Estimation of characteristic extreme response for mooring system in a complex ocean environment

Abstract As vital key parts of offshore floating structures, mooring systems should be able to withstand harsh environmental conditions. This study aims to assess the design loads of mooring lines subjected to random loads using various statistical models. A benchmark of extreme values is established using the global maxima method, and is based upon a Weibull distribution model for the maxima of 150 3-h time series generated by a given design sea condition with different random seeds. To reduce the computational cost, a peak-over-threshold (POT) method is combined with an asymptotic approach. Different variations are investigated, and a better choice for the number of individual top peaks from single time domain simulations is determined from a statistical error analysis. The average conditional exceedance rate (ACER) method and environmental contour (EC) approach are also applied and compared. The effects of various load combinations are considered, and a 3-D EC surface is established based on the Nataf transformation model. A simplified long-term extreme response analysis is conducted by accounting for the POT-based variabilities, so as to provide a more reliable design load estimation. Numerical results are presented and discussed for the tension estimation of a semi-submersible platform considering a long-term coupled analysis.