Experimental validation of soil–structure interaction of offshore wind turbines

Estimating the natural frequencies of a wind turbine system consisting rotor, nacelle, tower, foundation and surrounding soil is one of the important design considerations. This paper experimentally investigates the behaviour of a model wind turbine supported on a particular type of foundation called a monopile. Monopile is a single large diameter (2.5–4 m) long slender column inserted deep into the ground. This can be thought of as an extension of the wind turbine tower. In particular, the role of soil/foundation in the dynamics of wind turbines has been investigated. Analytical methods are developed incorporating the rotational and translation flexibility of the foundation. Novel experimental techniques have been developed to obtain the parameters necessary for the analytical model. The analytical model is validated using a finite element approach and experimental measurements. In total, results from 17 test cases is reported in the paper. Experimental results show that the natural frequencies and the damping factors of the wind turbine tower change significantly with the type of soil/foundation. Analytical results for the natural frequencies agree reasonably well to the experimental results and finite element results.