Numerical Simulation Of Fluid-structure Coupling Of Riser Vortex Induced Vibration

As the key equipment of offshore oil and gas development,offshore oil risers need to verify many indexes,among which service life and reliability are the most important.With the deepening of ocean exploration,many flexible risers with large aspect ratios have been put into use,which has encountered some difficult problems.For instance,when the riser is subjected to flow load,it will produce periodic vibration,especially when the natural frequency of the riser is close to the tail vortex shedding frequency,resonance will occur,which is also known as vortex induced vibration.This phenomenon will make the riser fatigue damage and cause economic losses.Firstly,this paper summarized the main of research methods of vortex induced vibration of riser at home and abroad.In addition,the subsequent work of riser's vibration has been carried out based on the computational fluid dynamics and fluid-structure interactions method.Secondly,the 2-D static cylinder and vortex induced vibration mathematical model of elastically supported cylinder have been established,and the computational fluid dynamics method has been applied to study the change laws of lift-drag coefficient,pressure coefficient under the subcritical layer range.Moreover,based on the user-defined function,the differential equation of structural vibration has been embedded into the computational fluid dynamics solver Fluent,and the fourth-order Rouge-Kutta method has also been used to calculate.Furthermore,analysis has been focused on the lift-drag coefficient,maximum lateral amplitude,vibration frequency under the change of mass ratio.In the same time,the difference between single and double degrees of freedom have also been compared.The results show that the increase of mass ratio will decrease the maximum lateral amplitude,but when the mass ratio is greater than 5.5,the maximum amplitude finished change.Besides,the effect of the mass ratio on the hydrodynamic coefficient is related to the response region,and the enhance effect of freedom for the lateral amplitude will disappear when the mass ratio greater than 5.5.Thirdly,the 3-D riser' s vortex induced vibration response has been carried out based on the System Coupling module of Ansys Workbench.With simplifying the ocean undulate inflow as a sine pulsating flow,the effect of pulsating frequency on the riser's vortex induced vibration also have been explored.Meanwhile,a pulsating frequency has been selected as the velocity inlet of the model of tandem double risers and staggered four risers to study the vibration response of riser groups.The results show that the pulsating flow will reduce the vibration response,and there is a critical pulsating frequency which makes the lateral amplitude reach the maximum value.Besides,the flow direction vibration frequency is mainly performed the pulsating frequency,the lateral vibration frequency remains unchanged,and the vibration frequency of double and four risers is consistent with the single ones.As for tandem double risers,the upstream riser will reduce the amplitude of the downstream riser,and there is also a phase difference between the lateral vibrations.However,as for staggered four risers,the upstream risers will enhance the amplitude of the downstream risers,and the amplitude of the upper and lower risers will be greater than the single ones,because the existence of the upper and lower risers will reduce the phase difference between the vibration of the left and right risers.The research content of this paper can be applied to the prediction of vortex induced vibration response of oil riser in which provide a certain reference for the future study of vibration phenomena under more complex incoming flow conditions.