Continuous strain prediction for fatigue assessment of an offshore wind turbine using a joint input-state estimation algorithm and a modal interpolation algorithm

Offshore wind turbines exhibit high periodic stresses and strains at critical locations, due to their continuous subjection to cyclic loading caused by wind and wave excitation. This cyclic loading may lead to structural failure. Continuous fatigue assessment is therefore an issue of essential importance. The stress-strain response time histories can be directly obtained from sensor data at points accessible to measurements. For some critical locations which are not accessible for direct measurements, however, this is not possible. Direct measurements of the stresses at the e.g. the mud-line, 30 meter below the water level are expensive and practically unfeasible, although this location is a hotspot in the fatigue assessment of an offshore wind turbine. When direct measurements of the stresses are impossible, response estimation techniques can be used to estimate the response at unmeasured locations from a limited set of response measurements (accelerations, strains, etc.) and a system model. This paper compares two algorithms for the estimation of stresses at the mud-line of an offshore wind turbine in the Belgian North Sea [1], a joint input-state estimation algorithm [2] and a modal expansion algorithm [3]. Both algorithms make use of a Finite Element (FE) model of the structure and a limited number of response measurements for the prediction of the stress-strain responses. The FE model has been validated using a set of modal characteristics obtained from an Operational Modal Analysis of the structure, hereby using acceleration time histories recorded at several points on the tower. The response used in the response estimation procedure consists of acceleration and strain measurements.status: accepte