The invention relates to an offshore
natural gas liquefying method and an offshore
natural gas liquefying device. The method comprises the following steps of: 1) pretreating
natural gas extracted from
seabed, and inputting into a heat exchange cold box; 2) continuously reducing pressure and temperature of preset
carbon dioxide gas in a
carbon dioxide pre-cooling circulating
system, and introducing the
carbon dioxide gas into the heat exchange cold box; 3) compressing preset
nitrogen in a double
nitrogen expansion
refrigeration circulating
system into
high pressure nitrogen, introducing the
high pressure nitrogen into the heat exchange cold box, precooling by carbon dioxide, directly reducing pressure and temperature of one path of nitrogen by using an expander, continuously precooling the other path of nitrogen and reducing pressure and temperature by using the expander, returning the two path of low pressure nitrogen to the heat exchange cold box, exchanging heat, raising the temperature, lifting the pressure, reducing the temperature, and returning to the heat exchange cold box; and 4) precooling the pretreated natural gas by using the carbon dioxide gas in the heat exchange cold box, introducing into a heavy
hydrocarbon separation unit, gradually cooling the natural gas by the returned nitrogen, removing the heavy
hydrocarbon by using a gas-liquid separator and a rectifying column, cooling, throttling and reducing temperature by using a
throttle valve, introducing a gas-liquid mixture into a
buffer tank, introducing a
liquid phase into a
storage tank for storage, discharging a
gas phase from the top of the
buffer tank and collecting the
gas phase.