Risk-based maintenance planning of offshore wind turbine farms

A risk-based maintenance planning for offshore wind farm installations is developed. Initially, the optimal number of monopile offshore wind turbines to be installed in an offshore wind farm is estimated, targeting a minimum levelised cost of energy. To this end, a sufficient number of design combinations for monopile support structure is generated by the design of experiment technique and then analysed based on the fatigue limit state. Following the fatigue reliability analysis performed for the monopile designs, an analytical relation between manufacturing cost and the structural safety regarding fatigue is developed to be used in the life-cycle cost analysis. The offshore wind farm is considered here as a system consisting of correlated components. The system reliability is estimated by using Ditlevsen bounding technique, which uses a time-variant correlation matrix of offshore wind turbines. The event tree method is employed to assess the expected cost of failure to be included in the capital investment as the structural risk premium, and the total expected cost to be included in the operational cost. Furthermore, different inspection policies are studied, and the most cost-effective inspection and maintenance policy is found for each studied wind farm. The present study also develops a novel framework for inspection and maintenance planning that maximises the benefits of performing inspections for a multi-unit system. Finally, the developed framework is applied to an offshore wind farm with sixty installations, and the detailed description of the planned inspections are discussed.