Renewable forms of energy, and especially wind energy, have become an integral part of the energy infrastructure around the globe. However, the constant increase of installed wind capacity has put a strain on the selection of potential onshore locations for wind turbine installation. As a result, the wind industry is focusing on offshore applications, mainly with the use of bottom-fixed sub-structures. Nevertheless, research community has started shifting their interest in floating wind turbine designs to harvest the abundant wind potential of far-offshore locations, sited in water depths where monopiles and jacket sub-structures are not economically feasible. Despite the progress on the design of such floating units, there is still no consensus upon their optimum installation and maintenance strategies, activities which comprise one third of the lifetime cost of a floating wind farm.

The aim of this research is to gain understanding on the optimum installation and maintenance strategies for semi-submersible and spar buoy floating units based on their cost and duration. Towards this direction, four different installation strategies are investigated: semi-submersible installation with wind turbine assembly at quayside, semi-submersible installation with wind turbine assembly at wind farm site, spar buoy installation with wind turbine assembly in a single lift with a heavy lift vessel (HLV) and spar buoy installation with wind turbine assembly with multiple lifts with a crane barge. Furthermore, based on the type of the required maintenance activities, the onsite minor and major repairs of wind turbines with the use of a crew transfer vessel (CTV) or a walk-to-work (W2W) vessel are examined, as well as the onsite or offsite replacement of major wind turbine components for wind turbines installed on both semi-submersible and spar buoy floating sub-structures. In the context of this thesis, a deterministic installation and O&M model was developed. This model relies on various input data, including weather time series for near-shore and far-offshore locations, vessels and facilities costs, personnel and components costs and an electricity price for the calculation of the lost revenue due to downtime.

The analysis of the installation and maintenance strategies is based both on a base case scenario and a sensitivity analysis of the main factors influencing the cost and duration of offshore activities: distance from shore/port and weather limits of activities. The results show that semi-submersible installation with wind turbine assembly at quayside is the most cost-effective and fastest of all implemented strategies, followed by the spar buoy installation with HLV. Furthermore, the use of a CTV for onsite repairs is more cost-effective, albeit slower, approach than the W2W one, for all examined distances and weather limits. Finally, the offsite major replacement strategy for semi-submersibles is the most cost-effective of the examined strategies. No definitive conclusion can be drawn on the best major component replacement strategy for spar buoys, as the cost difference between the onsite and offsite approach is small. When the cost difference of different installation and maintenance strategies is low, the final choice depends on the volatile vessel rates and the electricity market prices.