Research On The Flow-Induced Vibration Characteristics And Influence Factors Of The Marine Riser

As an important part of underwater oil and gas production system,marine risers act as a connection between offshore platforms and ocean floor producers.It is known as the lifeline of offshore oil and gas fields with cables,but at the same time it is the weakest link in the whole production system.During service,the riser is always under dynamic load caused by floater motions,ocean current,wave and two-phase flow in the pipe,resulting in fluid-induced vibration,which may lead to overstretch,bending deformation,fatigue damage and other problems,seriously endangering the safety of pipeline operation.In view of this,this paper studies the characteristics of marine riser flow induced vibration and its influencing factors.The main research contents and conclusions are as follows:Based on the similarity criterion,the two-dimensional and three-dimensional calculation models of marine riser is established,and the fluid-structure coupling computation of two-phase flow phase field and solid mechanical physical field is carried out.The longitudinal analysis of the gas-liquid two-phase flow characteristics and the dynamic response characteristics of the pipeline is carried out by combining the two models.The flow state in the riser is divided into the filling stage and the periodic generation stage of the liquid plug,and the riser is in the two stages of bending deformation aggravation and relatively stable flutter state respectively.The inflow induced riser flutter is mainly caused by longitudinal vibration,and the inclined upward zone of the riser is the flutter transition zone.The form of flutter will gradually change from longitudinal vibration to lateral vibration.The fixed end at the bottom of the riser has a large bending deformation,while the one at the outlet has a large tensile deformation.A total of 45 sets of simulation calculations are carried out by changing fluid flow parameters(superficial velocities of gas-liquid phase,dynamic viscosity of liquid phase,liquid holdup)and riser structure parameters(riser deflection angles,riser thickness)and current parameters(uniform flow,linear shear flow and ocean current velocities)respectively based on the control variable method.The effect of various factors on the dynamic response of the riser induced by two-phase flow is analyzed horizontally.The liquid superficial velocity and liquid holdup show a significant positive correlation with the degree of deformation of the riser.In contrast,the gas superficial velocity and liquid dynamic viscosity have less influence on the dynamic response of the riser.Increasing the deflection angle of the riser will increase the fluid force on the pipe wall and increase the deformation of the riser.Increasing the wall thickness enhances the ability of the riser to resist deformation,thereby eliminating vibration.Under the synergistic action of ocean current and the two-phase flow,the internal and external walls of the riser will be deformed asynchronously.And the synchronization of internal and external walls deformation in cross-flow direction is better than that in in-line direction.The research results are conducive to further understand the flow-induced vibration characteristics of the marine riser.And it provides theoretical basiss for the design of restraining measures for vibration and flow instability in the offshore oil industry,and ensures the safe operation and maintenance of submarine pipelines.