| With the development of offshore oil industry into the deep sea, thelength of the riser is increasing from a few hundred meters to severalkilometers. The security of the riser is of extreme importance for oil industryas well as marine environment. Vortex induced vibration (VIV) due to thevortex shedding form both sides of the riser is the main cause for the fatiguredamage. Therefore, it has been an essential issue in ocean engineering toaccurately predict the hydrodynamic force and response of the riser in realmarine environment and optimize the design of the structure.Based on the demands above, a series of riser model experiments wereconducted. By the research on the hydrodynamic characteristics of rigidcylinder under high Reynolds number, we can construct a complete lift forcecoefficient database for the frequency and time-main prediction method, whichcan, to a great extent, improve the precision on predicting the VIV response ofthe full scale riser. From the experiments of flexible risers under uniform andshear current, we mainly focus on the time domain property of VIV. Then, westudied the applicability and limitation of the frequency method by comparingits prediction results with experiment results. Finally, a time domain methodfor predicting the VIV response of marine risers was proposed in this paper,and compared with results of experiment and frequency method. The maincontents and contributions of this thesis can be summarized as follows:1. Hydrodynamic characteristics of a bluff cylinder oscillating intransverse direction in steady current were experimentally investigated atReynolds number of2×105. The effects of non-dimensional frequency, oscillating amplitude and Reynolds number on drag force, lift force and phaseangle are studied. Within subcritical regime, compared with low Reynoldsnumber, high Reynolds number can result in low mean drag coefficient whilehas little impact on lift coefficient and phase angle. Generally to say, liftcoefficient and phase angle are the key parameters in the prediction of VIVresponse of rises. Thus it is reasonable to utilize the vortex induced forces ofthe oscillating cylinder under low Reynolds number into the prediction of thevortex induced vibration of risers in real sea state when the Reynolds numbersare in the same subcritical regime.2. Towing and rotating experiments were conducted to simulate the VIVresponse of flexible riser model under uniform and shear current. We used themode superposition method and wavelet method to analysize the oscillatingamplitude and time domain property of the dominating frequency. We find thatthe lock-in frequency in IL direction is always two times of CF direction evenwhen the nature frequency in still water of the dominating mode in IL directionis not two times of CF direction. In addition, we find the dominating mode hasa characteristics of intermittency both in uniform and shear current. Frequencymethod may no long be applicable for such cases.3. Numerical calculating is conducted in the thesis to study thecharacteristics of flexible risers according to freqeuncy theory. We find that thenumerical results match very well with experimental result only for singlemode response in shear current. The prediction for uniform current andmulti-mode response in shear current is not satisfying. In addition, thenumerical simulation shows that the pre-tention make a great difference in theexcited mode and oscillating amplitude. Therefore, we can tell that thevariation of axial force would influence the VIV response greatly. It is verynecessary for a time domain method to take such the non-linear factor intoconsideration.4. A semi-emperical time-domain method, taking the axial force variationinto cosideration, is proposed in the thesis. We predicted the VIV response of marine risers under uniform, step and shear current, which match very wellwith experiment results. This is a strong proof of the reliability of thetime-domain method. In theory, it can be used for solving most of thenonlinear problems in pipelines. |
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