Reliability of rotor blades of tidal stream turbines under extreme and fatigue loading

Since tidal stream turbines are a new technology there are still no standards or other clear guidelines for their design in general and the design of their rotor blades in particular. There are two types of limit states that are of concern for the design of the rotor blades - ultimate and fatigue. The paper will consider how reliability issues can be addressed in the design in the context of both limit states. Loads acting on the rotor blades depend on the rotor orientation and a control system of the tidal stream turbine. In this paper only horizontal-axis pitch-controlled turbines will be considered. Uncertainties associated with tidal current velocities (mainly due to turbulence), the blade resistance and the model used to calculate internal forces in the blades will be taken into account in the reliability analysis. Based on results of the analysis, importance of each of the limit states for the design of rotor blades will be discussed. The paper will also demonstrate how results of the reliability analysis can be applied to derive values of the partial safety factors for the design of tidal turbine rotor blades. Finally, issues related to the assessment of the probability of fatigue failure of rotor blades are briefly considered.