Accurate assessment of the fatigue life of tidal stream turbines and componentsrequires prediction of the unsteady loading of turbine componentsover a wide range of frequencies. This study focuses on the influenceof ambient turbulence, velocity shear and the approach taken tomodel wave kinematics, on the variation of thrust load imposed on therotor shaft and supporting tower. Load cycles are assessed based on seastateoccurrence data taken over a five month period for a case study site.The influence of each environmental parameter on component loadingis evaluated and the impact on material design parameters assessed. Alternativeapproaches are considered for modelling turbulent loading andwave loading. The frequency variation of loads due to turbulence arescaled from experimental data from trials of a three-bladed horizontalaxis turbine of 1.2 m diameter on a bed-mounted supporting structure.Frequency dependent wave loading is estimated by a relative form of thedrag term of the widely used equation of Morison et al. (1950), with thedepth decay of kinematics modelled by linear wave theory. Over the fivemonth interval considered a ten year design life can be obtained with alower design load by accounting for variation of turbulence intensity thatoccurs during each tidal cycle. This is expected to vary further with theapproach taken to model the onset turbulence. A component can also bedesigned for lower loads over the same time period if irregular waves aremodelled instead of regular.
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