Vertical Riser VIV Simulation In Sheared Current

Abstract

This paper studied a vertical riser VIV under sheared current using numerical simulation and presented the results and their comparisons to published experimental data. The riser was made of a 9.63m brass pipe with an OD of 0.02m (L/D=482), and mass ratio of 1.75. In the experiment the riser was positioned inclined with top tension of 817N, and pinned at its two ends to the test rig. Rotating the rig in the wave tank would simulate the sheared current. In our numerical simulation we pinned the riser’s ends to the ground, and imposed a linearly sheared far field incoming current. The riser and its surrounding fluid were discretized using 1.5 million elements. The flow field was solved using an unsteady Reynolds-Averaged Navier-Stokes (RANS) numerical method in conjunction with a chimera domain decomposition approach with overset grids. The riser was also discretized into 250 segments. Its motion was predicted through a tensioned beam motion equation with structural damping. The external force terms were obtained by integrating viscous and pressure loads on the riser surface. We then processed the critical parameters including riser VIV a/D, vorticity contours, response histories and spectra, and VIV induced fatigue. Finally, comparisons were made to the experimental data and conclusions were drawn. In general the VIV simulation results agree well with the experimental data. It is concluded that the present CFD approach is able to simulate the vertical riser VIV in sheared current. Furthermore, it can also predict the VIV induced fatigue damage as well.