Page 467 - Marine Structural Design

Part IV

Structural Reliability

Chapter 25 Reliability of Ship Structures

25.1 General

Since researchers first began to apply probabilistic methods in the structural design of ships
(Mansour, 1972, Mansour and Faulkner, 1973), a significant amount of achievement has been
accomplished. The earliest applications of reliability methods to ship structures focused on
overall hull girder reliability subjected to wave bending moments (Mansour, 1974, Stiansen et
al., 1980, White and Ayyub, 1985, Guedes Soares, 1996). Recent work in applying reliability
methods to the ultimate strength of gross panels using second moment methods (Nikolaidis, et
al., 1993) has shown considerable promise. Casella and Rizzuto (1998) presented a second-
level reliability analysis of a double-hull oil tanker. Frieze and Lin (1991) assessed reliability
for ship longitudinal strength. There is still a continuing effort, which is looking at how these
methods and procedures can be used in a system analysis.
There has been a tremendous amount of effort to develop statistical models for load effects
(e.g., Guedes Soares and Moan, 1985, 1988, Ochi, 1978, Sikora et al., 1983, Mansour, 1987).
Recent research includes the uncertainties associated with loads and load effects (Nikolaidis
and Kaplan, 1991), and on loads and load combinations (Mansour et al, 1993).
FPSOs have been used worldwide as an economic solution for the development of offshore oil
and gas. Actually many FPSOs are sited at locations with dynamic components of their
loading that are less than those arising from unrestricted service conditions. The reliability of
FPSO hull girders for the specific-site conditions are quite different with that of oil tanker for
unrestricted service conditions. Therefore it is necessary to assess the reliability of FPSO hull
girders in order to develop rational design criteria.
As the ocean-going cargo ships, the most catastrophic event of FPSOs is structural failure of
hull girders due to extreme bending moments. During its service life, FFSO hull girders
predominantly withstand stillwater and wave-induced bending moments. The former is caused
by the action of the self-weight, the cargo or deadweight. The latter is a result from the wave
action at the specific installation locations. The “Environmental Severity Factors(ESFs)”
should be introduced in order to accounting for the specific-site conditions in the wave-
induced bending moments (ABS 2000). Because the maximum values of the stillwater and
wave-induced bending moments don’t occur at the same instant, the stochastic combination
method should be used in order to more rationally determine the maximum value of the
combined load e.g. Guedes Soares (1990), Mansour (1994) and Wang et a1 (1996).
In carrying out the reliability assessment relating to the failure of progressive collapse, the
limit state fhction is very complex and may only be expressed implicitly. Among of the
methods available for solving such a problem, the response surface method is an effective and