Experimental Investigation Of Vortex-Induced Vibration Of Marine Riser

Marine risers are the main connecter between floating platform and subsea equipment, which are exposed to various marine environmental conditions such as wind, wave and current. When the current or wave flows across the risers at a certain velocity, there will appear vortex shedding behind the riser cross-section and as a result, the riser will experience vortex-induced vibration (VIV); if the vortex shedding frequency approaches the natural frequency of the riser, oscillation will make the vortex shedding frequency fixed nearby to the structural natural frequency, reaching a state called'lock-in', which will cause fatigue damage. Therefore, the study of marine riser's VIV and its suppression methods not only has scientific significance, but also has great potential economic value in engineering practice.To study marine risers'VIV characteristic, suppress the vibration induced by vortex-shedding and alleviate the fatigue damage, the paper conducted an experiment in the physical oceanography laboratory of Ocean University of China, using a 1.45m plexiglass as a bare riser, another of 1.2m length equipped with suppression devices. When the current velocity varies, time history strain response of the bare riser and those equipped with different suppression devices, in both the in-line and cross-flow direction, were measured. The data were analyzed using the DASP software. The results show that the vibration frequency of the bare riser increases as the current velocity increases, verify that 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 exposed to the same current velocity, the vibration amplitude of risers equipped with suppression devices is conspicuously smaller than that of the bare riser, and VIV suppression devices suppressed VIV at different levels. In addition, employing the relevant module of software DASP, statistical analysis of alternate load the riser endured was performed by the rain-flow counting method which conformed to the riser material's stress-cycle characteristic and the fatigue life was calculated.