Structural Health Monitoring Considerations on Offshore Wind Turbine Models

The growth of wind turbines in terms of power capacity and rotor diameter size has driven the research toward new studies and ideas. Nowadays high potential sites on land have already been taken and offshore locations need to be exploited taking advantage of the stronger wind. In 2009 a 5 MW concept has been proposed by NREL in order to be considered as the reference wind turbine for the next years. In 2013 a new reference model has been introduced by DTU Wind Energy, a 10 MW wind turbine. Both offshore wind turbines have been modeled by using an aeroelastic code and several simulations have been carried on with the main objective of developing advanced health monitoring techniques. As the first step to perform Structural health Monitoring (SHM) is the identification of the most sensitive parameters to structural degradation, advanced data analysis were developed. Methods such as the Multi-Blade Coordinate (MBC) transformation and the Harmonic Power Spectrum (HPS) have been validated and combined with Operational Modal Analysis to identify the modal parameters of the turbine in operational conditions, including sensors on the tower, blades and nacelle. Once the model and analysis techniques have been validated, damages have been introduced by adding lumped masses and by reducing the blades and/or the tower stiffness. Some SHM considerations have been drawn by analyzing the so-called whirling modes. They are due to the rotation of the blades and they can be observed only after that a pre-processing technique has been applied. They have been highlighted because they seem to be very sensitive to the introduction of small damages. doi: 10.12783/SHM2015/350