Separation of oil and gas phases in wellhead fluids

Claim 1

Apparatus for separating an oil/liquid phase from a gas phase contained in a wellhead fluid from a hydrocarbon production system, the apparatus comprising: a vessel (14) capable of pressurization and having a wellhead fluid inlet (16) for connection to the hydrocarbon production system, for entry of wellhead fluid (12), a gas export outlet (18) for exit of export gas (20) separated from the wellhead fluid, and an oil/liquid export outlet (22) for exit of oil/liquid (24) separated from the wellhead fluid; a main oil/liquid inventory compartment (31) at a lower end of the vessel (14), the oil/liquid export outlet (22) communicating with the inventory compartment (31), a secondary compartment (58) at an upper end of the vessel, and means (52) for connecting the inventory and secondary compartments (31, 58) to each other; a primary centrifugal separator (30) in the vessel (14) for separating a majority of the oil/liquid phase from the wellhead fluid (12) to leave a wet gas phase, the primary centrifugal separator (30) comprising a riser tube (32) with an open lower end communicating with the wellhead fluid inlet (16) for receiving upwardly flowing wellhead fluid (12), the riser tube (32) having a closed upper end, a plurality of curved-arms (34) spaced around the riser tube (32) for causing a majority of the oil/liquid phase to separate from the wellhead fluid (12) to leave the wet gas phase, each curved-arm (34) having an axially extending curved wall, curving away from the riser tube (32) between a root edge of the curved wall at the riser tube (32) and an outer edge of the curved wall spaced outwardly from the riser tube (32), at least one radial partition in the curved-arm (34), for dividing an inner space defined by the curved-arm (34) into multiple levels, the primary centrifugal separator (30) also comprising a return cylinder (36) around the riser tube (32) and curved-arms (34), for receiving oil/liquid phase moving outwardly by centrifugal force from the outer edge of the curved-arms (34), the return cylinder (36) having an open lower end extending into the inventory compartment (31) for carrying the oil/liquid phase downwardly to the inventory compartment (31), the return cylinder (36) having an open upper end for passing the wet gas phase; and a secondary centrifugal separator (50) in the vessel (14) spaced above and axially aligned with the primary centrifugal separator (30) by an open interstage region (44), the secondary centrifugal separator (50) receiving the wet gas phase and comprising a plurality of tangential inlet vanes (54) into which the wet gas phase passes for further separating oil/liquid from the wet gas phase to leave a dry gas phase, the secondary centrifugal separator (50) including skimmer means (56) defining skimmer slots above the inlet vanes (54) for receiving the dry gas phase and for channelling the further separated oil/liquid downwardly into the secondary compartment (58), the further separated oil/liquid passing from the secondary compartment (58) to the inventory compartment (31) through the means (52) for connecting the inventory and secondary compartments (31, 58), the skimmer means (56) having an open upper end (57) communicating with the gas export outlet (18) for passing the dry gas phase to the gas export outlet (18).