Managed pressure drilling system and method

1. A method for controlling pressure in a well, comprising:

pumping fluid into a riser extending between a drilling vessel and a wellhead;

pumping fluid out of the riser to the drilling vessel by operating a first jet pump disposed in a conduit extending from the riser to the drilling vessel;

pumping gas into a power fluid inlet of a second jet pump having a working fluid inlet in communication with a working fluid outlet of the first jet pump, the second jet pump having a working fluid outlet in fluid communication with the conduit; and

wherein a rate of pumping power fluid into a power fluid inlet of the first jet pump is adjusted to maintain a liquid level in the drilling riser at a selected elevation.

2. The method of claim 1 further comprising pumping gas into a mud return line extending from a working fluid outlet of the first jet pump to the drilling vessel.

3. The method of claim 1 further comprising connecting an auxiliary line associated with the riser to a power fluid inlet of the first jet pump and pumping power fluid through the auxiliary line.

4. The method of claim 3 further comprising adjusting a rate of pumping the power fluid to maintain the liquid level at a selected elevation.

5. The method of claim 1 wherein the selected elevation corresponds to a selected equivalent circulating density.

6. The method of claim 1 wherein the selected elevation corresponds to a selected pressure in the wellbore.

7. The method of claim 1 further comprising adjusting a setting of an iris type annular pressure control device disposed in the riser in an annular space between the riser and a drill string disposed in the riser to increase back pressure on the well.

8. The method of claim 1 further comprising filtering cuttings exceeding a selected size from fluid entering a working fluid inlet of the first jet pump.

9. The method of claim 1 further comprising adjusting a rate of the pumping gas to maintain the liquid level at the selected elevation.

10. The method of claim 1 wherein the pumping fluid into the riser comprises pumping fluid into a drill string extending through the riser into the well below the bottom of the riser such that the pumped fluid exits the drill string and enters an annular space between the riser and the drill string.

11. The method of claim 1 further comprising closing an iris type annular pressure control device disposed in the riser in an annular space between the riser and a drill string disposed in the riser to maintain fluid pressure in the well during connecting and disconnecting of segments of a drill string extending through the riser.

12. The method of claim 1 further comprising inducing reverse fluid flow in the first jet pump to remove obstructions therefrom.

13. A managed pressure drilling system, comprising:

a riser extending from a subsea well to a platform on the surface of a body of water, the riser having a fluid port at a selected position above the subsea well and below the surface, the fluid port in fluid communication with a working fluid inlet of a first jet pump;

a second jet pump having a fluid inlet in fluid communication with a fluid outlet of the first jet pump, an outlet of the second jet pump in fluid communication with a fluid processing system on the platform;

a power fluid pump disposed on the platform and in fluid communication with a power fluid inlet of the first jet pump; and

a gas source disposed on the platform and in fluid communication with a power fluid inlet of the second jet pump, wherein the power fluid pump and the gas source are controllable such that a fluid level in the riser is maintained at a selected elevation.

14. The system of claim 13 further comprising a fluid level sensor in the riser.

15. The system of claim 14 wherein the fluid level sensor comprises a pressure sensor.

16. The system of claim 14 further comprising a controller in signal communication with the fluid level sensor, the controller providing control output to operate the power fluid pump and the gas source in response to signals from the fluid level sensor to maintain the fluid level at the selected elevation.

17. The system of claim 13 further comprising a drilling fluid pump disposed on the platform and connected at an outlet to a drill string extending into the riser.

18. The system of claim 13 further comprising valves connected to the power fluid inlet, the working fluid inlet and the fluid outlet of the first jet pump, the valves operable to cause fluid to flow into the fluid outlet of the first jet pump, the valves operable to bypass the first jet pump and the valves operable to direct fluid flow from the port to the working fluid inlet of the first jet pump and fluid flow from the fluid outlet of the first jet pump to the working fluid inlet of the second jet pump.

19. The system of claim 18 further comprising at least one valve connected between a riser kill line and the power fluid inlet of the first jet pump wherein power fluid to operate the first jet pump is moved through the kill line.

20. The system of claim 18 further comprising at least one valve disposed in a choke line extending from the subsea well to the fluid outlet of the first jet pump, and at least one valve disposed in a return line extending from the fluid outlet of the first jet pump to the working fluid inlet of the second jet pump, wherein the choke line is operable as a drilling fluid return line from the riser port to the platform.

21. The system of claim 13 further comprising a rock catcher and separator disposed in a fluid line connecting the port and the working fluid inlet of the first jet pump.

22. The system of claim 13 further comprising an annular control device operable to close an annular space in the riser between the riser and a pipe string disposed in the riser, wherein the power fluid pump is operable to maintain a selected pressure in the subsea well when a drilling fluid pump in pressure communication with a pipe disposed in the riser is switched off.