Influence of soil properties on the shift in natural frequencies of a monopile-supported 5MW offshore wind turbine under scour

Abstract

This paper studies the change in the natural frequencies of the NREL 5MW monopile-supported offshore wind turbine (OWT) due to scour. The natural frequencies are derived from accelerations arising under wind and wave loads, for a range of different soil properties. A model of the wind turbine is developed using OpenFAST and SESAM software which considers aerodynamic and hydrodynamic loading with soil-structure interaction. The superstructure behaviour is modelled using OpenFAST and the foundation behaviour is modelled with SESAM, where soil-structure interaction is incorporated using p-y springs. The influence of soil density and scour hole depth on the obtained natural frequencies are studied. Accelerations are extracted at nine locations along the tower resulting from applied wind and wave loads. The first two system natural frequencies are estimated using frequency domain decomposition (FDD) applied to the acceleration time-histories for each scour depth and soil type. It is shown that the increase in scour depth around the foundation of the OWT leads to a decrease in natural frequencies when compared to the case where there is no scour.