Innovation in the wind energy sector has opened numerous possibilities, including the building of floating offshore wind turbines in greater depths of water. At Stuttgart Wind Energy (SWE), an affiliate of the Institute of Aircraft Design at the University of Stuttgart, my fellow researchers and I are advancing preliminary research in this area with the help of ANSYS simulation software. Specifically, we are using the software’s algorithms to perform an in-depth, hydrodynamic analysis of the hull shapes and a structural optimization analysis of the floating foundations for offshore wind turbines. We’ve conducted much of our research while working on the LIFES50+ (www.lifes50plus.eu) and INNWIND.EU (www.innwind.eu) projects.
Within the LIFES50+ project, we established an optimization framework and methodology for floating wind turbine design. Using ANSYS Aqwa, we created hull geometries based on external inputs and evaluated the hydrodynamic parameters needed to run the time domain simulation model. Additionally, we combined the frequency-dependent wave excitation forces with a given incident wave spectrum to calculate the resulting forces on the floating platform. In this way, we were able to simulate the loads impacting the turbines and platform due to changes in the wind and wave environment. The results show the effect of wave cancellation for different platforms designs and the trade-off between performance and material costs (Figure 1).
Figure 1. Results from platform design optimization.
Working on the INNWIND.EU project, we also mapped the time domain wave pressure by making calculations based on linear wave theory. To do this, our researchers created a finite element model (FEM) within ANSYS Mechanical from the solid model of the TripleSpar floating platform (Figure 2). This FEM model uses inputs of the pre-processed time domain pressures for each panel of the mesh. These pressures simulate the wave excitation pressure and diffraction pressures, and are only calculated for the wet surface under the mean water level (Figure 3).
Figure 2: Model of the floating platform created with ANSYS APDL.
Figure 3. Mesh of the underwater part of the TripleSpar platform (left) and map of the wave excitation pressures of one load step from a regular sinusoidal wave (right).
Our SWE team has also worked to validate the TripleSpar turbine models, using the results from the hydrodynamic analysis performed in ANSYS Aqwa. This validation process was accomplished by comparing the models with the measurement data from a wind and wave tank test (Figure 4). As a result, we were better able to design the controller of the turbine which, in turn, controls the actuators of rotor blade pitch. To increase the stability of the overall system, we needed to ensure the wave excitation frequencies were damped sufficiently.
Figure 4. Model and wave tank test of floating wind turbine.
With ANSYS’ highly versatile software, we are now able to investigate several aspects of floating offshore turbines, including optimization, controller stability and loading. To learn more about our research at SWE, please visit http://www.ifb.uni-stuttgart.de/windenergie.