Boil-off gas (BOG) is the vapour that forms when liquefied natural gas (LNG) absorbs heat and evaporates inside a ship’s cargo tanks. This guide explains what causes boil-off gas, how the boil-off rate is measured and how LNG carriers manage, burn or reliquefy it. It is written for gas carrier officers, cadets and surveyors.
What Is Boil-Off Gas (BOG)?
Boil-off gas is natural gas vapour produced when LNG, carried at about -162°C near atmospheric pressure, warms slightly and evaporates. No insulation stops all heat ingress, so every LNG cargo tank generates BOG throughout the voyage.
BOG stands for boil-off gas. It is mostly methane, but the first vapour to leave the liquid is richer in nitrogen, because nitrogen boils at a lower temperature. As the voyage goes on, the remaining cargo “ages” and becomes slightly heavier and richer in energy.
Ship’s staff distinguish two kinds of boil-off:
- Natural boil-off (NBOG): vapour produced by heat leaking through the insulation.
- Forced boil-off (FBOG): LNG deliberately vaporised in a forcing vaporiser when the engines need more gas than natural boil-off supplies.
LNG is not the only liquefied gas that boils off. LPG and ammonia cargoes also produce vapour, but they are carried warmer and handled differently, as explained further down.
What Causes Boil-Off Gas on an LNG Carrier?
Boil-off gas is caused by heat reaching the cargo. Most comes through the tank insulation. Sloshing, cargo pump work, pressure changes and loading into warm tanks add more.
- Heat ingress: the main source. Warm sea water, air and sunlight heat the hull, and some heat passes through the insulation.
- Sloshing: cargo moving in partly filled tanks in a seaway adds energy and wets warmer tank surfaces.
- Cargo pump work: submerged pumps turn electrical energy into heat inside the liquid.
- Pressure reduction: lowering tank pressure lets part of the liquid flash into vapour.
- Loading and cool-down: warm lines and tanks vaporise LNG until they reach cargo temperature, and incoming liquid displaces vapour.
Weather matters too. Boil-off is higher on a laden voyage in warm, rough water than on a calm, cool passage.
What Is the Boil-Off Rate (BOR)?
The boil-off rate (BOR) is the share of cargo that evaporates each day, stated as a percentage of tank volume. Modern membrane LNG carriers have design boil-off rates of about 0.07–0.10% per day.
Containment designers quote a guaranteed BOR for a standard ship, usually a 174,000 m³ carrier. Bigger tanks boil off a smaller share of their cargo, because volume grows faster than surface area. Actual daily boil-off on a voyage depends on sea and air temperature, sea state and how full the tanks are.
| Containment system | Design BOR (% of volume per day) | Source |
|---|---|---|
| GTT NO96 L03+ | 0.10 | GTT |
| GTT NO96 Super+ | 0.085 | GTT |
| GTT Mark III Flex (2017 version) | 0.07 | GTT |
| Three-tank Mark III Flex, 177,000 m³ | 0.08 (guaranteed) | GTT, BW LNG order |
A simple example shows the scale. A 174,000 m³ carrier with a BOR of 0.085% boils off about 148 m³ of LNG a day. Over a 20-day laden voyage, that is roughly 3,000 m³.
A 2023 study of laden voyages reported actual rates of 0.10–0.15% per day and 0.06–0.10% per day in ballast. Design figures are therefore a floor, not a guarantee of what the crew will see in bad weather.
Why Do Three-Tank LNG Carriers Have a Lower BOR?
Three-tank LNG carriers have a lower BOR because three longer tanks hold the same cargo with less insulated surface than four. Less liner area per cubic metre means less heat gets in.
- GTT’s three-tank concept removes one cofferdam, one pump tower and the cryogenic equipment that goes with them.
- DNV and BV gave approvals in principle in October 2022, and Lloyd’s Register followed for a Samsung Heavy Industries design in May 2023.
- Hudong-Zhonghua’s 174,000 m³ three-tank design with NO96 Super+ cut the insulation area by over 7%, according to the yard.
- BW LNG ordered four 177,000 m³ three-tank carriers with Mark III Flex tanks in November 2025 and May 2026. The builder is HD Korea Shipbuilding & Offshore Engineering.
The BW LNG ships add 3,000 m³ of capacity over a standard four-tank 174,000 m³ design, with a guaranteed BOR of 0.08% per day. Deliveries start in late 2028, so no three-tank carrier is trading yet.
How Does the Containment System Affect Boil-Off?
The containment system sets the boil-off rate through its insulation type, thickness and the surface area of the tanks. Membrane systems dominate new LNG carriers; spherical Moss tanks and prismatic SPB tanks are the main alternatives.
- Membrane tanks (GTT Mark III and NO96): a thin metal primary barrier supported by insulation fixed to the inner hull. Thicker insulation and better foam have pushed design BOR down.
- Moss spherical tanks: self-supporting aluminium spheres. A sphere has the least surface for its volume, but the shape wastes hull space.
- Prismatic (SPB) tanks: self-supporting tanks shaped to the hull, built in small numbers.
- Q-Max carriers: Qatar’s 266,000 m³ ships, the largest LNG carriers, were built with slow-speed diesel engines and reliquefaction plants instead of burning BOG.
The IGC Code sets the design rules for these tanks, including their secondary barriers. The types and sizes of LNG carriers guide compares the ship designs themselves.
How Is Boil-Off Gas Managed on LNG Carriers?
LNG carriers burn boil-off gas as fuel, reliquefy it, burn surplus in a gas combustion unit, or let tank pressure rise within limits. The IGC Code allows venting to atmosphere only in emergencies.
IGC Code Chapter 7 requires tank pressure and temperature to stay within design limits. The accepted methods are reliquefaction, thermal oxidation, pressure accumulation or liquid cooling, and the code itself is mandatory under SOLAS Chapter VII.
- Use as fuel: the main method on most LNG carriers. Compressors send BOG to the boilers or engines.
- Reliquefaction: a full or partial plant turns BOG back into LNG and returns it to the tanks.
- Gas combustion unit (GCU): burns surplus BOG when the engines cannot use it, for example at anchor or at low speed.
- Pressure accumulation: tank pressure is allowed to rise slowly, within the design limit, for short periods.
What Equipment Handles Boil-Off Gas?
A BOG system typically includes compressors, heaters, a forcing vaporiser and a GCU, controlled from the cargo control room.
- Low-duty (LD) compressors: take boil-off from the vapour header and send it to the engine room as fuel.
- LD or BOG heater: warms the cold vapour to about +25°C before it reaches the engines or boilers.
- High-duty (HD) compressors: move large vapour volumes, such as vapour returned to shore during loading or warm gas for tank warm-up.
- Forcing vaporiser: vaporises LNG to make up the fuel supply when natural boil-off is not enough.
How Is Boil-Off Gas Used as Fuel?
Boil-off gas is burned in the ship’s boilers, dual-fuel diesel generators or two-stroke gas engines. One study puts BOG at 70–90% of propulsion energy on a laden voyage and 40–50% in ballast.
IGC Code Chapter 16 sets the safety rules for using cargo as fuel. The propulsion type decides how the gas is used:
- Steam turbine: boilers burn BOG, fuel oil or both. This was the standard LNG carrier plant for decades.
- Dual-fuel diesel-electric (DFDE): four-stroke dual-fuel generators burn low-pressure gas and drive electric propulsion motors.
- ME-GI: MAN two-stroke engines inject gas at about 300 bar, with very low methane slip. They need a high-pressure compressor or pump.
- X-DF: WinGD two-stroke engines burn low-pressure gas on the Otto cycle, which is simpler to supply but lets more unburned methane escape.
Our page on dual-fuel engines on ships explains the engine side. Unburned methane, called methane slip, now carries a cost: the EU Emissions Trading System covers methane from shipping from January 2026.
Why Do LNG Carriers Keep a Heel?
LNG carriers keep a small quantity of cargo, called the heel, after discharge. The heel keeps the tanks cold on the ballast voyage and supplies boil-off gas as fuel.
Without a heel, the tanks would warm up and need cooling down with LNG before the next loading. That costs time and cargo, so charterers usually agree the heel quantity in advance.
How Does Boil-Off Differ on LPG Carriers?
LPG carriers also produce boil-off, but fully refrigerated ships reliquefy it rather than burn it. Propane is carried at about -42°C, so a refrigeration plant can condense the vapour and return it to the tanks.
- Fully refrigerated LPG carriers: reliquefaction plants compress and cool the vapour, then return the condensate to the tanks.
- Pressurised LPG carriers: small ships keep cargo liquid under pressure, so vapour stays in the tank.
- LPG as fuel: since 2020, some very large gas carriers burn LPG cargo in dual-fuel engines. BW LPG’s BW Gemini was the first VLGC converted.
The difference between LNG and LPG page covers the two cargoes in more detail.
What Are the Safety Risks of Boil-Off Gas?
The main safety risks of boil-off gas are tank over-pressure, fire or explosion, cold burns and asphyxiation. Rollover, a sudden release of vapour from layered cargo, is a rarer but serious hazard.
- Over-pressure: if BOG is not removed, tank pressure rises until the relief valves lift and vent cargo vapour.
- Flammability: methane burns in air between about 5% and 15% by volume.
- Cold, heavy vapour: methane vapour colder than about -110°C is denser than air and spreads at deck level until it warms.
- Asphyxiation and cold burns: vapour displaces oxygen in enclosed spaces, and liquid or cold gas causes cryogenic burns.
- Rollover: layers of LNG with different densities mix suddenly, releasing a large volume of BOG in a short time.
Gas detection, relief valves set at the tank’s maximum allowable pressure and emergency shutdown systems manage these risks. Crews also monitor tank pressures and temperatures continuously from the cargo control room.
What Do Inspectors Check on BOG Systems?
Class surveyors, port State control officers and vetting inspectors check that the BOG system keeps tank pressure within limits. They also check that alarms and relief valves work and that records match the operation.
- Class surveys: annual IGC surveys examine the cargo pressure and temperature control arrangements, including GCUs and refrigeration plants. See types of ship surveys.
- Port State control: the International Certificate of Fitness, relief valve settings, gas detection and emergency shutdown tests. Deficiencies can lead to detention under the port State control MoUs.
- Vetting: a vetting inspection asks officers to explain BOG handling, tank pressure limits and the GCU.
- Company systems: under the ISM Code, compressors, heaters and the GCU sit in the planned maintenance system.
Boil-off is also commercial. Charter parties often state a maximum daily boil-off, so the crew records tank pressures, BOG consumption and cargo figures carefully.
Does Bio-LNG Change Boil-Off Gas?
Bio-LNG boils off in the same way as fossil LNG, because both are liquefied methane. Its lower lifecycle emissions do not make venting harmless, since methane is still a strong greenhouse gas.
Under the IPCC’s latest assessment, methane warms the climate about 28–30 times more than CO₂ over 100 years. Venting or slipping bio-methane therefore still adds to warming, even if the gas came from waste.
Bio-LNG mainly helps on paper and in lifecycle accounting, as part of the wider move towards sustainable shipping. For the crew, BOG handling stays the same.
Related Topics
Frequently Asked Questions
What does BOG stand for in LNG shipping?
BOG stands for boil-off gas. It is the natural gas vapour produced when LNG in a cargo tank absorbs heat and evaporates.
What is BOG fuel?
BOG fuel is boil-off gas burned in a ship’s boilers or engines. LNG carriers send it from the cargo tanks through compressors and heaters to the engine room instead of wasting it.
Is boil-off the same as regasification?
No. Boil-off is unwanted evaporation caused by heat leaking into the tanks. Regasification is the deliberate conversion of LNG back into gas at a terminal or FSRU for delivery into a pipeline.
What is a typical LNG boil-off rate?
Modern membrane LNG carriers have design boil-off rates of about 0.07–0.10% of cargo volume per day. Actual laden-voyage rates are often higher in warm or rough weather.
What is BOG combustion?
BOG combustion is the burning of surplus boil-off gas in a gas combustion unit (GCU) when the engines cannot use it. It keeps tank pressure in limits without venting methane.
What do dual-fuel LNG carriers do with boil-off gas?
They burn it in their engines as the main fuel. If natural boil-off is short they add forced boil-off; if it is surplus they reliquefy it or burn it in the GCU.
Is boil-off gas from LNG dangerous?
It can be. Methane is flammable between about 5% and 15% in air, cold vapour can spread at deck level, and gas displaces oxygen in enclosed spaces. Tank over-pressure is the main operational risk.
Why does boil-off gas contain more nitrogen at first?
Nitrogen boils at a lower temperature than methane, so it leaves the liquid first. The early boil-off on a voyage is therefore richer in nitrogen and slightly lower in heating value.
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