A fully-coupled method is deemed desirable for global response analyses of deep water floating structures. However, even with ever increasing computing power, the fully-coupled method is still time consuming, especially when a large number of different load cases need to be considered. In contrast, the computational cost is much lower for a semi-coupled method at the expense of analytical accuracy. The benefits of a fully-coupled method include taking into account the coupling effects between damping due to the slender structures and motions of the FPSO. Previous studies have indicated that the total damping can be significantly influenced by the mooring line damping. The semi-coupled method can be improved if this damping effect is properly modelled and included.

It is known from previous studies that mooring line damping can be significantly increased by wave-frequency (WF) motions. However, typically only harmonic WF motions were considered in these studies. In this paper, the effect of random WF motions on mooring line damping is investigated. It is found that, statistically, the effect of random WF motions on the mooring line damping can be represented by an equivalent harmonic WF motion. Based on this, the mooring line damping can be estimated and used in the semi-coupled analysis. A comparison of the semi-coupled method with a fully-coupled method is made by performing a global analysis of a turret moored FPSO under an extreme sea-state with a 100-year return period. It is shown that with the system damping properly estimated, the excursion results of the moored FPSO by the semi-coupled method are in very good agreement with those obtained by the use of the fully-coupled method.

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