Numerical simulation of vortex-induced vibration of a marine riser with a multiphase internal flow considering hydrate phase transition

Abstract

To investigate the effect of a multiphase internal flow with hydrate phase transition on the vortex-induced vibration (VIV) of a marine riser, a numerical model is established that first solves the flow parameters, and the natural frequencies of risers with different gas intake ratios are obtained. Then, a VIV equation that considers the effect of the multiphase internal flow is established. The Newmark - β method is used to solve the VIV equation in the time domain. Finally, the effect of the gas intake ratio on the natural frequencies and the vortex induced vibration response is studied. The results show that the natural frequencies increase with an increase of the gas intake ratio, and the changing trends of higher modes are quite obvious. When the riser is subjected to a linear shear flow, the VIV show a multi-mode response, and the structural response changes from high-order mode dominant to low-order mode dominant with an increase of the gas intake ratio. This can be explained in terms of the natural frequency of riser, which increases with the increase of the gas intake ratio. Moreover, the spanwise reduced velocity decreases, which makes frequency locking occur more easily in the low-order modes.