This paper presents a methodology of defining the best maintenance strategies for offshore wind farms (OWF) regarding downtime due to the unavailability at system and turbine level. The methodology takes into account a set of turbines, their systems, probability of failure, maintainability characteristics and logistic aspects. The later ones are very important for this type of installation as vessel availability, as well as weather conditions or shore distance can have a large impact on downtime periods. The methodology uses the Analytical Network Process (ANP) approach to identify the best maintenance strategies at each critical system level. Regarding an offshore wind farm the outcome of the proposed methodology clearly shows the best maintenance strategy for each wind turbine system and how to deal with the results to create an OWF maintenance planning. The paper presents a demonstrative example related to wind turbine gearboxes.