This paper deals with the fatigue damage of catenary mooring chain under combined low frequency (LF) and wave frequency (WF) loads which are both non-Gaussian. The distribution of quasi-static LF mooring line tension is obtained by representing the slowly-varying wave force acting on the vessel as a second-order Volterra series and by taking into account the nonlinear relationship between the vessel motion and the resulting line tension. The dynamic WF tension distribution is estimated when both the drag and inertia forces on the mooring line are considered. Finally, with the narrow-band assumption for both the LF and WF response spectra, the total fatigue damage is approximated by the sum of the fatigue damages due to the WF tension process and the sum process of the WF tension envelope and the LF tension. The procedure is illustrated by considering of the mooring system of a semi-submersible. Short-term distributions of LF and WF line tensions are given and compared with the results from the linearized analyses with Gaussian tension. Relative LF and WF long-term contributions to the total fatigue damage from different sea states are also provided.

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