A design-oriented method for calculating the reliability of offshore structures under correlated, multiple wave load effects with due account of the long-term variation of sea state is presented. Ultimate limit states are formulated in terms of an interaction ratio (IR), such that IR greater than 1 means failure. The long-term distribution of IR and the corresponding failure probability is first determined conditioned upon other time-independent random variables z, such as geometrical and material parameters, model uncertainties etc. Then the unconditional failure probability is determined by FORM or MOnte Carlo simulation. The long-term distribution of IR is obtained by combining the distribution for each short-term sea state. The short-term response of the generally nonlinear IR, is determined by time domain simulation, and fitted by using Weibull distribution. Determination of conditional long-term distributions of IR is the time-consuming effort involved in this approach. Various measures to make the approach computationally efficient are introduced. The feasibility of the proposed method is demonstrated by applying it to cylindrical members, unstiffened curved plates and stiffened cylindrical panels of the North SeaFPS Semi-submersible PM-4.
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