Rotating continuous flow sub

1. A method for drilling a wellbore, comprising:

drilling the wellbore by rotating a tubular string using a top drive and advancing the tubular string longitudinally into the wellbore;

rotationally unlocking an upper portion of the tubular string having a side port from a rest of the tubular string;

adding a tubular joint or stand of joints to the upper portion while injecting drilling fluid into the side port and rotating the rest of the tubular string using a rotary table;

rotationally locking the upper portion to the rest of the tubular string after adding the tubular joint or stand of joints; and

resuming drilling of the wellbore after rotationally locking the upper portion.

2. The method of claim 1 , further comprising:

opening the side port before injecting drilling fluid into the side port; and

stopping injection of drilling fluid into the side port after adding the tubular joint or stand of joints; and

closing the side port after stopping injection of drilling fluid into the side port.

3. The method of claim 2 , further comprising:

engaging the tubular string with a clamp before opening the side port; and

disengaging the clamp from the tubular string after closing the side port.

4. The method of claim 3 , wherein the side port is opened and closed by operating an electric, pneumatic, or hydraulic actuator.

5. The method of claim 4 , wherein:

the actuator is part of the tubular string, and

the clamp provides electrical, hydraulic, or pneumatic power to the actuator.

6. The method of claim 5 , wherein the actuator opens and closes the side port by longitudinally moving an internal sleeve of the tubular string to expose and cover the side port.

7. The method of claim 1 , further comprising stopping drilling by holding the tubular string longitudinally stationary.

8. The method of claim 1 , wherein rotation of the rest of the tubular string is at a substantially reduced angular velocity.

9. The method of claim 1 , further comprising:

closing a portion of a bore of the tubular string between the side port and a top of the tubular string before adding the tubular joint or stand of joints; and

opening the bore portion after adding the tubular joint or stand of joints.

10. The method of claim 1 , further comprising stopping rotation of the tubular string before adding the tubular joint or stand of joints.

11. The method of claim 10 , further comprising halting rotation of the rest of the tubular string before locking.

12. A continuous flow sub (CFS) for use with a drill string, comprising:

a tubular housing having a central longitudinal bore therethrough and a port formed through a wall thereof and in fluid communication with the central bore;

a bore valve operable between an open position and a closed position, wherein the bore valve allows free passage through the central bore in the open position and isolates an upper portion of the central bore from a lower portion of the central bore in the closed position;

a port valve having a sleeve longitudinally movable relative to the housing between an open position for allowing flow through the port and a closed position for covering the port; and

a locking swivel for longitudinally and torsionally connecting the housing to the drill string in a locked position and for allowing rotation of the drill string relative to the housing in an unlocked position while sealing an interface between the drill string and the housing.

13. The CFS of claim 12 , wherein the locking swivel comprises:

an upper housing having one or more lugs extending from an outer surface thereof;

a lock ring disposed around an outer surface of the upper housing, longitudinally movable relative thereto, and having a key for each lug;

a fastener for connecting the lock ring to the upper housing in the locked position; and

a lower housing having: a keyway for receiving a respective lug, a shoulder for engaging the respective lug once the lug has been inserted into the keyway and rotated relative to the lower housing, and a seal mandrel for engagement with an inner surface of the upper housing.

14. The CFS of claim 13 , wherein the locking swivel further comprises a seal carried by the lower housing for sealing an interface between the upper and lower housings and a bearing for accommodating the rotation of the lower housing relative to the upper housing.

15. A continuous flow system, further comprising:

the CFS of claim 12 ; and

a clamp for operation of the port valve between the positions and having an inlet for injecting fluid into the housing port.