System for controlling the discharge of drilling fluid

1. A drilling system for drilling a borehole into an earth formation, the drilling system comprising pump means for pumping drilling fluid into the borehole and discharge means for discharging drilling fluid from the borehole, wherein the discharge means comprises two or more pressure chambers arranged to be alternately filled with the drilling fluid from the borehole for temporarily accommodating drilling fluid being discharged from the borehole, and control means for controlling the fluid inflow into each pressure chamber, wherein the pressure chambers contain an expel fluid which is arranged to expel from a first one of the pressure chambers by the inflow of drilling fluid in that pressure chamber, into another of the pressure chambers, and wherein the control means is arranged to control the outflow of expel fluid from the first pressure chamber.

2. The drilling system of claim 1 , wherein said control means is arranged to control the fluid pressure in the first pressure chamber.

3. The drilling system of claim 1 , wherein the control means comprises a controllable throttle opening.

4. The drilling system of claim 1 , wherein the control means comprises a throttle opening.

5. The drilling system of claim 1 , wherein the first and the other said pressure chambers are interconnected by an expel fluid conduit for transporting the expel fluid between the pressure chambers.

6. The drilling system of claim 5 , wherein the control means comprises a control valve in the expel fluid conduit.

7. The drilling system of claim 6 , wherein the control valve is a choke valve for controlling the resistance in the expel fluid conduit.

8. The drilling system of claim 5 , wherein each pressure chamber is provided with first and second compartments separated by a movable wall, whereby one of the compartments is filled with drilling fluid and the other compartment contains said expel fluid.

9. The drilling system of claim 8 , wherein said movable wall includes a flexible membrane.

10. The drilling system of claim 8 , wherein the compartments for expel fluid are interconnected by the expel fluid conduit for transporting the expel fluid between said compartments, and wherein said compartments together with the expel fluid conduit and the control means form a closed system.

11. The drilling system of claim 8 , wherein the system further comprises inlet valve means to direct the drilling fluid to be discharged alternately to one of said drilling fluid compartments and with outlet valve means to remove drilling fluid from the other drilling fluid compartment.

12. A method of drilling a borehole into an earth formation, comprising pumping a drilling fluid into the borehole and discharging the drilling fluid from the borehole and transporting the discharged drilling fluid to two or more pressure chambers, alternately filling the two or more pressure chambers with the drilling fluid from the borehole whereby an expel fluid is expelled from a first one of the pressure chambers by the inflow of drilling fluid in that pressure chamber, into another of the two or more pressure chambers, and controlling the inflow of drilling fluid into the first pressure chamber by controlling the outflow of expel fluid which is expelled from the first pressure chamber by the inflow of drilling fluid in that pressure chamber.

13. The method of claim 12 , whereby said inflow of drilling fluid is controlled by controlling the fluid pressure in the first pressure chamber.

14. The method of claim 12 , further comprising transporting the expel fluid from first pressure chamber to the other pressure chamber and controlling the transporting by a control valve.

15. The method of claim 12 , further comprising controlling the inflow of drilling fluid into each of the pressure chambers.

16. The method of claim 12 , whereby each pressure chamber is provided with a membrane to form a drilling fluid compartment and an expel fluid compartment, both having a variable content, the expel fluid compartments being interconnected by the expel fluid conduit, the method further comprising alternately directing the drilling fluid to be discharged to one of said drilling fluid compartments, while drilling fluid from the other drilling fluid compartment is removed.

17. The method of claim 12 , comprising controlling the discharge of the drilling fluid from the borehole by throttling the expel fluid.

18. The method of claim 12 , further comprising directing the expel fluid which is expelled from the first pressure chamber through a throttle opening.

19. The method of claim 18 , comprising controlling the throttle opening, thereby controlling the inflow of the drilling fluid into the first pressure chamber.