Experimental Investigation Of Vortex-induced Vibration Of Long-flexible Marine Risers

The primary objective of this thesis is to improve the understanding of vortex-induced vibrations(VIV) for cases with very high order of responding modes,analysis the effect of suppression device,and try to improve the ability of modeling long-flexible risers at high-mode numbers.As part of this thesis work,two separate experiments have been planned and executed in SLCOE(the State key Laboratory of Coastal and Offshore Engineering) of DUT(Dalian University of Technology).The first test was in a flume,length-to-diameter ratio is 120;and the other test was in Ocean Basin,length-to-diameter ratio is 1700.In the first part of the thesis,a systematic framework used for experiment setup is developed,which allows experiment operators choose the number of sensors and address these sensors effectively.In additional,a perfect reconstruction method is developed,which allows us to reconstruct the riser motion from a limited number of sensors placed along its length,including the resonant mode,vibration frequency and displacement associated with the riser responses subjected to steady flows.In the second part of the thesis,laboratory tests were conducted to investigate the effect of mass ratio on the VIV response of marine risers at high-mode numbers by filling air,water and sand in the model tube.It was found that at higher reduced velocity,the vibration frequencies for the three type model tubes are between the vortex-shedding frequency and the natural frequency,and the vibration frequency of hollow tube becomes higher as the reduced velocity increases;with the same velocity in this domain,the higher mass ration yields the lower vibration amplitude,and for hollow tube the lager-amplitude vibrations can be observed in a wide range of velocities.In additional,we find that with the same velocity the higher mass ration yields the higher modal number.The last part describes measurements of the vortex-induced vibrations of a model with an L/D(length-to-diameter ratio) of 1700 which was conducted at SLCOE's Ocean Basin. The riser was tested without VIV suppression and with a suppression device conducted by relative researchers in SLCOE.Testing was performed on an innovative test rig that could simulate uniform currents,with a steel model that featured a dense array of high-quality fiber brag grating(FBG).In line and cross-flow responses were considered.This study reinforces recently published results about the response of bare risers and obtains a lot of valuable date which could be used for numerical prediction models.The findings also indicate that this suppression device can be effective in mitigating VIV response of long risers,and more evident at high velocities.