WIT Press

Time-frequency Analysis Of Spar Motions


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WIT Press


A. E. Altenberg, J. M. Niedzwecki & J. M. Roësset


In the study of the dynamic response of spars to wave excitation it is normal to look not only at the time histories and maximum amplitudes but also at the frequency content of the motions in order to isolate various nonlinear effects. When dealing with wave basin tests or the results of dynamic analyses of relatively small duration the response may not have reached a steady state condition over most of the duration and it is necessary to look also at the variation of the frequency content with time. In this paper data from wave basin tests on a Spar are analyzed using a time-frequency decomposition with a continuous wavelet transform in order to show their evolutionary nature. Keywords: spars, wavelets, evolutionary response to waves. 1 Introduction The dynamic response of Spars has been an area of considerable interest in the past decade. Extensive experimental studies were carried out at the Offshore Technology Research Center's (OTRC's) wave model basin [1] on two models tested under regular monochromatic and bichromatic waves, as well as irregular waves. Numerical studies were conducted by Mekha et al. [2, 3] and Johnson et al. [4] using an extension of Morison’s equation in the time domain, commonly known as slender body theory. Niedzwecki et al. [5] showed that some nonlinear effects are of similar magnitude but act in opposite directions and thus need to be carefully considered in the analysis. Including some of these terms while neglecting others may lead to widely varying and incorrect numerical predictions. Another approach for the dynamic analysis of floating structures is the use of nonlinear (usually second order) diffraction-radiation theory. Weggel and Roësset [6, 7] carried out analyses with this procedure in the frequency domain. Ran et al. [8] investigated the behavior of spars using second order diffraction in the time domain. This effort was further extended by Ran and Kim


spars, wavelets, evolutionary response to waves.