Page 290 - Practical Design Ships and Floating Structures

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change A significantly. However, if also most of the transverse bulkheads are omitted, then the value
of A decreases substantially. Thus, for the present vessel, as soon as the transverse bulkheads are fixed
then a very good estimate of A can be obtained. It should be noted that the margin line does not enter
into the probabilistic damage stability criteria, but that down-flooding points are included. These
down-flooding points oRen have a significant influence on A.

The two different regulations yield qualitatively the same tendencies. The Attained Index obtained
from SLF 43/3/2 is slightly larger than for SOLAS Part B-I. Careful study of the results reveals that
the main reason is that the survivability index s generally becomes about 15 per cent larger using SLF
43/3/2 in the cases where O<s<l, The differences in the probability distributions for the damage loca-
tion and extent are of lesser importance.

Another important parameter in the calculation of the Attained Index is the centre of gravity as given
by GM in the intact conditions. A change of GM is easily included in the analysis as it only influences
the index s and only through a change in the residual stability curve GZ:

GZ(8) = GZ(8) f GG'sin(6) (1)
where G and G are the old and the new centres of gravity, respectively, and 6 the heeling angle. For
the subdivision defmed in Table 1, the effect of changes of GG' in the range f Im is given in Figure 3
using SOLAS Part B-1. For layout #3 a strong influence by such variation is seen because O<s<l for
damages to the main deck and either of the four large compartments below the main deck. For the lay-
outs # 1 and #2 s = 1 and for layout #4 s = 0 for similar damages for the range of GG' shown in Figure
3 and hence the changes in GZ given by Eqn. 1 do not change A.

-1 0 GG(m) 1

Figure 3: Variation of the Attained Index A with change in the vertical position GG' of the centre of
gravity. SOLAS Part B-1

3 SENSITMTY OF THE ATTAINED INDEX WITH BASIC DESIGN PARAMETERS
To obtain the required Attained Index A the naval architect may need to change some design parame-
ters. It is hence important to identify the parameters with most influence on A. This can be done by
sensitivity analyses as shown in Figure 3 for the centre of gravity. Other important design parameters
have been identified ffom the responses to a questionnaire to leading Danish naval architects regarding
Ro-Ro femes. Two main parameters are