Mooring system fatigue analysis for a semi-submersible

T-N curves, S-N curves, and fracture mechanics (FM) based mooring system fatigue analyses for a semi-submersible are presented. Stress ranges are calculated based on the tension ranges of mooring lines subjected to the combined loading process induced by the motions of wave frequency (WF) and low frequency (LF). A comparison between T-N curves, S-N curves, and FM based mooring fatigue analyses for the semi-submersible is made and the results show that the fatigue lives predicted by the three approaches are in general comparable if the safety factors suggested by API and DNVGL are considered in the T-N and S-N curves based approaches. In addition, the crown section of a mooring chain is prone to fatigue damage compared to bend and weld sections without considering the SCF. A parametric study to investigate the impact of initial crack shape, critical crack depth, and initial crack sizes on fatigue life of a mooring chain is also conducted and the results show that fatigue life of a mooring chain predicted by the FM approach is generally sensitive to initial crack shape and initial crack sizes, however, it is relatively insensitive to the critical crack depth.