Page 471 - Marine Structural Design

Chapter 25 Reliability of Ship Structures 447

that the long-term VWBM can be modeled as a Poisson process and the peak of each
individual VWBM, M,, can be well approximated by a Weibull distribution.

[ (ZT]
FMW(Mw) I - exp - In(v,Jo) - (25.16)
=

where v, is the mean arrival rate of one wave cycle, h, is the shape parameter varying from
0.9 to 1.1 and reasonably taking 1 .O as a representative value, M,,is the maximum VWBM in
the reference design period T~ =20years,

- 0.1 t(ESF), C,L2B(CB + 0.7) (sagging) (25.17)
MW.0 =
O.l9(ESF), C,L2 BC, (hogging)

where (ESF), and (ESF)h are the environmental severity factors for sagging and hogging
conditions, which can be taken from 0.5 to 1.0 consistent with the specific installation site
(ABS, 2000).

Similar to that of SWBM, the CDF for the largest of the individual VWBMs can be found for
a total of the v, Trepetitions,

where X, = M,/M,, , the parameters a,and p,,, are given by

Based on Ferry-Borges method, the CDF of the combined bending moment is expressed as
following (Wang, et al, 1996)

(25.19)

The combined bending moment in a given time Tis obtained by

F(M,)=I-I/v,T (25.20)

For the practical design considerations, load combination factors for SWBM and VWBM,
denoted as and p,., are introduced
(25.21)