Foundation modelling to assess dynamic behaviour of offshore wind turbines

The dynamic behaviour of wind turbines at offshore locations is more complex than that of both onshore wind turbines and offshore platforms used in the oil and gas industry. In order to reduce the computational burden, the work presented in this paper aims at simplification of the dynamic model of the foundation, while maintaining sufficient accuracy. A stiffness matrix at the mudline is found to be the best solution for monopiles. With respect to the required accuracy, the sensitivity of dynamic behaviour to variations in several parameters is investigated. An inaccuracy of about 4% can be expected for the first natural frequency. Finally, experimental data is used to determine whether expected accuracy is met in practice and whether modelling techniques, which are commonly used for offshore structures, can be used for wind turbines on monopile foundations. For five wind turbines in an offshore wind farm the results corresponded with expectations, but two wind turbines in another farm gave unexplained higher errors.