Spectral analysis for nonlinear wave forces

The spectral analysis of nonlinear wave forces is investigated based on the recently introduced Dynamic Morison equation. This equation accounts for nonlinearities associated with history or vortex shedding effects accurately, in contrast to the standard Morison equation which has undesirable features in this respect. An expression for the response power spectrum is initially derived in terms of the higher-order nonlinear frequency response functions and the spectrum of the input velocity. It is then shown how this can be evaluated for the Dynamic Morison model to yield an expression for the output power spectrum in terms of the coefficients of the time-domain differential equations and properties of the input. A comparison of the output spectra computed using traditional methods and the new expression of the response spectrum over several data sets is included. An interpretation of the Morison equation response spectrum is finally given. (C) 1998 Elsevier Science Ltd. All rights reserved.