Abstract: A subsea primary separating vessel (2,22) is installed close to the wellhead (1,31) of an oil producing well to effect primary separation of the liquid and gas phases of the fluids produced. A line (3,23) connected to the top of the separating vessel allows separated gas to flow to a gathering vessel (8,28) located at any gathering center. At least one flow line (4,5,24) connects the lower part of the separating vessel to a vessel (9,29) located at any gathering center. When the volume of liquid phase which separates out within the at least one flow line begins to exert a back pressure which adversely affects production from the well, pressurized gas can be injected into the flow line(s) for a specific period of time in order to promote the flow of liquid gas to the vessel (9,29). If it is desired that flow efficiency should be increased a mechanical interface driven by the pressurized gas may be used to encourage flow of the liquid phase.