Observations of wave-structure interaction for a multi-legged concrete platform

Abstract

This paper presents the results of a physical model study of waves around the Brent Bravo gravity based structure. The work arose as a result of a recent incident in which significant amounts of water were projected upwards, thus causing damage to ancillary steelwork and pipework. Measurements of the water surface elevation in the vicinity of the model structure show that the three large, closely spaced legs give considerable wave-structure interaction. In particular, if the incident waves are relatively short and steep, this wave-structure interaction produces 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 explains the very high water surface elevations experienced at Brent Bravo. Although the present data specifically relate to this structure, similar behaviour may arise in other types of structures with large, closely spaced legs. The observed nonlinear wave-structure interaction is also relevant for the study of ‘ringing’, or the highfrequency response of relatively flexible structures, but in the case of very stiff structures such as Brent Bravo, this leads to excitation of the fluid rather than the structure. For structures where nonlinear interaction with waves may occur, it cannot be assumed that the crest elevation in the open sea provides a good estimate for the air-gap. However, it must be stressed that given the nature of the interaction, any wave impact will be local rather than global and will therefore not threaten the overall integrity of the structure. On the basis of the test results, practical guidance was developed for the assessment of loads on items in the air-gap of Brent Bravo.