Abstract
A fracture mechanics analysis for the mooring system of a semi-submersible accounting for out-of-plane bending (OPB) is presented in this paper. Stress ranges acting on the mooring chain links are calculated based on tension and OPB of mooring chain links induced by motions of wave frequency (WF) and low frequency (LF). The narrow-banded method is used for predicting the combined mooring loading process. Initial cracks are assumed to propagate from surfaces of chain links and stress intensity factors are then calculated in terms of stress ranges determined by a finite element analysis. The influence of the OPB on the remaining service life of mooring chain links is investigated and the results show that the remaining service life of mooring chain links connecting to fairleads is significantly reduced due to the OPB effects.
Highlights
Fatigue failure is one of critical failure modes of offshore permanent mooring systems
For fatigue analysis of mooring chain links connecting to chain wheels at fairleads, chain links passing over bending shoes, or chain linkers provided by chain hawse or chain stoppers, the effects of the of-plane bending (OPB) on the mooring fatigue should be considered [2, 3]
Stress ranges acting on the mooring chain links are calculated based on tension and OPB of mooring lines induced by motions of wave frequency (WF) and low frequency (LF)
Summary
Fatigue failure is one of critical failure modes of offshore permanent mooring systems. Lassen et al [8] and Lassen et al [9] performed a mooring chain fatigue life using an S-N curves based approach, and the behaviour of chain links subjected to tension and OPB was investigated by both experimental and FE methods. Stress ranges acting on the mooring chain links are calculated based on tension and OPB of mooring lines induced by motions of wave frequency (WF) and low frequency (LF). 2. Mooring system A semi-submersible with 16 mooring lines operated at OWA (Offshore Western Africa) is utilized for the mooring analysis in this paper [10]. In the fairlead of the mooring system, seven-pocket chain wheels are installed to guide the top chains operating around the semi-submersible. Ochi-Hubble spectrum [13] is used for the simulation of the semi-submersible motion induced by wave and swell, and Harris spectrum [14] is used for the simulation of the vessel motion induced by wind
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