Abstract: A double wall tank for the transportation of cryogenic materials such as liquefied natural gas by ship. The inner wall of the tank is the primary barrier and defines the cargo space. The primary barrier is substantially planar and is liquid and gas-impervious. A plurality of primary structural support webs are welded to the inner surface of the primary barrier and extend into the cargo space. The primary structural support webs preferably comprise a plurality of web rings extending between the centerline longitudinal bulkhead and the sides of the inner tank and having plurality of tension trusses extending transversely in the inner tank and connecting opposite sides of the web rings. A plurality of stiffeners are welded to the outer surface of the primary barrier and extend substantially perpendicular to the plane of the primary structural support webs. The outer wall of the tank is the secondary barrier, is spaced from the primary barrier and is supported by the stiffeners.