Numerical Analysis Of Vortex-induced Vibration Of Top-tensioned Risers Under Platform Heave Motions

The development technology of marine oil and gas resources has long been widely concerned by a large number of experts and scholars.As an important part of the marine oil and gas production system,the marine riser system is an important channel connecting the sea platform and the subsea wellhead,and is also one of the weakest links in the platform structure.Under the action of marine environmental load,the vortex-induced vibration(VIV)may occur in the marine riser.With the large number of floating platforms,the top platform has a relatively larger range of motion.The platform motion has a great influence on the VIV characteristics of the riser itself.This paper focuses on the platform heave motion,and carries out numerical analysis on the vortex-induced vibration of the top tension riser based on the viv-FOAM-SJTU solver,which is developed by the open source CFD framework OpenFOAM.This paper first expands the existing viv-FOAM-SJTU solver and introduces the influence of platform heave.In this paper,the effect of platform heave motion is transformed as the varying axial tension,which is assumed to change in sinusoidal form with time.The variation of the axial tension is defined by three parameters: the heave amplitude,the heave frequency and the initial phase angle.After completing the expansion of the solver,this paper selects three standard model experiments and sets the same conditions as the experiments for calculation.The calculation results are in good agreement with the experiments,and the accuracy of the solver is verified.After that,this paper selects a typical condition with variable tension and two corresponding constant tension conditions through the equivalent principle.Numerical simulations are carried out to analyze the differences of vibration displacement,frequency component and vibration mode.The results are compared from the displacement and curvature in both in-line and cross-flow directions.The results show that the average displacement and the average curvature mainly depend on the pre-tension,while the standard deviation of the displacement and the standard deviation of the curvature are determined by the platform heave.Finally,this paper analyzes the parameters of the studied problem and discusses the influence of the amplitude and the frequency of the platform heave motion on the VIV of the riser.The results show that the platform heave significantly amplifies the VIV of the riser itself.The amplitude of the vibration increases with the increase of the amplitude of the heave,and the influence of the heave of the platform gradually decreases with the increase of the flow velocity.The low-frequency heave of the platform can directly control the in-line vibration frequency,exhibiting obvious periodic vibration and obviously amplifying the vibration displacement.The high-frequency excitation can suppress the displacement amplitude to a certain extent,but it will significantly enlarge the curvature of the riser and aggravate the influence of fatigue damage.