Explainers
What Is Boil-off Gas and Why Does It Matter in LNG Shipping?
A practical guide to vapour generation, tank pressure control, fuel use, gas combustion units and reliquefaction.
What boil-off gas actually is
LNG is held at around −162 °C. Insulation slows heat ingress but cannot stop it, so part of the cargo continuously returns to vapour.
That vapour is boil-off gas. Managing it is first and foremost about keeping tank pressure inside design limits, not about eliminating the phenomenon.
What changes the rate
Insulation type and condition, filling level, ambient and sea temperature, voyage length and sea state all influence how much vapour forms.
For this reason a single boil-off percentage should always be read as indicative rather than a specification.
Three ways to handle it
Using vapour as fuel turns an unavoidable process into propulsion energy, and is the default on most modern designs.
A gas combustion unit safely disposes of surplus vapour when generation exceeds demand. Reliquefaction instead returns vapour to liquid so cargo is retained, at the cost of added machinery, energy demand and maintenance.
Why it shows up in commercial discussion
Retained cargo is delivered cargo, so boil-off behaviour appears in charter terms and voyage planning. The engineering choice and the commercial arrangement are linked.
Vessel-specific values belong in project documentation, not in general educational material.
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- Status
- Educational overview — editorial review required
- Seeded educational summary awaiting editorial and technical review.
- Last reviewed
- 15 September 2026
- Source status
- Sources pending editorial review.
Technical parameters are indicative and may vary by vessel design, generation, shipyard, operator and intended service.