On The Bionics-Based Vortex-Induced Vibration Mitigation Technique For Marine Risers

When fluids past a cylinder,vortices are shed on both sides of cylinder.The vortexshedding alters the local pressure around the cylinder and exerts alternating forces which will cause the riser to experience vortex-induced vibration(VIV).VIV is one of the important reasons for fatigue damage of marine risers and circular structure.In order to reduced or eliminate the damage caused by VIV to marine risers and prolong the service life of risers,the vortex-induced vibration suppression device is applied.Inspired by the giant saguaro cactus,different types of cactus-shaped cross-section suppression devices were designed to reduce the impact of vortex-induced vibration on the marine riser.In this study,based on dynamic gird technology and user-defined functions of FLUENT,the two-degree-of-freedom vortex-induced vibration of circular and cactus-shaped crosssection riser was simulated by ANSYS FLUENT,and the vortex-induced vibration response of cactus-shaped cross-shaped risers with cross-sectional shapes was analyzed,and the crosssectional shape with better suppression effect was selected.Then,the suppression effect of the cactus-shaped cross-sectional on the vortex-induced vibration of the single cylinder was verified by the water tunnel test.At the same time,the pitch of the cactus-shaped cross-section was changed,and the best pitch for suppressing vortex-induced vibration was determined by the water tunnel test.Finally,the water tunnel test was used to study the suppression effect of the spiral cactus-shaped suppression device on the vortex-induced vibration of the flexible cylinder array.The study results that:(1)The two-degree-of-freedom vortex-induced vibration of flexible cylinder can be accurately simulated by dynamic grid technology and user defined function of FLUENT.(2)Water tunnel test results show that the vortex-induced vibration of single pipe and riser array can be well suppression by the R8-0.04 suppression device,and the R8-0.04 flexible cylinder has a lower drag coefficient.(3)the water tunnel test shows that the vortexinduced vibration of upstream cylinder of flexible cylinder array can be suppression by the R8-0.04 sleeve with a pitch of 0 D and 15 D.However,the R8-0.04 sleeve with a pitch of 0 D and15 D have poor suppression effect on the vortex-induced vibration of the downstream cylinder in the flexible cylinder array.In this study,it should be noted that the cactus-shaped crosssection can be further optimiaed to improve its suppression effect on vortex-induced vibration of the cylinder,especially the downstream cylinder in the flexible cylinder array.In addition,the preliminary conclusions obtained in this study,which need to be verifird by a large number of tests.