Continuous lifetime monitoring technique for structural components and main bearings in wind turbines based on measured strain and virtual load sensors

Abstract. Decisions on the lifetime extension of wind turbines require evaluating the remaining useful life of major load-carrying components by making a comparison to the design lifetime. This work focuses on the lifetime assessment of two fundamentally different components: a structural component in the form of the tower and rotating components in the form of the main bearings. A method is presented that combines high-frequency SCADA, accelerometers, minimally intrusive strain gauge at blade and tower, and limited design information for continued estimates of the component loads and their subsequent fatigue damage accumulations. The work is applied to a highly instrumented DTU research turbine, a Vestas V52 model, where strain gauges in the blade root and in the tower bottom are calibrated for nearly 10 years using continual calibration methods without the need for operator input. The lifetime estimates of the tower bottom and front and rear main bearings were found to be 1770 years and 166–333 years, respectively, reflecting the low average wind speed of the turbine site compared to the wind turbine design wind class IA. Secondly, it was investigated whether virtual load sensors can replace tower strain gauges and if one can use only uptower sensors for lifetime evaluation. Consistent tower bottom strain signal estimate and long-term damage accumulation were achieved with ±5 % lifetime variability once SCADA, nacelle accelerometers, and blade root strain gauges were combined for the deployment of a long short-term memory (LSTM) neural network. A systematic underprediction of the accumulated damage of the tower bottom was observed for the virtual load sensors, and a correction method was proposed. Finally, the impact of environmental conditions, including turbulence intensity and shear exponent of the incoming wind, on the main bearing lifetime was investigated using 10 years of measurements. A simple drivetrain thermal model was used to evaluate the modified lifetime L_10m of the main bearings, depending on the measured ambient temperature and the grease cleanliness assumptions. Higher fatigue loads are observed on the main bearings at rated wind speeds with low turbulence intensity and low shear. Changes of ±5 °C in the ambient temperature around 15 °C caused a 10-year difference in the operational life of the main bearings at rated wind speed. It was also found that the specification of the gearbox mounting stiffness can lead to a 60 % overprediction of the main bearing loads.