An optimal group maintenance policy for multi-unit offshore wind turbines located in remote areas

Many offshore wind farms are planned to be built in remote sea locations due to high wind resources. However, the difficulties in accessing and maintaining wind turbines lead to increased operation and maintenance (O&M) costs and subsequently, increased cost of energy (COE). In order to share the maintenance set-up costs and reduce the power production losses, the group maintenance policies are widely used in offshore wind energy industry. In this paper, we investigate an optimal group maintenance policy for a multi-unit wind turbine system located in cold, icy or remote offshore areas. Each individual unit is exposed to a gradual degradation phenomenon that takes place in different forms of wear, corrosion, erosion, fatigue and crack generation. The whole system undergoes an unplanned maintenance action when the degradation level of one of units reaches its critical size; otherwise a planned group maintenance task is conducted at the operational age T (>0). We propose an optimization model to determine the optimal time T* such that the average long-run maintenance cost per unit time is minimized. Finally, the proposed model is applied to a three-bladed rotor system and the results are compared with the individual maintenance of blades.