Liquefied natural gas, or LNG, is natural gas that has been cooled until it becomes a liquid at about -162 °C. In liquid form, natural gas takes up far less space, so it can be shipped across oceans to places without pipelines. LNG carriers are specialized ships built to keep this extremely cold cargo safely contained during long voyages.
They are important in global energy transport because they connect gas-producing regions with distant ports and power systems.
An LNG carrier uses heavily insulated cryogenic tanks inside a strong outer hull to slow heat transfer from the ocean and air. Even with insulation, a small amount of LNG warms and evaporates into boil-off gas, which can be managed or used as fuel by the ship. The tanks may be spherical or membrane-style, but both designs must handle very low temperatures without cracking.
Safe LNG transport depends on thermal insulation, pressure control, gas detection, and careful loading and unloading at specialized terminals.
Key Facts
- LNG is mostly methane cooled to about -162 °C at near atmospheric pressure.
- Liquefying natural gas reduces its volume to about 1/600 of its gas volume.
- Heat transfer into the tanks causes boil-off gas, even with strong insulation.
- Density formula: ρ = m/V, where ρ is density, m is mass, and V is volume.
- Heat transfer relation: Q = mcΔT, where Q is thermal energy, m is mass, c is specific heat, and ΔT is temperature change.
- Pressure and temperature are monitored because warming LNG can increase vapor production in the tank.
Vocabulary
- LNG
- Liquefied natural gas is natural gas cooled to a very low temperature so it becomes a liquid for easier storage and transport.
- Cryogenic tank
- A cryogenic tank is a container designed to store materials at extremely low temperatures.
- Boil-off gas
- Boil-off gas is vapor that forms when a small amount of LNG absorbs heat and evaporates during storage or transport.
- Insulation
- Insulation is material that slows heat transfer between the cold LNG tank and the warmer surroundings.
- Double hull
- A double hull is a ship structure with an outer and inner barrier that improves protection against leaks and collision damage.
Common Mistakes to Avoid
- Thinking LNG is stored under extremely high pressure, which is wrong because LNG carriers usually keep it cold at near atmospheric pressure rather than mainly compressing it.
- Ignoring boil-off gas, which is wrong because some evaporation happens even in well-insulated tanks and must be controlled safely.
- Confusing LNG with LPG, which is wrong because LNG is mostly methane while LPG is mainly propane and butane stored under different conditions.
- Assuming insulation stops all heat flow, which is wrong because insulation only reduces heat transfer and cannot make it zero.
Practice Questions
- 1 Natural gas takes up 600 times less volume when liquefied. If a carrier holds 170,000 m³ of LNG, what volume would the same natural gas occupy as a gas?
- 2 An LNG tank contains 2.0 x 10^7 kg of LNG. If 0.10% of the cargo becomes boil-off gas in one day, how many kilograms evaporate in that day?
- 3 Explain why an LNG carrier needs both strong insulation and systems for handling boil-off gas, even though the cargo tanks are designed to stay very cold.