Dynamic modeling and load analysis of the 20MW INNWIND.EU reference wind turbine
1, Daniel Kaufer2, Martin Kühn1
1ForWind-Center for Wind Energy Research, Oldenburg, Germany, 2Rambĝll Offshore Wind, Hamburg, Germany
At present the largest wind turbines have a capacity of 8-9 MW. The ongoing research at the joint European project INNWIND.EU presented the first design basis for developing future very large offshore wind turbines of 20 MW and with rotor diameter of approximately 252 m. The preliminary design of such a wind turbine is the result of direct extrapolation from the 10 MW reference wind turbine. Therefore, the initial upscaled model requires developed innovations to enable this basic design to be significantly improved. For large offshore wind turbines with a jacket structure, a common problem is dynamic excitations due to resonances between the blade passing frequencies and the fundamental frequency of the support structure. In the early stage of the support structure design, the influence of the natural frequency and blade passing frequencies on the support structure and wind turbine loading should be assessed.
In this paper, dynamic analyses of the 20 MW upscaled wind turbine are carried out. A semi-active magnetorheological (MR) damper model is developed and integrated into the tower top to mitigate tower bending moments. Furthermore, a control algorithm is proposed to optimize the performance of the damper system in all operating conditions. Damage Equivalent Loads (DEL) are calculated with and without the damping system. Finally, the initial dimension of the jacket structure is estimated and optimized by including the innovative damping system.
Load calculations and dynamic structural analysis are performed in Bladed simulation tool. The relevant Design Load Cases (DLC) are performed according to the design standards and interface loads are calculated. Interface loads at tower base are generated and used in the fatigue and ultimate limit state analyses of the jacket structure. The initial dimensions of the jacket structure are then estimated from these analyses. The structure below the tower base and wave impact are modelled using a superelement. The resulting loads at the tower base can be mitigated when a semi-active MR damper is integrated into the initial reference model. The control system is based on acceleration feedback for controlling the MR damper to reduce structural responses.
The full paper will report results of both the dynamic analysis and load mitigation approach. DELs of the tower base are determined at different wind speeds. The Campbell diagram which shows the interaction between the entire system and blade passing frequencies will be discussed. The numerical modelling and verification of the MR damper and the design of the control algorithm will be described. Furthermore, the improved jacket design achieved by application of the damping system will be explained and the overall size and dimensions of the optimised jacket will be quantified. The integrated optimized design will be compared with the initial design and design improvements will be described.
The preliminary results show that the semi-active MR damper can effectively alleviate external loads within the whole operational range. The improved tower base loads are obtained and compared with the reference design. The integration of the semi-active dampers in the early stage of the jacket design could significantly alleviate the interface loads which can result in an optimized and economic jacket structure.
The main learning objectives of this paper can be summarized as below: