Page 480 - Marine Structural Design

456 Part IV Structural Reliability

Figure 25.7 shows the influence of “Environmental Seventy Factors” to the conditional
reliability, where the number denotes the value of ESFs. Accuracy measurement of the
specific-site installation conditions is very important for the FPSO hull girder design and
inspections.
The transition time tj is another important parameter relating to the reliability and its effect of
different value is shown in Figure 25.8, where the number on the curve denotes the transition
years. A relatively larger transition time will keep reliability at relatively higher reliability and
postpone the happening of unsteady propagation of fatigue cracks. This result was also
obtained by Guedes Soares and Garbatov (1999) who canied out the reliability analysis of
maintained, corrosion protected plates.

1.00 I

0.99

0.98
5
a:
0.91

0.96

0.95
0 5 10 15 20 25 30 35
t, years

Figure 25.8 Influence of Transition Time on Conditional Reliability

Corrosion wastage depends on many factors, including coating properties, cargo composition,
inert gas properties, temperature of cargo, and maintenance systems and practices, and spot-
checks may not measure the same location in subsequent spot-checks in normal thickness
measurements. This evident makes theoretically constructing corrosion model quite difficult.
In the numerical analysis, the mean steady corrosion rates were used, which correspond to the
permissible values of corrosion wastage for oil tankers in classification society rules. The
coefficient of variation for corrosion rates typically increase with time from 10% at loth
service year to 100% or even larger at 20th service year, but the contribution to the total
uncertainty of hull girder ultimate strength is very limited based on the sensitivity analysis (see
Figure 25.9).