An Investigation On Vortex-induced Vibration Of A Flexible Riser With Severe Slugging

As an important equipment for connecting subsea oil wells with offshore platforms,risers are usually affected by waves and currents when transporting oil and gas resources.And the risers and currents affected by waves and currents may perform complicated dynamics,such as Vortex-Induced Vibration(VIV).In addition,the gas-liquid two-phase flow transported in the marine riser is prone to severe slugging.The coupling effects of VIV and severe slugging may induce more complex dynamics of the riser.Therefore,there is a need to investigate the VIV characteristics of marine risers under the influence of severe slugging.This paper mainly investigates the phenomenon of VIV of risers under the influence of severe slugging.Firstly,a classical van der Pol oscillator is employed to simulate the dynamic characteristics of the effect of VIV coupled with severe slugging.Then a 2D Computational Fluid Dynamics(CFD)model is developed for capturing the dynamic characteristics of severe slugging type I and II.Based on the mass and velocity of internal fluid from the CFD analysis,the governing equation of the riser with internal flow is discretized by the finite difference method and it is then solved by the Runge-Kutta method.Then,the VIV characteristics of the riser with severe slugging in the uniform outflow are analyzed,and the dynamic behavior in different stages with severe slugging is considered.Finally,the VIV characteristics of the top tension riser with severe slugging in sheared flow are investigated.The effects of different sheared flow,mass ratio and top tension on the response characteristics of the top tension riser with severe slugging are analyzed.The results show that the vibration of the riser with severe slugging exhibits extremely strong periodicity in IL and CF directions because of the periodic characteristics of severe slugging;The dominating modes in CF direction of the VIV of the riser coupled with severe slugging are switched due to the excitation of new mode responses;There is a multi-frequency vibration phenomenon in IL and CF directions of the riser due to the severe slugging,and some of the frequencies are close to the dominant frequency.