Improved in-line VIV Prediction for combined in-line and Cross-Flow VIV Responses
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Original versionASME 2017 36th International Conference on Ocean, Offshore and Arctic Engineering 10.1115/OMAE2017-61715
Slender marine structures are subjected to ocean currents, which can cause vortex-induced vibrations (VIV). Accumulated damage due to VIV can shorten the fatigue life of marine structures, so it needs to be considered in the design and operation phase. VIV prediction tools are based on hydrodynamic coefficients, which are obtained from forced motion experiments on a circular cylinder. Most of the forced motion experiments apply harmonic motions in either in-line (IL) or cross-flow (CF) direction. Combined IL and CF forced motion experiments are also reported. However, measured motions from flexible pipe VIV tests contain higher order harmonic components, which have not yet been extensively studied. This paper presents results from conventional forced motion VIV experiments, but using measured motions taken from a flexible pipe undergoing VIV. The IL excitation coefficients were used by semi-empirical VIV prediction software VIVANA to perform combined IL and CF VIV calculation. The key IL results are compared with NDP flexible pipe model test results. By using present IL excitation coefficients, the prediction of IL responses for combined IL and CF VIV responses is improved CF responses. Forced motion tests with two dimensional harmonic motions have been carried out by . Realistic orbits measured from a flexible beam VIV model tests were applied in forced motion VIV model tests . This experimental method was first applied by , and further used by  and . Both non-periodic time history of the motions and representative periodic motions were used in the experiments, see Figure 2. The hydrodynamic coefficients from the periodic motion tests were calculated and presented in . The sensitivity of the hydrodynamic force and vortex shedding modes on the different realistic orbits were investigated in . It was found that harmonic orbits had larger uncertainties to predict VIV than realistic orbits, and that IL motions can result in large higher order force components. Results from non-periodic and periodic forced motion VIV tests were compared in . Depending on the response types, for quasi-periodic VIV responses, periodic orbits are representative for non-periodic time histories; while when the responses are partly or fully chaotic, the hydrodynamic coefficients calculated from tests with selected periodic orbits have larger uncertainty or fail to represent the entire time history.