This study proposed a new suction bucket (SB) foundation model for offshore wind turbines (OWT) suitable for a shallow muddy seabed, using more than three single buckets through kinetic derivation. The performance of new optimal foundation was evaluated by its horizontal displacement capacity and compared with a conventional SB composed of three buckets. Under external loads such as earthquakes, wind, and the combination of the both, the stability of this novel SB foundation was verified. The seismic fragility curve was also evaluated at some scour depths. These results were compared with the response of a tripod suction bucket (TSB) foundation, which was also designed for a shallow muddy seabed. The results indicated that scour significantly changed the dynamic response of this novel SB foundation but it had a better bearing capacity than the TSB foundation, despite its smaller size and weight. The fragility of TSB is always higher than the developed foundation in the same environmental condition. With reasonable volume and size, this novel SB foundation has great potential for future industrialization and commercialization.

For the offshore wind turbines (OWTs) located in a seismically active region, the occurrence of earthquakes combined with scour is a highly possible multi-hazard event. This study developed an alternative fragility analysis framework to assess the seismic performance of suction bucket-supported OWTs under the action of scour. First, the probabilistic approach was applied to calculate the occurrence probability of scour depth (SD) and earthquake events. Then, the possible combinations of these two events were considered in the analysis model to establish the fragility surface of the suction bucket foundation. Finally, by integrating the product of scour and earthquake hazard, as well as fragility curves, the suction bucket foundation failure probability was obtained. The developed framework provides a reliable approach to risk assessment for OWT-supporting structures in extreme event situations and can be applied to other complex natural hazards.

Scour risk assessment considering reaction force at foundation was proposed and applied to newly developed pentapod suction bucket support structures for a 5.5 MW offshore wind turbine under ultimate limit state environmental load. Scour hazard was obtained according to scour depth by using an empirical formula, which is the function of marine environmental conditions such as significant wave height, significant period, and current velocity. Fragility of the pentapod support structure was evaluated using the bearing capacity limit state criterion under ultimate limit state load case. Scour risk was assessed by combining the scour hazard and the fragility. Finally, scour risk of the developed pentapod suction bucket support structure under ultimate limit state has been assessed.

Kim, D.-H. and Kim, Y.-J., 2020. Scour risk assessment of offshore wind turbine suction bucket foundation. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 237–241. Coconut Creek (Florida), ISSN 0749-0208.Risk assessment procedure for offshore wind turbine support structures induced by scour was proposed. Probabilistic distribution of scour depth, a hazard curve, was calculated by using the C.S.U formula with parametric uncertainties. To get fragility curves of support structure, dynamic analysis was done with a certain assumed scour depth, given significant wave height and return period. Scour induced failure probability of support structure for a given wave height then can be found by integrating the multiplication of hazard and fragility. It is repeated for possible range of scour depths to form a risk curve. In numerical example, scour induced structural risk of 3MW offshore wind turbine supported by suction bucket foundation is evaluated. The proposed approach can be used in determining wind farm location where scour is significant.

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.