A Volterra-model method is used to create the time series of second-order nonlinear wave force by employing the frequency response function, which forms the Fourier transform pair with the impulse response function. The linear and quadratic frequency response functions for the wave forces are hydrodynamically computed using HOBEM. The convolutions of the impulse response functions with the given random wave and random wave*wave yield the nonlinear wave forces in the time domain. As an example, this method is applied for the simulation of nonlinear responses of a TLP in the long-crested seas.
This paper presents a practical method for the time-domain simulation of nonlinear wave forces on a floating structure in random seas. Time histories of nonlinear wave forces are essential for the simulation of nonlinear responses of offshore structures. For instance, the slowly and rapidly varying dynamic responses of a Tension-Leg-Platform (TLP) are due to the above nonlinear hydrodynamic wave forces. Conventional methods to generate the time series of wave elevation employ the given wave spectrum and linear superposition theory (random phase). These may lose natural randomness and groupness as discussed by Burcharth (1981) and Tucker et al. (1984). Besides, it needs enormously dense frequency resolution to produce a long Gaussian wave time series. To avoid the foregoing difficulty, frequency-disturbance method was introduced by Shinozuka and Jan (1972), but it failed to produce Gaussian waves. These disadvantages can be avoided if we use a linear filter with Gaussian random signals, where the linear filter is derived from the specified wave spectrum. The wave time series derived in this manner will have practically infinite repeating period with randomness and groupness resembling the natural waves. Zhao (1996) developed an authentic Volterra method recently.