An LNG carrier is a special ship used to transport liquefied natural gas (LNG). These ships carry LNG at extremely cold temperatures, about -162°C (-260°F). They use special insulated containers, such as membrane or spherical tanks, to keep the gas in liquid form. This process makes the gas 600 times smaller, allowing it to be safely and efficiently transported by sea from liquefaction plants to import terminals. This is especially important for areas not connected by pipelines, like sending gas from producing regions to Asia and Europe.

Modern LNG carriers often use the natural gas they carry as fuel, which helps improve efficiency. A Gas Combustion Unit (GCU) is frequently used to handle gas that turns into vapor during the journey.

The first ocean-going LNG tanker was called Methane Pioneer. It started operating in 1959 and could carry 5,500 cubic metres (190,000 cubic feet) of LNG. Since then, larger LNG carriers have been built. Today, huge Q-Max LNG ships can carry up to 266,000 cubic metres (9,400,000 cubic feet) of LNG each. As of 2023, there were 772 active LNG carriers worldwide.

History

The first LNG carrier, Methane Pioneer (5,034 DWT), carried 5,500 cubic metres (190,000 cu ft) of LNG. It was built by Bureau Veritas and left the Calcasieu River on the Louisiana Gulf coast on 25 January 1959. The ship delivered the world's first ocean cargo of LNG to the UK. The success of the modified ship, originally named Normarti and later renamed Methane Pioneer, led the Gas Council and Conch International Methane Ltd. to order two new LNG carriers: Methane Princess and Methane Progress. These ships had Conch aluminum cargo tanks and began carrying LNG from Algeria in 1964. Each had a capacity of 27,000 cubic metres (950,000 cu ft).

In the late 1960s, Alaska began exporting LNG to Japan. In 1969, two ships, Polar Alaska and Arctic Tokyo, each with a capacity of 71,500 cubic metres (2,520,000 cu ft), were built in Sweden. In the early 1970s, the U.S. government supported U.S. shipyards in building LNG carriers, resulting in 16 ships. By the late 1970s and early 1980s, plans for Arctic LNG ships were studied.

As LNG cargo capacity increased to about 143,000 cubic metres (5,000,000 cu ft) at a cost of $250 million, new tank designs were developed, including Moss Rosenberg, Technigaz Mark III, and Gaztransport No.96. LNG carriers now have a capacity of 170,000 cubic metres (6,000,000 cu ft) and can cost $200 million.

Since 2005, Qatargas introduced two new classes of LNG carriers: Q-Flex and Q-Max. These ships carry between 210,000 and 266,000 cubic metres (7,400,000 and 9,400,000 cu ft) of LNG and are equipped with re-liquefaction plants.

Today, there is growing interest in small-scale LNG bunker carriers. Some must avoid the life rafts of cruise ships and Ropax vessels. Examples include the Damen LGC 3000 and the Seagas.

By 2005, 203 LNG carriers had been built, with 193 still in service. Increased U.S. natural gas production, enabled by hydraulic fracturing ("fracking"), led to more LNG exports starting in 2016. This growth raised the need for more LNG carriers.

At the end of 2016, the global LNG shipping fleet had 439 vessels. In 2017, about 170 vessels were in use at any one time. By the end of 2018, the fleet had grown to about 550 vessels.

The 2022 Russian invasion of Ukraine increased global demand for LNG shipping. U.S. shipments to Europe more than doubled in 2022, reaching 2.7 trillion cubic feet. In 2021–2022, an LNG shipment from the U.S. to Europe could earn $133–200 million in profit. Shipping rates reached $100,000 per day for 5-year contracts, though they varied between $60,000 and $250,000.

As of 2023, there were 772 active LNG carriers worldwide. However, this number includes floating storage units.

New building

In 2021, 90 new LNG carriers were ordered. By 2022, high demand caused the delivery of these new orders to be moved to 2027.

In November 2018, South Korean ship builders secured contracts for three years of large-scale LNG carrier projects, including more than 50 orders valued at $9 billion. In 2018, South Korean builders received 78% of all LNG-related shipbuilding contracts, with 14% going to Japanese builders and 8% to Chinese builders. These new contracts would increase the global LNG fleet by 10%. Historically, about two-thirds of all LNG ships have been built by South Koreans, 22% by Japanese builders, 7% by Chinese builders, and the rest by builders in France, Spain, and the United States. South Korea’s success is due to innovation and cost-effective pricing. South Korean builders created the first ice-breaking LNG ships and have met growing customer demand for Q-max vessels instead of Moss-type ships.

In 2018, South Korea’s Hyundai Mipo Dockyard (HMD) delivered the world’s first LNG-fueled bulk carrier. This ship has the largest capacity of any of its kind, holding 50,000 deadweight tons.

According to SIGTTO data, in 2019, there were 154 LNG carriers being built and 584 operating LNG carriers.

In 2017, Daewoo Shipbuilding & Marine Engineering delivered the Christophe de Margerie, an ice-breaking LNG tanker with a capacity of 172,600 cubic meters. This amount of gas is enough to supply Sweden for one month. The ship completed its first revenue voyage from Norway through the Northern Sea Route in the Arctic Ocean to South Korea. The shipyard has 14 more of these ships planned.

For small-scale LNG carriers (those with less than 40,000 cubic meters of capacity), the best size of a ship depends on the project it is built for. Factors like the amount of gas, the destination, and the ship’s design are considered.

List of small-scale LNG carrier builders:
• Hanjin Heavy Industries and Construction
• STX Offshore & Shipbuilding
• Damen Shipyards Group

Cargo handling

A typical LNG carrier has four to six tanks placed along the center of the ship. Around these tanks are ballast tanks, cofferdams, and voids, creating a double-hull design that helps protect the ship.

Building LNG carriers is very complex, similar to building aircraft carriers. It can take up to 30 months to complete one.

Inside each tank, there are usually three submerged pumps. Two main pumps are used to move cargo during unloading, while a smaller pump, called the spray pump, has other roles. The spray pump can remove liquid LNG to make fuel, cool the tanks, or help empty the last remaining cargo during unloading. All these pumps are inside a structure called the pump tower, which hangs from the top of the tank and extends to the bottom. The pump tower also holds the tank gauging system and the tank filling line, both near the bottom of the tank.

In membrane-type ships, there is an empty pipe with a spring-loaded foot valve. This is the emergency pump tower. If the main cargo pumps fail, the top of the pipe can be removed, and an emergency pump lowered inside. The top is then reattached, allowing the pump to push down on the foot valve and open it. This lets cargo be pumped out.

All cargo pumps send their output to a shared pipe that runs along the ship’s deck. This pipe branches to the sides of the ship, connecting to cargo manifolds used for loading or unloading.

The vapor spaces in all cargo tanks are connected through a vapor header that runs parallel to the cargo header. This header also links to the sides of the ship near the loading and unloading manifolds.

Typical cargo cycle

A typical cargo cycle begins with the tanks in a "gas free" condition, meaning the tanks are filled with air. This allows workers to perform maintenance on the tanks and pumps. Cargo cannot be loaded directly into the tank because oxygen in the air could create an explosive atmosphere. Also, the extreme cold of LNG at −162 °C (−260 °F) could damage the tanks if loaded directly.

First, the tank must be "inerted" to reduce the risk of explosion. An inert gas plant burns diesel in air to create a gas mixture (usually less than 5% oxygen and about 13% carbon dioxide plus nitrogen). This gas is blown into the tanks until the oxygen level drops below 4%.

Next, the vessel goes to port to "gas-up" and "cool-down." Loading cannot happen directly into the tank because carbon dioxide could freeze and damage the pumps, and the sudden temperature change could harm the tank’s pump column.

LNG is brought onto the vessel and sent through a spray line to a main vaporizer, which turns the liquid into gas. This gas is then heated to about 20 °C (68 °F) in gas heaters and blown into the tanks to replace the inert gas. This continues until all the carbon dioxide is removed. At first, the inert gas is released into the air. Once the gas contains 5% hydrocarbons (the lower flammability range of methane), the inert gas is sent to shore through a pipeline using high-duty compressors. The shore terminal burns this gas to avoid the risk of explosions.

Now, the vessel is gassed up and warm. The tanks remain at ambient temperature and are filled with methane.

Next, the vessel cools down. LNG is sprayed into the tanks through spray heads, which turns the liquid into gas and begins cooling the tank. Extra gas is sent to shore to be re-liquified or burned at a flare stack. Once the tanks reach about −140 °C (−220 °F), they are ready for bulk loading.

Bulk loading begins, with liquid LNG pumped from shore storage tanks into the vessel’s tanks. Gas displaced during this process is sent to shore by high-duty compressors. Loading continues until the tanks are about 98.5% full (to allow for temperature changes in the cargo).

The vessel then travels to the discharge port. During the journey, boil-off gas (gas that forms as LNG warms) can be burned in boilers to power the ship or re-liquified and returned to the cargo tanks, depending on the vessel’s design.

At the discharge port, cargo is pumped ashore using cargo pumps. As the tank empties, the empty space is filled with gas from shore or by vaporizing some cargo in the cargo vaporizer. The vessel can be pumped out completely, with the last small amount of cargo removed using spray pumps, or some cargo may remain on board as a "heel."

It is common to leave 5% to 10% of the cargo in one tank after discharge. This remaining cargo, called the "heel," is used to cool the other tanks before loading new cargo. This must be done slowly to avoid damaging the tanks if they are loaded directly into warm tanks. Cooling down can take about 20 hours on a Moss-type vessel and 10–12 hours on a membrane-type vessel. Keeping a heel allows cooling to happen before reaching port, saving time.

If all cargo is pumped ashore, the tanks warm up to ambient temperature during the return journey, returning the vessel to a gassed-up and warm state. The vessel can then be cooled again for future loading.

If the vessel needs to return to a "gas free" state, the tanks are warmed using gas heaters to circulate warm air. Then, the inert gas plant removes methane from the tanks. Once the tanks are free of methane, the inert gas plant switches to producing dry air, which is used to remove all remaining inert gas until the tanks have a safe working atmosphere.

Transporting natural gas as LNG or through pipelines produces greenhouse gas emissions, but in different ways. For pipelines, most emissions come from making steel pipes. For LNG, most emissions come from the process of liquefying the gas. Both methods produce additional emissions during transportation, such as pressurizing pipelines or moving LNG tankers.

Containment systems

Today, there are four systems used to store liquefied natural gas (LNG) on new ships. Two of these systems are self-supporting, and the other two are membrane types. The patents for the membrane systems are owned by Gaztransport & Technigaz (GTT).

There is a growing preference for membrane systems over self-supporting ones. This is because prismatic membrane tanks fit better inside the ship’s hull, leaving less empty space between the cargo tanks and ballast tanks. As a result, a Moss-type ship, which uses a different design, would cost more to pass through the Suez Canal compared to a membrane ship of the same size. However, self-supporting tanks are stronger and better at handling the movement of liquid inside the tank. These may be used in the future for offshore storage, where rough weather is common.

The spherical IMO type B LNG tanks, named after the company Moss Maritime, are round in shape. Most ships with this design have four or five tanks.

The outside of the tanks has a thick layer of foam insulation. This insulation is either attached in panels or wrapped around the tank in newer designs. A thin layer of "tinfoil" covers the insulation, keeping it dry by using a nitrogen atmosphere inside the tank. This nitrogen is regularly tested for methane, which would signal a leak. The outside of the tank is also inspected every three months for cold spots, which might indicate damaged insulation.

The tank is supported around its edges by a ring called the equatorial ring. This ring is held in place by a circular skirt known as a data-couple, made from a mix of aluminum and steel. This skirt allows the tank to expand and contract when the temperature changes. During cooling or heating, the tank can move up to 60 cm (24 inches). To allow this movement, all pipes connected to the tank are attached at the top and use flexible bellows to link to the ship’s systems.

Inside each tank, there are spray heads. These are placed around the equatorial ring and spray LNG onto the tank walls to lower the temperature.

LNG tanks typically operate at a pressure of up to 22 kPa (3.2 psi). If the main pumps fail, the pressure can be increased to 100 kPa (1 bar) to allow emergency removal of cargo. In this case, the filling line at the bottom of the tank is opened, along with lines from other tanks on the ship. The pressure from the tank with the failed pumps pushes the LNG into the other tanks, where it can be removed.

The self-supporting prismatic type B (SPB) tanks, designed by Ishikawajima-Harima Heavy Industries, are used in only two ships. These tanks reduce sloshing, which is a problem that can damage membrane tanks and the ship’s hull. This makes SPB tanks important for FPSO LNG (or FLNG) vessels.

IMO type B tanks can also handle internal damage, such as from equipment failures. This feature was added to the design after several accidents in membrane tanks.

The membrane tanks designed by Technigaz are made of stainless steel with a "waffle" pattern to absorb thermal changes when the tank cools. The primary barrier, which is in direct contact with the LNG, is made of corrugated stainless steel about 1.2 mm (0.047 in) thick. This is followed by primary insulation, then a secondary barrier made of a material called "triplex," which is a metal foil between glass wool sheets. This is covered by secondary insulation and supported by the ship’s hull.

From the inside of the tank outward, the layers are:
– LNG
– Primary barrier (1.2 mm thick corrugated/waffled 304L stainless steel)
– Primary insulation (interbarrier space)
– Secondary barrier (triplex membrane)
– Secondary insulation (insulation space)
– Ship’s hull structure.

The tanks designed by Gaztransport use two thin membranes made of Invar, a material with little thermal expansion. The insulation consists of wooden boxes filled with perlite and flushed with nitrogen gas. The condition of both membranes is checked by detecting hydrocarbons in the nitrogen. A newer version, proposed by NG2, replaces nitrogen with argon, which provides better insulation and could reduce boil-off gas by 10%.

CS1 stands for Combined System Number One. It was created by Gaztransport & Technigaz by combining the best parts of the MkIII and No96 systems. The primary barrier is made of Invar 0.7 mm (0.028 in) thick, and the secondary barrier is triplex. Both primary and secondary insulation use polyurethane foam panels.

Three ships with CS1 technology were built by one shipyard. However, established shipyards have chosen to continue producing MkIII and No96 systems instead.

Reliquefaction and boil-off

Natural gas is cooled to about −163 °C (−261 °F) at atmospheric pressure to turn it into a liquid. Special tanks on an LNG carrier act like large thermoses to keep the liquid cold during storage. However, no insulation is perfect, so the liquid continuously boils during the journey.

According to WGI, on a typical voyage, about 0.1–0.25% of the cargo turns into gas each day, depending on the insulation quality and voyage conditions. Over a 20-day voyage, this may result in a loss of 2–6% of the original LNG volume.

LNG tankers are usually powered by steam turbines with boilers that can use methane, ammonia, or bunker oil. Gas from boiling LNG is traditionally directed to the boilers as fuel. Before use, the gas must be warmed to about 20 °C using gas heaters. The gas is either sent to the boiler through tank pressure or compressed to higher pressure using Low Duty compressors.

The type of fuel used depends on factors such as voyage length, the need to carry a reserve for cooling, the cost of oil versus LNG, and port requirements for cleaner exhaust.

There are three main operating modes:

Low Boil-Off, High Oil Use: In this mode, tank pressures are kept high to minimize boiling and reduce gas loss. Most energy comes from fuel oil. This maximizes the amount of LNG delivered but may raise tank temperatures, causing storage and unloading challenges.

High Boil-Off, Low Oil Use: Here, tank pressures are kept low, leading to more boiling but still using a large amount of fuel oil. This reduces the amount of LNG delivered but keeps the cargo cold, which some ports prefer.

100% Gas Use: Tank pressures are similar to the high boil-off mode. However, if this pressure is insufficient for the boilers, additional LNG is forced to vaporize. A small pump sends LNG to a vaporizer, where it is warmed and turned back into gas for the boilers. No fuel oil is used in this mode.

Recent technological improvements allow reliquefaction plants to be added to ships, enabling boil-off gas to be turned back into liquid and returned to the tanks. This has made it possible to use more efficient slow-speed diesel engines instead of traditional steam turbines. Exceptions include the LNG carrier Havfru (originally named Venator, built in 1973) and its sister ship Century (originally Lucian, built in 1974), which used dual fuel diesel engines or gas turbines before switching to diesel systems.

Ships with Dual or Tri-Fuel Diesel Electric (DFDE/TFDE) propulsion systems are now in use.

Recent interest in using boil-off gas as a fuel source has grown due to the IMO 2020 regulation, which bans the use of marine fuel oil with more than 0.5% sulfur unless ships have flue-gas scrubbing systems. Space and safety concerns often prevent installing such systems on LNG carriers, forcing them to stop using cheaper, high-sulfur fuel oil and switch to more expensive, less available low-sulfur fuels. In these situations, boil-off gas may become a more practical option.

Spillage risk

Compared to oil, there is less worry about accidental spills from ships carrying Liquid Natural Gas (LNG) because the gas quickly turns into vapor and rises into the air as methane.

From the start of LNG shipping until 2004, no major accidental spills happened during nearly 80,000 times that LNG ships traveled through ports while fully loaded.

A study of several spherical LNG ships showed that these ships can handle a side collision with another similar LNG ship traveling at 6.6 knots (12.2 km/h; 7.6 mph), which is about half the usual speed in ports, without losing any LNG. If a fully loaded 300,000 dwt oil tanker collided with an LNG ship at the same speed, the LNG ship might not stay intact. The report also says such collisions are very uncommon.

HAZID did a risk assessment for an LNG spill. Considering safety steps, training, rules, and technology improvements over time, HAZID estimates the chance of an LNG spill to be about 1 in 100,000 trips.

If the tank on an LNG transport ship is damaged, the natural gas inside could catch fire, leading to an explosion or fire. This happened in 2026 when a ship carrying 62,000 tons of LNG fuel partially caught fire in the Mediterranean Sea. Two of the ship’s tanks likely held thousands of tons of LNG. The ship was abandoned and left floating freely.