Visualisation of mechanisms governing suction bucket installation in dense sand

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Abstract
Suction buckets are an increasingly considered foundation option for offshore wind turbines. Although the required suction can be predicted well using existing methods, uncertainty remains around some input parameters, because the effects of suction installation on the soil state are not understood in detail. This paper visualises the mechanisms governing both initial self-weight penetration and following suction-assisted installation in dense sand. Pioneering particle image velocimetry measurements in a centrifuge environment underpinned the investigation, with details of the experimental apparatus offered in the paper. Changes in the deformation mechanisms governing the installation process and in the soil properties are revealed. The findings have an impact on the understanding of the formation of internal plug heave – the cause of premature refusal – and the prediction of the installation response. Revealing changes in void ratio and permeability also present implications on the accumulated displacements under the metocean loading, which may conflict with serviceability requirements. This chapter discusses visualisation of mechanisms governing both initial self-weight penetration and following suction-assisted installation in dense sand. It offers pioneering particle image velocimetry measurements in a centrifuge environment underpinned the investigation, with details of the experimental apparatus. Changes in the deformation mechanisms governing the installation process and in the soil properties are revealed. The chapter focuses mainly on the experimental apparatus and the PIV analyses, reporting the results of a representative test only. The suction bucket technology is nowadays regarded with interest for the foundations of offshore wind farms, in relatively shallow waters. The initial self-weight penetration resembles other penetration processes, such as jacked installation of large diameter piles, although neglecting the vertical stress enhancement through the action of the downward friction has been flagged as non-conservative for the prediction of suction bucket installation. However, no guidance is provided on the size of this down-drag enhancement.

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