Experimental Apparatus Design And Its Effect On Vortex-induced Vibration Of Marine Riser

Marine risers are the main connecter between a platform and the mouth of a wellin the seabed. They are in very harsh environment in the deep ocean and they aresubjected to many kinds of loads including waves, currents, floating platform motionat the upper end, ice, etc. When current or wave flows across the risers at certainspeed, there will appear vortex shedding, and if the frequency of the risers is close tothat of the vortex shedding, oscillation will make the vortex shedding frequency fixednearby the structural natural frequency and cause large amplitude vibration and thefatigue failure of the riser, which is called ‘lock-in’ state.In order to study the VIV characteristics of large length-to-diameter-ratio riserwith internal flow, external flow, different boundary conditions subjected to marinecurrent, tests of VIV were conducted in a large wave-current pool. Model materialswere made of copper, with external diameters of20mm and a total length of6.2m. Asupporting structure, a hinged support, a fixed support and a system which can applytension and convey internal flow are designed based on the size of the pool and theriser model. And the designs’ static analysis and dynamic analysis are been carried on.The supporting structure consists of three sections which connected by high strengthbolts and a bottom made of steel plate. The section at the top whose side wall is madefrom PVC gusset plates is also connected to the side wall of the pool with highstrength bolts. The hinged support can rotate freely both in-line and cross-flowavoiding its axial rotation while the fixed support guarantees free rotation does notoccur. The top tension applied system which connects the weight farmar and thesupport adopts double crown blocks used with high strength steel strand. The internalflow applying system connecting the support deliveries flow by high pressure waterpump and control flow velocity by electronic meter. The experimental data fromoptical fiber grating strain gauge attached to different kinds of risers under22different external flows and different boundary conditions are comparatively studied considering dynamic response of amplitude and frequency. It can be concluded thatthe amplitude of the riser with a hinged-hinged support is larger than the riser with afixed-fixed support under same top tension, internal flow velocity and external flowvelocity. With the increase of external flow velocity, the riser with a hinged-hingedsupport is earlier to undergo ‘lock-in’ phenomena than the riser with a fixed-fixedsupport.