Motion prediction of a self-aligning floating wind turbine
The accurate prediction of the motion behavior of a floating wind turbine is a crucial issue in the design and development of new platforms. This applies in particular to the design of a self-aligning system, as the evaluation of the self-aligning ability requires the consideration of many influences. The self-aligning ability depends not only on the seaway and current forces, but also on aerodynamic loads, which are essentially induced by the turbine rotor and tower. In addition, the tension of the anchor lines, which keep the system in position, contributes significantly to the forces acting on the floating structure. Furthermore, additional forces are induced due to different dynamic effects. Small roll and pitch motion amplitudes of the platform induce large motion amplitudes of the nacelle and the rotor in the longitudinal (along the rotation axis) and transversal directions. The motion along the longitudinal direction induces a complicated interaction between the rotor and its wake. The motion transverse induces a gyroscopic moment. A reliable simulation of the platform motion requires the consideration of all acting forces and moments. In the design process, the loads at different off-design conditions must be accurately evaluated. The applied numerical simulation method should be able to calculate the forces of the rotor, when its axis is not aligned with the wind direction. The unsteady hydrodynamic loads are strongly depending on the instantaneous wetted surface of the platform. Therefore, the relative position between the actual waterline of the platform and the wave elevation has to be updated at each instant of time. In particular for self-aligning floating wind turbine case, the drag force must be calculated with sufficient accuracy as it has a strong influence on the platform motion.
The new simulation method panMARE is developed at the Institute for Fluid Dynamics and Ship Theory of the Hamburg University of Technology and is applied within the HyStOH-Project to simulate the comprehensive loads on turbine and platform. The method is based on potential theory and it is used to calculate the three dimensional flow field. The computation comprises the aerodynamic and the hydrodynamic flow fields. In the aerodynamic domain, the flow on the rotor, the wake shape of the rotor blades and the dynamic rotor-wake-interactions are included. In the hydrodynamic domain, the flow field on the three-dimensional geometry of the underwater body of the floating structure is computed. The simulation allows the computation of the instantaneous added mass matrix. Additional elements are included to consider the hull drag. Furthermore, a dynamic mooring model is used to capture loads acting along the mooring line. The strong coupling of aerodynamic and hydrodynamic domain and the consideration of the mooring lines forces ensures synchronized motions. The developed method panMARE is suitable especially for predicting the motions in many extreme load conditions and in particular for the analysis of the self-aligning capability. The passive aligning of platform and rotor can be simulated for different wind, current or wave angles. Dynamic conditions with changing flow directions or velocities can also be simulated.
A verification study is carried out within the research project. The simulation results were compared with those of well-established numerical methods. For the investigated load cases, a good agreement was achieved. Wind tunnel experiments have been conducted to validate the aerodynamic model. The experimental investigations have been carried out with the entire platform-rotor-configuration in the wave tank. A good agreement has been achieved between the measured and the calculated results.