Systems reliability of offshore structures including fatigue and extreme wave loading

Traditionally, the focus of systems reliability analysis has been on estimating the reliability under either fatigue loading or extreme environmental loading. However, for many offshore structures, a combination of these two sources of risk may be more critical, e.g. initial failures may occur in fatigue and subsequent collapse under an extreme wave. In this paper, a probabilistic formulation for such a sequence of combined failures is presented. A formulation for a sequence of failures under an extreme wave, with realistic failure criteria for offshore structures, is also presented. With these formulations, it is possible to account for both fatigue and extreme wave failures. An application of such a unified approach to a steel jacket platform is presented. It is found that, for an individual member, the most probable cause of failure is fatigue, but for overall structural failure, overload and a combination of fatigue and overload are more important.