The offshore wind industry is a rapidly growing sector, and it is estimated that the number of offshore Wind Farms (WF) will increase in the future. At present, offshore WF is based at close-by onshore substations or port facilities where power transmission, construction, and operation and maintenance (O&M) are performed. This is reasonable for offshore area close to coastal area. However, it is no more justified for far offshore areas such as EEZ. In the case of such offshore WF, the construction of offshore base is required since it can reduce the trip time between the base port and the offshore WF site and increase the workability. Therefore, this study examines a very large floating structure with substation and O&M functions by numerical simulations. Similar projects such as the North Sea Wind Power Hub are reported, but they are assumed to be deployed at a water depth of less than 50 m. In our companion paper, we propose a new concept of an offshore base which is a kind of a very large floating structure installed at a water depth more than 50 m in Japan’s EEZ. The proposed offshore base has a complex geometry and properties. It is semi-submersible and rectangular shape, but with a few cut-outs for its functionalities such as the port function as a base, and with heavy concentrated-mass objects mounted on its upper deck. In this study, the elastic response of a very large floating structure with non-uniform mass and with complex geometry, is analyzed by using numerical calculations.