This paper presents an interpretation of boundary conditions using a case study of a deep-draft concrete spar supporting a floating offshore wind turbine. The presented assumption aims to establish the design bending moment for a spar’s hull concrete during a preliminary design stage; thereby, a prestressed concrete design could be carried out. The design bending moments were determined based on the structural mechanics theory as well as the strain distribution obtained by finite element analysis, which was performed to verify the design results and observe stress and strain distribution. The influence of boundary conditions on the structural design of the floating body was investigated considering two scenarios: fixed bottom and pinned bottom. The study results indicated that the simplified boundary condition adopted at the preliminary stage resulted in a highly conservative design, as expected. The boundary condition drastically impacted the structural design of the floating body and needs to be considered carefully to achieve a cost-effective structure.