Operations and Maintenance Cost Comparison Between 15 MW Direct-Drive and Medium-Speed Offshore Wind Turbines

Abstract. Determining offshore wind energy operational expenditures relies on acquiring reliability data, particularly as turbine power ratings increase. The uncertainty surrounding operational costs and optimal drive train configurations for larger turbines persists. While previous research addressed reliability data for 3 MW offshore wind turbines, this study reviews and collates updated failure data for 15 MW turbines, comparing direct drive and medium-speed configurations. It employs an Operations and Maintenance (O&M) modeling tool to calculate total operational costs. The study concurs with existing literature, showing that direct drive turbines have lower operational costs than medium-speed turbines in three case studies. However, the cost and availability differences between configurations are smaller than previously suggested. For 15 MW turbines, the analysis reveals that the cost disparity between direct drive and medium-speed turbines is significantly smaller than for smaller-rated turbines, with percentages of 1.59 %, 1.58 %, and 5.78 % for the three ScotWind sites selected. Previously, the absolute % difference in cost between direct drive and medium speed turbines was estimated at 29.79 %. Sensitivity analyses explore the influence of three factors—failure rates, accessibility limits, and major replacement times—on total operational costs. These analyses demonstrate that medium-speed configurations exhibit more significant cost fluctuations, and the cost gap between configurations is reduced if failure rates are lowered at the same rate for each configuration, accessibility increases or the major replacement time is reduced for the larger wind turbine components.