Abstract
Accurate assessment of the fatigue life of tidal stream turbines and components requires prediction of the unsteady loading of turbine components over a wide range of frequencies. This study focuses on the influence of ambient turbulence, velocity shear and the approach taken to model wave kinematics, on the variation of thrust load imposed on the rotor shaft and supporting tower. Load cycles are assessed based on sea-state occurrence data taken over a five month period for a case study site. The influence of each environmental parameter on component loading is evaluated and the impact on material design parameters assessed. Alternative approaches are considered for modelling turbulent loading and wave loading. The frequency variation of loads due to turbulence are scaled from experimental data from trials of a three-bladed horizontal axis turbine of 1.2 m diameter on a bed-mounted supporting structure. Frequency dependent wave loading is estimated by a relative form of the drag term of the widely used equation of Morison (1950), with the depth decay of kinematics modelled by linear wave theory. Over the five month interval considered a ten year design life can be obtained with a lower design load by accounting for variation of turbulence intensity that occurs during each tidal cycle. This is expected to vary further with the approach taken to model the onset turbulence. A component can also be designed for lower loads over the same time period if irregular waves are modelled instead of regular.