Page 474 - Marine Structural Design

450 Part IV Structural Reliabiliv

In this study, a response surface with a polynomial type function including squared terms but
not cross terms is adopted,
r I
G(~)=u+Cb,x, +Ccixiz (25.28)
i=l i=l
where Y is the number of basic random variables; x = (x,,x2,.-.,xr) is the basic random
vector; a, bi, and ci are the unknown coefficients that can be determined using 2rt1 sampling
points. The sampling points are selected to be located at the design point (T,,T~,...,?,) and
other 2r points kfui,...,Tr), wherefis a parameter determining the upper and lower
bounds of selection range. This process should be iterative to guarantee that the sampling
points chosen from the new design point include the information from the original failure
surface sufficiently. Once the response surface is defined, the failure probability is computed
using modified Monte Carlo simulation technique (Sun, 1997).

25.5 Time-Variant Reliability Assessment of FPSO Hull Girders

The time-variant reliability estimation of an FPSO hull girder subjected to the degradations of
corrosion and fatigue is studied in this sub-section. The relevant principal particulars of the
FF’SO are listed in Table 25.1. The midsection is shown in Figure 25.1. Table 25.1 summarizes
variable measurements in this assessment. The determination of the various variables is based
on the previous studies of FPSOs.

Depth, molded, including box keel
Depth, molded, from base line
Block coeffrcient
Transverse web kame spacing