Current offshore foundation technology is being transferred successfully to the renewable energy sector. Still, there is clearly scope for developing foundations that are more tuned to the needs of the renewable power systems such as wind turbines. One such approach is the hybrid monopile-footing system with a proven record of improving the ultimate lateral resistance, particularly in cohesionless soils. This paper builds on to the previous studies by investigating the behaviour of the hybrid system, such as the effect of footing size, the magnitude of pre-loading and its significance in developing sufficient contact pressure beneath the footing, and the importance of the degree of rigidity.

1. Introduction

Due to the needs of ongoing developments in the oil and energy sector, the design of offshore foundations is constantly evolving. In the hydrocarbon extraction sector, exploration and development is moving into ever deeper water, resulting in extremely challenging geotechnical conditions. The development of sites for offshore wind farms (such as Round 2 and 3 in the UK) is also extending into deeper water. The increase of wind turbine generator capacity is requiring significant development in foundation design to generate economic and practical solutions to the installation of these deepwater wind farms. Offshore foundations are generally subject to combined loading conditions consisting of self-weight of the structure (V), relatively high horizontal loads (H) and large bending moments (M). The preferred foundation system to date has been the monopile, which has been successfully employed for the majority of the offshore wind turbines installed. The advantage of the monopile is that it can be installed in a variety of different soil conditions even when loading conditions are very high. For instance, in many of the proposed offshore wind farm locations superficial seabed deposits are often underlain by weak rocks, such as mudstones and chalk.

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