State-independent apparent aero-elastic properties of wind turbine rotors: A method for the preliminary design of offshore wind support structures

In the previous decade, offshore wind undeniably took off as an important player in the European energymarket, which resulted in continuously enhancing turbine sizes and foundation structures pushing the boundaries of engineering, with the purpose of minimizing the levelized cost of energy to the range of optimal competitiveness. Regarding the foundation structure – or support structure – an important trade-off exists with respect to optimization and differentiation within an offshore wind farmon the one hand, and the required computational effort at an early stage of the design on the other. This effort comprises the extensive set of environmental conditions that require evaluation and the level of complexity of the modelling of the different environmental interactions, be it with wind, waves or soil.
Concerning the modelling of the environmental interactions with the structure, a decoupling of the turbine and the support structure is commonly applied, allowing the turbine manufacturer and the offshore contractor to develop their designs separately. The analysis of the support structure, however, has to account for the effect of the aerodynamic force, particularly for the aerodynamic damping, as this is known to affect the structural response to wave actions substantially. In this respect, the shared information usually concerns the damping ratio of the first fore-aft mode of vibration. This damping ratio does not explicitly express its dependency on the operational conditions of the turbine, e.g., the mean wind velocity and the rotor speed. Moreover, the provided ratio is only valid for the fore-aft motion in the first mode or vibration, and can therefore not be applied for higher fore-aft modes, or modes describing different motions, such as side-to-side or torsional.