A reduced order analytical model to reconstruct tower bending moments time series of onshore wind turbines validated with experimental data

• Fatigue damage is critical for end-of-life decision making in onshore wind turbines • Direct estimates of internal forces require laborious installation of strain gauges • Analytical formulations can be used to map accelerations into bending moments • A new methodology to estimate bending moments from accelerations is presented • The methodology was validated using experimental data of a full-scale structure With many onshore wind turbines reaching their design lifetime, typically conditioned by fatigue, a proper evaluation of the actual damage accumulated is a key element for any life extension decision, and a proper knowledge of the stresses time series is needed. In this work, a methodology to estimate the bending moments at an arbitrary tower section from the tower accelerations based on a reduced order analytical model is presented and its theoretical framework derived. The proposed approach is firstly validated using aero-elastic numerical simulation in OpenFAST, a reference open-source software for multi-physics simulation of wind turbines dynamics, and two different levels of instrumentation are tested. The same methodology is then evaluated with 6 months of monitoring data from an onshore 1.8MW wind turbine. The results obtained show that this methodology is capable of estimating the bending moments time series, at selected tower sections, for all operation and environmental conditions, that can then be used to estimate fatigue damage. The computational efficiency of this approach, that does not require large amounts of data or computationally expensive models, makes it suitable for a wider implementation in current wind farms, using simple monitoring systems that might be already available in modern wind turbines.