Mechanism of torque induced by chain twist and its effects on the predicting fatigue lives of mooring chains of a semi-submersible operated in OWA (Offshore Western Africa) are investigated in this paper. The fatigue lives of mooring chains are estimated based on a fracture mechanics analysis. Stress ranges on the mooring chains induced by tension ranges acting on the twisted chains are achieved by a finite element analysis. The low frequency (LF) and wave frequency (WF) tension processes induced by the motions of LF and WF are regarded as two random loading processes and the combined tension process of LF and WF is predicted by a dual narrow-band method. The mooring chains are treated as round bars and initial surface cracks are assumed to propagate at the surface of mooring chains. The influence of the chain twist on the fatigue lives of mooring chains is investigated and the results show that when the twist angle is less than 20 degree, fatigue lives of mooring chains are increased with the increase of the twist angle. However, when the twist angle approaches lock-up angle of 30 degree, fatigue lives of mooring chains are significantly decreased due to the torque effects induced by the chain twist.

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