Scour assessment around offshore wind monopile using EFA testing and Delft-3D numerical modeling

Owing to the increasing demand for offshore wind energy, the use of monopiles, which are vulnerable to the scouring caused by hydrodynamic forces, has rapidly increased. The stability against scour around offshore structures becomes important, whilst research focusing on the surface erosion mechanism of sandy and silty in-situ offshore seabed is limited. This study used an erosion function apparatus (EFA) to assess the erosion mechanism of Japanese offshore seabed soils followed by Delft-3D numerical modeling to predict the scour depth around a monopile structure (diameter = 10 m). Additionally, empirical equations were applied under the same soil characteristics and fluid dynamic conditions to derive the predicted values. The estimations based on the empirical studies were compared with the numerical results, and the relationship between the soil characteristics and the erosion parameters was analyzed. The EFA test results showed that the uppermost layer of the sandy soil layers had the critical shear stress of 10.11 Pa, whereas the over-consolidated silty soil layers showed no erosion even at an extreme hydraulic shear stress of 106.47 Pa. The numerical results indicated that even under typhoon-simulating conditions, no scouring was observed under the considered hydrodynamic conditions because the bed shear stress remained below the critical shear stress of the upper layer. However, empirical formulas tended to overestimate scouring in the offshore environment; the estimated scour depth varied widely from 1.33 to 23.48 m. Correlation analysis showed that the mean diameter, percentage finer, and water content of soil affected the critical shear stress, and the erosion behavior changed when the boundary mean diameter was 0.156 mm. These results provide experimental and numerical evidence for understanding the erosion behavior of sandy and silty seabed and offer practical insights for a more reliable scour assessment and monopile foundation design in offshore environments.