Risk-based Operation and Maintenance for Offshore Wind Turbines

For offshore wind turbines, costs to Operation and Maintenance (OM) are substantial, and can be expected to increase when wind farms are placed at deeper water depths and in more harsh environments. Traditional strategies for OM include corrective and preventive (scheduled and condition-based) maintenance strategies. This paper describes a risk-based life-cycle approach for optimal planning of OM and design of offshore wind turbines. Damage accumulation models used to describe deterioration mechanisms such as fatigue, corrosion, wear and erosion are associated with significant uncertainty and are often the driving mechanisms for failures / faults that need maintenance. Observations of the degree of damage can increase the reliability of predictions and decrease the costs of OM if integrated in a risk-based framework theoretically based on pre-posterior Bayesian decision theory. The mathematical formulation is presented in the paper and can be applied for OM planning for main components such as generator, rotor blades, blade pitch mechanism, yaw mechanism, main shaft, tower / support structure (jacket): cracks, corrosion, etc. The different OM strategies are compared for application for offshore wind turbines. Some of the OM strategies only need basic information on failure rates and costs. Other methods need much more detailed information about damage accumulation, stochastic models for uncertain parameters and processes, reliability of inspections, indicators and monitoring systems, weather statistics and costs of the different operations. The different OM strategies are described and compared in an illustrative example with focus on which types of information that are needed. Special focus is on comparison between risk-based maintenance strategies and the conventional maintenance planning methods.