The SOLAS regulations on subdivision and damage stability,
as contained in part B-1 of SOLAS
chapter II-1, are based on the probabilistic concept which takes the
probability of survival after collision as a measure of ship's safety
in the damaged condition, hereinafter referred to as the "attained
subdivision index A".
This is an objective measure of ship safety and therefore
there is no need to supplement this index by any deterministic requirements.
These new regulations, therefore, are primarily based on the probabilistic
approach, with only very few deterministic elements which are necessary
to make the concept practicable.
The philosophy behind the probabilistic concept is that
two different ships with the same index of subdivision are of equal
safety and therefore there is no need for special treatment for specific
parts of the ship. The only areas which are given special attention
in these regulations are the forward and bottom regions which are
dealt with by special rules concerning subdivision, provided for the
cases of ramming and grounding.
In order to develop the probabilistic concept of ship subdivision,
it is assumed that the ship is damaged. Since the location and size
of the damage is random, it is not possible to state which part of
the ship becomes flooded. However, the probability of flooding a space
can be determined if the probability of occurrence of certain damages
is known. The probability of flooding a space is equal to the probability
of occurrence of all such damages which just open the considered space.
A space is a part of the volume of the ship which is bounded by undamaged
watertight structural divisions.
Next, it is assumed that a particular space is flooded.
In addition to some inherent characteristics of the ship, in such
a case there are various factors which influence whether the ship
can survive such flooding; they include the initial draught and GM,
the permeability of the space and the weather conditions, all of which
are random at the time when the ship is damaged. Provided that the
limiting combinations of the aforementioned variables and the probability
of their occurrence are known, the probability that the ship will
not capsize or sink, with the considered space flooded, can be determined.
The probability of survival is determined by the formula
for entire probability as the sum of the products for each compartment
or group of compartments of the probability that a space is flooded
multiplied by the probability that the ship will not capsize or sink
with the considered space flooded.
Although the ideas outlined above are very simple, their
practical application in an exact manner would give rise to several
difficulties. For example, for an extensive but still incomplete description
of the damage, it is necessary to know its longitudinal and vertical
location as well as its longitudinal, vertical and transverse extent.
Apart from the difficulties in handling such a five-dimensional random
variable, it is impossible to determine its probability distribution
with the presently available damage statistics. Similar conditions
hold for the variables and physical relationships involved in the
calculation of the probability that a ship with a flooded space will
not capsize or sink.
In order to make the concept practicable, extensive simplifications
are necessary. Although it is not possible to calculate on such a
simplified basis the exact probability of survival, it is possible
to develop a useful comparative measure of the merits of the longitudinal,
transverse and horizontal subdivision of the ship.