Measurement of the Instantaneous Characteristics of Natural Response Modes of a Spar Platform Subjected to Irregular Wave Loading

In this work, wavelet analysis is applied to measured responses of a 1:55 scale model spar platform in order to measure instantaneous amplitude and frequency fluctuations of surge and pitch natural modes. The analysis was performed for a spar subjected to irregular wave loading representative of operational conditions, a 10 year storm, and a 100 year survival storm. The first part of this study focuses on the evaluation of the frequency response of wavelet ridges, curves extracted from a continuous wavelet transform for measuring amplitude and frequency fluctuations of a response mode. Using amplitude modulated (AM) and phase modulated (PM) test signals to perform a numerical "swept sine" test, it is found that wavelet ridges for measuring instantaneous amplitude and frequency fluctuations have a frequency response characterized as a low-pass filter with zero phase shift. In the second part of the study, wavelet analysis of the experimental data reveals that the surge and pitch natural frequencies of the spar fluctuate for each of the sea states examined. Furthermore, the wavelet ridge frequency response results indicate that the attenuation of the measured instantaneous amplitude and frequency fluctuations is significant for the spar natural response modes and should he accounted for in quantitative studies. Finally, surge responses were examined in detail and it was found that they have a maximum frequency excursion between 5% and 10% of the average frequency and a phase modulation index of 0.33 to 0.53 with a modulation period of about 1800 s. Pitch natural frequency fluctuations exhibited similar levels of fluctuation for the three sea states and did not appear to have a characteristic modulation period.