Apparatus and method for degassing drilling fluids

1. An apparatus for degassing drilling fluid comprising a primary separator and a secondary separator, the primary separator comprising a vessel body having an inlet port, a liquid outlet port located at a lower end of the vessel body, and a gas vent port located at an upper end of the vessel body, the secondary separator having an inlet port which is connected to the gas vent port of the primary separator, a gas outlet port, and a liquid outlet port which is connected to the interior of the vessel body of the primary separator via a liquid return line, wherein gas leaving the vessel body of the primary separator via the primary separator gas vent port enters the secondary separator via the inlet port of the secondary separator.

2. An apparatus according to claim 1 wherein the inlet port of the secondary separator is connected exclusively to the gas vent port of the primary separator.

3. An apparatus according to claim 1 wherein the liquid outlet port of the second separator is directly connected to the interior of the vessel body of the primary separator via a liquid return line.

4. An apparatus according to claim 1 wherein the inlet of the primary separator is provided with a primary diverter assembly which splits the incoming fluid into a lower density stream which is predominantly gas with entrained solids and liquids, and a higher density liquid stream which is predominantly liquid with entrained gas and solids.

5. An apparatus according to claim 1 wherein the inlet of the primary separator is located in an upper region of the vessel body.

6. An apparatus according to claim 1 wherein the primary separator is further provided with a liquid seal which is connected to the liquid outlet port.

7. An apparatus according to claim 1 wherein the primary separator is provided with a liquid level sensing system which is operable to provide an indication of the surface level of liquid collected in the bottom of the vessel body.

8. An apparatus according to claim 1 wherein the secondary separator comprises a cyclone separator which has a housing which is configured to divert the flow of a gas stream entering the housing via the inlet of the cyclone separator so that the gas stream it flows in a generally circular flow path around a vortex chamber within the housing.

9. An apparatus according to claim 8 wherein the inlet and gas outlet port of the cyclone separator are arranged along a longitudinal axis of the secondary separator, whilst the liquid outlet port is provided in a portion of the housing which extends generally parallel to the longitudinal axis of the secondary separator.

10. An apparatus according to claim 1 wherein the liquid return line extends down the interior of the vessel body to end at a point in the lowermost portion of the vessel body.

11. A drilling system including an apparatus for degassing drilling fluid according to claim 1 wherein the drilling system includes a riser which extends between an subsea blowout preventer mounted on a wellhead at the top of a wellbore, and a diverter which has a returns port which is connected to the inlet of the primary separator via a returns line, the returns line containing a diversion flow valve which is operable to permit or substantially prevent flow of fluid from the returns line to the primary separator.

12. A drilling system according to claim 11 further comprising a drilling fluid reservoir, wherein the liquid outlet port of the primary separator is connected to the drilling fluid reservoir via a liquid seal.

13. A drilling system according to claim 12 wherein a liquid level control valve is provided in a line between the liquid seal and the drilling fluid reservoir, the liquid level control valve being operable to stop, or restrict to a variable extent, flow of fluid from the liquid outlet of the primary separator to the drilling fluid reservoir.

14. A drilling system according to claim 11 wherein the drilling system also includes an overboard diversion line which extends from the returns line to a discharge point where riser fluid may be discharged relatively safely, and an overboard diverter valve is provided in the overboard diversion line, the overboard diverter valve being operable to either permit or substantially prevent flow of fluid along the overboard diversion line.

15. A drilling system according to claim 11 wherein a further overboard diversion line extends from the liquid seal to a further discharge point wherein fluid from the primary separator may be discharged relatively safely, and a further overboard discharge valve is provided in the further overboard diversion line, the further overboard discharge valve being operable to either permit or substantially prevent flow of fluid along the further overboard diversion line.

16. A drilling system according to claim 11 wherein the returns line is also connected to the drilling fluid reservoir via a main diverter flow line, the main diverter flow line including a diverter valve which is operable to either permit or substantially prevent flow of fluid from the returns line to the drilling fluid reservoir.

17. A drilling system according to claim 11 wherein the drilling system further includes a rig choke and kill manifold which is connected to a wellbore annulus below the subsea BOP via a well control choke line, the choke and kill manifold being, in turn, connected to the inlet of the primary separator via a choke valve which is operable to either permit or substantially prevent flow of fluid from the choke and kill manifold to the inlet of the primary separator.

18. A method of operating a drilling system including a separator comprising a vessel body having an inlet port, a liquid outlet port located at a lower end of the vessel body, a liquid level control valve operable to control or prevent flow of fluid out of the vessel body through the liquid outlet port, a gas vent port located at an upper end of the vessel body, and a liquid level sensing system that measures a surface level of liquid collected in the bottom of the vessel body and provides an output representing the surface level, the method including monitoring the output from the liquid level sensing system to monitor the rate of change in liquid level in the vessel body over time to determine a flow rate trend over time.

19. The method according to claim 18 wherein the method further comprises closing the liquid level control valve prior to monitoring the output from the liquid level sensing system.

20. The method according to claim 18 wherein the method further comprises using the liquid level control valve to regulate automatically the liquid level in the separator.

21. The method according to claim 18 wherein the method further includes initiating an alert procedure if the liquid level sensing system detects that the liquid level has exceeded a predetermined level.

22. A method according to claim 18 wherein the drilling system is also provided with a drilling fluid reservoir and the liquid outlet port of the separator is connected to the drilling fluid reservoir via a liquid seal, the liquid level control valve being provided in a line between the liquid seal and the drilling fluid reservoir, and being operable to stop, or restrict to a variable extent, flow of fluid from the liquid outlet of the primary separator to the drilling fluid reservoir.

23. A method according to claim 18 wherein the inlet of the separator is provided with a primary diverter assembly which splits the incoming fluid into a lower density stream which is predominantly gas with entrained solids and liquids, and a higher density liquid stream which is predominantly liquid with entrained gas and solids.