Experimental Investigation On The Dynamic Responses Of A Free-Hanging Water Intake Riser Under Vessel Motion

As the offshore gas production and gathering system develop to the deep sea resently,the application of water intake riser(WIR)is more andmore widely.WIRproduced severe in-line dynamic responses under thevessel motion,and formed relative oscillation between fluid and riser.The vortexinduced vibration(VIVs)of riser might be stimulated under the relative oscillation flow.A large-scale model test of a free-hanging WIRwas performed in an ocean basin to investigate the riser complexed responses.Fiber Brag Grating(FBG)strain sensors were used to measure the WIR stain of In-Line(IL)and Cross-Flow(CF)directions.Thewavelet analysis,modal analysis are used to analysis the data.Experimental results firstly confirm that the freehanging WIR would experience stable out-of-planeVIVseven for the case with a KC number as low as 5.Meanwhile,comparison between numerical results and experimental measurements suggests a significant drag amplification by out-of-plane vessel motion-induced VIV.What's more,further study on WIR response frequencies and cross section trajectories reveals a strong correlation between vessel motion-induced VIV and KC number,owing to the small KC number effect.The presented work provides useful references for gaining a better understanding on VIV induced by vessel motion,and for the development of future prediction models.