This study explores the feasibility of offshore floating bases for wind farms deployed in Japan's Exclusive Economic Zone (EEZ), aiming to achieve the national ambitious goal of installing 30–45 GW of offshore wind power by 2040. Construction of energy islands for wind farms in European waters is seriously discussed. Given the deep waters exceeding 200 m in Japan's EEZ, floating solutions are deemed more suitable than the artificial islands, contrasting with European practices. The research investigates three scenarios at varying distances from the coast (50 km, 100 km, and 300 km, respectively), considering environmental factors such as sea conditions, water depths, and wind characteristics. Referencing to the foregoing research of the energy islands, functional requirements for each floating offshore base are discussed and outlined, guiding the layout, area, and dimensions of the facilities. A semi-submersible structure supported by multiple vertical columns is finally selected from the viewpoint of its response characteristics in high waves and suitability for open ocean installation. The performance and structural safety under the design wave conditions are confirmed by numerical hydroelasticity simulations based on the finite element (FE) model.