This article presents a preliminary assessment of a subsea buoyancy and gravity energy storage system (SBGESS). The storage device is designed to power an off-grid subsea water injection system to be installed at the Libra oil field in Brazil at 2000 m below sea level. Two 12MW floating wind turbines provide the energy supply. The system performance is evaluated according to historical wind data from reanalysis models, the water injection pumps’ power curves, the required daily water flow rate, and the maximum number of shutdowns allowed per year. A control strategy with three different operation modes and one energy-save sub-mode was implemented to optimise the size of the proposed energy storage system.