Systems for transporting liquefied gas in bulk.

A free-standing prismatic tank is a self-supporting rectangular tank built of cryogenic nickel-steel or aluminium, carried in a hold and supported on the hull. Sketch: a rectangular tank with flat or corrugated plates, a top dome carrying the cargo piping, safety and pressure/vacuum valves, access hatch and level gauging; the tank is supported on load-bearing seatings (wood/mild-steel chocks) at the bottom, with side hoppers and a roof, and is surrounded by insulation (polyurethane foam) to keep the cargo cold and reduce boil-off. It is a self-supporting (type A or B) tank with a secondary barrier (drip tray/gas-tight) so any leakage is contained. The tank is designed to withstand liquid sloshing and cryogenic temperature, with corrugated internal stiffening and notch-tough welded joints. Venting systems, dry-pipe vent risers and flame arresters prevent vapour accumulation; high-level and emergency shut-down (ESD) systems are fitted.

A membrane tank is a thin, non-self-supporting membrane (a thin metallic or non-metallic lining) that is supported by the hull structure through insulation. Sketch: the hull forms the outer structure, and a thin membrane (e.g. stainless steel or Invar) is laid over a layer of insulation (polyurethane foam) on the inner surface of the hull. The membrane contains the liquid, and the insulation keeps the cargo cold and transfers the loads to the hull. The membrane is not self-supporting and relies entirely on the hull for its strength; it is used in large LNG carriers. The membrane is corrugated to accommodate thermal contraction, and a secondary barrier is provided. The system gives a high volumetric efficiency (the tank fills the hull) but requires very careful construction and inspection.

A free-standing spherical (Moss) tank is a self-supporting spherical tank built of aluminium or cryogenic steel, carried in the hull and supported on a cylindrical skirt (or equatorial ring) at the equator. Sketch: a sphere supported on a vertical cylindrical skirt attached to the hull, with the tank top carrying the cargo piping, relief valve, access hatch and level gauging; the tank is insulated externally. The spherical shape gives the minimum surface area for a given volume (low boil-off) and is structurally efficient (the sphere is strong in all directions), and it is self-supporting. The tank is supported at the equator, and the skirt transfers the loads to the hull. The spherical tank is used in LNG carriers and gives good structural efficiency but a lower volumetric efficiency than the membrane tank.