Fatigue life assessment of FSRU mooring system

Abstract

KORDI (Korea Ocean Research and Development Institute) carried out FSRU (Floating Storage Re-gasification Unit) project during 2008∼2011 and developed core technologies of FSRU engineering. As parts of the project, time domain coupled solver for body & mooring and fatigue analyzer for mooring lines were developed and they were applied to fatigue assessment of an FSRU system based on coupled analysis. This paper summarizes the results. The FSRU system has a 315 meter long floating body. A turret system with nine catenary steel chains is considered as a mooring type. In coupled analysis, HOBEM (Higher Order Boundary Element Method) is applied to calculate hydrodynamic coefficients and forces of floating body and time domain equation is formulated using convolution. FEM (Finite Element Method) was applied to formulate nonlinear equation of catenary mooring system and Newmark integration is used in time marching. Coupled solution is obtained by iteratively finding compatibility at fairleads. After solving the coupled equation, mooring line tensions are obtained. Then, fatigue life can be evaluated by analyzing those tension components in a statistical manner based on design rules. This paper applied API-RP-2SK and DNV-OS-E301 as a fatigue design rules. In irregular sea states, mooring line tensions generally have two main components such as low frequency component and wave frequency component. So, total fatigue damage should be obtained by combining those components. Three methods such as SS (Simple Summation), CS (Combined Spectrum) and CSDN (Combined Spectrum with Dual Narrow Band) are applicable in combining strategies. This study applied the three methods in evaluating fatigue life of FSRU mooring line and those three methods are compared and discussed from the results. Copyright ？ 2012 by the International Society of Offshore and Polar Engineers (ISOPE).