Assessing heavy maintenance alternatives for floating offshore wind farms: towing vs. onsite replacement strategies

This paper presents the first comparative study evaluating towing and onsite replacement strategies for heavy maintenance of floating offshore wind (FOW) turbines. The towing maintenance strategy is characterised by a Markov chain and implemented within a computationally-efficient operation and maintenance (O&M) model. This model includes all key phases of the towing strategy: transit-to-site, turbine disconnection, towing-to-port, component replacement, towing-to-site, turbine connection, and transit-to-port. Additionally, the paper provides the first spatial assessment of heavy maintenance for FOW turbines in the North Sea. Evaluation across the ScotWind area shows that onsite replacement can reduce turbine downtime, especially for quick heavy maintenance operations like blade and gearbox replacements. However, for longer operations, such as generator and pitch and hydraulic system replacements, onsite solutions are more effective than towing only when O&M vessels can operate in wave heights over 1.5 metres. Otherwise, a mixed heavy maintenance strategy is recommended, combining onsite replacements for blades and gearboxes with towing for generators and pitch and hydraulic systems. The average turbine availability reduction with the mixed strategy is 0.39%, followed by the fully towing strategy at 0.43%, and the fully onsite replacement strategy at 0.46%.