Reliability of Offshore Wind Turbine Drivetrains based on Measured Shut-down Events

The key objective of this paper is to investigate the frequency of normal shutdowns to be used in the design stage of wind turbines, based on measurements at an offshore wind farm and thereby quantify the irimpact on the fatigue loads on the drivetrain and tower top. The measured shut-downs observed on a fully instrumented multi megawatt wind turbine located in an offshore wind farm are correlated with corresponding observations of shutdowns on surrounding windturbines. The observed wind turbines have multiple shut-downs at high mean wind speeds. The normal shutdown is brought about by initiating blade pitching to feather and also sometimes using the generator torqueas a brake mechanism. The shutdowns due to wind speed variation nearcut-out are predicted using an Inverse First Order Reliability Model(IFORM) whereby an expected annual frequency of normal shutdownsat cut-out is put forth.
A simulation model of the wind turbine is set up in the aeroelastic software HAWC2, based on which the observed shut-downs are simulated, along with normal operation. The simulated tower topmoments are compared with the measured loads, thereby quantifying the amplification in the loads due to the shutdown action. The IFORM determined frequency of shutdowns at cut-out mean wind speed is usedas an input to the fatigue load computations in the drivetrain, by which, the resulting damage equivalent loads are analyzed to quantify their coefficient of variation for varying site specific wind conditions underboth normal operation and with shutdowns. The maximum coefficient of variation (CoV) due to varying wind conditions was found on the low speed shaft torsion, but the shutdowns by themselves were not seento significantly change the fatigue loads.