METHOD OF LIQUEFYING NATURAL GAS DURING DEVELOPMENT OF UNDERWATER FIELDS Russian patent published in 2018 - IPC F25J1/00 

FIELD: oil and gas industry.

SUBSTANCE: invention relates to methods for liquefying natural gas and can be used for underwater development of gas and gas condensate fields. Consistently carry out two independent refrigerant liquefaction cycles of natural gas. First independent cycle is carried out in one step in a flexible underwater pipeline connecting the producing underwater structure with a subsea plant for liquefying natural gas, with use of Arctic seawater as a refrigerant and the process of liquefaction isobaric at a pressure of not less than 4.0 MPa, with the cooling of natural gas to a temperature below plus 10 °C. Gas is then fed to a second independent liquefaction cycle at a subsea plant for liquefying natural gas. It includes four stages with interstage separation and supply of liquefied natural gas in countercurrent with the refrigerant, which is used as air in the liquid phase, having a pressure of at least 4.0 MPa, which is fed to the last fourth stage along the course of natural gas. Gas is cooled in the first stage of this cycle to a temperature of -15…-20 °C, in the second stage – to -35…-45 °C, in the third stage – up to -60…-75 °C, on the fourth stage – to -161.5 °C with a decrease in pressure at this stage to 0.15 MPa with production of liquefied natural gas.

EFFECT: technical result consists in simplifying the technological scheme of the method, reducing the refrigerant flow, increasing the output of the target LNG and eliminating the need to remove gaseous nitrogen under the level of ice formations or into the atmosphere, in improving the environmental factor.

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