3D VIV fatigue analysis using CFD simulation for long marine risers
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Copyright 2016 by the International Society of Offshore and Polar Engineers (ISOPE). Computational Fluid Dynamics (CFD) offers a powerful tool to simulate the fully three-dimensional Vortex Induced Vibration (VIV) around flexible bodies such as marine risers. In this paper two riser geometries with L/D = 1400 from Norwegian Deepwater Program and L/D = 4200 from Miami experiments in Gulf of Mexico are studied. The former case includes the VIV fatigue prediction of a horizontal riser in uniform currents and shear currents. The latter case has the VIV simulation of a vertical riser in non-uniform current. The results from both studies are validated using field experimental data. The motion of the riser is solved using a tensioned beam motion equation discretized with a finite difference scheme. The flow field around the riser is obtained by unsteady Navier-Stokes equation along with Large Eddy Simulation (LES) method. The flow field solver is coupled with the riser motion solver using a feedback loop to get the Fluid Structure Interaction. An overset (chimera) grid system is applied to decompose the structure and flow field in blocks of overlapping grids. Both inline and crossflow motion are resolved and processed to obtain the corresponding fatigue damage. The instantaneous displacements obtained at different axial locations on the risers are converted to bending stress time series which are used in rainflow counting algorithm with Palmgren-Miner's rule to obtain fatigue damage.
