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Feature 5 | CAD/CAM UpDAte
The probability of survival
Damage stability application eases complexity of probabilistic
methodology, writes Bill Plice, president, Creative Systems, Inc.
S
everal variations of the probabilistic to the person running the calculations.
approach to stability-while-damaged It offers the potential of a high degree
have been developed over the of automation and time-saving for the
years, but newer regulations are focusing designer.
on the method adopted in 2005 by the The methods of determining damage
International Convention for the Safety of and survival probabilities must address the
Life at Sea (SOLAS), known as Resolution many possible intricacies of ship subdivision
MSC.194(80). in a general manner. The approach that
Two versions of this method are has been taken by the formulators of these
prescribed: one for passenger ships and methods is to envision an idealised set of
one for cargo vessels. The passenger-ship features which is assumed to represent
version escalates the complexity of the all of the important features of any real
required calculations to a new level since it probabilistic approach to stability. ship, and then design rules that address
adds a requirement to evaluate stability with the idealisation. However, it is not always
heeling moments due to wind, passenger possible to address real subdivisions, using
crowding, and lifeboat deployment and, in set in consideration of the type of ship, its such rules, without ambiguity. Therefore,
addition, requires checking intermediate length and, at least for passenger ships, the when actual computational procedures
flooding to determine whether lesser numbers of people onboard. are laid down in computer code, the verbal
stability might be encountered as flooding The probabilistic approach to stability rules have to be augmented in order to
progresses. has much to recommend it. It takes into make the software work. In other words,
All probabilistic methodologies are account the fact that there is no such thing methods must be programmed that take
based on a thorough analysis of the vessel’s as enough stability to meet the most severe the features of the real ship and fit them
response to damage or flooding where damage or the most severe weather. As a into the idealisations upon which the rules
single and multiple compartments are practical matter, ship design must aim for are built. In most cases this transformation
assumed to be flooded one at a time and an acceptable probability of survival. Since is straightforward; but in some cases a
in combination. A range of damage extents all of the design features affecting stability choice has to be made between alternatives,
is considered, where higher probability are rolled into one probability, the designer which seem to satisfy the wording of the
of damage is generally assigned to lesser is free to make tradeoffs without being rules. In such cases, the software might well
extents of damage. hurt by unnatural features of rigid stability have a means of user input so that decisions
For each damage scenario, the probability criteria, on the one hand; or, on the other about how to treat these features can be
of survival is calculated using certain hand, being tempted to exploit loopholes made intelligently and deliberately by the
formulas that are ultimately based on in the rule to increase loading limits at the naval architect.
statistics from experience with ships at sea. expense of real safety. With the probabilistic For example, wing tanks or
The product of the probability of damage approach, design tradeoffs, insofar as they compartments are effective in limiting the
times the probability of survival is a partial affect stability, are realistically represented flooding due to horizontal penetration
probability that contributes to an overall in the attained subdivision index. through the side shell of the vessel. The
measure of stability when added together From the standpoint of the computational extent of such penetration is linked to its
with all the other partial probabilities procedures, the great advantage of the probability: a greater extent of penetration
from all of the extents of possible damage probabilistic method is that it embraces requires more energy and is therefore less
along the length of the vessel. This grand the problem of damage extents as well likely. In concept, a given wing-tank design
summation is called a subdivision index. as the problem of survivability after will limit flooding to the wing tank when
Actually it is only a partial subdivision damage, combining the two in one elegant the penetration is less than a certain value.
index, because under other loading measure. This formalises and standardises In order to get the highest probability out
conditions or draughts the calculations the generation of damage scenarios and of this damage case (that is, to contribute
will produce different results. Therefore, makes it possible to present the results in most to the attained index) the designer
partial indices from multiple draughts a compact format. Since the probabilistic will want the penetration value used in
are combined to arrive at the attained methodologies directly address specific the calculations to be as large as possible.
subdivision index that is then compared features of ship subdivision, there are (at But what is this penetration value that just
to a required index. The required index is least theoretically) fewer decisions left misses rupturing the wing bulkhead? Is it
The Naval Architect February 2008 101
NA Feb 08 - p101+103.indd 101 05/02/2008 09:56:00
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