Damage detection in laboratory scale model of the offshore support structure using two different measurement techniques

The paper presents a comparison of two vibration-based methods for the damage detection in a laboratory scale model of a tripod. Tripods are a part of the support structures for offshore wind turbines. The damage being tested was a circumferential crack of the tripod upper cylindrical brace. The influence of the measurement uncertainty is accounted to prevent "false alarm" due to the inherent scatter in the test results of the object under investigation. The first damage detection method addresses the use of acceleration signals in a genuine experimental modal analysis (i.e. input-output modal analysis) and in an operational modal analysis (i.e. output only modal analysis). The progressive damage is monitored by the calculation of the modal parameters and following their deviations. The second method is based on the application of Fibre Bragg Grating (FBG) strain sensors. Both methods were performed on the undamaged and damaged structure for different support conditions and excitations (shaker, hammer, in water basin under wave excitation). The results suggest that the method based on modal analysis can be considered a useful tool for damage detection in dry and in-water conditions. Furthermore, the FBG and Frequency Domain Decomposition (FDD) based damage detection method shows its utility to detect damage (totally unfixed flange) for all analyzed cases, although its localization is possible for a wave excitation only.