Time-frequency Domain Characteristics on the Dynamic Response of a Moored Floater Under a Freak Wave by Wavelet Analysis

Regarding freak wave actions on structures, the majority of existing studies focus on the time-domain statistical characteristics of floaters. For the limited frequency-domain studies available, they were analyzed by adopting a conventional Fourier transform, which cannot provide the variation of energy in a time series. However, a freak wave is a spike in a random wave series, and hence the local characteristics in a time domain are of key importance. Compared with Fourier transfer, the wavelet analysis method is more effective in obtaining the energy spectral density as well as energy distribution of each frequency in time domain, especially the instantaneous physical changes under freak waves. By adopting wavelet analysis, this study manages to figure out the time-frequency domain characteristics on the dynamic responses of the moored floater under a freak wave through extensively experimental investigations. The results show that the wavelet analysis method is effective in obtaining the energy spectral density and the energy distribution of each frequency in time domain. There is a significant variation on the time-frequency domain characteristics of the dynamic response under a freak wave. The energy parameter delta(E) of surge, heave, and pitch in freak waves can be up to 2.48, 1.88, and 1.65 times of those under random waves, respectively.