The energy transition towards carbon neutrality requires a rapid electrification of all energy sectors by 2050. In the EU-27, the REPowerEU strategy, initiated in March 2022, is pursuing this target even more ambitiously. However, in order to achieve the aforementioned goals, the increasing demand for green hydrogen will cause an increase of the renewable energy needs as early as 2030. In this way, offshore wind can be a solid supplier of the renewable energy for green hydrogen production. However, since nearly 80% of the worldwide wind energy potential is situated in deep waters, floating offshore wind turbines (FOWT) can be used to cover those energy needs. In addition, the literature review showed that when green hydrogen production via FOWT is considered, the most economically and energy efficient layout is the in situ topology, where hydrogen is locally produced on the FOWT. Although FOWT and green hydrogen production via FOWT have been lately examined in literature, a literature gap was found. More specifically, no attention was given on the performance change of a turbine when it is adapted from a bottom fixed (BF) application to a FOWT. Also, the effect of the in situ hydrogen plant to the FOWT performance was not considered. Thus, the aim of this project is to highlight if FOWT for in situ hydrogen production should be aerodynamically redesigned to improve their performance or to tackle possible energy losses. In this direction, several sub-questions should be answered, including the selection of turbine, floater type and site to be investigated, the effect of the floater design on the FOWT performance, the performance change of a turbine due to its adaptation as FOWT and the effect of the added mass of the hydrogen plant on the FOWT performance. Lastly, in case of a proven FOWT performance deterioration, solutions should be provided so as to regain performance.  Aligned with the previous goals, the IEA 15MW reference wind turbine on top on the UMaine Volturn US-S semi-submersible floater is simulated in OpenFAST under steady and turbulent wind fields, according to wind & wave conditions of a typical US East Coast site. In addition, the in situ hydrogen plant is incorporated in the model using a simplified approach. The results suggest that in the whole partial load region, the FOWT experiences power losses due to the static floater pitch angle, which reduces the inflow wind speed seen by the rotor. However, between 9 and 12 m/s, a peak shaving routine is incorporated in the FOWT controller, which results in early power shedding and contributes, together with the static floater pitch, to considerable power losses. Furthermore, the simulations conducted using the OpenFAST FOWT model, which incorporated the hydrogen plant, suggest that it has a negligible effect on the FOWT performance and can be omitted from the model.  Finally, the comparison of the FOWT and of the BF turbine under turbulence, highlights the fact that the FOWT exhibits a spanwise aerodynamic torque reduction and a spanwise airfoil aerodynamic inefficiency in terms of angle of attack, that can be solved via an aerodynamic redesign. As a result, a variety of blade twist angle and airfoil chord length profiles are developed, tested and evaluated using the FOWT annual energy production (AEP) as an indicator. The results point out that all solutions result in a slight FOWT AEP gain, compared to the original FOWT design, at the expense of increased rotor loading, which effectively increases the static floater pitch. Thus, the aerodynamic redesign approach requires a more cautious approach.