Structural health monitoring of major wind turbine components

Abstract

Fraunhofer IZFP in Dresden is developing technologies for monitoring major components of wind turbines operating off- and onshore. Different structural health monitoring (SHM) techniques have been developed for monitoring rotor blades by acoustic methods, drive trains by operational modal analysis and welding seams in the foundation of offshore wind turbines by guided ultrasonic waves. Rotor blade monitoring technologies have been realized and tested at full scale under fatigue loading on a ground test bed and on a rotating wind turbine. The SHM-based techniques were compared to results obtained from conventional NDT techniques such as ultrasonics and thermography where validations have been made at damage critical locations of the different components considered. After completion of the full scale test selected damage critical parts were cut out of the rotor blade to allow for further inspection based on X-ray tomography and microwave technology. Acoustic emission monitoring results performed on an SHM basis showed comparatively good correlation when compared to the results obtained by conventional NDT. Drive train monitoring was realized through operational modal analysis. Generated through the excitation of the rotating blades the drive train started to resonate in its eigenfrequencies resulting from the eigenfrequencies of the drive train components such as bearings and gears. These oscillations caused by the different eigenmodes had to be determined through clear separation. Monitoring of foundation structures is a dedicated issue in offshore applications. In the range between low and high water levels welding seams become specifically prone to corrosion and cracking. In such cases a guided wave based monitoring system turns out to be valuable. Such a system is based on piezoelectric transducer arrays where the acoustic beam can be focused on the welding seam. This is currently realized after first tests have proven validity of the concept.