A method for reliability analysis of drag-dominated offshore structures is outlined. Stochastic dynamic response analyses are performed in the time domain by a stepwise integration procedure. Probability distributions of the Wei bull type are fitted to the samples of response maxima which are obtained from the simulated time series. These form the basis for calculation of long term response distributions from which extreme response can be estimated. Samples of the time-invariant parameters describing the structure and the environment are generated by simulation based on importance sampling. For each sample of these parameters, a long term analysis is carried out. Reliability analysis of a jack-up platform is performed as an illustration of the present approach.
A fundamental difference between reliability analysis of static and dynamic structures is the type of quantities which enter into the failure criteria. For a static structure, the criteria can be expressed in terms of external loads or internal load effects at convenience. For dynamic structures, the statistical properties (e.g. distribution functions) of the internal load effects are unavoidably required. Determination of these properties demands that some kind of dynamic response analysis is performed. For structures where both the loading and the structure ifself are linear, frequency domain procedures can typically be applied. However, if nonlinearities are present, other methods of response analysis will frequently be required. This approach is here employed for analysis of offshore platforms with nonlinear drag-dominated loading where also dynamic effects play an important role. For these types of structures, the extreme response may either occur for extreme sea states or for lower sea states where significant dynamic amplification of structural response occurs. By performing response analyses for a number of sea states, this effect can be accounted for by the so-called long term analysis procedure.