Numerical study on the stiffening properties of scour protection around monopiles for Offshore Wind Turbines

An active marine seabed causing scour around monopile foundations has to be addressed as a geological hazard. These structures are often protected with rock armour to prevent foundation failure. However, scour protection also increases the confining pressure and embedment length, providing additional stiffness to the soil–pile system. This study focuses on the stiffening effect of scour protection to optimise foundation design. A parametric analysis of the dimensions and materials of scour protection is carried out with more than 100 simulations to assess the stiffening effect of scour protection. Therefore, small-diameter and large-diameter monopiles are investigated. Numerical analysis with the Finite Element method is conducted to estimate the natural frequencies and the static capacity through moment and lateral load ($M-H$) curves and the Load Utilisation method. These methods, which are new to the study of scour protection, are proposed for the quantification and assessment of scour protection in foundation design. The results show that rock fill restores the initial foundation conditions independently of the pile dimensions. While for small-diameter monopiles, scour protection fulfils its double purpose of preventing scour and providing stiffness to the foundation, for large-diameter monopiles the contribution to the stiffness is limited and should only be considered for heavy rock armour and significant scour protection heights. The parametric analysis indicates that a thicker and heavier scour protection increases the static capacity by 10%, whereas the width or a densification through sand accretion have negligible effects ($<$1%). The $M-H$ curves and the Load Utilisation method have shown to be effective in assessing the static capacity of monopiles supporting Offshore Wind Turbines.