Boil-off gas balanced method of cool down for liquefied natural gas tanks at sea

Abstract

Cooling down the cargo tanks of liquefied natural gas carriers (LNGC) prior to the ships arrival at discharge and loading ports follow various widely adopted operating procedures. The tank cool-down procedures typically followed cannot though be considered as best practice, because they consume considerable boil-off gas while reducing tank temperatures. An alternative tested method, described here, consumes considerably less liquefied natural gas (LNG) during the tank cool-down process, which is beneficial, particularly during the ballast voyages. In certain circumstances, LNGC consume liquid marine fuels so that they are able to preserve enough LNG heel to complete tank cool down at sea to required low temperatures before reloading can be commenced. The novel method devised for cooling down LNGC, particularly those fitted with membrane cargo tanks, at sea prior to arrival at a loading terminal involves much lower LNG heel consumption than conventional methods. The boil-off gas (BOG)-balanced tank-cool-down method applies continuous spraying, at very low rates, of the tanks with LNG extracted from the heel. This procedure enables the ship’s engines to consume all excess BOG without the need to pass some of it as waste for combustion in the gas combustion unit or steam dump. It also ensures that the LNG cargo tanks are maintained at stable and constant pressure and reduces the coolant LNG quantity consumed. The BOG-balanced tank-cool-down is straightforward to implement and monitor, simplifying tank pressure control. Test results demonstrate that tank cool-down rates of 4 to 5 ◦C/per hour can be maintained such that tank temperatures can be reduced from +30 to -130 ◦C within 37 hours. The method could work on LNGC with Moss-type tanks but is likely to be less effective as they are typically fitted with fewer tank spraying nozzles. Cited as : Kulitsa, M., Wood, D.A. Boil-off gas balanced method of cool down for liquefied natural gas tanks at sea. Advances in Geo-Energy Research, 2020, 4(2): 199-206, doi: 10.26804/ager.2020.02.08