The exploitation of the wind as a renewable energy source has increased over the last years. Transitioning to offshore wind means an increased wind energy potential. Bottom-founded wind turbines are suitable for shallow waters, but floating solutions are needed for greater water depths. Barge-type floating wind turbines can offer advantages compared to other competing alternatives in terms of manufacturability, logistics, and installation in shallower water depths. Combining the barge with a single mooring line layout enables a reduction in material costs, utilizing the weathervane principle, and decreasing the wake effects in the wind farm, therefore increasing power output. The WindBarge from Green Floating Marine Structures (GFMS) was studied in this thesis, which integrates, as a first design approach, a 15 MW upwind turbine on a barge-type structure with vertical side walls, horizontal bottom, and one mooring line including internal redundance.

Few floating wind turbine designs exist with a barge-type floater, and the design of the inner structure has not been investigated thoroughly in academic research. The inner design of the WindBarge is established by the WindBarge project, however, further modifications of the inner structure are proposed in this study. Its structural response is associated with the external loading and eigenfrequencies, providing an insight into the stress field of the inner structure for these types of structures based on ultimate limit state load cases. The simulation procedure using DNV's software suite SESAM was implemented.

Apart from extreme loads, fatigue can be a driving design factor due to the dynamic wind and wave loading. A focus was given on the tower transition region in this study. The external loading and the complex geometry can lead to a multiaxial stress state, and only a few studies in academic research have addressed this topic for floating wind turbines. A multiaxial screening method was utilized based on the 2-dimensional stress space representation and a minimum volume enclosed ellipse (MVEE) to identify the level of non-proportionality. Multiaxial conditions were observed at the deck, and the fatigue damage was calculated based on fatigue limit state load cases.

Three different approaches were implemented and compared for the fatigue assessment: the nominal uniaxial fatigue based on the forces/moments from the beam model of the tower, the hot spot plate fatigue, and the hot spot plate multidirectional fatigue, based on an effective stress according to DNVGL-RP-C203. The multidirectional approach considers different stress directions, therefore, different crack planes to identify the critical plane. For the fatigue damage calculation using the shell model of the floater, the response reconstruction method with SESAM Core was utilized.

Supplementary studies were carried out on how to reduce the observed difference in fatigue damage around the tower base caused by the roll motions, focusing on the cause of these motions, the coupling of the different degrees of freedom of the floater, and the effect of increased damping, roll stiffness, and the yaw controller. A conclusion is made that the design of such a floater is mainly fatigue rather than ultimate limit state driven.