Page 470 - Marine Structural Design

446 Part IV Structural Reliabiliq

to loading pattern and human action and needs to be considered as stochastic process in long
term.
For an FPSO, a Poisson rectangular pulse process in time domain is applied to model the
SWBM. Its cumulative distribution cab be fitted by a Raleigh distribution for the sagging
condition and by an exponential distribution for the hogging conditions according to Wang,
Jiao and Moan (1996), e.g.:

(25.1 1)

for sagging SWBM and
(25.12)

for hogging condition, where M, is the SWBM of an individual load condition and V, is the
mean arrival rate of one load condition. The specified maximum SWBM in a design lifetime
of ~,=20 years, MSgis (IACS, 1995)

-0.065C,LzB(CB + 0.7) (sagging)
(25.1 3)
C,L2B(0.1225-0.015CB) (hogging)
in which L, B and C, are the ship length, breadth and block coefficient respectively, and c, is
the wave coefficient given by
10.75 -((300- L) / 100 < L I 300
C, = 10.75 300 < L I350 (25.14)
i 10.75 -((L- 350)/1S0)3’2 L > 350

The CDF (Cumulative Distribution Function) for the largest of the individual SWBMs can be
found for a total of the v, Trepetitions using the extreme theory,

where A‘, = M,/M,,, , the parameters a, and ps are given by

Many research efforts have been made on the prediction of VWBM experienced on the ship
hulls using linear and nonlinear methods for unrestricted service conditions. VWBM is a
naturally stochastic process and can be described by either short-term or long-term statistics.
The long-term VWBM is based on the weighted short-term statistics. It is generally accepted