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 ℃/per hour can be maintained such that tank temperatures can be reduced from +30 to -130 ℃ 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.

Floating storage and regasification units (FSRU) are in high demand globally forming a specialized and growing fleet of ships to service a steadily expanding liquified natural gas (LNG) industry. Ship-to-Ship (STS) LNG cargo transfers are an inevitable part of the FSRU day-to-day operating patterns. It is little more than a decade since FSRU operations began and a lack of knowledge persists concerning their relatively complex tank pressure behaviour during STS transfers. In this work,based on details from more than thirty STS operations,FSRU tank pressure trends and influencing factors are explained. The interactions between the vapor space and liquid LNG are strongly influenced by the thin surface film between those phases in an LNG tank. The saturated vapor pressure (SVP) of the LNG and the volumetric balance of gas and liquid movements in and out of the tank are also influential. These factors control the rate and extent of evaporation and condensation occurring at the surface film,thereby impacting tank pressure changes. This paper presents observations that reveal a duality in FSRU tank pressure behaviour that in over-pressured conditions allows tank pressure to rise significantly and rapidly. On the other hand,in under-pressured conditions tank pressure stabilizes at certain levels and inhibits significant reduction below those levels. Here,we explain the processes involved in determining FSRU tank pressure trends during STS transfers,and,based on observations of actual STS transfers,provide rules of thumb and empirical equations that can be used to estimate tank pressure behaviour in a range of operating conditions. These are supported by several generic cases of tank pressure,temperature and SVP trends reflecting the various tank scenarios commonly associated with routine operating patterns of FSRU during STS transfers. The principles described in this paper apply to LNG Carriers but tend to be disrupted by sloshing and wave movements also in the tanks during transit.