Liquefied gas treatment method for vessel

Abstract

A liquefied gas treatment method for a vessel is performed by a liquefied gas treatment system for the vessel including a cargo tank storing LNG, and a main engine and a sub engine using the LNG stored in the cargo tank as fuel. The liquefied gas treatment system includes a compressor line configured to compress BOG generated in the cargo tank by a compressor and supply the compressed BOG to the engines as fuel, and a pump line configured to compress the LNG stored in the cargo tank by a pump and supply the compressed LNG to the engines as fuel. In a laden condition in which an amount of the LNG stored in the cargo tank is larger than in a ballast condition, the BOG generated in the cargo tank is supplied as fuel to at least one of the engines through the compressor line.

What is claimed is: 
     
         1 . An LNG tank ship comprising:
 an LNG tank containing therein an LNG composition in liquid phase and gas phase;   a gas fuel combustion device configured to consume gas phase LNG;   a supercritical fuel injection engine configured to consume supercritical state LNG;   a gas discharge port located inside the LNG tank at a higher portion thereof and configured to discharge gas phase LNG from the LNG tank;   a gas-to-supercritical pathway for processing gas phase LNG from the gas discharge port to generate supercritical state LNG, the gas-to-supercritical pathway comprising a gas phase LNG transfer line for receiving gas phase LNG from the gas discharge port, a heat exchanger downstream of the gas phase LNG transfer line, and a multi-stage compressor downstream of the heat exchanger, the multi-stage compressor comprising a plurality of serially connected compressors configured to pressurize the gas phase LNG for generating supercritical state LNG having a pressure of 150-400 bara;   a supercritical-to-liquid pathway for processing supercritical state LNG to generate liquid phase LNG for returning to the LNG tank, the supercritical-to-liquid pathway comprising a supercritical state LNG transfer line for receiving supercritical state LNG from the gas-to-supercritical pathway, an oil filter configured to filter lubricant oil added to an LNG stream in the multi-stage compressor, the heat exchanger downstream of the supercritical state LNG transfer line, a decompressor downstream of the heat exchanger and a LNG return line downstream of the decompressor;   the heat exchanger configured to heat-exchange between the gas phase LNG of the gas-to-supercritical pathway and the supercritical state LNG of the supercritical-to-liquid pathway such that the gas phase LNG of the gas-to-supercritical pathway is heated while the supercritical state LNG of the supercritical-to-liquid pathway is cooled sufficient to form liquid phase LNG, wherein the LNG tank ship does not comprise a refrigeration cycle of a coolant for cooling the supercritical state LNG in the supercritical-to-liquid pathway;   the decompressor of the supercritical-to-liquid pathway, being configured to depressurize the liquid phase LNG from the heat exchanger;   the LNG return line of the supercritical-to-liquid pathway being in fluid communication with the LNG tank for returning the depressurized liquid phase LNG to the LNG tank;   the multi-stage compressor of the gas-to-supercritical pathway being in fluid communication with the supercritical fuel injection engine for supplying at least part of the supercritical state LNG from the multi-stage compressor to the supercritical fuel injection engine; and   a valve system configured to control connection of the gas-to-supercritical pathway to the gas discharge port and connection of the supercritical-to-liquid pathway to the gas-to-supercritical pathway for selective operation of one or more of the gas-to-supercritical pathway and the supercritical-to-liquid pathway such that the supercritical-to-liquid pathway is to operate only when the gas-to-supercritical pathway is operating to generate supercritical state LNG.   
     
     
         2 . The ship of  claim 1 , wherein the plurality of serially connected compressors comprise at least one non-lubricated compressor configured to operate without using lubricant oil and at least one lubricated compressor configured to operate with lubricant oil, the at least one lubricated compressor disposed downstream of the at least one non-lubricated compressor such that the heated gas phase LNG is pressurized by the at least one non-lubricated compressor and further pressurized by the at least one lubricated compressor for generating the supercritical LNG to which the lubricant oil is added. 
     
     
         3 . The ship of  claim 2 , wherein the multi-stage compressor is configured such that the at least one non-lubricated compressor is to operate for generating pressurized gas phase LNG which is to be supplied to the gas fuel combustion device while not operating the at least one lubricated compressor for generating supercritical state LNG. 
     
     
         4 . The ship of  claim 3 , wherein no oil filter is provided for pressurized gas phase LNG generated by the at least one non-lubricated compressor to be supplied to the gas fuel combustion device. 
     
     
         5 . The ship of  claim 2 , further comprising:
 a liquid discharge port located inside the LNG tank at a lower portion thereof and configured to discharge liquid phase LNG from the LNG tank; and   a liquid-to-supercritical pathway for processing liquid phase LNG from the liquid discharge port to generate supercritical state LNG for supplying to the supercritical fuel injection engine, the liquid-to-supercritical pathway comprising a pump and a heater downstream of the pump,   wherein, in the liquid-to-supercritical pathway, the pump is configured to pressurize liquid phase LNG from the LNG tank to a pressure of 150-400 bara,   wherein, in the liquid-to-supercritical pathway, the heater is configured to heat the pressurized liquid phase LNG for generating supercritical state LNG, wherein the heater is in fluid communication with the supercritical fuel injection engine for supplying at least part of the supercritical state LNG to the supercritical fuel injection engine,   wherein the valve system is further configured to control connection of the liquid-to-supercritical pathway to the liquid discharge port for operation of the liquid-to-supercritical pathway such that the multi-stage compressor of the gas-to-supercritical pathway is not to operate for generating supercritical state LNG while the liquid-to-supercritical pathway is operating to generate supercritical state LNG.   
     
     
         6 . The ship of  claim 5 , wherein the LNG tank ship is configured such that while the liquid-to-supercritical pathway is operating to generate supercritical state LNG, the at least one lubricated compressor is not to operate for generating supercritical state LNG but the at least one non-lubricated compressor is to operate for generating pressurized gas phase LNG which is to be supplied to the gas fuel combustion device. 
     
     
         7 . The ship of  claim 1 , wherein the decompressor is configured to provide a liquid-gas mixture of LNG, wherein the LNG return line is configured to return the liquid-gas mixture to the LNG tank. 
     
     
         8 . The ship of  claim 1 , wherein the decompressor is configured to provide a liquid-gas mixture of LNG, wherein the supercritical-to-liquid pathway further comprises a liquid-gas separator downstream of the decompressor and upstream of the LNG return line, wherein the liquid-gas separator is configured to separate liquid phase LNG from the liquid-gas mixture and to supply the separated liquid phase LNG to the LNG return line for returning to the LNG tank. 
     
     
         9 . The ship of  claim 1 , wherein the valve system is configured to control connection of the gas-to-supercritical pathway to the gas discharge port to selectively operate the gas-to-supercritical pathway based on a rate of boil-off gas generation within the LNG tank and further based on a rate of a total amount of LNG combustion in the LNG tank ship. 
     
     
         10 . The ship of  claim 9 , wherein the valve system is configured to control connection of the gas-to-supercritical pathway to the gas discharge port to selectively operate the gas-to-supercritical pathway when the rate of boil-off gas generation within the LNG tank is smaller than the rate of the total LNG combustion in the LNG tank ship. 
     
     
         11 . The ship of  claim 1 , wherein the valve system is configured to control connection of the gas-to-supercritical pathway to the gas discharge port to operate the gas-to-supercritical pathway only during a laden voyage, wherein the valve system is configured to control connection of the supercritical-to-liquid pathway to the gas-to-supercritical pathway to operate the supercritical-to-liquid pathway only during a laden voyage. 
     
     
         12 . The ship of  claim 1 , wherein the gas fuel combustion device comprises an engine configured to run on gas phase LNG, wherein the supercritical fuel injection engine comprises an MEGI engine. 
     
     
         13 . The ship of  claim 1 , wherein the LNG tank ship comprises only one unit of the multi-stage compressor for the gas-to-supercritical pathway and does not comprise a backup multi-stage compressor for the gas-to-supercritical pathway. 
     
     
         14 . A method of processing LNG in the LNG tank ship of  claim 1 , the method comprising:
 running the gas-to-supercritical pathway, which comprises the multi-stage compressor comprising the plurality of serially connected compressors, which comprise at least one non-lubricated compressor and at least one lubricated compressor which are disposed, wherein gas phase LNG discharged from the LNG tank is pressurized by the at least one non-lubricated compressor and further pressurized by the at least one lubricated compressor for generating supercritical LNG having a pressure of 150-400 bara, wherein lubricant oil is added to an LNG stream in the multi-stage compressor; and   running the supercritical-to-liquid pathway, which comprises cooling supercritical state LNG to form liquid phase LNG, subsequently depressurizing the liquid phase LNG, and returning at least part of the liquid phase LNG to the LNG tank, wherein, in the supercritical-to-liquid pathway, the lubricant oil added to the supercritical LNG is filtered with the oil filter.   
     
     
         15 . The method of  claim 14 , wherein the at least one non-lubricated compressor operates for generating pressurized gas phase LNG which is to be supplied to the gas combustion device while the at least one lubricated compressor is idle. 
     
     
         16 . The method of  claim 15 , wherein the pressurized gas phase LNG has a pressure of 6 bara to 10 bara. 
     
     
         17 . The method of  claim 15 , wherein no oil filter is provided for pressurized gas phase LNG generated by the at least one non-lubricated compressor for supplying to the gas fuel combustion device. 
     
     
         18 . The method of  claim 14 , wherein the multi-stage compressor operates based on a rate of boil-off gas generation within the LNG tank and further based on a rate of fuel consumption at the supercritical fuel injection engine. 
     
     
         19 . The method of  claim 18 , wherein when boil-off gas generation is smaller than fuel consumption at the supercritical fuel injection engine, the at least one lubricated compressor is not operating to generate supercritical state LNG but the at least non-lubricated compressor is operating to generate pressurized gas phase LNG. 
     
     
         20 . The method of  claim 18 , wherein the multi-stage compressor does not operate when the rate of boil-off gas generation within the LNG tank is smaller than the rate of fuel consumption at the supercritical fuel injection engine.