Advanced control for floating offshore wind turbines

• DIRECTORS: Santi Alonso (UPV/EHU) and Carlos Calleja (IKERLAN)
• UNIVERSITY: UPV/EHU

Abstract:

Wind energy is becoming the real green energy alternative to the conventional fossil fuel sources, as the increment of the new wind farm projects worldwide demonstrates. The offshore wind presents many advantages compared to its onshore counterpart, but the current bottom-fixed technology is limited by the water depth since the supporting structures can only be installed in shallow water coastal areas. Therefore, a solution to overcome this limitation has been developed by mounting the wind turbines on floating structures, i.e. the Floating Offshore Wind Turbines.

Early studies of Floating Offshore Wind Turbines have shown that control plays an important role for the dynamic behaviour of the system due to the possible platform negative damping effect. Therefore, several control techniques have been proposed in the last years in order to avoid such a negative effect and improve the performance of the Floating Offshore Wind turbines.

In this thesis, an advanced control technique has been designed and tuned to improve the overall performance of Floating Offshore Wind Turbines in terms of power regulation and global mechanical loadings, as well as to reduce the impact of the waves in the floating platform motion. Two platform concepts have been studied with more detail, (1) the ITI Energy's barge concept with 5-MW wind turbine and (2) the 10-MW wind turbine on a TripleSpar concept. The first model has been chosen because its basic and economic design, the fabrication and installation advantages, and the challenges posed to the turbine control system by less stable and more compact platforms. Furthermore, the relationship between the fundamental platform dimensions and the operating performance, especially in terms of challenges posed to the turbine control system, is investigated with more compact barge models. Besides, the second model has been chosen to prove the scalability of the designed advanced control technique in a higher power rated wind turbine mounted on a hydrodynamically more stable platform. Furthermore, an optimisation methodology to automatically tune the advanced controller has been developed based on the Damage Equivalent Loads results, improving the manually tuned controller outcomes.

Two alternative linearisation strategies for Floating Offshore Wind Turbines are proposed. The first one uses the generator torque trimming while the second applies the chirp signal methodology. The generator torque trimmed linear models show acceptable results, while the chirp signal methodology delivers the highest fidelity results respect to the identification of the system modes.

For the hydrodynamic analysis of floating platforms, the open source code NEMOH has been proposed to obtain the hydrodynamic matrices required for the simulation code FAST. The obtained results have been compared with those obtained with WAMIT code, validating the approach from the method development point of view.

The obtained results suggest a significant effectiveness of the designed advanced control technique for reducing the mechanical loads suffered in tower and blades while improving the wind turbine performance, contributing to achieve a cost effective solution for the Floating Offshore Wind Turbine technology.