Full-field strain prediction applied to an offshore wind turbine

Fatigue life is a design driver for the foundations of offshore wind turbines (OWT’s). A full-scope structural health monitoring strategy for OWT’s needs to consider the continuous monitoring of the consumption of fatigue life as an essential part. To do so, the actual stress distribution along the entire length of the structure and predominantly at the fatigue hotspots needs to be known. However installation of strain sensors at these hotspots is not always feasible since these hotspots are mainly situated beneath the water level (e.g. muddline). In practice this implies the installation of strain gauges on the monopile prior to pile driving and difficulty in maintaining these submerged sensors throughout the operational life of the turbine. Therefore, an effective and robust implemented technique using the more reliable accelerometers and very limited strain sensors at few easily accessible locations integrated within a new analytical structural dynamic approach is preferred. In this paper, a novel multi-band implementation of the well-known modal expansion approach, a.k.a. full-field strain prediction, is introduced. This technique utilizes the limited set of response data derived during a monitoring campaign and a calibrated Finite Element Model (FEM) to reconstruct the full field response of the structure. The obtained virtual responses are compared with measurements from an ongoing measurement campaign on an offshore wind turbine.