Abstract: A subsea tank element comprises at least one tank section (1), the tank section(s) forming a cylindrical concrete-tank (2) closed at its opposite ends by two end caps (12). The tank element further comprises a rectangular structure (3, 4, 5) surrounding the cylindrical tank (2), and a connection (150, 160) between the rectangular structure (3, 4, 5) and the cylindrical tank (2) permitting a motion of the wall of the cylindrical tank (2) within predetermined limits for deflection of the rectangular structure (3, 4, 5). Permanent ballast (6, 7) and a ballast tank (8) control buoyancy and ensure static stability. Post tensioning cables through channels 9 tie the parts into a tank element, and the tank elements into a system. Several applications, including a subsea barge, a subsea hydro-electric plant and a hovering storage tank assembly are disclosed.