have not been immediately introduced into the regulations due to operational characteristics (for example, offshore supply vessels); pollution (tankers); or basic hull design (offshore drilling units, high speed craft). These aspects need to be further analysed before harmonisation can take place.
In applying the probabilistic method, the Required Subdivision Index, R, level has generally been based on calculations of the attained index of sample ships to provide an equivalent level of safety to that achieved under the previous SOLAS 90 requirements. It is a simple formulation based on ship length and passenger numbers.
The accepted principle is that survivability increases with length and passenger numbers. Therefore, the new regulations have two formulations; one for cargo ships and the other for passenger ships. Cargo ships, for example, have an increase in the required index from 0.394 to 0.717 in the ship length range between 80 and 300 metres. Passenger ships follow a similar trend, but with passenger numbers included in the formulation.
Some difficulties were encountered in designing the R factor for passenger ships due to the limited amount of actual damage case data and the impact of a defined, as opposed to non dimensional damage, length used in the previous SOLAS regulations. During the development of the
new framework, some inconsistencies in the
deterministic requirements were found and this resulted in a mandate from the IMO’s Maritime Safety Committee (MSC) to raise the safety level for passenger ships.
There are many factors relating to the risk of a ship sustaining and surviving a collision at sea; for example, loading, sea state, permeability, internal configuration and cargo shift. With good judgement and analysis the affect of these factors can be quantified to formulate an index. The location and extent of the damage is random but the probability can be defined by experience from damage statistics. The ability to survive is assessed on the characteristics of the
ship form itself. This is
represented by the attained Subdivision Index, A, calculated using measures of subdivision such as the probability that a collision will result in the flooding of a certain compartment and/or adjacent compartments, plus the ability of the vessel to display sufficient residual buoyancy to survive.
1 2 3 4 Ls
Figure 3: Representation for probability of along the ship’s length with seven zones.
flooding
The formula for the survivability factor, si, is a general formulation designed to represent the probability that the ship may survive after flooding of a compartment or group of compartments. The factor si is a generalised expression of the probability of representing all risks, including transient, intermediate stages of flooding and progressive flooding effects. To a certain degree it also implicitly covers characteristics such as the probability of ship survival with water accumulation on deck relevant
A-2 ©2011: The Royal Institution of Naval Architects 5 6 7
Maximum Damage Length
Attained subdivision index 0.4As+0.4Ap+0.2Al A ≥ R
Required subdivision index A = pi . si
Passenger
Minimum attained index at specific draughts
Cargo
As Ap Al ≥ 0.9R As Ap Al ≥ 0.5R
Figure 2: Description of the probabilistic method.
The index is obtained by the sum of indices calculated for the ship at three arbitrary conditions at the subdivision, light service and partial draughts.
A = 0.4As + 0.4Ap +0.2Al The non-dimensional
damage length, or pi factor
represents the probability that a compartment or groups of compartments may be flooded, disregarding horizontal subdivision. It is based on the examination of collision cases in which information, on both damage penetration and damage longitudinal extent, was available.
Ls
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