Vortex Induced Motions (VIM) of a Spar platform have significant impact on the performance of mooring and riser systems in terms of both strength integrity and fatigue, especially when the platform is located in high current speed environment. With A/D curves obtained from VIM model tests or CFD analysis, nonlinear time domain analysis tool can accurately predict the motion and mooring tension time history of Spar platforms when subjected to VIM effects. This paper will discuss the time domain analysis of Spar long term and short term VIM fatigue using nonlinear quasi-static program MLTSIM. One advantage of quasi-static time domain program is its high computational efficiency, and since the line dynamics effect is found to be small for a taut leg mooring system, the accuracy is considered to be comparable to fully coupled programs. For mooring analysis, with the tension time history obtained for each environmental bin, the fatigue damage can be calculated using T-N curve and rainflow counting method. For riser/ pull tube analysis, the motion time history may be used in more detailed riser models to obtain stress history, and the fatigue damage can be then obtained using S-N curve and rainflow counting method. Detailed discussion is also given on the impact of polyester stiffness on the mooring fatigue analysis. Example of VIM fatigue analysis of a Truss Spar using cycle counting methods will be presented and the results are compared to those obtained from a spectrum method.
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