Ever since the commissioning of the first offshore wind farm in October 1991, the offshore wind market has experienced exponential growth all around the world. As more wind farms will be built offshore in the future, onshore grid connection and transmission capacity may not be sufficient to integrate this growing amount of offshore wind farms. On top of that, current wind energy is being dissipated due to a mismatch between offshore energy supply and onshore demand. This combined with the current search for renewable sourced hydrogen poses the question of directly producing hydrogen offshore. Historically, the monopile foundation has been an important factor in cost reduction due to its ease of fabrication, transportation, and installation and therefore accounts for 80 % of bottom-founded offshore wind foundations. The question arose if decentral hydrogen production on a monopile-based support structure of an offshore wind turbine would be structurally feasible. The goal of this research is to define the differences in support structure geometry and assess the changes in the design methodology of an offshore wind turbine support structure, including a decentralized hydrogen production platform. Looking into future developments, a 15 MW reference turbine is selected for a water depth of 45 m in the F3 sector of the North Sea. To obtain platform mass, dimensions, and rotational inertia, all required systems are selected, listed and an optimized platform layout and mass estimation are made. For the design of the platform support beams, gravitational loads and extreme wind gust loads were taken into account. The selection of the support structure concept is performed using a Multi-Criteria analysis. To obtain the monopile design of a support structure with and without the production platform, a static monopile geometry optimization tool is constructed in Excel. The tool optimizes support structure geometry by locating structures' first natural frequency to a set target frequency. For this, it uses maximum ULS loading of aligned wind and waves and checks 3 governing ULS criteria (Global- and local buckling, and Von Mises Yield). The validation of this model is checked by 3 checks, including the redesigning of a previously made support structure by Enersea and by an Euler-Bernoulli beam simulation model in Maple.
For fatigue assessment, an analytical fully dynamical model is constructed in Maple. The structure is simulated by the equations of motions, including airy wave force, rotor damping, topside and platform mass and rotational inertia, embedded length and homogeneous soil stiffness. The maple model is used to simulate dynamic behavior of both structures, determine first and second natural frequency, and present displacements and overturning moments in these two mode shapes. Finally, a fatigue damage calculation including 500 combinations of wave height and period is performed, for a 25-year lifetime. A SCF for weld misalignment and can thickness variation of 1.06 is taken into account. A new support structure design is made for both support structures, optimized to a fatigue damage of 85 %, accounting for 15 % driving fatigue. The model is verified and validated by re-evaluating a fatigue damage calculation of a previous Enersea project, of which results only differ by 8 %. Lastly, a sensitivity analysis is performed to assess the impact of soil capacity, water depth, and platform mass on the dynamic behavior and fatigue damage of a structure including a hydrogen production platform. Platform design of an off-grid 15MW hydrogen production platform results in a hollow platform to accommodate a turbine tower of 25x25x6 m weighing 900 t. A monopile support structure with a separate transition piece and platform is the option of choice, selected by its ease of fabrication, transportation, installation, and O&M. In conclusion, no technical showstoppers were found in the monopile-based support structure design for a 15 MW OWT in 45 m water depth, provided 12 % more steel can be added to the support structure, thus being more costly. The addition of the production facility functions as a dynamic response amplifier at frequencies below first natural frequency and reduces dynamic response of the structure above first natural frequency. The dynamic response amplification of the platform around low excitation frequencies makes critical limiting factors for a conventional offshore wind support structure like water depth and soil capacity increasingly crucial for the design of a support structure including a hydrogen production platform.