Seabed scour assessment for offshore windfarm

Whitehouse, R.J.S. and Sutherland, J. and O'Brien, D. (2006) Seabed scour assessment for offshore windfarm. In: ICSE 2006 (3rd International Conference on Scour and Erosion), 3-5 July 2007, Gouda, The Netherlands.

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Abstract

The prediction of scour at offshore windfarm foundations in areas with mobile seabeds is a challenging topic. In areas with strong currents and wave action, and in areas with shallow water with the additional process of wave breaking to consider, it is necessary to complete laboratory testing. The work described in this paper examined the scour at foundations for a coastal site with waves and strong currents crossing at an oblique angle. The scour in the sand bed was tested for a range of current dominated and extreme wave conditions. The detailed scour profiles were used to determine the depth and extent of scour for a range of water levels, currents and boundary conditions for waves. The testing confirmed the need for scour protection to be installed. A careful design of the scour protection was required to ensure performance of the foundations. The information on scour depth and extent for the design conditions was used to value engineer the scour protection design. A range of options was tested and the most appropriate one was selected based on a quantified and acceptable level of damage and the degree of interaction of the scour protection design with the surrounding seabed. Following installation of the offshore wind turbines data was collected on the depth and extent of scour in the field. A reasonable agreement was found between the laboratory results and site observations of the scour hole. This added confidence to the scour protection design that was selected and installed. The OPTI-PILE design tool gave the correct form of behaviour for rock stability but should be recalibrated for use in such shallow water with such fast currents.

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