Dependability Assessment of a Blade Pitching Mechanism in Tidal Stream Turbines

To achieve sustainable energy goals, marine renewable resources particularly tidal stream turbines are expected to contribute to the next generation ocean energy systems. The reliability and performance assessment of tidal stream turbines require an integrated modelling framework that captures both mechanical component degradation and environmental variability. The study aims to analyse the effects of seal-bearing coupling on the dependability of the blade pitching mechanism in tidal stream turbines. The operational behaviour of the pitch bearing and seal assembly, critical components of the simplified blade pitching mechanism that experience continuous marine environmental stresses, is modelled by a Markov model. The model explicitly captures the physical dependency between the bearing and seal processes, acknowledging that these subsystems interact rather than degrade independently. The seal prevents seawater ingress and lubricant loss, white the bearing supports cyclic mechanical loads and enables controlled blade rotation. When the seal degrades, contamination and lubricant depletion can accelerate bearing deterioration. Therefore, seal repair is usually prioritized to prevent cascading degradation; however, if the bearing has already sustained major damage, it becomes the priority for maintenance and structural integrity concerns. A numerical study of the proposed unified stochastic framework is developed and presented, illustrating the dependability assessment of the simplified blade pitching mechanism. Results show that energy losses persist even at high tidal current velocities and the energy produced by the system is shown to increase with tidal current speed, rising steeply in the sub-rated region (2.2-2.4 m/s), yet never attaining the theoretical maximum due to degradation.

Keywords: Tidal Stream Energy, Marine energy systems, Markov modelling, Dependability assessment.