Abstract
An efficient method to compute the seismic response of an offshore platform in still water is presented. The structure is modelled by a three-dimensional space frame with lumped parameters and the dynamic response is determined by the mode-superposition method. Both the effects of the nonlinear drag force and the mode-damping coupling are also taken into account. The random nonlinear drag force is linearized by the method of the least square in statistical sense, and the mode-damping coupling is uncoupled by use of the concept of the generalized complex damping. As a consequence some important spectral bandwidths for all vibrational modes can be derived simply in terms of the generalized complex damping ratios and fhe generalized coordinates at resonances.
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