However, even with advanced insulation, a small portion of LNG naturally evaporates during the journey due to heat ingress.
This process is known as Boil-Off Gas (BOG).

• Approximately 0.10% – 0.15% of LNG cargo evaporates daily.
• This vapor is continuously generated due to unavoidable heat leakage.
• If not managed, it increases tank pressure.

• Lighter components like methane and nitrogen evaporate first.
• This changes the composition of LNG over time.
• The remaining LNG becomes denser with higher calorific value.
• This process is called “aging” or weathering.

As LNG evaporates, tank pressure rises. To prevent structural risks:

• Low-duty compressors remove BOG continuously.
• This maintains safe pressure levels inside cryogenic tanks.

Modern LNG carriers smartly use BOG as an onboard energy source:

• Steam Turbines: BOG is burned in boilers to produce steam for propulsion.
• Dual-Fuel Engines (DFDE): Engines run on both LNG and diesel.
• ME-GI Engines: High-pressure gas injection (~300 bar) provides maximum efficiency.

• Advanced LNG carriers use reliquefaction plants.
• BOG is cooled and converted back into liquid LNG.
• This allows delivery of nearly 100% cargo.

• Excess gas is burned safely in a GCU.
• Used when engines cannot consume all BOG.

• Occurs when layering forms inside tanks.
• Heavier LNG settles at bottom, lighter stays on top.
• Sudden mixing can cause rapid vapor release.

• LNG is lighter than water and floats.
• Rapidly vaporizes forming a cold vapor cloud.
• Initially, vapor is heavier than air, then disperses.
• Does not mix with water and evaporates completely.

Flammability

• Flammable only between 5%–15% concentration in air.
• Outside this range, it cannot ignite.

Explosivity

• Rapid Phase Transition (RPT) can occur without fire.
• Sudden expansion may create pressure waves.

Asphyxiation

• High gas concentration displaces oxygen.

Cryogenic Damage

• Extreme cold can freeze tissues and damage materials.

• LNG evaporates quickly, leaving minimal long-term pollution.
• Main risks are fire, explosion, and cold exposure.
• Economic impacts may include port disruption and repair costs.

Studies show that when LNG contacts water, it may undergo:

• Film Boiling: Vapor layer forms, slowing evaporation.
• Rapid Phase Transition: Sudden explosive vaporization.

Boil-off in LNG transport is not a flaw but a well-managed engineering advantage.
Through advanced systems like dual-fuel engines, reliquefaction units, and pressure control mechanisms, LNG carriers convert a natural physical process into an efficient energy solution.