The effects of wave grouping on the slow drift oscillations of floating moored structures

Murray, John James (1984) The effects of wave grouping on the slow drift oscillations of floating moored structures. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

An original combined analytical and experimental study of the effects of wave grouping on the slow drift oscillations of floating moored structures has been presented. The work constitutes the first incorporation of existing techniques of simulating wave groups and the slow oscillating response of a moored model of a rectangular barge. The analytical model assumes that each pair of frequencies in a discrete wave spectrum will give rise to a regular wave group. Each of these regular wave groups will produce a low frequency force that is proportional to the product of the wave amplitudes in the ·group and varies at a frequency equal to the difference in the frequency pair. Furthermore, this low frequency force is related to a modified SIWEH of the regular wave group by a constant phase angle. Therefore the resultant force is determined by superposition of these regular slow-varying forces. Subsequently the ensuing slow oscillating motions can be established and related to the SIWEH spectrum. This relation is expressed in the form of a transfer function. The analytical model is then evaluated by means of a set of detailed experimental tests conducted on a rectangular barge. A JONSWAP spectrum was modelled for a range of grouping characteristics and the results compared to the analytical model. Results indicate that the slow drift motions are highly influenced by free motion effects. An empirical transfer function which included these free motion effects produced good agreement between subsequent predicted and measured slow drift responses. The technique developed in this study shows that the second-order response to linear waves can be related to the wave envelope when the effects of these linear waves are dominant.

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