Effect of Large-Diameter Foundation on Scour Risk of Offshore Wind Turbines

Offshore wind turbines (OWTs) with higher capacity are typically associated with larger structural dimensions, such as increased hub height, tower diameter, and rotor diameter. Consequently, they require support structures with large-diameter piles, particularly when employing suction buckets, a type of large-diameter foundation. These large-diameter structures exhibit a distinct scour mechanism compared to the conventional mechanisms observed in smaller-diameter piles. This study investigates the scour risk of OWTs while explicitly accounting for the large pile effect. First, a scour fragility analysis is developed to evaluate the vulnerability of suction bucket foundations under scour, represented in terms of fragility curves. Then, the probability density function (PDF) of scour depth is derived from the PDF of the Keulegan–Carpenter number, a key parameter for estimating scour depth that incorporates the large pile effect. Ultimately, scour risk is quantified by integrating the PDF of scour depth with the corresponding scour fragility curve. Comparative results show that, with a safety factor of 1.0, the reliability indices considering the large-diameter pile effect are 2.509, significantly lower than 5.115 for cases that neglect this effect, representing a decrease of less than 51%. For a safety factor of 1.75, the difference is 43%. These results suggest that ignoring the large-diameter pile effect not only underestimates the scour risk of OWTs, but also demonstrates a nonlinear effect of the safety factor on OWT risk. Ignoring this effect could also compromise the sustainability of offshore wind turbine systems. This highlights the importance of considering the unique scour mechanisms associated with large-diameter OWT foundations to avoid overestimating structural risk.