Continuous mud circulation during drilling operations

1. A method for continuous mud circulation during a drilling operation, comprising:

delivering a first flow of drilling mud from a mud supply, through a drilling device and a drill string, into a well bore, and through a blowout preventer, wherein the drill string is received through the blowout preventer;

delivering a second flow of drilling mud into the blowout preventer and into the well bore, wherein the second flow is not delivered through the drilling device;

stopping the first flow; and

removing or adding a tubular to or from the drill string when the first flow is stopped and while continuing to deliver the second flow, wherein removing or adding the tubular to or from the drill string comprises preventing the drill string from rotating using a tubular lock of the blowout preventer.

2. The method of claim 1 , wherein, while delivering the first flow, the first flow of drilling mud is received from a rotating control device coupled to the blowout preventer and into a first flow line connected to the rotating control device, the rotating control device being configured to seal and rotate with the tubular.

3. The method of claim 2 , wherein, while delivering the second flow, the second flow is delivered into the blowout preventer and received out of the blowout preventer into a second flow line connected to the blowout preventer.

4. The method of claim 3 , wherein the rotating control device at least partially defines a first chamber above the well, and the blowout preventer at least partially defines a second chamber above the well, wherein the first flow line is in communication with the first chamber, and the second flow line is in the communication with the second chamber, the method further comprising:

receiving fluid from the first chamber into the first flow line, while delivering the first flow; and

receiving fluid from the second chamber into the second flow line, while delivering the second flow.

5. The method of claim 4 , further comprising preventing the second chamber from communicating with the first chamber while delivering the second flow.

6. The method of claim 4 , further comprising supporting the tubular string below the first and second chambers and in the wellbore.

7. The method of claim 1 , further comprising:

after removing or adding the tubular:

stopping the second flow; and

again delivering the first flow at least while the second flow is stopped.

8. The method of claim 1 , wherein delivering the first flow further comprises delivering the first flow to the drill string via a drilling device coupled to the drill string, wherein the second flow is not circulated through the drilling device.

9. The method of claim 8 , wherein, at least partially during adding or removing the tubular, the tubular and the drilling device are disconnected from one another, or the tubular and the drill string are disconnected from one another, or both.

10. The method of claim 9 , further comprising:

moving a lower connection of the tubular below a rotating control device coupled to the blowout preventer and configured to seal with and rotate with the tubular, and above an annular seal of the blowout preventer;

closing the annular seal;

bleeding pressure inside the rotating control device; and

removing the tubular out of the rotating control device.

11. The method of claim 8 , further comprising positioning the drilling device, while delivering the second flow, such that a connection between a shaft of the drilling device and the tubular is above a seal of a rotating control device through which the drill string is received, the rotating control device being configured to seal with and rotate with the tubular.

12. The method of claim 11 , further comprising:

positioning a connection between the tubular and a subjacent tubular of the drill string below the rotating control device and below an annular seal and above a pipe ram of the blowout preventer; and

breaking the connection between the tubular and the subjacent tubular while the connection is positioned below the rotating control device.

13. The method of claim 12 , further comprising:

applying a first layer of grease to threads of a lower connection of the tubular, and applying a second layer of grease over the first layer of grease; and

lowering the tubular at least partially into the blowout preventer and into engagement with the drill string after applying the first and second layers of grease.

14. The method of claim 12 , wherein removing or adding the tubular comprises adding the tubular, and wherein adding the tubular comprises connecting the tubular to the drilling device, the method further comprising:

connecting a lower end of the tubular to a drill string supported in a blowout preventer, while delivering the first flow;

lowering the tubular partially through the blowout preventer;

supporting the tubular, attached to the drill string, in the blowout preventer;

disconnecting the drilling device from the tubular when the tubular is supported in the blowout preventer; and

raising the drilling device relative to the tubular, to accept another tubular.

15. The method of claim 14 , further comprising applying a first layer of grease to threads of the lower end of the tubular, and applying a second layer of grease over the first layer of grease, prior to connecting the lower end of the tubular to the drill string, wherein the first layer of grease has a higher viscosity than the second layer of grease.

16. The method of claim 1 , further comprising raising an annular seal of a rotating control device away from the blowout preventer when raising the drilling device, wherein the annular seal is configured to rotate with and form a seal with a shaft of the drilling device, and wherein at least a portion of the rotating control device coupled to the blowout preventer.

17. A system for drilling a wellbore, comprising:

a blowout preventer configured to be disposed above a wellbore, wherein the blowout preventer is configured to receive a drill string therethrough;

a rotating control device coupled to the blowout preventer, such that the blowout preventer is configured to be positioned between the wellbore and the rotating control device, wherein the rotating control device is configured to receive the drill string therethrough;

a drilling device configured to rotate the drill string and lower the drill string through the blowout preventer and the rotating control device, wherein the drilling device has a conduit that is configured to communicate with an inner bore of the drill string;

a first mud supply line fluidly coupled to the drilling device, so as to deliver a first mud flow into the drill string via the drilling device; and

a second mud supply line coupled to the blowout preventer, so as to deliver a second mud flow thereto, wherein the second mud flow does not extend through the drilling device,

wherein the blowout preventer comprises a tubular lock configured to engage the drill string and prevent the drill string from rotating relative to the wellbore, so as to permit forming or breaking a connection between the drilling device and the drill string within the blowout preventer.

18. The system of claim 17 , wherein the first and second mud supply lines are coupled together, wherein the system further comprises a first valve coupled to the first mud supply line, and a second valve coupled to the second mud supply line, and wherein the first and second valves are configured to selectively permit and block flow through the first and second mud supply lines, respectively.

19. The system of claim 17 , further comprising:

a first flow line coupled to a first sealable chamber defined at least partially by the rotating control device and configured to receive the first mud flow therefrom; and

a second flow line coupled to a second sealable chamber defined at least partially by the blowout preventer and configured to receive the second mud flow therefrom.

20. The system of claim 19 , further comprising a choke for managed pressure drilling, wherein the first flow line and the second flow line deliver the first and second mud flows, respectively, to the choke.

21. The system of claim 20 , further comprising a third valve coupled to the first flow line, and a fourth valve coupled to the second flow line.

22. The system of claim 20 , further comprising a pressure bleed line coupled to the rotating control device, and a fifth valve coupled to the pressure bleed line.

23. The system of claim 20 , wherein the blowout preventer further comprises a pipe ram configured to support the drill string, wherein the second flow line is connected to the blowout preventer between the pipe ram and the tubular lock.

24. The system of claim 17 , wherein the drilling device comprises a top drive.

25. The system of claim 17 , wherein the rotating control device comprises an annular seal that is configured to rotate with and form a seal with a shaft of the drilling device, and wherein the annular seal is configured to be lifted away from the blowout preventer.