Assessment of Existing Structures and Repairs - Offshore Structures: Design, Construction and Maintenance - page 512

1.0E-02

1.0E-03

1.0E-04

Minimum
RSR

1.0E-05

1.0E-06
1.0

2.0 3.0 4.0 5.0 6.0

RSR

FIGURE 7.23 Relation between RSR and the probability of failure.

approximated and plotted. It should be noted that the approach is an approxima-
tion and assumes that the load on the structure remains constant. In fact, it may
be argued that increasing RSR would perhaps also increase load; assuming
larger members and therefore increased fluid drag loading

— and conversely

reducing the RSR may reduce the load. If this effect were judged to be signif-

icant for the structures in question, the effect on the load could also be estimated
and included by adjustment of the estimated mean load.
Figure 7.23 illustrates the relation between RSR and probability of failure. As
shown in the figure, if the probability of failure is defined to be equal to 1

—

10 − 3

×

the corresponding minimum RSR can be obtained.

7.9.5 Reliability Analysis
For platforms that experience storm waves from a dominant approach direction,
probability of failure may be calculated for the dominant direction. However, it
may be necessary to calculate the failure probability for up to eight wave-
approach directions (four orthogonal directions and four diagonal directions)
for platforms that do not have a dominant wave-approach direction. In this
case, failure in each direction is treated as a failure element, and the multiple-
direction failure forms a series system for reliability calculations.
For each wave direction, a system reliability approach is also required for
estimation of the annual probability of platform failure. During the system relia-
bility analysis, a platform is modeled in terms of two basic subsystems:

The jacket, including foundation

●

The topsides.

●

The platform fails if either the jacket subsystem or the deck subsystem fails.
The platform system reliability analysis models the two correlated subsystems