Numerical Estimation and Experimental Comparison of Vortex-induced Vibration Fatigue in Marine Risers

Abstract

Advancement in computational sciences has made Computational Fluid Dynamics an attractive alternative to simulate fully three dimensional VIV motion for very long marine risers (L/D > 1000). In this study, an unsteady Finite Analytic Navier-Stokes (FANS 3D) method has been applied in conjunction with Large Eddy Simulation (LES) to simulate the flow field around the risers. The riser has been modeled as a tensioned beam and a beam motion equation is employed to calculate the riser displacements. This equation has been discretized using finite difference scheme and is coupled with the FANS 3D solver using a feedback loop to achieve Fluid Structure Interaction (FSI). The riser and flow field are decomposed in two structured grids and the chimera (overset) grid technique is utilized to achieve interpolation between the overlapping grid regions. A modal strain calculation method is developed to extract the strain time series from riser displacements and the rainflow counting algorithm is employed to estimate fatigue damage associated with different riser and current profiles.

VIV fatigue analysis of two riser geometries with L/D ≈ 1400 from NDP 2003 experiments and L/D ≈ 4200 from DeepStar Miami2 2006 experiments, has been conducted. The riser from NDP experiments is placed horizontally and is subjected to uniform and linearly sheared current profiles. A highly-sheared (non – uniform) current is specified to the riser from Miami2 experiments. The results obtained from CFD simulations are compared to the experimental results and published data to verify the accuracy of the flow field solver, riser motion solver and fatigue calculation module.