Abstract: The invention relates to an improvement in partial secondary barriers for self-supporting, axi-symmetrical cargo tanks having a layer of thermal insulation thereon, the cargo tanks mounted on board marine vessels having a double-bottom construction with an inner shell by means of an annular support supporting the cargo tank of the vessel. The improvement in the partial secondary barrier comprises a secondary barrier tank, preferably cylindrical in shape. The secondary barrier tank is installed under each cargo tank wherein the cross-section of the secondary barrier tank is substantially less than the maximum horizontal cross-section of the cargo tank. The barrier tank, in one embodiment, is structurally joined to the inner shell of the double bottom of the vessel. The tank comprises a stiff external wall, and internal thermal insulation, and an inner impermeable layer for withstanding the low temperatures of the liquid being collected in the secondary barrier tank.