Application of ply-by-ply fatigue analysis methodology in the design of a full-scale tidal turbine blade

Tidal turbine blades are subjected to varying intensities of fatigue loads in their installation environment. In comparison with wind turbines, the flow experienced by tidal turbines is more turbulent in nature. Auxiliary factors such as the presence of waves, and alternating tidal currents contribute to the generation of significant fluctuation in the loads experience by a tidal turbine blade.

The high speed of rotation of the turbine ensures that the blades experience a high number of fluctuating cycles (order of 107) during its service life. Consequently, the estimation of fatigue life of a tidal turbine blade is a significant concern of the blade designer. The European research project Real Tide aims to tackle advanced monitoring, simulation and control of tidal devices in unsteady, highly turbulent realistic tide environments.

With a vision to improve the reliability prediction of tidal turbine blades, velocity characterization has been performed at the commercial site of Ushant, off the western coast of France. Within this paper, a fatigue analysis methodology is proposed that aims to simulate the fatigue strength of the blade using results from quasi-static strength analysis.

Realistic operating conditions are estimated using the site measurement data. Blade Element Momentum Theory (BEMT) is used to transform the realistic velocity data into blade forces. Using composite macro-mechanics, the blade forces are converted into ply-by-ply stresses on the laminate. A formulation, based on laminate failure theories, is proposed to convert the macro-mechanical ply-by-ply stresses into equivalent micro-mechanical stresses on the fibres and the matrix.

By means of the S-N curve data for uni-directional fibres, the quasi-static micro-mechanical stresses are used to predict the fatigue life of the blade under the test conditions, and the failure model.