Liquefied gas reliquefier, liquefied-gas storage facility and liquefied-gas transport ship including the same, and liquefied-gas reliquefaction method

A liquefied-gas reliquefication method for reliquefying BOG resulting from evaporation of liquefied gas in a plurality of liquefied-gas storage tanks, the method comprising:

• liquefying a secondary refrigerant using a cooling unit, the cooling unit being disposed in a secondary-refrigerant circulating channel through which the secondary refrigerant circulates, the secondary refrigerant having a melting point lower than a condensation temperature of the BOG, and the cooling unit including a plurality of pulse-tube refrigerators,
• feeding the liquefied secondary refrigerant cooled by the cooling unit through the secondary-refrigerant circulating channel using a liquefied-secondary-refrigerant feeding unit;
• condensing the BOG by heat exchange between the BOG and the liquefied secondary refrigerant fed by the liquefied-secondary-refrigerant feeding unit using a heat exchange unit disposed in the secondary-refrigerant circulating channel;
• controlling the number of pulse-tube refrigerators in operation and/or the cooling capacities of the individual pulse-tube refrigerators based on a measurement result from at least one of thermometers, pressure gauges, and pump discharge flow meters installed in the liquefied gas storage tanks;
• and
• precooling the secondary refrigerant supplied into the secondary-refrigerant circulating channel with cold energy possessed by the BOG using a precooling unit,
• wherein the heat exchange unit is disposed inside a channel of a header pipe disposed above the plurality of liquefied-gas storage tanks.

A liquefied-gas storage facility comprising:

• a plurality of liquefied-gas storage tanks;
• and
• a liquefied gas reliquefier for reliquefying boil-off gas (hereinafter abbreviated to “BOG”) resulting from evaporation of liquefied gas in the plurality of liquefied-gas storage tanks to prevent a rise in the internal pressure of the liquefied-gas storage tanks, the liquefied gas reliquefier comprising:
• i. a cooling unit for liquefying a secondary refrigerant which is a liquid having a melting point lower than a condensation temperature of the BOG, the cooling unit being disposed in a secondary-refrigerant circulating channel through which the secondary refrigerant circulates;
• ii. a liquefied-secondary-refrigerant feeding unit for feeding the liquefied secondary refrigerant cooled by the cooling unit through the secondary-refrigerant circulating channel;
• iii. a heat exchange unit disposed in the secondary-refrigerant circulating channel to condense the BOG by heat exchange between the BOG and the liquefied secondary refrigerant fed by the liquefied-secondary-refrigerant feeding unit;
• iv. a header pipe disposed above the plurality of liquefied-gas storage tanks;
• v. a precooling unit for precooling the secondary refrigerant supplied into the secondary-refrigerant circulating channel with cold energy possessed by the BOG,
• wherein the header pipe defines a channel which connects the liquefied-gas storage tanks to each other,
• wherein the heat exchange unit is disposed inside the channel of the header pipe,
• wherein the cooling unit includes a plurality of pulse-tube refrigerators, and
• wherein the number of pulse-tube refrigerators in operation and/or the cooling capacities of the individual pulse-tube refrigerators are controlled based on a measurement result from at least one of thermometers, pressure gauges, and pump discharge flow meters installed in the liquefied gas storage tanks.