Numerical study on influences of water scouring on dynamic characteristics of mono-pile foundation for offshore wind turbines

The structure dynamic characteristics determine the response, vibration behavior, and long-term usability of the foundation to external loads; quantitative identification of its driving factors is crucial for precise design, safety optimization, and fatigue life prediction. In harsh marine environments, local erosion around offshore foundations can significantly alter dynamic performance. Previous studies linked the depth of scour to changes in pile fundamental frequency, but largely overlooked the additional effects of scour pit morphology and hydrodynamic forces. This study established a fully coupled fluid soil pile interaction model with dynamic grids to capture real-time erosion evolution and validated it through comprehensive flume measurements. Then, the system analyzed and quantified how current velocity, water depth, and scour pit geometry collectively affect the natural frequency of the foundation. The results indicate that for maximum scour depths below 1.8 times the pile diameter, it is not sufficiently accurate to explain the frequency attenuation by considering only the maximum scour depth while ignoring scour-pit morphology. Neglecting hydrodynamic effects may result in an estimated fundamental frequency difference of up to 20% for scour pile foundations.