Long-term monitoring of existing wind turbine towers and fatigue performance of UHPFRC under compressive stresses

An increasing number of onshore wind turbines in Europe will reach the end of their relatively short service duration, currently limited to 20 years by the design codes and many owners already wish to extend the service duration of their turbines. Recently published recommendations require the coupled visual inspection and complex aero-elastic models for the safety verification of aging turbines. These methods however still lack accuracy and rely on generic numerical models that are subject to uncertainties. The direct monitoring of the wind loading effects on the structure can overcome many limitations and provide accurate data on the fatigue loading. Although such structures are subject to heaving loading, the site-specific wind conditions are likely to be below the design assumptions, and the effective fatigue loading endured by the structural components of the wind turbines is lower than initially planned. This thesis aims to develop a robust and economical setup for the long-term monitoring of wind turbine towers that can be combined with the SCADA data and operated by wind farm owners. The proposed methodology for the measurement of the tower deformation provided solutions concerning the temperature and long-term effects on the strain gauges. This setup was found particularly stable over the three years of continuous high-frequency measurement. Extreme events and accidental loads were recorded during the extended measurement period, and extreme values theory was used for the safety verification at ultimate limited state. Recommendations are provided on sufficient monitoring duration. The fatigue damage and extreme acting forces were found to be highly dependent on seasonal effects, with a majority of the damage produced during few high-wind storms, mainly occurring in winter. A complete year of measurement was found to be sufficient for the extrapolation of extreme loading events. Based on these results, recommendations are given to the owner to improve the management of the wind farm during storms, thereby allowing to decide between full power production and increased risk of damage. Monitored data, in combination with the SCADA data, are used to estimate the endured fatigue load spectrum. The estimated remaining service duration takes into account the changing wind conditions by correctly taking into account the variable annual frequency of storms. Results show that the monitored turbine tower could be safely extended significantly beyond the 20-year design limit. Another approach for the extension of the service duration of future towers is the use of Ultra-High Performance Fibre Reinforced Cement-based Composites (UHPFRC). UHPFRC is a high-strength cementitious material reinforced with fine steel fibres and it exhibits high mechanical and excellent durability properties. Constant amplitudes uniaxial compressive fatigue tests were conducted on thin UHPFRC plates under high fatigue stresses and very high number of cycles. The fatigue endurance limit of UHPFRC under compressive stresses at 10 million cycles was determined to be around 60 % of the ultimate limit strength. UHPFRC is found to be particularly suited for offshore turbines support structures as it could overcome the severe durability concerns of steel corrosion and allow for a more economical and durable solution for both onshore and offshore wind turbines.