Extreme dynamic, non-linear response of fixed platforms using a complete long-term approach

The general theoretical basis for application of the direct long-term approach to offshore structures with non-Gaussian response is outlined. The key to the method is the determination of the short-term response distribution. Special considerations should be taken, depending on the application, and the Weibull distribution with a special fitting procedure is suggested as a short-term probability model for fixed offshore platforms. The efficiency of the method is improved by introducing smoothing functions for the variation of the Weibull parameters with the seastate.

The method is applied to a jacket and jackup platform. The results show that smoothing functions increase the efficiency considerably. The design storm method compares well with the long-term results for the selected structures, but only the long-term approach yields realistic estimates of the design extreme response when both extreme and resonant seastates contribution to the 100-year response.