Continuous Fatigue Assessment of an Offshore Wind Turbine Using a Limited Number of Vibration Sensors

Abstract : Offshore Wind turbines are exposed to continuous wind and wave excitation that leads to high periodic stresses and strains at critical locations. This makes the structures prone to structural failure due to possible crack initiations and propagations. The continuous monitoring of the Wind Turbine is of utmost importance in order to assess the remaining lifetime and accumulative fatigue damage of the structure. Health monitoring of wind turbines is usually performed by collecting real-time operating data on a limited number of accessible locations using traditional sensors such as accelerometers and strain-gauges. When dealing with Offshore Wind Turbine though, most of the fatigue sensitive spots are inaccessible for direct measurements, e.g. at the muddline 30 meters below the water level. Response estimation techniques can then be used to estimate the response at unmeasured locations from a limited set of response measurements and a Finite Element Model. This paper makes use of a modal decomposition and expansion algorithm that allows for successful response prediction. The algorithm is validated using data obtained from a monitoring campaign on an offshore Vestas V90 3 MW wind turbine on a monopile foundation.