The Ostend Declaration (2023) sets out the ambition to transform the North Sea into Europe’s green power plant by significantly increasing offshore wind generation. While essential to meet Europe’s rising energy demand and sustainability targets (North Sea Energy, 2020), this also raises challenges. Transmitting such large volumes of electricity faces technical bottlenecks, including grid congestion and limited storage, and economic barriers such as high cable costs (Beaubouef, 2024). Hydrogen offers a promising pathway. As a flexible, storable energy carrier, it balances the system and reduces reliance on costly grid expansions (Dute et al., 2024; Farahmand et al., 2024). Offshore electrolysis—in turbines, hubs or energy islands—can lower pressure on onshore grids and land use (Ramboll, 2025; Janssen et al., 2025). However, large-scale offshore electrolysis remains underdeveloped: no full-scale projects exist, optimal sites are unclear, and deployment requires cross-border coordination (European Commission, 2025a; Van Wingerden et al., 2023).

Realizing this potential requires collaboration among North Sea countries (Van Wingerden et al., 2023). This involves connecting national offshore systems and integrating electricity and hydrogen into a supranational North Sea energy system (North Sea Energy, 2020; One North Sea, 2021). Energy islands can be strategic assets by linking offshore wind farms to a hydrogen backbone, enabling economies of scale and cross-border flows (Arteaga et al., 2024). Yet the system design faces uncertainties: the hydrogen economy is still developing, national governments mainly plan domestically, and ecological areas, military zones and shipping routes impose spatial constraints, demanding careful multi-use planning (North SEE, n.d.; Staeb, 2025).

Against this background, this thesis develops a conceptual system design for 2050 that minimizes overall costs while accounting for spatial constraints and infrastructure reuse. The societal relevance lies in showing that energy islands can enable large-scale offshore wind and hydrogen production and that a multinational approach is vital for Europe’s climate goals. Academically, it contributes to the still limited literature on multi-energy, multinational offshore systems. The guiding research question is: What is a system design with minimal overall costs for the North Sea, in which energy islands integrate offshore wind farms in an offshore hydrogen network, while accounting for other uses?

Three sub-questions are assessed. The first concerns how many energy islands are needed to balance efficiency and costs. Wind farms are grouped by distance, and cost implications analysed to identify the lowest-cost grouping. The second considers island locations, taking into account spatial constraints. Alternative layouts are compared to identify technically feasible and economically attractive sites. The third examines how these islands can be connected into a cost-efficient hydrogen backbone enabling cross-border flows.

The results show that eight energy islands balance construction and cabling costs, with an estimated €36.4 billion investment. Island locations are shaped by spatial constraints, but accounting for them reduces cabling costs from €14 billion to €0.3 billion by enabling shorter connections. The analysis also shows that the total capacity connected to islands is a key driver of costs, stressing the need for balanced capacity flows.

A hydrogen backbone is then designed. It could integrate 93 GW of hydrogen capacity, requiring 186 GW of electricity, at €8.9 billion. Existing natural gas pipelines can partly be reused, but new pipelines are still required. The combined system—energy islands, offshore wind connections, and a hydrogen backbone—amounts to €31.7 billion. This reflects a coordinated multinational approach; if countries plan separately, costs will be higher and integration weaker.

In sum, integrating offshore wind, hydrogen and energy islands into one North Sea system is technically feasible at substantial but necessary investment costs. These should be seen as strategic opportunities: without them, climate targets may be missed, energy supply less secure, and Europe more dependent on external sources.