A floating hydraulic energy storage plant, designed according to the Buoyant Energy principle, consists of a structural shell that is made of as much concentrated mass as possible and encloses at least one empty space (water reservoir). Similar structures have been built before for other applications, i.e. concrete ships1 and immersed tubes2. Because of this well tested and proven construction methods are already available. Modern shipbuilding focuses mainly on lightweight construction, whereas robustness and great mass are crucial for a floating hydraulic energy storage plant.