Riser VIV Induced Fatigue Assessment by a CFD Approach
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This paper presents a CFD based fatigue analysis approach for riser VIV induced fatigue assessment in uniform current. The riser has a L/D of 1400. It is horizontally positioned and constant tensioned. The flow field around the riser was discretized into 1.5 million elements, and was solved using an unsteady Navier-Stokes numerical method on an overset (Chimera) grid system. The drag and lift coefficients at each time step were then calculated and applied to the riser structural FEA model through load mapping. Runge-Kutta scheme was employed to integrate the riser beam motion equation in time domain and determine the riser dynamic responses. The bending stress time histories were then generated based on the riser's instantaneous displacements, and the fatigue damages were predicted through Rain Flow Counting technique and Palmgren-Miner's rule. The predicted fatigue results were compared to the previous publications and shear 7's results. General agreements were observed. We also investigated the convergence of the fatigue results, the stress histogram characteristics, and the relation between in-line and cross flow VIV induced fatigue. It is concluded that the presented numerical method is valid and feasible for riser VIV induced fatigue calculation. Copyright 2008 by The International Society of Offshore and Polar Engineers (ISOPE).
