NOVELTY - During transfer of methane-rich fraction from the storage tank (S1) into the transport storage container (S2), the hydrocarbon-rich fraction (1) to be liquified, is sub-cooled (E). It is taken to a temperature of 1, preferably 2 - 5,degreesC beneath that reached in intervals when there is no transfer of methane-rich fraction from the storage tank (S1) into the transport storage tank (S2).    USE - To operate a liquefaction process, especially for natural gas, separating-off a nitrogen rich fraction used as fuel gas by condensation or fractionation, supplying the methane rich fraction to a storage tank and thence intermittently to a tanker (ship). Vaporized methane-rich fractions are admixed with the fuel gas fraction.    ADVANTAGE - The process recovers gas evaporated during tanker filling. It avoids both the large compressors required to re-liquefy it, and the waste occasioned by venting and flaring it off. Instead it is sub-cooled as described, below temperatures reached when the tanker is not being filled.    DETAILED DESCRIPTION - Preferred features: At least during transfer of methane-rich fraction from storage tank (S1) to transport storage tank (S2), a hydrocarbon-rich fraction, preferably a partial stream (8) of the hydrocarbon-rich fraction to be liquified is admixed with the fuel gas fractions (6, 7). This process is controlled using at least one control valve (V). Vaporized methane-rich fraction(s) of the fuel gas fraction (7) supplied, arising in the storage tank (S1) and/or the transport storage vessel (S2), are compressed (C) in a single- or multistage process, to or above the pressure of the fuel gas fraction (7). Between storage tank (S1) and transport storage tank (S2) a pressure differential of 10-100 mbar, preferably 20-50 mbar is set, pressure in the storage container (S2) being the higher.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of the installation.    Single- or multistage compressor (C)    Sub-cooling heat exchanger (E)    Storage tank (S1)    Transport storage container (S2)    Hydrocarbon-rich fraction to be liquified (1)    Fuel gas fractions (6, 7)    Partial stream of hydrocarbon-rich fraction (8)