A Simplified Method for Analyzing the Fatigue life of a FPSO Mooring System

Abstract

This paper deals with problems in connection with the evaluation of fatigue damage for mooring lines. Fatigue of the mooring system of ship-shaped FPSO units is assed here. The main objective of this paper is to find a new method for estimating the fatigue life with relatively simpler and less time consuming method, but without compromising accuracy. Using this method, it will be possible to check whether the mooring systems satisfy fatigue limit state (FLS) design criteria, especially for FPSO and also for MODUs which are usually not designed against fatigue. It will also give an idea whether a detail fatigue analysis is needed or not. In this method, the approximate fatigue damage of mooring line will be calculated from Weibull equation. But the challenge is to find the appropriate Weibull parameter, (h) value which will give the accurate fatigue damage. After conducting a study it is found that, using the Weibull parameter (h) value of 0.559 for windward line and 0.575 for leeward line; it might be possible to estimate the fatigue damage using the proposed simplified method. Information from the ultimate limit state (ULS) design results and the long term distribution of line tensions will also be used here. Since all FPSOs are designed based on ULS condition, all these information are available beforehand. Therefore, it will reduce the time required for fatigue calculation.