Resonance of steel wind turbines: problems and solutions

This paper deals with an experimental and numerical dynamic characterization of two steel wind turbines currently located in Italy. The considered steel turbines are onshore and were originally built more than 20 years ago with 65 m of height (from the ground level to the hub). Five years ago, they were unmounted and reassembled in the present location. During this phase, due to local and national environmental prescriptions, they were shortened to about 45 m (hub height). Unfortunately, this procedure generated a huge problem in their structural behaviour: the fundamental frequency of vibration of both the “shorter” steel turbines become close to the working frequency of their rotors, causing a resonance problem. To evaluate exactly the turbine frequencies and modal shapes, structural monitoring involving several accelerometers has been implemented. Then, numerical finite element models (FEM) have been developed to investigate the influence of the soil-structure interaction (SSI) on the dynamic behaviour of these structures. Finally, two practical solutions to overcome the resonance problem are proposed and discussed. Since the investigated problems can arise for any worldwide wind turbine, the proposed solutions are of general validity and can be extended to many other different cases