Reliquefaction and Boil-off
In order to facilitate transport, natural gas is cooled down to approximately −163 °C at atmospheric pressure, at which point the gas condenses to a liquid. The tanks on board an LNG carrier effectively function as giant thermoses to keep the liquid gas cold during storage. No insulation is perfect, however, and so the liquid is constantly boiling during the voyage.
According to WGI, on a typical voyage an estimated 0.1–0.25% of the cargo converts to gas each day, depending on the efficiency of the insulation and the roughness of the voyage. In a typical 20-day voyage, anywhere from 2–6% of the total volume of LNG originally loaded may be lost.
Normally an LNG tanker is powered by steam turbines with boilers. These boilers are dual fuel and can run on either methane or oil or a combination of both.
The gas produced in boil off is traditionally diverted to the boilers and used as a fuel for the vessel. Before this gas is used in the boilers, it must be warmed up to roughly 20 °C by using the gas heaters. The gas is either fed into the boiler by tank pressure or it is increased in pressure by the LD compressors.
What fuel the vessel runs on is dependent on many factors which include the length of the voyage, desire to carry a heel for cooldown, price of oil versus price of LNG.
There are three basic modes available.
Minimum boil-off/maximum oil:- In this mode tank pressures are kept high to reduce boil off to a minimum and the majority of energy comes from the fuel oil. This maximises the amount of LNG delivered but does allow tank temps to rise due to lack of evaporation. The high cargo temps can cause storage problems and offloading problems.
Maximum boil-off/minimum oil:- In this mode the tank pressures are kept low and you have a greater boil-off but still there is a large amount of fuel oil used. This decreases the amount of LNG delivered but the cargo will be delivered cold which many ports prefer.
100% gas:- Tank pressures are kept at a similar level to max boil off but this is not enough to supply all the boilers needs so you must start to "force". A small pump is started in one tank to supply LNG to the forcing vaporiser, where the LNG is warmed and vaporized back into a gas that is usable in the boilers. In this mode no fuel oil is used.
Recent advances in technology have allowed reliquefication plants to be fitted to vessels, allowing the boil off to be reliquefied and returned to the tanks. Because of this, the vessels' operators and builders have been able to contemplate the use of more efficient slow-speed Diesel engines (previously most LNG carriers have been steam turbine-powered). Exceptions are the LNG carrier Havfru (built as Venator in 1973), which originally had dual fuel diesel engines, and its sister-ship Century (built as Lucian in 1974), also built with dual fuel gas turbines before being converted to a diesel engine system in 1982. Vessels using dual or tri-fuel diesel electric propulsion systems are now in service.
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