Analysis Of Vortex Induced Vibration Characteristics Of Riser Group Under Wake Interference

With the increasing demand for offshore oil and gas resources in China,riser group is widely used in offshore oil and gas development engineering for its characteristics of low cost and small space occupation.When the riser is lashed by the sea current,it will lead vortex-induced vibration of the riser.Especially in the riser group,the downstream riser is affected by the wake interference effect of the upstream riser,and the vortex-induced vibration of the downstream riser will be more complex.Therefore,it is necessary to analyze the vortex-induced vibration under the wake interference effect of the riser group.In this paper,Fluent and Transient Structure software in the ANSYS Workbench platform are used as solvers and System Couping is used for data interaction between flow field and structure field.The problem about vortex-induced vibration of marine risers is simulated,and the hydrodynamic coefficients obtained from the simulation are compared with the data from other scholars to prove the feasibility of numerical simulation technology in the study on vortex-induced vibration.Based on that,B.sannati's experimental parameters are taken as the reference of modeling,a calculation model of the string and parallel riser group is established.For the established calculation model,first of all,calculate the first six natural frequencies and modes of the riser under different top constraints,and analyze the influence of the top constraints on the natural frequencies and modes of the structure itself;secondly,carry out three-dimensional fluid structure coupling numerical simulation for different top tensions,different riser intervals,parallel risers and other conditions,and analyze the top constraints on the opposite pipe group The influence law of vortex induced vibration response.The simulation results show that for the series riser,the amplitude of the downstream riser is larger than that of the upstream riser due to the influence of the upstream tail flow,and the shield effect of the upstream riser in the downstream direction,the vibration amplitude is smaller than that of the upstream riser,but the amplitude fluctuation is larger.At the same time,increasing the top tension will increase the rigidity of the riser structure and reduce the vibration amplitude.For the series riser with different riser spacing,increasing the riser spacing will reduce the interference effect of upstream riser wake on downstream riser.For parallel risers,the vibration law of the upper and lower risers is similar,but because the upper riser is affected by the clearance flow,the vibration clothing has a small range of fluctuation,and the amplitude of the upper and lower risers near the clearance flow positionappears a "suppression" phenomenon,which is the same as that of the series risers.Increasing the top tension can effectively reduce the vibration amplitude of the riser,which is most obvious when the top is fully constrained.