Study On The Characteristics Of Fluid Structure Interaction Of A Marine Riser Conveying Internal Flow With Pulsating Velocity

Marine riser is one of the most important parts of the offshore oil production and can be used to convey gas,oil as well as transfer information.In the actual working process,the riser can be vibrated by a variety of environmental loads,resulting in cumulative fatigue of the structure,and even breach in extreme cases.The common vibration forms are the vortex induced vibration caused by the tube outflow,the parametric vibration by the fluctuating internal flow in the tube or the variable axial force on the top end.In recent years,the mechanism and the characteristics of vortex-induced vibration of the marine riser have been paid much attention by the engineering and academic world,and nice research results have been obtained.A large number of research results have been obtained for the nonlinear dynamic characteristics of the pipeline under the influence of internal fluid.However,there are few reports on the characteristics of fluid-structure interaction of a marine riser,considering the fluid in the riser and out of the riser.Based on the above deficiencies.In this paper,the dynamic characteristics of a marine riser under the influence of external vortex force,fluctuating internal flow and variable axial forces.The motion control equation of the riser conveying the fluid is established through the analysis of the force of the structural element and the fluid element.Then the equation is changed into a dimensionless equation.The governing equation is discretized by the Galerkinmethod.The autonomous differential equation is transformed to the non-autonomous differential equation by averaging method.The stability of the time-varying system is analyzed.Finally,considering the coupling effect of the vibration of transverse flow and in-line flow,the coupling response characteristics of the parametric vibration and the vortex excited vibration are analyzed.The following important conclusions are obtained:(1)Pulsating internal flow can cause dynamic instability of the pipe conveying fluid,and fluid damping,axial force,mass ratio and other parameters will affect the instability characteristics in varying degrees;(2)Under certain conditions,the fluctuating internal flow and the variable axial force may seriously affect the vortex induced vibration response of the marine riser,and the influence mainly exists in the frequency locked region;(3)The Quasi-periodic motions of the marine riser may occur under the influenceof fluctuating internal flow and variable axial forces.But the periodic motion of the marine riser will occur under some values of flow speed out of theriser.The periodic motion of the marine riser will occur more easily under the variable axial forces than under the pulsating internal flow.