Estimation of short-term extreme responses of a semi-submersible moored by two hybrid mooring systems

This paper concerns the estimation of 3-h short-term dynamic responses of semi-submersible system moored by two hybrid mooring systems. The global maximum method, which is applied by fitting 50 individual maximum observations by Gumbel distributions, is used to obtain extreme surge motion and mooting tensions. The accuracy of the numerical model has been validated by the model tests. The most probable maximum extreme (MPME) values of Gumbel distribution fitted by L-moments are adopted as the benchmarks and used to check the accuracy of the extrapolation methods. The average conditional exceedance rate (ACER) method as well as optimized peaks-over-threshold (POT) method combined with Generalized Pareto distribution (GPD), three parameters Weibull distribution and Generalized Extreme Value distribution (GEV) are applied to study 3-h return levels. The effects of threshold, ensemble size and simulation duration on extreme values are investigated, and suitable empirical thresholds are suggested. A method to predict short-term extreme values by combing 20 individual realizations is given, the effects of parameters estimation method, threshold and duration are discussed.