Gas carriers (LNG and LPG): the English of the ship
Gas carriers carry liquefied gases: LPG such as propane and butane, fully refrigerated or under pressure, and LNG at about minus 162 degrees C. On a gas ship everything is about temperature, pressure and boil-off.
A gas carrier keeps its cargo liquid by cooling it, by pressurising it, or both. The tanks are insulated, the pipes are cold enough to shatter ordinary steel, and the vapour that boils off has to go somewhere: back into the tank through the reliquefaction plant, or into the engine as fuel. The crew talks about tank pressure, cargo temperature, boil-off rate and nitrogen all day.
Gas ships are highly automated and highly procedural. Emergency shutdown systems, gas detection in every space around the tanks, water curtains over the hull during cargo transfer: the safety systems are part of the routine, and the English of a gas carrier is precise because a wrong figure can mean a cold leak.
Tests for gas carrier positions expect the IGC Code words: containment system, primary barrier, cofferdam, inerting, purging, custody transfer, MARVS. Learn them as a set.
Words you will use every day (26)
- Liquefied gas
- A gas turned into liquid by cooling or pressure so that it takes a fraction of the space.
- LNG
- Liquefied natural gas, mostly methane, carried at about minus 162 degrees C near atmospheric pressure.
- LPG
- Liquefied petroleum gas, propane and butane, carried fully refrigerated (propane about minus 42 degrees C) or under pressure at ambient temperature.
- Boil-off gas (BOG)
- The vapour produced as heat leaks into the tank and some cargo evaporates.
- Reliquefaction plant
- The refrigeration plant that turns boil-off gas back into liquid and returns it to the tank.
- Containment system
- The complete arrangement of tank, insulation and barriers that holds the cargo: membrane, Moss spherical, or independent type C tanks.
- Primary and secondary barrier
- The layer in contact with the cargo and the second layer that contains a leak for a limited time.
- Interbarrier space
- The space between the two barriers, kept inert with nitrogen and monitored for gas.
- Cofferdam
- The empty space between two cargo tanks or between a tank and the hull; heated on LNG ships so the hull steel stays warm.
- Cooldown
- Lowering the temperature of a warm tank slowly, by spraying liquid cargo, before loading.
- Heel
- A small quantity of LNG kept in the tanks on the ballast passage to keep them cold.
- Inerting
- Replacing air in a tank or line with inert gas (nitrogen) so that no flammable mixture can form.
- Purging
- Displacing one gas with another, for example cargo vapour with nitrogen before opening a line.
- Gassing-up
- Replacing the inert gas in a tank with cargo vapour before the first loading.
- Cargo compressor
- The machine that moves cargo vapour, for pressure control and for the reliquefaction plant.
- Deepwell pump
- A submerged cargo pump inside the tank, with the motor on deck; on LNG ships the pump and motor are both submerged.
- Vapour header
- The main pipe that connects the vapour spaces of all cargo tanks.
- Emergency shutdown (ESD)
- The system that closes the cargo valves and stops the pumps within seconds when triggered, on ship and shore together.
- Ship-shore link
- The electrical or fibre connection that lets the ship and the terminal trigger each other's ESD.
- Water curtain
- Water sprayed down the ship's side under the manifold during cargo transfer, so that a cold leak does not crack the hull.
- Cryogenic
- Extremely cold: LNG temperatures. Contact causes burns; ordinary steel becomes brittle.
- Brittle fracture
- Steel breaking suddenly without bending because it has been made cold by cargo contact.
- MARVS
- Maximum allowable relief valve setting: the highest pressure the tank relief valves may be set to.
- Custody transfer
- The official measurement of the cargo quantity at loading and discharge, from tank gauges and temperature.
- Gas detection system
- Fixed sensors in the hold spaces, cofferdams, compressor room and accommodation intake that alarm at a set percentage of the lower flammable limit.
- IGC Code
- The IMO code for the construction and equipment of ships carrying liquefied gases in bulk.
Ten things you will hear
Start cooldown of the cargo tanks with the spray pumps.
Cargo engineer before loading a warm tank.Tank pressure in number two is rising, start the reliquefaction plant.
Cargo watch to the engine control room.ESD test completed, all valves closed within thirty seconds.
Chief Officer reporting to the terminal before transfer.Cargo temperature minus one six one degrees, pressure one zero five kilopascals.
Custody transfer readings, said figure by figure.Purge the manifold lines with nitrogen before disconnecting.
Chief Officer to the deck crew after discharge.Water curtain on before the arms are connected.
Standard step in the pre-transfer checklist.Boil-off gas is going to the engine as fuel; the boilers are on standby.
Engine watch handover on a dual-fuel LNG carrier.Keep two thousand cubic metres of heel for the ballast voyage.
Master to the Chief Officer at the end of discharge.Gas detected in the interbarrier space, alarm on, start the investigation procedure.
Cargo control room to the bridge and engine room.Nitrogen in the cofferdam is at the set pressure, temperature normal.
Routine round report.
Numbers are said digit by digit on board: "one decimal five", "channel one six", "zero seven three zero". Train your ear in listening practice.
Safety on gas carriers (lng and lpg)
- Cryogenic burns and brittle fracture: liquid cargo must never touch bare skin or the hull. Water curtain on during transfer, drip trays under the manifold, cryogenic gloves and face shield when working on cargo lines.
- Gas detection everywhere: hold spaces, interbarrier spaces, compressor rooms, air intakes. An alarm is treated as real until proved otherwise.
- Nitrogen kills silently. A space purged with nitrogen has no oxygen; enclosed space entry rules apply with gas measurement.
- The ESD system is tested before every cargo transfer, together with the ship-shore link.
- Tank pressure is controlled at all times; relief valves lift at MARVS, and rising pressure on passage is handled by the reliquefaction plant or by burning boil-off.
- No source of ignition in the cargo area; on LPG ships the vapour is heavier than air and collects low down.
- Emergency response: cold leak, fire on deck, gas in the accommodation; the emergency plan says who goes where, and drills practise it.
What recruitment tests ask
- Carriage temperatures of LNG and refrigerated LPG, and why the cargo is kept cold or under pressure.
- What boil-off is and where it goes.
- The purpose of nitrogen, inerting, purging and gassing-up.
- ESD, ship-shore link and the water curtain.
- Containment systems and the IGC Code vocabulary: barriers, cofferdam, interbarrier space, MARVS.
Sit the gas carrier knowledge test: 30 questions for your rank
The full test mixes up to eight questions on gas carriers (lng and lpg) with the STCW areas for your rank group, under a 30-minute clock. The names of commercial tests are used on this site only to describe what our training prepares you for.
Quiz: 10 questions on gas carriers (lng and lpg)
One question at a time, feedback after every answer, score at the end. Keys 1 to 4 answer, Enter goes on.
Sources: IGC Code (under SOLAS VII), STCW V/1-2 (liquefied gas tanker training), SOLAS II-2; SIGTTO publications are the industry guides.