DEM modelling of screw piles as foundations for floating offshore wind turbines

Screw piles are efficient foundations for offshore renewable energy devices. Guidance recommends that they are installed in a perfect (pitch-matched) manner, in order to limit the soil disturbance. However, this necessitates the application of a vertical compressive (or crowd) force at their head, whose magnitude for upscaled offshore foundations is impractical or costly to impose in the field. It was shown that screw pile overflighting during installation in sand, i.e. applying more rotations than recommended for a given downwards displacement, has the potential to reduce the vertical load without degrading the foundation resistance. DEM simulations were undertaken to explore this seemingly counter-intuitive outcome. The soil domain was truncated to reduce the computational effort and the soil particles were enclosed in 7 radial rings whose displacement was independently servo-controlled. Different boundary conditions (constant stiffness radial rings, fixed walls or constant pressure) were tested. Screw piles were installed by applying constant rotation and displacement rates, then measured force and torque were compared against centrifuge experiments. The DEM simulations were shown to capture well the centrifuge results when a constant stiffness was applied. In addition, the origin of the torque and penetration resistance during installation (shaft, helix…) were identified.