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.
Understanding Ships and Submarines: LNG Carriers
At the molecular level, cooling methane removes energy from its particles. Their motion slows until the attractive forces between particles can keep them close together as a liquid. Heat entering a tank has an important effect.
Much of that heat can go into changing some liquid into vapor instead of quickly making the remaining liquid warmer. This is called a phase change. The vapor takes up far more space than the liquid it came from.
If it is trapped, tank pressure rises. Engineers must therefore control both heat flow and the route taken by the vapor.
The cargo containment system is designed for severe cold and constant movement. Materials shrink when cooled, so pipes, supports, seals, and tank walls must allow controlled thermal contraction. A material that is strong at room temperature may become brittle in cryogenic conditions.
The outer hull provides strength against waves and impacts, while the inner cargo system focuses on insulation and containment. Spaces around the tanks are monitored for gas. Inside a partly filled tank, the liquid can surge as the ship rolls.
This sloshing places force on tank surfaces, especially in rough seas. Cargo levels and sailing conditions affect how this risk is managed.
Loading begins with a careful cool-down process. Sending extremely cold liquid into warm equipment too quickly can create large temperature differences and damage metal parts. At the terminal, transfer arms connect shore pipes to the ship.
Operators watch liquid level, pressure, temperature, and gas concentration throughout the transfer. Boil-off gas may be sent to the ship's engines, returned to shore, reliquefied, or burned safely in controlled equipment when necessary.
Emergency shutdown systems can rapidly stop the flow if a leak, fire signal, or unsafe pressure is detected. At the receiving terminal, LNG is warmed in equipment called vaporizers until it becomes pipeline gas again.
Students can connect this topic to particle models, thermal energy, pressure, density, and forces. The density idea explains why a given mass needs much less storage volume after liquefaction. The heat relation shows that a large cargo can absorb a lot of thermal energy because its mass is large, even when its temperature changes only a little.
Pay close attention to units when solving problems. Celsius temperature changes have the same size as kelvin temperature changes, but absolute temperature matters in gas laws. It is useful to separate liquid behavior from gas behavior.
A tank may hold mostly liquid while its pressure is controlled by the vapor space above it. This distinction is central to safe cryogenic transport.
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.