A method of and apparatus for reliquefying of boil-off gas stream obtained from liquefied gas in a gas storage of a floating storage unit

A method of reliquefying of boil-off gas stream obtained from liquefied gas in a gas storage of a floating storage unit, the method comprising:

• leading boil-off gas from the gas storage to a gas compressor and increasing pressure of the boil-off gas, providing pressurized boil-off gas;
• leading the pressurized boil-off gas to a first condenser, and reliquefying at least part of the pressurized boil-off gas forming a first reliquefied and pressurized gas portion and remaining portion of the pressurized boil-off gas;
• reliquefying at least part of the remaining portion of the pressurized boil-off gas in a second condenser forming a second reliquefied and pressurized gas portion;
• expanding the first and the second reliquefied and pressurized gas portions to lower pressure and introducing a majority of the reliquefied gas portions back into the gas storage, wherein the second condenser includes a thermoelectric cooler including a first heat exchanger through which a remaining portion of the pressurized boil-off gas stream flows and reliquefies therein at least partially, and a second heat exchanger through which a heat transfer medium flows, and wherein condensing power of the thermoelectric cooler is controlled by controlling electric power input to the thermoelectric cooler.

Method according to claim 23, wherein electric power input to the thermoelectric cooler is controlled by:

• measuring pressure of the gas upstream to last compressor stage;
• defining a pressure difference between the pressure of the gas upstream to last compressor stage and upstream the thermoelectric cooler;
• setting a target value to the pressure difference;
• comparing the measured value to the target value;
• and
• controlling electric power input to the thermoelectric cooler based on a signal obtained using the measured and target pressure difference value.

An apparatus for reliquefying of boil-off gas stream obtained from liquefied gas in a gas storage of a floating storage unit, the apparatus comprising:

• a gas compressor for increasing pressure of the boil-off gas providing pressurized boil-off gas;
• a first condenser for reliquefying at least part of the pressurized boil-off gas;
• a second condenser for reliquefying at least part of the remaining portion of the pressurized boil-off gas;
• and
• at least one valve for expanding reliquefied and pressurized gas portions to lower pressure;
• wherein the second condenser includes a thermoelectric cooler including a first heat exchanger in which the remaining portion of the pressurized boil-off gas stream is arranged to reliquefy, and a second heat exchanger in which a heat receiving transfer medium is arranged to flow, and a power management system for supplying electric power to the thermoelectric elements of the thermoelectric cooler in controllable manner.