The problem both systems solve
A tank of LNG has to do three things at once. It must stay leak-tight at minus 162 degrees. It must survive the metal contracting as it cools, which shrinks a large tank by tens of centimetres. And it must cope with the liquid sloshing as the ship rolls, which can hit the structure hard enough to damage it.
The two families answer those demands in opposite ways. Membranes are thin and flexible, and lean on the hull for strength. Free-standing tanks are thick and rigid, and lean on nothing.
Membrane systems
In a membrane ship the cargo touches a metal skin about a millimetre thick. Behind it sits insulation, and behind that the inner hull. The membrane is not structural in the usual sense: the weight of the cargo passes through the insulation into the hull, and the membrane’s only job is to be leak-tight while accommodating thermal movement.
Two designs dominate, both licensed by GTT.
NO96 uses flat Invar, a nickel-iron alloy that barely expands or contracts with temperature, in two layers with plywood boxes of insulation between them. Because Invar hardly moves, the membrane can be flat.
Mark III uses corrugated stainless steel. Stainless does move with temperature, so the corrugations act as expansion joints, folding and unfolding as the tank cools and warms. Behind it sits reinforced polyurethane foam.
KC-1, from KC LNG Tech, is the Korean alternative: corrugated stainless steel as both primary and secondary barrier, with a single insulation structure rather than a double one. It entered service in 2018 aboard two 174,000 cubic metre ships.
Membranes carry two big advantages. They fit the hull’s shape, so a membrane vessel carries appreciably more cargo than a spherical one of the same length and beam. And the flat deck above them is easier to work with. The cost is complexity: building one means welding hundreds of thousands of precise seams inside a cold, confined space, and only a few yards do it well.
Free-standing tanks
Moss spherical tanks are aluminium spheres supported at the equator by a cylindrical skirt, so the cold cargo is thermally isolated from the hull. The design is old, proven and forgiving. Because the sphere is a full structural pressure boundary, classification rules allow the full secondary barrier to be omitted, replaced by a partial drip tray. They are strikingly recognisable, with the tank tops standing proud of the deck. Since the first Moss carrier in 1973, around 145 have been built.
Their weakness is geometry. Spheres do not tile. Fitting them into a hull leaves large voids, so a Moss ship is bigger and heavier for the cargo it carries, and the tank tops raise the air draught and windage.
SPB tanks from IHI answer that objection. They are self-supporting like a Moss tank but prismatic, so they fill the hull. Internal bulkheads break up the liquid surface, which suppresses sloshing so effectively that the tanks can be operated partly full at any level, something membrane ships must handle carefully. The trade-off is cost and weight, and they remain uncommon.
Why sloshing matters
Sloshing is the discipline that separates the two families. In a membrane tank the liquid presses directly on a thin barrier. A partly filled tank in a beam sea can generate impact pressures capable of damaging the insulation, so membrane vessels usually avoid intermediate fill levels, either running nearly full or nearly empty with a heel of cargo left to keep the tanks cold.
That constraint is a real operational limit. It shapes how ships can be used as floating storage, how they can discharge partial cargoes, and how they trade in rough waters. Free-standing tanks with internal structure do not face it in the same way, which is precisely where SPB and Moss designs still win work.
What the fleet actually uses
The modern order book is overwhelmingly membrane, and mostly GTT’s two families with their variants. Older steam-turbine vessels are more likely to be Moss. Small bunkering vessels are different again, often using Type C pressure tanks, which are thick-walled cylinders or bilobes that hold pressure and need no secondary barrier at all, practical at small volumes and impossible at large ones.
Our fleet directory records the containment system for each vessel where the tracker reports it, and normalises the wording into these families while keeping the original text on the vessel page. The containment hub lists the firms behind each system, including the alloy makers whose Invar and nickel steels make the thin barriers possible.
The concentration risk
It is worth stating plainly why this page exists on a site about infrastructure. A single French licensor’s designs sit inside most of the world’s LNG carriers. The alloy for one of those designs comes from a small number of producers. The yards able to install either are concentrated in three countries.
None of that is a prediction of trouble. It is a description of a supply chain with very few nodes, and it is the reason a directory of LNG assets is incomplete without a directory of the firms behind them.