Patent EA-003655-B1 (EA)
Title: Method and system for suppressing and controlling slug flow in a multi-phase fluid stream
Inventor: HAANDRIKMAN GRITIENUS
IPC: E21B43/36
Filed: 2000-11-07
Abstract: 1. A method for suppressing and controlling liquid slugs and gas surges in a stream of multiphase fluid flowing from a flowline (1) into a gas/liquid separator (9) which has a liquid outlet (13) provided with a liquid flow control valve (15) and a gas outlet (14) provided with a gas flow control valve (16), the method comprising - measuring at least one control variable selected from the group of the liquid level (LLIQ) in the separator (9), the liquid flow rate (QL) in the liquid outlet (13), the gas flow rate (QG) in the gas outlet (14), the sum (QL+QG) of the liquid flow rate in the liquid outlet (13) and the gas flow rate in the gas outlet (14), and the fluid pressure (P) at or near the separator (9); - adjusting said liquid and gas flow control valves (15,16) so as to reduce a difference between a selected control variable (QL+QG, QL, QG, P, LLIQ) and a pre-set reference value of the selected control variable, characterized in that the method further comprises - changing the selected control variable (QL+QG, QL, QG, P, LLIQ) from time to time automatically if one or more control variables reaches a pre-set value, wherein during normal operation the selected control variable is the sum (QL+QG) of the liquid flow rate in the liquid outlet (13) and the gas flow rate in the gas outlet (14), and wherein the selected control variable is changed into the liquid flow rate (Q L) in the liquid outlet (13) if the liquid level (LLIQ) in the separator (9) or the liquid flow rate (QL) in the liquid outlet (13) reaches a pre-set value and wherein the control variable is changed back again into the sum (QL+QG) of the liquid flow rate in the liquid outlet (13) and the gas flow rate in the gas outlet (14) when said liquid level (LLIQ) in the separator (9) or liquid flow rate (Q L) in the liquid outlet (13) is below said pre-set value. 2. The method of claim 1, wherein during normal operation the liquid flow control valve (15) is adjusted such that variations of the liquid level (LLIQ) in the separator (9) are minimized, and the gas flow control valve (16) is adjusted such that variations in the sum of the liquid and gas flow rates (QL+QQ) in said outlets (13,14) are minimized. 3. The method of claim 1, wherein during normal operation the sure (QL+QG) of the liquid flow rate in the liquid outlet (14) and the gas flow rate in the gas outlet (13) is controlled by dynamically adjusting the position of the gas flow rate control valve (16) by means of a mixture flow controller (30B) which is set to maintain a total volumetric flow set-point and by dynamically adjusting the position of the liquid flow rate control valve (15) by means of a liquid level controller (30C). 4. The method of claim 3, wherein, if the liquid flow rate (QL) is selected as the control variable, the gas flow control valve (16) is substantially closed and the liquid flow control valve (15) is dynamically adjusted by a liquid flow controller (30D) which is set to maintain a liquid flow set-point. 5. The method of claim 4, wherein, if the liquid flow rate (QL) is selected as the control variable, the gas flow control valve (16) is substantially closed by closing off or choking the gas flow control valve (16) until the gas pressure reaches an unsafe level at which the gas flow control valve (16) is operated as a pressure relief valve. 6. The method of any one of the previous claims, wherein during a start-up period the liquid valve (15) is closed until the liquid level in the separator (9) reaches a selected level, whereas the gas-valve (16) is adjusted dynamically such that the pressure measured by a pressure gauge (27) of the separator (9) is maintained at a selected level. 7. A system for suppressing and controlling liquid slugs in a stream of multi-phase fluid flowing from a flowline (1) into a gas/liquid separator or slugcatcher (9) having a liquid outlet (13) provided with a liquid flow control valve (15) and a gas outlet (14) provided with a gas flow control valve (16), the system comprising a control system (30) for dynamically adjusting the position of said gas and liquid control valves (15,16) in response to measurement of at least one control variable selected from the group of the liquid level (LLIQ) in the separator (9), the liquid flow rate of (QL) in the liquid outlet (13), the gas flow rate (QG)in the gas outlet (14) and the sum of the liquid and gas flow rates (QL+QG) in said outlets (13,14), characterized in that the control system (30) is adapted to change from time to time the control variable (LLIQ, QL, QG, QL+QG) in response to which said gas and liquid control valves (15,16) are adjusted if a selected control variable has reached a pre-set value, wherein during normal operation the control variable is the sum (QL+QG) of the liquid flow rate in the liquid outlet (13) and the gas flow rate in the gas outlet (14), and wherein the control variable is changed into the liquid flow rate (QL) in the liquid outlet (13) if the liquid level (LLIQ) in the separator (9) and/or the liquid flow rate QL in the liquid outlet (13) reaches the pre-set value and wherein the control variable is changed back again into the sum (QL+QG) of the liquid flow rate in the liquid outlet (13) and the gas flow rate in the gas outlet (14) when said liquid level (LLIQ) in the separator (9) or liquid flow rate (QL) in the liquid outlet (13) is below said pre-set value. 8. The system of claim 7, wherein the control system (30) is set such that during normal operation the liquid flow control valve (15) is adjusted such that variations of the liquid level (LLIQ) in the separator (9) are minimized and the gas flow control valve (16) is adjusted that variations in the sum of the liquid and gas flow rates (QL+QG) in said outlets (13,14) are minimized, and wherein the control system (30) substantially closes the gas flow control valve (16) and the liquid flow control valve (15) is adjusted such that variations in the liquid flow rate (QL) in the liquid outlet (13) are minimized if either the liquid level (LLIQ) in the separator (9) reaches a threshold level or if the liquid flow rate (QL) in the liquid outlet (13) reaches a threshold value. 9. The system of claim 7 wherein the flow line (1) forms part of a hydrocarbon fluid production system through which a mixture of crude oil, condensate, water and/or natural gas is produced from one or more hydrocarbon fluid production wells. 10. The system of claim 9 wherein the separator (9) is a primary separator and wherein the liquid and gas outlets (13,14) are fluid inlet conduits for a secondary a-separator or slugcatcher which has a larger volume than the primary separator (9). 11. The system of claim 10 wherein the separators are mounted on an offshore platform (5), on the seabed (3) onshore, or downhole in an oil and/or gas production well.