- Primary Barrier: A thin membrane made of INVAR (36% Nickel, 64% Iron) with a thickness of 0.7 to 1.2 mm, forming the primary containment layer for the liquefied gas.
- Primary Insulation box: A 230 mm thick layer of granulated Perlite insulation packed in plywood boxes, surrounding the primary barrier. Perlite is siliconised to make it impervious to moisture.
- Secondary Barrier: A secondary membrane made of the same INVAR material is installed to prevent cargo leakage in case of primary barrier failure.
- Secondary Insulation box: Another 300 mm thick layer of granulated Perlite is placed above the secondary barrier to provide additional insulation and prevent thermal transfer.
Invar (36% Ni, 64% Fe) is used for both the primary and secondary barriers. The reason for this choice is its exceptionally low coefficient of thermal expansion. This eliminates the need for expansion joints or corrugations in the membrane design. In addition, Invar remains strong and does not become brittle at the very low temperatures experienced by the liquefied gas. The thin and lightweight nature of Invar maximizes the cargo-carrying capacity of the tank.
, meaning they don't form part of the ship's hull and don't contribute to the ship's structural strength. They can be spherical, cylindrical, or prismatic (box-shaped). Prismatic tanks usually have internal stiffeners like bulkheads, webs, girders, and stiffeners for added structural integrity.
(i) Longitudinally:
Tanks are positioned longitudinally within the ship's cargo hold. The tanks are supported longitudinally by anti-roll chocks and anti-lift chocks that resist forces caused by ship motions. These chocks ensure the tank remains stable even in rough sea conditions. They also act as thermal barriers, preventing the transfer of heat between the hull and the tank.
The chocks also serve as thermal barriers between the hull and cargo and are often constructed of wood or plastic materials.
(ii) Transversely: Transverse support is provided by anti-pitch chocks and support chocks, which stabilize the tank against lateral forces. These chocks are typically made of plywood or plastic and help prevent thermal stress between the tank and the ship’s structure.