Fatigue Damage Analysis Of Unbonded Flexible Riser Caused By Platform Vortex Induced Motion

In this paper,the Spar platform and unbonded flexible risers are selected as the research objects.The vortex-induced motion of the platform under ocean currents and the fatigue damage of the lower risers caused by the platform are studied.Simulation of the dynamic response process,the influence of vortex-induced motion related parameters on the fatigue damage of unbonded flexible risers was studied,in order to provide new ideas and methods for the suppression of vortex-induced motion and the structural optimized design of unbonded flexible risers.Firstly,using numerical calculation as a research method,Fluent software as a numerical calculation tool,overlapping grid technology as a way to update the flow field information,combined with the vortex-induced motion characteristic equation subroutine,numerical simulated the response of the platform's vortex-induced sway motion under different incoming velocity.It was found that the dimensionless amplitude ratio presents"increasing-decreasing"with the increasing of reduced velocity U_r,when the reduced velocity U_ris between 3.34 and 7.8,the vortex-induced motion enters the lock-in state.At this time,the maximum dimensionless amplitude ratio is 0.7D,which is within 10%of the error of the model test results,indicating that the overlapping grid technology can correctly update the flow field information in the vortex-induced motion..Secondly,based on the ABAQUS/AQUA module,the time-domain dynamic analysis of the unbonded flexible risers under the vortex-induced motion of the platform was carried out,and the stress,tension and displacement of the flexible risers under different reduced velocity were obtained.The research results show that:when the platform is affected by the vortex-induced motion to reach the near point,the fatigue life of the ground contact area is at least 384 years,and the fatigue life at the far point is greater than that at the near point.the platform vortex-induced motion enters the lock-in state.The risers is located at the near-point and the far-point,and the reduced velocity does not have a great influence on the life of the overhanging section.The fatigue life of the ground contact area shows a trend of decreasing and then increasing with the increase of reduced velocity.Finally,a local model of an eight-layer unbonded flexible riser was established by shell elements and solid elements.Local mechanical properties were analyzed by ABAQUS/Explicit slover.It was found that the stress value of the tensile armor layer was greater than other structural layers,and the stress value of the tensile armor layer is the largest.