In model testing of offshore platforms subjected to irregular waves, it is customary to generate waves to match a desired power spectrum. Tests may be conducted on one or a few sets of these irregular waves. One must consider however that the results obtained for different realizations of a random sample matching the desired spectrum will be generally different. It is important therefore to have not only the results for a few tests but also to have an idea of the variability inherent in these results. This paper presents results of studies conducted at the Offshore Technology Research Center intended to investigate the statistical variation in the response of a Spar platform subjected to irregular waves generated to match a specified wave spectrum. The results presented include time histories of response as well as the Fourier transforms of these motions. Of particular importance in these spectra are the peaks at the natural frequencies of the platform associated with nonlinear (difference frequency) effects. The analyses were conducted in the time domain using slender body theory with a number of modifications intended to account for various diffraction effects. Results were obtained including or omitting different sources of nonlinear behavior.
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