Inclusion of low-frequency cycles on tower fatigue lifetime assessment through relevant environmental and operational conditions

Structural Health Monitoring (SHM) campaigns aiming at accurate consumed fatigue damage estimations play a pivotal role in the decision-making of wind turbines approaching their design lifetime. This study focuses on developing a sensor-based fatigue damage counting approach for wind turbines to calculate the consumed lifetime of the tower. The reliability of strain gauges for long-term operation is studied for an onshore V52 and an offshore turbine Adwen AD 5-116. Interestingly, the offshore strain gauges showed more variability compared to the onshore case. Moreover, an investigation on the coupling of low-frequency fatigue damage (LFFD) with Environmental and Operations Conditions (EOCs) through damage matrices highlighted that not only wind speed variability but varying operational conditions of a turbine increase tower fatigue consumption and should be accounted for in the design phase and optimized operational strategies aiming at lifetime extension.