Optimisation of screw anchor lateral capacity in sand for offshore renewable energy applications

Screw piles or screw anchors are a promising solution to anchor floating offshore renewable energy devices, such as wind turbines or tidal turbines. The installation generates limited noise (driven piles are noisy) and can be undertaken in all soil conditions. Although they are mainly used for their large uplift capacities, screw anchors can also be designed to provide significant lateral resistance. The optimisation of screw anchor design does not rely only on the geotechnical assessment of the uplift capacity based on soil strength, but also on operational (installation requirements) and structural (helix bending, core section stress, limiting steel plate thickness) constraints. This paper develops a methodology for the design optimisation of screw anchors under lateral loading in dense sand, incorporating all of these constraints, based on simplified analytical or semi-analytical approaches. The results show that it is possible to optimise the anchor design, maximising the anchor lateral capacity, whilst minimising the anchor weight. The maximum embedment depth and then the anchor capacity is mainly limited by the maximum torque available during installation and the short-pile to long-pile failure mechanism transition respectively.