Experimental Research On Suppression Of Vortex-Induced-Vibration Of Marine Risers Considering Wake Interference

With energy crisis and the depletion of oil resources on land,the exploitation and development of offshore oil and gas has become an inexorable trend.Marine riser is a key structural system for offshore oil and gas exploitation,it has far-reaching significance in the research of the vortex-induced vibration(VIV)of marine riser to ensure the exploit of offshore oil and gas.Riser arrays have been widely used in offshore oil engineering due to its economy and flexibility.Compared with an isolated riser,wake interference makes VIV more complicated.Installation of vibration suppression devices is an effective way to suppress VIV and improve fatigue life of a riser.Therefore,the study on marine riser's VIV and different VIV suppression devices not only has important theoretical significance,but also has great economic value in engineering practice.To investigate tandem and parallel marine risers' VIV characteristics and the efficiency of different VIV suppression devices,in this study a series of experiments are conducted in a water tank on both bare risers and risers with VIV suppression devices attached in tandem and sideby-side arrangement,which is made of PMMA.Strain time history responses of the bare riser and those equipped with different suppression devices in both in-line and cross-flow direction are measured.The results show that the bare riser's transverse vibration amplitude increases as the current velocity increases,so as to the vibration frequency,at last reaches the state-“lock in”,it can be seen the riser's in-line vibration frequency is twice as that in the cross-flow direction,and analyze that the vibration characteristics and the dynamic response of bare riser;when two riser are arranged in tandem configuration,the vibration amplitude of the upstream riser is larger than that of the downstream riser due to wake interference,and shedding effect makes the drag load of the downstream riser much smaller than that of the upstream riser;when two risers are in parallel arrangement,the vibration characteristics of two risers are different with each other because of wake interference.When exposed to the same current velocity,the vibration amplitude of risers equipped with suppression devices is conspicuously smaller than that of a bare riser,and VIV suppression devices suppress VIV at different level.Fairing has a good effect on suppressing riser transverse vibration,it can reduce the drag load for upstream riser but enlarge that of downstream riser in tandem arrangement,and fairing has a good performance in reducing drag load for risers in parallel arrangement.Splitter plate performs well in suppressing parallel risers VIV,as well as reducing drag load.Control rods can effectively suppress riser's transverse vibration,for inline vibration,control rods can only take effect for upstream riser in tandem arrangement.It is noteworthy that four control rods have a better performance in suppressing transverse VIV than three control rods.And fairing with caudal horn of 60°,four control rods and splitter with length L/D=1.0 are recommended in this paper.