Integrated design of an offshore wind turbine by simul- taneous redesign of the support structure and controller parameters

In order for wind energy to become a well established source of alternative energy that is able to compete with the traditional forms of energy production, the costs of (offshore) wind energy should be lowered. This can be achieved by either increasing the energy output of a turbine or by reducing its production and maintenance costs. Current research on lowering the costs of energy of wind energy is conducted by multiple fields of engineering. In the field of structural engineering great effort is put into creating more accurate structural models of the turbine and the surrounding environment, such as the soil-structure interaction. Having more accurate models of the turbine potentially lowers the turbines costs by using a less conservative design when optimizing the system or by extending the lifetime by better understanding the applied loads to the system. In the field of control engineering there is a focus on control techniques which increase the energy output of the turbine or actively reduce the forces which act on the turbine. Integrating the controller and structural design into a single optimization routine is a heavily investigated area in other, less conservative, fields of engineering. Currently, little research focuses on applying this principle to wind turbine design. This research was partly conducted in collaboration with the SIEMENS wind power Centre of Competence in The Hague, to provide a framework for simultaneously redesigning the controller and structural parameters of their state of the art, multi-megawatt, offshore wind turbine and to define the key limitations of this framework. In order to meet this goal a framework has been presented that allows for simultaneously varying both structural and controller parameters of the wind turbine design. The framework allows one to find an combination of the controller and structural design parameters which optimize the overall performance of the turbine. This is achieved by evaluating the overall system performance in the frequency domain, while simultaneously optimizing over all tunable parameters. The capabilities of the method have been demonstrated on two design cases. In the first case a simple academic problem was optimized an the performance of the framework was analysed. The framework of simultaneously varying the structural and controller parameters is also applied to a commercial, state of the art, multi-megawatt, offshore wind turbine. The integrated design framework allows for varying physical properties of the structural model, in this case the wall thickness of the support structure. The derivatives of the mass and stiffness matrices of the turbine’s equations of motion are computed. Thereby obtaining a sensitivity description of the structural dynamics to the wall thickness. These derivatives are used to relate a change in wall thickness of tower elements to a change in the structural properties, and thereby the dynamical behaviour, of the entire wind turbine model. With the number of inand output channels in the framework being linearly dependent to the number of states of the turbine model, the full turbine model description complicates the application of the integrated design framework. Therefore, the overall model of the wind turbine is reduced to capture only the relevant structural dynamics of the system. By using the reduced basis of the original system the derivative matrices are reduced, while keeping the dependency to the change in wall thickness parameters. This makes it possible to change the system dynamics, through wall thickness changes, while using a reduced number of inand outputs in the framework. The integrated framework is used to design the rotor-speed controller parameters of the system, which is limited by the dynamics of the structural turbine model. The structural dynamics limited the overall performance of the system in two ways. First, a further increase in the frequency peak of