| A single point soft yoke mooring system is composed of a turret single point mooring structure and a soft yoke which provides resilience for moored floating platforms. Then, the floating offshore platforms can keep long-term operating in a relatively fixed position. The motion responses of platforms in irregular waves are very important and must be considered during the design, operation and maintenance processes. Physical modeling tests are effective tools for studies on the motion responses of platforms in ocean waves. It can simulate the motions of platforms as realistic as possible. In the real ocean, the motions of platforms are always non-stationary owe to the nonstationary characteristics of waves. Therefore, methods based on Fourier transform are not optimally suitable for the analysis motion responses. However, the wavelet transform is a popular tool for analysis non-stationary singles due to its multi-resolution characteristics. In the field of ocean engineering, the wavelet transform is applied successfully in many studies. However, studies on analyzing motions of floating platforms are rare. In the present study, a physical model experiment on the motion responses of a single moored floating platform is carried out. Then, the motion responses are analyzed using the wavelet transform. This dissertation concentrates on three aspects of the motions: de-coupling free decay motions, the correlation between waves and the induced motions of the platform and the influence of the wave groupiness on the motions of platform under circumstances of combination of winds, waves and currents.First of all, the experiment is introduced in detail on the following five aspects:the layout of the experiment, the platform and the mooring system, the instruments, the experimental conditions and experimental contents. Then, the basic principles of one dimensional continuous and the discrete wavelet transform are presented. Furthermore, some methods based on wavelet transforms are illustrated.In the free decay test, the main analyzing work is focusing on the free-roll decay experiment. Using the wavelet coherence spectrum, it is demonstrated that the heave and pitch will significantly affect the roll in time-frequency domain. Then the discrete wavelet transform is used to de-couple the raw signals and found that the db20 wavelet is the most suitalbe mother wavelet.For the motion responses of platform under action of irregular waves, the discussion is focused on the time-frequency coherence between the waves and the surge, heave and pitch motions. It was found that the motions are closely related to the groupiness of waves. Local heave and pitch can be enhanced in wave trains with large local groupiness. The appearance of extreme surge motion can be ascribed to wave motions in both low frequency and wave frequencies. Furthermore, the local extreme pitch motion was mainly induced by the wave motions with frequencies that are close to the natural frequency of the pitch motion of the platform. By comparing the results of wavelet transform and Fourier transform, it is proved that the wavelet transform has more advantages in the analysis of non-stationary waves and the induced motions of the platform.Lastly, the influence of groupiness of waves on the surge, heave and pitch of the platform under the combination of wind, wave and currents are studied. In the present study, wave groupiness is quantized by two factors, group height factor GFw and group length factor GLw which are defined by continuous wavelet transform. The results show that the maximum wave height increases with an enhancement of GFw, but decreases with increasing of GLw. Hence, the same changes have occurred for the surge, heave and pitch of the platform. Constant winds and steady current with the same direction as the waves will both reduce the wave height and not affect the groupiness of wave trains. The influence of groupiness on the surge response of the platform will be strengthened because of the action of wind and current. |
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