Study on Vortex-induced Vibration Characteristics of Large Slenderness Ratio Tandem Risers

Abstract

The fluid mechanics software-Fluent was used to simulate vortex-induced vibration (VIV) of flexible tandem risers with large slenderness ratio. In this paper, VIV of a single riser and the flexible tandem risers with large slenderness ratio at a spacing of 2-16 times were numerically simulated, which was based on slicing principle for 2-DOF vortex-induced fluid-structure coupled vibration. The ratio of vibration frequency between upstream and downstream risers, dimensionless amplitude and the effects of reduced speed of the risers were discussed under different spacing conditions. The preliminary results show that the VIV characteristics of the upstream riser is similar to the single riser, "lock-in" occurs obviously, the frequency ratio of lift force and drag force is 1:2 over the entire reduced velocity range, and there is no obvious "lock-in" in the downstream riser due to the influence of the wake of upstream riser. The maximum cross-flow amplitude reaches 0.9 times the diameter of the riser, and complex coupling phenomena such as the 1:1.5 times frequency ratio and double "8" track were observed.