What Are LNG Carrier Ships?

An LNG Carrier, also known as a gas tanker or LNG tanker, is a specially designed vessel built to transport liquefied natural gas (LNG) via waterways.

There are around 700 active LNG Carriers in the world, including floating storage and regasification units (FSRUs) and floating storage units (FSUs), simply called floating LNG vessels.

Also sometimes referred to as a methane tanker, these LNG vessels are designed to handle the nature of LNG, as it cannot be shipped in regular cargo vessels.

This is because LNG is kept at either pressures far greater than atmospheric pressures or very low temperatures of around -162 degrees Celsius, or a combination of both to keep it in a liquid state.

LNG Carriers have highly insulated tanks, which keep the LNG in a liquid state.

Features of LNG Carriers

LNG Ships are around 300 m long and 43 m wide with a draft of around 12 m. They stand out from other bulk cargo carriers due to their unique features.

Special Tanks: The ships have 4-5 large insulated tanks made of aluminium or 9% nickel steel with several layers to maintain the required temperature and prevent leaks. The tanks function as a thermos flask to keep the LNG cold and prevent it from boiling off.

Pump systems: Every tank has internal pumps, including cargo pumps for discharge and spray pumps for cooling tanks, fuel use or stripping out the last of the cargo.

Double Hull: This is a safety feature where a second hull surrounds the cargo tanks, and the space between the inner and outer hulls provides protection against collisions and leaks.

Propulsion: They often utilise turbine-powered propulsion systems which run on ‘boil-off’ gas from the LNG itself, thus decreasing emissions in comparison to conventional fuels. These LNG Carriers also have dual-fuel engines, offering flexibility and decreased emissions.

Boil-off gas system: A gas combustion unit is a major component of these ships. It burns excess boil-off gas that is not used for propulsion and helps to prevent any pressure buildup in the tanks.

Emergency Systems: LNG vessels have emergency shutdown devices for cargo handling operations to prevent accidents.

Design of LNG Ships

The design of gas tankers is mainly influenced by the type of containment system they have. Two main designs include the Moss Type and Membrane Type.

The first uses independent spherical tanks made of aluminium alloy, supported by steel skirts. Tanks are separate from the hull with the void space filled with nitrogen for safety purposes and to provide insulation.

This design is highly resistant to pressure and external forces, is robust and handles partial loads well.

The membrane type comprises rectangular tanks integrated into the hull, which is more space-efficient and requires a multi-layered build.

The tanks have a primary and secondary barrier with an insulation layer of plywood boxes filled with perlite between them.

SPB Type is a less common design where independent tanks are integrated within the hull and have multiple spaces to tackle sloshing.

Cylindrical type C is a less prevalent design which uses cylindrical tanks.

Types of LNG Ships

LNG Vessels can be categorised into different types based on various factors.

1. On the basis of pressurisation

LNG ships can be classified as:

• Fully pressurised
• Semi-pressurised and refrigerated
• Fully refrigerated

Fully Pressurised Ships: Gas is shipped at surrounding temperatures by keeping it under high pressure, up to 17.5 to 19 bar. These are the simplest gas carriers with Type C independent tanks made of carbon steel with no insulation or reliquifaction plant as the gas remains liquid due to high pressure. It is used to ship LPG and Ammonia on short-sea and coastal routes. These vessels are small and have a capacity of 6,000 m³.

Semi-Pressurised and Refrigerated Ships: These vessels transport gas at a reduced temperature and intermediate pressure of 5 to 7 bars. This allows for lighter tanks and more volume. The tanks are insulated, and the ship is equipped with a refrigeration plant to maintain the temperature, down to -10 degrees Celsius for standard LPG or -104 degrees Celsius for Ethylene.

Type C independent tanks with thinner walls are used, made of low-temperature steel or special alloys. Vessels of this type have a capacity of 3000 to 20,000 m3 and can load or discharge at both pressurised and refrigerated terminals.

Fully-refrigerated ships: These vessels transport gas at its atmospheric boiling point by maintaining it at very low temperatures and near atmospheric pressure, which is 0.25 bar.

These vessels enable the transportation of large volumes over long distances, as lower-pressure tanks allow for much larger capacities. Tanks need extensive thermal insulation and highly reliable refrigeration/cargo containment systems.

Fully refrigerated LPG/Ammonia Ships use Type A tanks, which can withstand temperatures down to -48 degrees Celsius.

LNG Carriers use membrane tanks or type B spherical/prismatic tanks to handle the ultra-low temperature of -163 degrees Celsius.

Ships carrying LPG/Ammonia have a capacity of 100,000 m3, and LNG up to 266,000 m3.

2. Based on the hazard level of the material being transported

LNG carriers can be categorised as:

Type 1G – designed to carry the most hazardous cargo. For example, highly toxic gases such as Chlorine, Ethylene Oxide, and Methyl Bromide
Type 2G and 2PG – designed to carry cargo with a lesser degree of hazard, like Flammable gases, including LNG, LPG and Ammonia.
Type 3G – designed to carry the least hazardous cargo, including less hazardous gases like non-toxic, non-flammable gases like Nitrogen or Carbon Dioxide.

3. Based on the type of cargo tanks

Independent tanks: The tank is not a part of the ship’s hull and so is not crucial to hull strength.

Type A: They operate at pressure less than 700 mbar and have a full secondary barrier to hold leaks for at least 15 days.
Type B: spherical Moss-tanks or prismatic IHI SPB tank with pressure less than 700 mbar and a partial secondary barrier.
Type C: can operate at pressures above 2,000 mbar and does not have a secondary barrier. Sensors in the hold space can detect leaks by monitoring changes in gas composition.

Integral Tanks: The tanks form a structural part of the ship’s hull.

Membrane tanks are a type of integral tank and can be sub-categorised into:

TGZ Mark III (or GTT Mark III) – They are designed by Technigaz and have many layers, including a primary stainless steel barrier, primary insulation, a triplex membrane secondary barrier, secondary insulation, and the hull structure.

GT 96 (or GTT 96) – This tank has an invariable nickel-iron alloy for its primary and secondary membranes.

4. Based on Size

LNG vessels come in different sizes, from small-scale to large-scale carriers. The former have a cargo capacity of 1000 m3 to 40,000 m3. Medium-scale carriers have a cargo capacity of 40,000 m3 to 80,000 m3, while large-scale carriers, also known as Q-max or Q-flex ships, have a cargo capacity of 120,000 m3 to more than 260,000 m3.

5 Biggest LNG Carrier ships

The biggest LNG Carriers are Q-Max vessels that have a cargo capacity of up to 266,000 m3. These ships, owned by Qatar Gas/Nakilat, are the largest in active service and can carry more LNG than conventional carriers.

The first of these vessels was the Mozah, which was delivered in 2008 and represents a major step in the scale of LNG transportation.

There are 14 Q-Max ships in operation, built by Samsung Heavy Industries and Daewoo Shipbuilding & Marine Engineering.

1. Mozah

This LNG Carrier is 345 m long and 53 m wide with a 12 m draft, a 128,900 DWT and a gross tonnage of 163,922 tonnes. Such tonnage and size allow the cargo ship to carry 266,000 cubic metres (9,400,000 cu ft) of liquefied natural gas with a temperature of −163 °C.

2. Mekaines

This Q-Max carrier has a cargo carrying capacity of 266,476 cubic meters. It is 355 m long and 53 m wide with a 12 m draft. It has a deadweight of 143,309 tonnes and a gross tonnage of 163,922 tonnes.

3. Al Mafyar

This LNG Tanker has a carrying capacity of 266,000 m³, a deadweight of 130,441 tonnes and an LOA of 345 m, a width of 53.8 m to 54 m, a draft of 12.2 m and a gross tonnage of 163,922 tonnes.

4. Bu Samra

This LNG carrier also has a cargo capacity of 266,000 cubic meters. It has an LOA of approximately 345 meters, a width of 53.8 meters, a deadweight tonnage of about 143,581 tonnes and a gross tonnage of 163,922 tons.

5. Al Mayeda

This LNG vessel has a total carrying capacity of 266,000 cubic meters (cbm) of liquefied natural gas. It has a deadweight of 130,298 tonnes, a Gross Tonnage of 163,922 tonnes and is 345 m long and 52 m wide.

Conclusion

LNG Carriers, especially the Q-Max class, are an amalgamation of modern naval designs, cutting-edge propulsion technologies, and specialised material science. As the world transitions towards cleaner energy sources, the demand for natural gas and consequently these specialised gas carriers will only continue to increase, leading to the emergence of newer and more efficient designs to enhance safety, energy security and efficiency.

You might also like to read-

• Construction and Operation of Loading Arms Used for LNG Carriers
• Provalys – One of the Largest LNG Carriers in the World
• 5 Major LNG Terminals in the United States
• Biggest LNG Carrier Operating Today

About Author

Zahra is an alumna of Miranda House, University of Delhi. She is an avid writer, possessing immaculate research and editing skills. Author of several academic papers, she has also worked as a freelance writer, producing many technical, creative and marketing pieces. A true aesthete at heart, she loves books a little more than anything else.

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