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

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, tower bottom and blade root strain gauge bridges, 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 2952, 282, and 566 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. Consistent tower bottom strain signal estimates 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 with a reduced set of inputs, 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 based on load measurements. A simple drivetrain thermal model was used to evaluate the modified lifetime L<inf>10m</inf> of the main bearings. Fatigue loads in the locating main bearing are driven by the peak of the turbine thrust curve, with higher loads observed at rated wind speed. An effect of longer main bearing lifetime with higher turbulence intensity was observed at rated wind speed and can be explained by the turbulence averaging of the thrust loads.