The Study On Viv Responses Of A Free-Hang Riser

Free-hanging risers are widely used in deep-sea oil and gas development for its economy and efficiency.Due to the interaction of the vessel motion and the internal flow,the free-hang riser will have strong dynamic response in the inline direction(IL)and thus forming a severe relative oscillatory current with the surrounding water.This oscillatory flow is very likely to induce vibration of the riser in the cross flow(CF)direction(Vortex-induced Vibration,VIV).A large-scale model test of a free-hanging riser was performed in an ocean basin to investigate the riser responses under vessel motion and internal flow.In the test,Fiber Brag Grating(FBG)strain sensors were used to record the strain time history of some points in CF and IL direction of the riser.Then the wavelet analysis,modal analysis and other methods were used to analyze the data.In this paper,the distributions of strain,flow velocity,KC number and vortex shedding frequency were studied by both experiment and numerical simulation,the influence of internal flow was studied result confirmed that vortex induced vibration(VIV)will occur in large diameters free-hang risers under the influence of internal flow and oscillation current,but in the flow range studied in this paper,internal flow velocity have little effect on natural frequency,KC number,the distribution of amplitude and VIV,and the effect can be seen as negligible.Even if the internal flow had little effect on the WIR response,the WIR will undergo different response under the same vessel motion case,which proved that the VIV induced by vessel motion is uncertain.The fatigue damage of vortex-induced vibration(VIV)of the water intake riser under vessel motion is related to the KC number.The distribution trend of the inline fatigue damage values along the riser was similar to that of the in-line strain RMS values along the riser.The magnitude order of the fatigue damage value in the cross-flow direction was 10-2 times as large as that in the inline direction.This paper improves our understanding of the VIV of free-hanging risers under the coupling effect of vessel motion and internal flow,which is of great help and reference value for the future development of VIV prediction model.