This research examines the difference between two major component replacement strategies for the novel floating offshore wind turbines (FOWT), in terms of support vessel CO2eq emission and cost together with FOWT downtime. The first strategy is utilising a crane vessel to replace a component in offshore, this is called the heavy lift maintenance (HLM) strategy. The other strategy is the tow to port maintenance (TTPM) strategy, where the floating offshore wind turbine (FOWT) is towed to port to be serviced. Royal IHC is looking for a reliable and relatively quick method to determine the potential of both strategies and test new vessel and equipment concepts, which will allow a head start in this upcoming maintenance market.

To determine the three output parameters; CO2eq emission and cost originating from the support vessels and FOWT downtime, a calculation model has been developed, as this was not available. The calculation model compares seven different support fleet configurations for the two maintenance strategies. The configurations contain at least one heavy lift vessel (HLV), platform supply vessel or anchor handling supply vessel type, which are varied in this study on their empirical based parameterised designs. The calculation model also allows the possibility to include, amongst others, preventive maintenance, breakdown maintenance and utilization of different fuels. In total three FOWT farms, varying in size and distance from shore, were considered to determine the effect of those parameters on the different support fleet configurations.

The calculation model was verified and provides a reliable and relatively quick method to compare different support fleet configurations, concerning the two major component replacement strategies. With the use of this model it was found that the most sustainable support fleet configuration for both preventive and breakdown maintenance is a standalone HLV. However, this conclusion does not hold for full blade replacement campaigns. The cost of this HLM strategy is relatively high compared to the TTPM configurations, especially when considering preventive maintenance campaigns. Therefore, it is suggested that the TTPM strategy is preferred for preventive maintenance campaigns. It must be noted that this is also based on the assumption that the port of repair has unlimited facilities to replace the components as soon as the FOWT arrives. It should also be noted that the FOWT downtime is greater for the TTPM strategy, which could be a potentially important aspect of the repair. The calculation model also allows to test new innovative concepts, such as a quick mooring (dis)connection system and the climbing crane. Both show to have a beneficial effect on the replacement operation.

This research has shown that the developed calculation model is well suited, reliable and a relatively quick method determine the potential of both strategies. This research already provided Royal IHC insight into the new market, but can also be used to inform clients who request an advice. The flexibility of the model allows it to be used for different field, fleet and vessel parameters. Moreover, due to the modular programming method additional information can be added at a later stage. This also leads to some recommendations for further research. A suitable weather module has to be introduced in order to get a better estimation on the vessel downtime. After this is done the program should be optimised in terms of runtime efficiency, in such a way that a Monte Carlo method could be run. The added value of this method is that a more accurate support vessel downtime can be estimated. It is also recommended to include bunker time into the calculation as it is currently set to 0 h and potentially leads to an overoptimistic outcome. Lastly, two other floater types should be designed for a 15 MW turbine and included in the model. These floater types are the tension leg platform and barge floater.