Fracture Mechanics Based Mooring Fatigue Analysis for a Semi-Submersible Subjected to Triple Narrow-Band Loading Processes

In the general offshore operating locations, mooring systems are normally considered to be primarily affected by environmental loadings induced by waves, wind and current. WF motion induced by first-order waves, LF motion induced by second-order waves and wind would make the most contribution to the fatigue damage of mooring lines. However, it was reported recently that in the Gulf of Mexico (GoM), the fatigue life of mooring lines can be significantly affected by the vortex induced motion (VIM) induced by loop current. The aim of this presented paper is to address the influence of VIM on fatigue life of the mooring system operating at the central of the GoM through performing a fracture mechanics (FM) based fatigue analysis for an offshore mooring system. A frequency-domain mooring analysis for the semi-submersible is conducted where WF motion induced by first-order waves, LF motion induced by second-order waves and wind, and VIM induced by loop current are taken into account. WF motion, LF motion and VIM are treated as three independent loading processes. A wide-band loading combination method is then used for predicting the loading processes acting on the mooring system combining WF motion, LF motion and VIM. A fracture mechanics based analysis is performed to examine the fatigue life of mooring system, in which initial surface cracks in previous existence are assumed to grow from the surfaces of mooring chain links connecting to the fairleads. The stress intensity factor ranges to estimate the crack growth in the FM based analysis are obtained from a finite element (FE) analysis.