Fast second-order slender body force model calibration on a 20MW class floating wind turbine experimental campaign

Recently, significant attention has been drawn towards the holistic test and analysis of Floating Wind Turbines (FWT). Different approaches have been developed to address the complex interaction of the multiple phenomena that act simultaneously and at scale. The main problem when designing experimental campaigns on FWT is that there are two main fluid-structure interactions that scale with different laws. The first, is the wind turbine interaction with the wind flow that scales by Reynolds similitude. Secondly, the wave-floater interaction, where the Froude similarity is necessary and thus makes the aero- and hydro- time scales incompatible. Therefore, to overcome this, different approaches has been followed; generation of Froude scaled thrust by propellers, actuator systems to induce the expected motions and the re-designing of the wind turbine rotor to behave as the target turbine at such Froude scale. In this work the last of these is the selected approach for an experimental campaign that is part of the Danish FloatLab project. The experimental campaign will here be used as calibration for numerical models, with special emphasis on the hydrodynamic loading where the hydrodynamic damping is known to be highly dependent on the sea state. We have developed a new fast second-order slender body force model (paper under preparation) with improved motion-wave coupling terms and will validate it for the tested sub-structure. Since this model will be used within the test for control purposes, it must be able to deliver a fast response calculation, to ensure a fast calibration procedure. Further, as a key need for floating wind turbine response, the model also consider second order hydrodynamic loads, since they are directly related to the low-frequency response observed on FTW. Thus we present here a compact and efficient second-order slender body force model, directly applicable to platforms that are composed by slender elements (See Fig. 1 for reference.), and a method to quickly calibrate it with experimental data.