A Fatigue Life Assessment of Tidal Turbine Blades Subjected to Cyclic Yaw Misalignment from Diurnal and Semi-Diurnal Tidal Currents

A fatigue life assessment of horizontal-axis tidal turbine blades subjected to cyclic yaw misalignment from diurnal and semi-diurnal tidal currents was performed using integrated computational modelling. The research addresses the problem that turbine developers lack quantitative understanding of how daily current direction reversals affect blade durability, leading to uncertain design decisions about yaw mechanisms. Unlike previous studies that examined yaw as a fixed condition or tidal cycles only as speed variations, this research uniquely combines blade element momentum theory, finite element analysis, and extended finite element method with the Walker equation to capture the regular rhythm of direction changes. The loading analysis revealed that each tidal direction change produces rapid load reversals of approximately 400 KNM range at spring tides. At 3.0 m/s current speed, root bending moment increased from 414.3 KNM at zero yaw to 601.2 KNM at 20° yaw (45% increase). Finite element analysis showed maximum principal stress reached 83.5 MPa under cyclic yaw at 3.0 m/s, 33% higher than zero yaw conditions. Crack initiation analysis identified the pressure side near the leading edge at the blade root as the most critical location, with stress ranges of 78-85 MPa and criticality factor of 9.8. Crack growth analysis demonstrated that cyclic yaw produces growth rates of 2.8×10⁻⁷ mm/cycle at 10 mm crack length, 75% higher than fixed 10° yaw and 211% higher than fixed 0° yaw. For 0.5 mm initial defect, cyclic yaw gave 8.2 million cycles to failure compared to 18.7 million for fixed 0° yaw (56% life reduction). At 2.0 m/s current, cyclic yaw yielded 18.2 years life versus 41.2 years for fixed 0° yaw. Wave heights of 2.0 m reduced life by 45%, while 2.0 mm initial defects reduced life to 38% of 0.5 mm defect cases. A predictive model N_F = (58.3/U^2.8) ×F_YAW ×F_WAVE×F_DEFECT was developed, with penalty factors of 0.44 for cyclic yaw relative to fixed 0° operation. The research concludes that cyclic yaw significantly reduces blade life, with sites exceeding 2.0 m/s current and 1.0 m waves requiring special design measures to achieve 20-year design life. The findings provide quantitative tools for yaw mechanism decisions, inspection focusing, and design standards development.