Compared with conventional monopile foundations, a hybrid monopile with wings attached to the perimeter is expected to exhibit better performance for supporting offshore wind turbines under huge lateral loads. Nevertheless, local scour could compromise the performance of the winged monopile in a sandy seabed. This hazardous scour effect on the lateral response of winged monopiles has not been well understood and adequately evaluated yet. In this study, the local scour process around a winged monopile in a sandy seabed was firstly physically modeled in a water flume. It was observed that both the depth and the width of the local scour hole around a winged monopile can be larger than those of a conventional monopile. Therefore, the wings need to be mounted below the scour hole to avoid the intensified scour effect that impedes the effectiveness of the wings. A series of numerical simulations were then carried out to evaluate the lateral load response of a winged monopile in the sand under the local scour condition. The numerical results indicate that the deformation of a winged monopile can be significantly mitigated compared with conventional monopiles. Contours of displacement and plastic strain in the soil at the ultimate limit state are presented to reveal the mechanism of wings for enhancing the lateral bearing capacity of monopiles. The effects of the aspect ratio and the number of wings on the bearing capacity of the monopile are further examined under the premise of the same material consumption. Finally, the optimized design of wings is proposed to take full advantage of the merit of the winged monopiles under the local scour condition.
Offshore wind power has been drawing more attention due to the mass reserves, high-energy density, and minor disturbance than onshore wind power. By the end of 2020, the total cumulative installed capacity had increased up to 35 GW (GWEC, 2021). As the most commonly adopted foundation for offshore wind turbines (OWTs), monopiles have supported more than 80% of installed OWTs (Gupta & Basu, 2020). The construction and installation cost of the OWT foundations is high, accounting for 19% to 36% of the total project cost (Oh et al., 2018). To support larger-capacity wind turbines that operate in deeper waters at a reasonable cost, several new hybrid monopile foundation types for OWTs have been proposed recently (Qi & Gao, 2019).
