Investigating alternative foundation systems for offshore wind turbines in liquefiable soils

As the offshore wind industry expands into the Asia-Pacific (APAC) countries, seismic considerations become important in the design of foundations, especially when liquefaction of shallow layers can occur under seismic loading. This paper presents centrifuge modelling of three foundation systems for offshore wind turbines (OWTs) installed in liquefiable soil, specifically a monopile, a composite system with a monopile and an outrigger, and an outrigger-only system. The performance of the three foundations in supporting identical OWTs subjected to a combination of operational and earthquake loading is discussed. The rotational stiffness of these three foundations is investigated before, during and after seismic loading. The key findings suggest that the monopile experiences earthquake-induced rotation exceeding the allowable threshold recommended by the DNV standard from 2021, DNV-ST-0126, and the recommended practice of the same year DNV-RP-0585. Although the hybrid foundation is expected to perform better compared to the monopile and the outrigger as separate foundations, the outrigger as a stand-alone structure is the foundation that exhibits less rotation. However, this advantage comes at the cost of larger settlements. Overall, the outrigger-only foundation provided a reasonably consistent solution for supporting OWTs in areas prone to liquefaction induced by earthquakes.