Page 478 - Marine Structural Design

454 Part IV Structural Reliability

25.5.2 Timevariant Reliability Assessment
Figure 25.5 shows the conditional reliability as a time function considering the four cases of
degradations. Here the conditional reliability is defined by R(t)=l-Pdt). For the different
service years and mean steady corrosion rates, the reductive ratios of the corresponding
conditional reliability index compared to the initial value are listed in Table 25.3.

I .oo

0.99

0.98
e
v
% 0.97
0.96

0.95

0.94
0 5 IO 15 20 25 30 35
t, years
Figure 25.5 Conditional Reliability with Four Cases of Mean Steady
Corrosion Rates

Table 25.3 The Reduci ve Ratios of Conditional Reliability Index
10 15 20 25 30 35
Corrosion

~ ~~
91.50% 86.94% 82.39% 78.60% 62.66% 60.26%

88.15% 81.09% 72.35% 58.86% 57.99% 57.45%

The conditional reliability of the hull girder is also significantly decreased coincident with the
mean ultimate strength. If the reductive ratio 90% of conditional reliability index is selected as
reliability threshold in order to maintain the hull reliability level, then the inspections should
be made for the Case 3) and Case 4) at about loth service year. The degradation effect of
fatigue cracks seems not to be important to the hull girder reliability before they unsteadily