Analysis of mitigation of natural frequency reduction due to scour through inner water level control of offshore wind turbines

The natural frequency of offshore wind structures must avoid resonance with wind, wave, and rotor frequencies. Soil conditions and seawater significantly affect natural frequency, with scour and water level changes potentially altering it. This study analyzes the effects of inner water level and scour on the natural frequency of a 15 MW monopile-supported offshore wind turbine to ensure structural stability during the initial design phase. Under dense sand conditions, increasing water depth and monopile diameter expanded the controllable natural frequency range through IWL, with a maximum influence of 2.5 %. For scour depths of 1.5D, larger monopile diameters reduced natural frequency loss and enabled IWL to mitigate up to 0.5D of scour depth. In contrast, loose sand conditions with insufficient embedment depth amplified IWL's influence but worsened natural frequency reductions due to scour, reducing mitigation to 0.4D. Considering critical embedment depth allows greater mitigation of scour depths in deeper waters. These results highlight the importance of considering critical embedment depth and IWL interactions in the initial design phase to enhance structural stability and mitigate scour-induced natural frequency reductions, particularly in deep waters.