OBSERVATIONS OF WAVE-STRUCTURE INTERACTION FOR A MULTI-LEGGED CONCRETE PLATFORM

This paper presents the results of a physical model study of waves aro und the Brent Brave gravity based structure. The work arose as a resul t of a recent incident in which significant amounts of water were proj ected upwards, thus causing damage to ancillary steelwork and pipework . Measurements of the water surface elevation in the vicinity of the m odel structure show that the three large, closely spaced legs give con siderable wave-structure interaction. In particular, if the incident w aves are relatively short and steep, this wave-structure interaction p roduces high-frequency waves which radiate outwards from the centre of the structure. Nonlinear interaction of these waves with the incoming wave field leads to enhancement of their height and steepness, and ex plains the very high water surface elevations experienced at Brent Bra ve. Although the present data specifically relate to this structure, s imilar behaviour may arise in other types of structures with large, cl osely spaced legs. The observed nonlinear wave-structure interaction i s also relevant for the study of 'ringing', or the high-frequency resp onse of relatively flexible structures, but in the case of very stiff structures such as Brent Brave, this leads to excitation of the fluid rather than the structure. For structures where nonlinear interaction with waves may occur, if cannot be assumed that the crest elevation in the open sea provides a good estimate for the air-gap. However, it mu st be stressed that given the nature of the interaction, any wave impa ct will be local rather than global and will therefore not threaten th e overall integrity of the structure. On the basis of the test results , practical guidance was developed for the assessment of loads on item s in the air-gap of Brent Brave. (C) 1998 Elsevier Science Limited. Al l rights reserved.