Cargo containment splits into two families, and the split is structural rather than cosmetic. A membrane is a thin barrier that leans on the hull for support. A free-standing tank supports itself, and the hull merely carries it.
Everything else follows from that difference.
The Moss sphere
A Moss tank is an aluminium sphere, supported at its equator by a cylindrical skirt that transfers the weight into the ship’s structure. The lower half sits inside the hull; the upper half projects above the weather deck under a protective cover, which is why these ships are recognisable from a distance.
A sphere is the ideal pressure geometry: stress distributes evenly, there are no corners to concentrate it, and the tank can be analysed with confidence. Because the tank is self-supporting and independent, a leak can be detected in the space around it without ambiguity about where it came from, and thermal contraction is accommodated by the skirt rather than by the hull.
It is a design that inspires confidence, and its safety record is the reason it dominated for a period.
Why it lost
Geometry. A ship is a long box tapering at both ends; a sphere is a sphere. Packing spheres into that shape wastes an enormous amount of volume, so a Moss vessel needs a physically larger and more expensive hull to carry the same cargo as a membrane ship.
The exposed domes make it worse. They add windage, which matters when manoeuvring alongside a jetty. They obstruct the view forward from the bridge, which drives the accommodation block higher. And they increase the surface exposed to sun and air, working against the insulation.
Against that, membranes use nearly the whole cross-section of the hull. On cost per cubic metre of cargo, the contest was not close, and by the 2000s the fleet had largely moved.
What free-standing tanks kept
One real advantage survived: partial filling.
A membrane is supported by the hull, and liquid moving violently in a partly filled membrane tank can damage it. Membrane ships therefore carried filling restrictions — broadly, keep tanks nearly full or nearly empty, avoid the intermediate levels where sloshing loads peak. Modern membrane designs and better analysis have relaxed this considerably, but the constraint has not vanished.
A sphere does not care. Neither does a prismatic tank with internal bulkheads. That freedom matters for any vessel that does not simply load full and discharge empty: floating storage, bunker vessels, ships breaking a cargo into parcels, and units that sit at a mooring for years with the level rising and falling.
The SPB tank
IHI’s self-supporting prismatic tank, IMO Type B, is the design that tries to keep the free-standing advantages without paying the volume penalty.
It is prismatic rather than spherical, so it fits the hull form efficiently and leaves a flat weather deck with no domes. Internal bulkheads and a swash structure damp liquid movement, giving the same freedom of filling level as a sphere. Being Type B, it is designed so that any crack propagates slowly enough to be detected long before failure, which is why it needs only a partial secondary barrier rather than a full one.
It has been fitted to a small number of vessels and floating units. It has never been cheap, which is the recurring reason it has not displaced membranes on conventional carriers, but on floating facilities — where partial filling is constant and a flat deck is worth a great deal because process equipment has to go somewhere — the calculation is different.
Type C, for completeness
Small vessels use a third option entirely: Type C pressure vessels, essentially cryogenic tanks built to pressure-vessel rules. They are heavy and volume-inefficient, and at cargo scale they are absurd. At bunker-barge and small-scale scale they are ideal, because they tolerate pressure build-up and need no boil-off handling system at all for a short voyage.
How to read the fleet
Containment choice tells you what a vessel was built to do. Membrane on a conventional carrier means a ship optimised to move full cargoes point to point, cheaply. A free-standing tank suggests something else in the design brief: a vessel expected to sit, to part-load, to serve as storage, or to carry process equipment on deck.
The carrier directory records the containment system for each vessel alongside GEM’s raw description of it, because the normalisation into families is ours and the underlying text is theirs.