Offshore drilling system

• US 6,325,159 B1
• Filed: 03/25/1999
• Issued: 12/04/2001
• Est. Priority Date: 03/27/1998
• Status: Expired due to Term

• Current Assignee: Hydril USA Manufacturing LLC (GE Aerospace)
• Original Assignee: Hydril Company L P (General Electric Company)
• Inventors: Danna, John Mark, Mott, Keith C., Goldsmith, Riley G., Peterman, Charles P., Pelata, Kenneth L., Colvin, Kenneth W.
• Primary Examiner(s): SCHOEPPEL, ROGER J
• Application Number:
  US09/276,404
• Time in Patent Office: 985 Days
• Field of Search: 166/350, 166/359, 166/367, 166/374, 175/5, 175/7, 175/213, 175/214, 175/217
• US Class Current: 175/7
• CPC Class Codes: E21B 21/001 specially adapted for underwater drilling dual gradient drilling E21B21/082
• CPC Class Codes: E21B 21/01 Arrangements for handling drilling fluids or cuttings outside the borehole, e.g. mud boxes
• CPC Class Codes: E21B 21/08 Controlling or monitoring pressure or flow of drilling fluid, e.g. automatic filling of boreholes, automatic control of bottom pressure valve arrangements therefor E21B21/10
• CPC Class Codes: E21B 21/085 Underbalanced techniques, i.e. where borehole fluid pressure is below formation pressure
• CPC Class Codes: E21B 33/085 Rotatable packing means, e.g. rotating blow-out preventers
• CPC Class Codes: F04B 19/003 free-piston type pumps
• CPC Class Codes: F04B 43/06 Pumps having fluid drive
• CPC Class Codes: Y10T 137/86027 Electric
• CPC Class Codes: Y10T 137/86059 Hydraulic power unit

Abstract

A system for drilling a subsea well from a rig through a subsea wellhead below the rig includes a wellhead stack which is mounted on the subsea wellhead. The wellhead stack includes at least a subsea blowout preventer stack and a subsea diverter. A drill string extends from the rig through the wellhead stack into the well to conduct drilling fluid from the rig to a drill bit in the well. A riser which has one end coupled to the wellhead stack and another end coupled to the rig internally receives the drill string such that a riser annulus is defined between the drill string and the riser. A well annulus extends from the bottom of the well to the subsea diverter to conduct fluid away from the drill bit. A pump has a suction side in communication with the well annulus and a discharge side in communication with the rig and is operable to maintain a selected pressure gradient in the well annulus.

Claims

1. A system for drilling a subsea well from a rig through a subsea wellhead below the rig, comprising:

a wellhead stack mounted on the subsea wellhead, the wellhead stack comprising at least a subsea blowout preventer stack and a subsea diverter;

a drill string extending from the rig through the wellhead stack into the well, the drill string for conducting drilling fluid from the rig to a drill bit in the well;

a riser having one end coupled to the wellhead stack and another end coupled to the rig, the riser internally receiving the drill string such that a riser annulus is defined between the drill string and the riser;

a well annulus extending from the bottom of the well to the subsea diverter, the well annulus being separated from the riser annulus by the subsea diverter and adapted to conduct fluid away from the drill bit; and

a pump having a suction side in communication with the well annulus and a discharge side in communication with the rig, the pump being operable such that a selected pressure gradient is maintained in the well annulus, the pump comprising a first chamber in communication with the well annulus, the first chamber being provided to selectively receive fluid from and dispense fluid to the well annulus wherein the first chamber is defined in a vessel having a second chamber defined therein and a movable member disposed between the first and second chambers, the movable member being arranged to move within the vessel in response to pressure differential between the first and second chambers.

2. The system of claim 1, wherein the riser is filled with seawater.

3. The system of claim 1, wherein the pressure of the fluid flowing out of the well annulus is maintained at ambient seawater pressure.

4. The system of claim 1, wherein the first chamber is defined in the riser.

5. The system of claim 1, wherein pumping rate of the pump is controlled to maintain a predetermined amount of fluid in the first chamber such that the selected pressure gradient is maintained in the well annulus.

6. The system of claim 3, further comprising a boost pump for boosting the pressure of the fluid flowing into the suction side of the pump.

7. The system of claim 1, wherein pumping rate of the pump is controlled to maintain the movable member at a pre-selected position in the vessel.

8. The system of claim 7, wherein the pre-selected position corresponds to a condition when the pressures in the first and second chambers are substantially equal to the ambient seawater pressure.

9. The system of claim 7, wherein the pre-selected position corresponds to a condition when a selected pressure differential exists between the well annulus and the surrounding seawater.

10. The system of claim 1, further comprising a pressure sensor for monitoring pressure in the first chamber and a valve for preventing fluid flow from the well annulus to the first chamber when the pressure measured by the pressure sensor exceeds the pressure rating of the vessel.

11. The system of claim 1, wherein the first chamber is connected to receive fluid from a fluid source on the rig through a valve.

12. The system of claim 1, further comprising a device for controlling size of solid particles in the fluid flowing from the well annulus to the suction side of the pump.

13. The system of claim 12, wherein the device for controlling size of solid particles includes a rock crusher having rotating blades for crushing solid particles.

14. The system of claim 12, wherein the device for controlling size of solid particles comprises:

a housing having a port hydraulically connected to the suction side of the pump; and

a barrel disposed in the housing, the barrel having a bore hydraulically connected to the well annulus and a plurality of holes in fluid communication with the port, wherein solid particles having sizes larger than the holes are prevented from passing through the holes to the port.

15. The system of claim 1, further comprising a pressure-actuated valve disposed in the drill string for preventing drilling fluid from free-falling from the drill string into the well.

16. The system of claim 15, wherein the pressure-actuated valve comprises:

an elongated body having a bore running therethrough;

a flow nozzle disposed in the bore, the flow nozzle having at least one port for fluid communication between the drill string and the drill bit;

a flow cone interposed between the body and the flow nozzle, the flow cone being movable between an open position to permit fluid flow from the drill string to the port and a closed position to prevent fluid flow from the drill string to the port;

an orifice in the body for communicating pressure in the well annulus to the bore; and

a biasing mechanism for normally urging the flow cone to the closed position;

wherein the flow cone moves from the closed position to the open position when the pressure of the fluid pumped through the drill string reaches a predetermined pressure and returns to the closed position when the pressure of the fluid pumped through the drill string falls below the predetermined pressure.

17. The system of claim 1, wherein the pump is a positive-displacement pump.

18. The system of claim 1, further comprising at least one choke/kill line for fluid communication between the well annulus and the rig.

19. The system of claim 18, wherein the choke/kill line hydraulically connects the discharge side of the pump to the rig.

20. The system of claim 19, wherein the choke/kill line is hydraulically connected to the suction side of the pump through a valve and choke.

21. A system for drilling a subsea well from a rig through a subsea wellhead below the rig, comprising:

a wellhead stack mounted on the subsea wellhead, the wellhead stack comprising at least a subsea blowout preventer stack and a subsea diverter;

a drill string extending from the rig through the wellhead stack into the well, the drill string for conducting drilling fluid from the rig to a drill bit in the well;

a well annulus extending from the bottom of the well to the subsea diverter, the well annulus for conducting fluid away from the drill bit; and

a positive-displacement pump having a suction side in communication with the well annulus and a discharge side in communication with the rig, the pump being operable such that a selected pressure gradient is maintained in the well annulus, the pump comprising a first chamber in communication with the well annulus, the first chamber being provided to selectively receive fluid from and dispense fluid to the well annulus wherein the first chamber is defined in a vessel having a second chamber defined therein and a movable member disposed between the first and second chambers, the movable member being arranged to move within the vessel in response to pressure differential between the first and second chambers.

22. The system of claim 21, wherein a return line system for conducting fluid from a discharge end of the pump to the rig comprises:

a connector assembly affixed to the seafloor;

a return line extending from the connector assembly toward the rig;

a buoy coupled to the return line to keep the return line substantially vertical;

a first umbilical hydraulically connecting the return line to the rig; and

a second umbilical hydraulically connecting the return line to the discharge end of the pump.

23. A system for drilling a subsea well from a rig through a subsea wellhead below the rig, comprising:

a subsea blowout preventer stack having a first end coupled to the subsea wellhead;

a drill string extending from the rig through the subsea blowout preventer stack and wellhead into the well, the drill string for conducting drilling fluid from the rig to a drill bit in the well;

a rotating subsea diverter coupled to a second end of the subsea blowout preventer stack and adapted to slidingly receive and sealingly engage the drill string;

a well annulus extending from the bottom of the well to the rotating subsea diverter, the well annulus for conducting fluid away from the drill bit; and

a pump having a suction side in communication with the well annulus and a discharge side in communication with the rig, the pump being operable such that a selected pressure gradient is maintained in the well annulus, the pump comprising a device coupled to an inlet side thereof to control a size of solid particles entering the pump.

24. The system of claim 23, further comprising a pressure-actuated valve arranged in the drill string to prevent drilling fluid from free-falling from the drill string into the well.