Offshore wind farms are increasing in size and moving further from shore. Service operation vessels (SOVs) are used for offshore wind operation and maintenance (O&M) at these large far offshore sites. These large vessels typically have a smaller daughter craft (DC) on board that can assist them. This DC is however too small to provide the seakeeping capabilities needed at most far offshore sites, causing it to become essentially unusable. Previous studies at Siemens Gamesa Renewable Energy (SGRE) have looked into improving the capabilities of the DC while considering the constraints of the SOV, this was deemed insufficiently possible by Brans (2021). The second study looked at increasing the size of the DC, which saw significant improvements (Kamerbeek, 2022).

The work of Kamerbeek (2022) however raises new questions such as what is the optimum number of these larger DCs? Would another type of craft serve as a better mothership or DC? Is having the mothership perform maintenance the most efficient? SGRE is therefore interested in exploring mother-daughter concepts to perform offshore wind farm O&M activities at large far offshore wind farms, to see if these can outperform the status quo. A mothership is the home of the technicians offshore and the daughters are the craft that bring the technicians from the mothership to and from the turbines. The main research question is therefore:

What method can best be used to explore the design-space of mother-daughter
concepts for offshore wind farm O&M?

This research first focuses on understanding offshore wind farm O&M and finding the most important restrictions and challenges that need to be taken into account within a model. This has been done through a literature review and discussions with experts from SGRE. The work then focuses on selecting a modeling method and explaining the proposed method. This method is validated using a comparison with a real-life wind farm. A case study is done at the end using a dummy wind farm to demonstrate the workings of the method. The method uses a discrete-event simulation that simulates the transport of technicians to and from the turbines to estimate the performance of the concepts. Any wind farm, turbine failure rates, or fleet can be inserted into the model for analysis to ensure a wide range of applications. The performance of the fleets is assessed based on the estimated downtime/availability and emission estimates that the model produces. The financial and technical feasibility should be evaluated in the next stage when a selection of promising solutions has been made based on this first logistical analysis of the fleets. The visits are planned within the model based on the weather conditions, number of available technicians, craft availability, and the evacuation requirement.

The design space of mother-daughter concepts should be explored by running the model using the exploratory set of fleet configurations and inputting various wind farm layouts with varying realistic visit agendas and weather conditions. The output of each of these cases should then be analyzed by dividing all the fleet configurations into groups based on the craft each fleet contains. This grouping allows the performance of each type of fleet to be compared to one another, while the performance difference within the groups shows the effects of different transfer limits. The analysis should then focus on identifying cross-over points between different configurations and on selecting specific fleets based on performance and expected configuration cost.

The accessibility in far offshore wind farms during unplanned maintenance is reported to be insufficient. This system largely consists of daughter craft, small vessels that increase the multitasking capabilities of an SOV. The goal of this research is to improve the accessibility to ultimately increase turbine availability, which is proven in this work to be necessary. The research is carried out in cooperation with Siemens Gamesa, a major wind turbine supplier. This research builds off the basis laid by Brans et al., who applied a needs analysis on the daughter craft system. This research applies the next step in the systems engineering sequence: concept exploration. The scope of this research extends beyond that of the daughter craft to any system that can improve the unplanned maintenance of far offshore wind farms.

Because the subject matter is relatively little covered in the scientific field, this research lays great emphasis on the context of the problem. The status quo of the sector is described in terms of equipment, operations, regulations, forms of limitations, financial context, trends, and different stages performed in unplanned maintenance. By performing an analysis of alternatives a high potential for system improvement is found.

A set of performance requirements for the accessibility system is developed to structurally assess the system and possible improvements. These performance requirements are used to determine which alternative system holds the most potential for accessibility improvement. Increasing daughter craft dimensions is chosen as the most potent alternative. A feasibility study is performed on deploying CTV-sized vessels far-offshore for two weeks thereby significantly reducing transfer time and distance. These vessels are called far-offshore transfer vessels (FOTV).
Different configurations are tested for storing the FOTVs far-offshore when not in operation and interfacing with the SOV. Two principal concepts are identified: Enlarged daughter craft, where the FOTV is stored on the SOV, and the exposed principals, where the FOTV remains in the water. One configuration of the enlarged daughter craft principal is deemed feasible: The lifting launch configuration. Two configurations of the exposed principal are deemed feasible: The connected to the SOV configuration and the moored to designated platform configuration. Furthermore, a model is constructed to assess the logistic and economic merit of different combinations of FOTV, wind farm, and configuration.

This research concludes that the deployment of a FOTV combined with a lifting launch configuration is most profitable for any wind farm. Nonetheless, the exposed principal concepts outperform the current system significantly. The choice of FOTV depends mainly on the accessibility performance and day rate of the vessel but also on the wind farm. The model predicts that in the current market, the highest-performing CTVs are the most profitable options for FOTV deployment. Finally, the results show that the effects of improving accessibility vessel performance relates inverse exponentially to profit performance.