Geotechnical Aspects of Offshore Wind Turbine Dynamics Emerging from 3d Sand-Monopile Non-Linear Simulations

In recent years, offshore wind is becoming an increasingly growing market in Europe. Many resources are allocated towards ensuring its sustainable progress while one of the biggest uncertainties encountered lays in the offshore wind turbine's (OWT) foundations. In Europe, monopiles are the most widely adopted solution to found such structures in the subsoil. Owing to their large diameter and small L/D ratios, the response of these stocky monopiles remains unexplored. This work is exploiting the merits of 3D finite element (FE) modeling to shed light into the dynamic response of such structures. A detailed design of an 8MW OWT is implemented while soil description is achieved by employing a state of the art constitutive model which encompasses the critical state theory of soils. Long time-histories (10mins) followed by rotor stop tests provide critical insight on the dynamic response of large OWTs. The transient evolution of frequency content of the OWTs response, the evolution of pore pressures during storm events, the role of permeability and the contribution of soil in system damping are some of the issues that are addressed in detail. The effect of placing the monopile on soils of different capacity is examined by modeling sands of different relative density (DR=80%, 60%, 40%). Results show considerable dependence on the state of sand while for the loosest sand case, compliance of the structure to the exerted loads is recorded accompanied with a severe non-linear response. Furthermore, results indicate that permeability may have a non-negligible impact on the soil's contribution to damping at low loading conditions