Sea cargo reference

Gas Carriers

Gas held cold, under pressure, or both. The notes below cover the cargo, the way the hull is arranged, and the limit that puts a ship in this class.

Gas Carriers vessel
Ship's gear
Self-discharging by cargo pump
How capacity is counted
Cargo volume in cubic metres
Usual size
LNG carriers commonly 145,000-174,000 m³
Cargo type
Liquefied gases

Gas carriers

These ships carry gas as a liquid: cooled to its boiling point, held under pressure, or both. An LNG carrier keeps cargo at about minus 162 degrees Celsius. An LPG carrier works in less extreme conditions.

Capacity is counted in cubic metres, not deadweight, because the cargo is light for its volume. The tanks define the ship: a heavily insulated membrane, or independent spheres that show as domes on deck.

Some boil-off is unavoidable on a refrigerated cargo. Modern LNG ships are built to burn that gas as fuel rather than waste it, which also makes them among the cleanest-burning ships in service.

Cargo on board

  • LNG
  • LPG
  • Ammonia
  • Ethylene
  • Butane and propane

How the hull is arranged

  • Ethylene carriers are a third class beside LNG and LPG ships. They are built for smaller parcels and can handle cargo at the temperatures ethylene needs.
  • A double hull keeps cargo tanks off the sea, with ballast space on the sides and bottom. MARPOL requires this on oil tankers.
  • The cargo sits in several tanks. Longitudinal and transverse bulkheads limit how far the liquid can slosh.
  • An inert gas system holds the tank atmosphere below the oxygen level at which the vapour could catch fire.
  • Cargo pumps and a fixed pipeline meet a manifold amidships, so the ship pumps its own cargo ashore.
  • Tanks for cargo that must stay warm enough to pump carry heating coils.

Why this trade uses it

  • Large volumes of liquid move with no packaging.
  • The ship pumps itself out, so a terminal needs pipes and tanks rather than heavy cranes.
  • Separate tanks let several grades travel on one voyage without mixing.

Settle these points before you fix

  • Washing tanks between cargoes costs time and money. What the ship carried last decides what it may load next.
  • Oil majors vet ships on top of class and flag rules, and that vetting decides who will take the ship.
  • Pollution liability is heavy. Ship and terminal both follow strict transfer rules.
All ship types