T-N Curves and Fracture Mechanics Based Mooring Fatigue Analysis for a Semi-Submersible

Abstract

This paper is to perform T-N curves and fracture mechanics based fatigue analysis for mooring lines of a semi-submersible installed in Gulf of Mexico (GoM). The wave frequency (WF) and the low frequency (LF) load processes are regarded as two random processes and the load combination of the two processes is considered. Frequency-domain analysis is then conducted for calculating the tension ranges along hybrid-material mooring lines induced by motions of WF, LF and the combined WF and LF. The narrow-banded spectral method is used for calculating the mooring line tension and the fatigue damage of mooring lines is estimated by T-N curves and fracture mechanics based approaches. The fracture mechanics based analysis is combined with a finite element analysis to predict crack propagation at different locations of a studless chain link. It was found that the crown section of a mooring chain is the most critical location subjected to fatigue failure in a studless chain link. The most critical points prone to fatigue failure of the catenary and taut mooring systems designed for the semi-submersible are at the bottom end of top line and the top end of bottom line, respectively. Also, fatigue lives of mooring lines in the catenary mooring system are generally longer than those of the taut mooring system. In addition, a comparison between fatigue lives of mooring lines predicted by T-N curves and fracture mechanics based approaches shows that the results estimated by both approaches are generally comparable.