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Boil-off gas (BOG) is the vapour created when a small amount of liquefied natural gas (LNG) naturally evaporates inside a ship’s cargo tanks. It happens because LNG is stored at extremely low temperatures, and a small amount of heat from the surrounding environment always finds its way into the tanks.

Boil-off gas is a normal and an expected part of transporting LNG by sea. Rather than being a fault, it is something every LNG carrier is designed to handle, either by using the gas as fuel, turning it back into liquid or safely burning off any excess.

In this guide, we explain what boil-off gas is, why it forms, whether it can be used as fuel and how boil-off gas recovery systems help reduce cargo loss.

 

Why does boil-off gas form in LNG?

 

LNG is natural gas cooled to around −162°C*, turning it into a liquid that takes up far less space and can be shipped around the world.

LNG carriers use heavily insulated cargo tanks to keep the cargo cold. However, no insulation can keep heat out completely. As a small amount of warmth reaches the LNG, some of the liquid turns back into gas. This gas is known as boil-off gas.

 

How boil-off gas forms

 

  1. Heat from the air, sea and sun slowly reaches the cargo tanks.
  2. The LNG absorbs this heat.
  3. A small amount of the liquid turns into vapour.
  4. The vapour builds up, increasing pressure inside the tank.
  5. The ship manages this vapour by using it as fuel, turning it back into liquid or burning it safely.

 

Key boil-off gas terms

 

  • Boil-off gas (BOG) – The vapour created when LNG naturally evaporates inside cargo tanks.
  • Boil-off rate (BOR) – How quickly LNG turns into boil-off gas, usually shown as a percentage of the cargo per day.
  • Heat ingress – The small amount of heat that passes through a tank’s insulation and reaches the LNG.
  • Re-liquefaction – Cooling boil-off gas so it turns back into liquid LNG and can be returned to the cargo tanks.
  • Gas combustion unit (GCU) – Equipment that safely burns excess boil-off gas when it cannot be used as fuel or turned back into liquid.

 

What affects how much boil-off gas is produced?

 

The amount of boil-off gas created can change from voyage to voyage. Some of the main factors include:

  • Weather and temperature – warmer air and sea temperatures mean more heat reaches the cargo.
  • Voyage length – the longer LNG stays at sea, the more boil-off gas it produces overall.
  • Tank design and insulation – better insulated tanks let in less heat.
  • How the vessel is operated – including how full the tanks are and how the ship is run during the voyage.

 

How is boil-off gas managed on LNG carriers?

 

Because boil-off gas builds up pressure inside the cargo tanks, it must be managed throughout the voyage. LNG carriers generally use one or more of three main methods, often in combination, depending on the ship's design.

 

Option 1

Used as fuel

Option 2

Re-liquefaction

Option 3

Gas combustion unit (GCU)
What happens The gas is fed to the ship’s engines, generators or boilers to help power the vessel The gas is cooled back into liquid and returned to the cargo tanks Excess gas is burned safely in a controlled way
Main benefit Puts gas that would otherwise be lost to good use and reduces the need for other fuels Keeps more cargo in the tanks for delivery Keeps tank pressure safe when the gas cannot be used or turned back into liquid
Typically used On ships with engines designed to run on gas When the ship produces more gas than its engines need As a back-up safety option

Can boil-off gas be used as fuel?

 

Yes. Boil-off gas can be used as fuel, and on many modern LNG carriers this is the main way it is managed. The gas is supplied to engines that can run on both gas and conventional marine fuel (dual-fuel engines), as well as to onboard generators or boilers.

Using boil-off gas as fuel puts gas that would otherwise be lost to good use and reduces the amount of conventional fuel a ship needs. However, operators need to strike a balance:

  • Using too much leaves less LNG to deliver at the end of the voyage.
  • Using too little lets pressure build up in the tanks, so operators need another way to manage the extra gas.

 

What is a boil-off gas recovery system?

 

On LNG carriers, one method of recovering BOG is through a re-liquefaction unit, which cools the boil-off gas until it turns back into liquid and then returns it to the cargo tanks.

This is particularly useful on long voyages, or when the ship’s engines need less fuel than the amount of boil-off gas being produced. By recovering the gas, more LNG reaches its destination.

 

What happens to excess boil-off gas?

 

If boil-off gas cannot be used as fuel or turned back into liquid, it still needs to be dealt with to keep the tanks at a safe pressure. In these cases, modern LNG carriers use a gas combustion unit (GCU) to burn the excess gas safely and in a controlled way.

Burning the gas in a GCU is preferred to releasing it straight into the atmosphere, as methane, the main component of natural gas, is a powerful greenhouse gas.

 

Why does boil-off gas management matter?

 

Effective boil-off gas management is about more than keeping tanks at a safe pressure. It also affects:

  • Cargo – how much LNG is delivered at the end of a voyage.
  • Costs – how much fuel the ship needs.
  • Emissions – how much greenhouse gas is released.

 

For shipowners, charterers, and cargo owners, getting this balance right helps protect cargo value and improve each voyage's efficiency. As the industry focuses more on cutting emissions, efficient boil-off gas management is also an important part of shipping’s green transition.

 

Key takeaways

 

  • Boil-off gas is the vapour created when LNG naturally evaporates inside cargo tanks.
  • It forms because some heat always reaches the LNG, even through well-insulated tanks.
  • Boil-off gas can be used as fuel to help power the ship.
  • Boil-off gas recovery systems, such as re-liquefaction units, turn the gas back into liquid, reducing cargo loss.
  • Any excess gas can be burned safely in a gas combustion unit.

Frequently asked questions about boil-off gas

Boil-off gas is the gas that forms when a liquefied gas, such as LNG, naturally evaporates during storage or transport. It happens because a small amount of heat always reaches the very cold liquid.

Yes. Some boil-off gas is expected whenever LNG is stored or transported, and LNG carriers are designed to manage it safely.

As more LNG turns into vapour, the pressure inside the tank rises. Cargo tanks are designed to operate within set pressure limits, so crews must manage the gas throughout the voyage to keep the tanks safe.

Yes. Many LNG carriers use boil-off gas to power their engines, generators, or boilers, reducing the need for other marine fuels.

A boil-off gas recovery system captures boil-off gas so it isn’t wasted. On LNG carriers, this is usually a re-liquefaction unit that cools the gas back into liquid and returns it to the cargo tanks.

Sources:

*iea