Boil-off gas management
Natural and forced boil-off on gas carriers, reliquefaction energy cost, and the speed floor that a cargo-driven gas generation rate imposes on a laden passage.
Boil-off gas is the vapour generated continuously in a liquefied gas cargo tank by heat ingress through the containment system. Its rate is set by the cargo, the containment design and the ambient conditions, and it is essentially independent of ship speed, which makes it a sunk energy cost rather than a variable one.
That independence is what limits slow steaming on a gas carrier, and it is a different mechanism from the hotel load explanation usually offered. The classic design point sized combined gas consumption at a service speed of 19 to 21 knots so that propulsion demand matched natural boil-off. Below that speed the ship generates more gas than the machinery can burn, and the surplus has to be reliquefied, at roughly 1 kWh per kilogram, or otherwise disposed of. In one 150,000 cubic metre dual-fuel diesel-electric case with a reliquefaction plant, surplus boil-off for propulsion runs out at about 13.7 knots.
The full article will cover natural against forced boil-off, tank pressure management on laden and ballast passages, the reliquefaction plant and its energy demand, gas combustion units, and the methane slip and carbon accounting consequences of each disposal route. On an LNG-fuelled ship the same physics applies to a far smaller inventory, and the disposal routes are set by the onboard LNG fuel system against the 15-day holding time of IGF Code 6.9.1.1 rather than by cargo economics. See LNG carrier and slow steaming .