Numerical study of monopile offshore foundation dynamic behaviour using coupled simulation

Offshore wind turbines (OWTs) are highly dynamically loaded by sea waves and wind forces and they operate in a narrow range of rotation frequencies close to the eigenfrequencies of the support structure. The foundation stiffness and damping are therefore very important parameters for a reliable estimation of the dynamic behaviour and cost-effective design of a wind turbine's supporting structure. However, design guidelines for offshore structures usually ignore any soil damping. This paper describes the implementation of a simple geotechnical model in the software package Ashes, which is a servo-hydro-aeroelastic code for wind turbine simulations. In order to investigate the role of the foundation damping, a 20 m monopile supporting structure with a National Renewable Energy Laboratory 5 MW OWT was considered as a case study. The OWT was subjected to standard design load cases and the results were obtained at the mudline of the wind turbine. The parametric study undertaken showed that, when soil damping is considered, ultimate limit state loads can be reduced by 8% and the number of fatigue cycles can be reduced by 1·5%.