Responses of a spar platform in random waves and currents (experiment vs. theory)
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The response characteristics of a large slack-moored floating spar in regular waves, bichromatic waves, and unidirectional irregular waves with or without sheared currents are investigated by experiment and numerical method. The experiment with 1:55 scale model was conducted in the Offshore Technology Research Center (OTRC) three-dimensional wave basin at Texas A&M University. A time-domain nonlinear motion analysis computer program was developed to numerically simulate the performance of the spar for various wave and current conditions. It was found that the linear wave-body interaction theory alone was not adequate, and the second-order wave-body interaction theory including the effects of viscous and wave drift dampings had to be used for the reliable motion prediction of a spar. The complete first- and second-order diffraction/radiation programs for bichromatic, bidirectional waves were developed using the higher-order boundary element method (HOBEM), and the viscous drag forces were computed from the Morison drag formula based on the relative velocity squared. The wave drift damping was computed using the modified drift gradient method. The two-term Volterra series was used to calculate the time series of nonlinear potential excitations in random seas. The resulting numerical results agreed well with our measured data. It was seen that the low-frequency surge and pitch responses were in general greater than the wave-frequency responses, and the slowly varying responses were appreciably reduced in the presence of currents. The total response amplitudes were found to be practically acceptable in the survival condition characterized by a 100-year storm sea.