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 𝛿E 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.

Introduction

Many maritime accidents demonstrate that a freak wave is a serious hazard to offshore vessels and structures. Nowadays, more and more occurrences of freak waves have been observed in the oceanographic observations globally. Therefore, it is worth investigating the security of marine structures under freak waves. On the interaction between freak waves and structures, Clauss et al. (2003) investigated the motions behavior and splitting forces of the semisubmersible GVA 4000 under a freak wave through experimental measurements and time-domain simulation. It was found that the maximum response is subjected to freak wave height. Schmittner (2005) investigated the motions and bending moments of floating production storage and offloading (FPSO) and heavy lift vessels as well as the motions and splitting forces of a semisubmersible under a freak wave.

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