Scour protection assessment of monopile foundation design for offshore wind turbines

The driven monopile is the most common foundation type for all installed offshore wind turbines. These monopiles are often subject to scour phenomena, which cause a reduction in foundation strength and stiffness. In order to prevent the consequences of this phenomenon, scour protection systems are used to protect the seabed around a monopile structure against scour in practice. This paper presents a numerical model to investigate the effect of scour protection on the natural frequency and lateral responses of monopile-supported offshore wind turbines under the ultimate limit state (ULS), serviceability limit state (SLS) and fatigue limit state (FLS). This numerical model was validated through a series of model tests, and then applied to the National Renewable Energy Laboratory (NREL) 5-MW offshore wind turbine supported on an Offshore Code Comparison Collaboration (0C3) monopile with riprap rock scour protection system. The study concludes that the scour protection has a slight impact on the first natural frequency and monopile behavior under ULS and FLS. However, the scour protection significantly decreases the pile head rotation at mudline under SLS, which provides the opportunity for further optimization of monopile design by incorporating the contribution of scour protection, thereby leading to a more economical design.