Dynamics of Offshore Wind Turbines

must ensure that the natural frequency of the overall system (rotor-blade-tower-foundation assembly) does not come close to the forcing frequencies of the imposedenvironmental loads. A failure to do this may amplify the dynamic response of the structure, leading to larger tower deflections which can compromise the performance of the wind turbine. The dynamic response is dependent on the support condition i.e. the stiffness of the foundation, which relies on the strength on the surrounding soil. Under moderate to high cyclic loading, most soils degrade and the stiffness of the foundation changes. Therefore, there is a concern of long stability. High quality experiments, analytical and numerical studies were carried out to examine the long-term performance. Experiments were performed using a 1:100 scale model of real wind turbine (Vestas V90 3MW) supported on a monopile foundation. The experimental investigation was conducted using the facilities at the BLADE laboratories (Bristol Laboratory for Advanced Dynamics Engineering) at the University of Bristol. The different dynamic loadings acting on the real wind turbine have been simulated. In particular, the blades were rotated using an electrical motor and the wind shielding effects were modelled using an electro-dynamics actuator. Precise measurements of displacements, velocities and accelerations were carried out using: LVDT (Linearly Varying Differential Transformer), laser vibrometer and accelerometers respectively. The soils used were dry sand, saturated sand and soft clay. The mechanical properties of the soil were monitored during the tests. Signal-processing techniques were then implemented on the acquired data in order to assess the main dynamic properties of the wind turbine (i.e. natural frequency and damping). The results were compared with analytical solutions developed using Euler-Bernoulli's beam theory and numerical models using a commercial finite element program. The results showed that the dynamic properties of the system (natural frequency and damping) are sensitive to the support stiffness. In addition,the cyclic loading alters the stiffness of the foundation, which in is dependent on: (a) type of soil; (b) frequency of loading; (c) number of cycles; (d) intensity of loading; (e) instantaneous stiffness of the overall system. Generally, clayey soil showed frequency degradation behaviour due to repeated cyclic loading, which initially opens a cavity around the pile and is subsequently expanded by the jettingaction of the water due to cyclic movement of the pile. On the other hand, sandy soil showed stiffening behaviour, possibly due to the compaction of the soil. The results were compared with current codes of practice such as API (American Petroleum Institute) or DNV (Det Norske Veritas) which are currently being used for designing offshore foundations. Finally, practical design solutions are suggested, and the importance of predicting the long-term performance behaviour is highlighted.