*Article* **Reliability and Maintenance Management Analysis on OffShore Wind Turbines (OWTs)**

**Jose V. Taboada 1, Vicente Diaz-Casas <sup>1</sup> and Xi Yu 2,\***


**Abstract:** Due to the extreme marine operating environment, the remoteness from the maintenance base, and the expensive specialized accessibility and overhaul equipment needed (e.g., barges, boats, ships, and vessels), offshore O&M costs are greater than those for onshore-based installations. In the operation of wind farms, the main challenges are related to sudden and unexpected failures and downtimes. This paper has three main objectives. The first is to compare and optimize implementation techniques for maintenance strategies. The second is to analyze the cost-benefit of each maintenance strategy model. The third objective is to demonstrate the optimization and effectiveness of maintenance procedures and strategies recreated with stochastic and probabilistic life cycle cost (LCC) models, depending upon the degree of reliability and the maintenance process for offshore wind farms. The cost of operation and maintenance is directly dependent on failure rates, spare parts costs, and the time required by technicians to perform each task in the maintenance program. Calculations for each case study, with either light vessel/transfer boats (Alternative 1) or oilfield support vessels (Alternative 2), focused on the operational costs for transportation. In addition, each case study demonstrated which maintenance conditions and strategies are operational and optimal, and their corresponding cost–risk impacts. Results from this paper suggest that O&M costs are highly correlated with maintenance round frequency (offshore trips) and the operating costs for transportation by light vessel/transfer boat (CTV) and oil-field support vessel (FSV). The paper analyzes cumulative lifecycle costs and finds that for long-term life cycles (25 years), the implement of light vessels (Alternative 1) is more suitable and cost-effective. In contrast, oilfield support vessels (Alternative 2) are more expensive to operate, but they guarantee major capabilities, as well as the advantage of achieving the access levels need to efficiently operate. According to the results obtained by the outcome analysis, it can be concluded that the implementation of light vessels (Alternative 1) shows a lower overall LCC (<million \$), which is mainly due to the fact that corrective maintenance and minor repairs are less costly. It should be noted that the cost of major repair operations with light vessels in Alternative 1 is still less than the high costs for minor repairs in Alternative 2 (with FSV).

**Keywords:** accessibility; O&M plan; O&M strategies/alternatives; condition-monitoring; LCC; condition base monitoring (CBM); integrity maintenance reliability (IMR); levelized cost of energy (LCOE); cumulative lifecycle costs; risk-based inspection and maintenance analysis (RIMAP)
