Pull-down method and equipment for installing well casing

1. An apparatus for performing well construction operations with a drilling rig having a top drive, comprising:

a pipe gripper having a mandrel with an upper end for connection to and rotation with a drive string extending downward from the top drive, the pipe gripper having gripping elements that move radially into engagement with a string of pipe;

a pull-down mechanism adapted to be mounted to the rig and secured to a non-rotating portion of the pipe gripper for selectively exerting a downward force on the mandrel, wherein the pull-down mechanism comprises a pair of winches, each adapted to be mounted to the rig below a rig floor; and

a cable wrapped around each of the winches, each of the cables adapted to pass through a hole or respective holes in the rig floor into operative engagement with opposite sides of the non-rotating portion of the pipe gripper.

2. The apparatus according to claim 1 , further comprising:

a sensor adapted to be operatively coupled to the top drive to sense weight being supported by the top drive; and

a controller linked to the sensor and the pull-down mechanism for controlling the downward force exerted on the mandrel by the pull-down mechanism in response to the weight sensed by the sensor.

3. The apparatus according to claim 1 , wherein the drive string is a quill of the top drive.

4. The apparatus according to claim 1 , wherein the string of pipe comprises a drill bit attached to a lower end of the string of pipe.

5. The apparatus according to claim 1 , wherein gripping elements comprise grapples.

6. The apparatus according to claim 1 , wherein the top drive comprises an anti-rotation mechanism.

7. The apparatus according to claim 6 , wherein the anti-rotation mechanism is slidably coupled with one or more guide rails.

8. An apparatus for performing well construction operations with a drilling rig having a top drive, comprising:

a pipe gripper having a mandrel with an upper end for connection to and rotation with a drive string extending downward from the top drive, the pipe gripper having gripping elements that move radially into engagement with a string of pipe;

a pull-down mechanism adapted to be mounted to the rig and secured to a non-rotating portion of the pipe gripper for selectively exerting a downward force on the mandrel;

a sensor adapted to be coupled into the drive string to sense axial forces in the drive string between the top drive and the pipe gripper; and

a controller linked to the sensor and the pull-down mechanism for causing the pull-down mechanism to exert a downward force to the non-rotating portion of the pipe gripper if the axial forces sensed become compressive while the string of pipe is being lowered into the well.

9. The apparatus according to claim 8 , wherein:

the pull-down mechanism comprises at least one winch; and

a cable wrapped around the winch and in operative engagement with the non-rotating portion of the pipe gripper.

10. An apparatus for performing well construction operations with a drilling rig having a top drive, comprising:

a pipe gripper having a mandrel with an upper end for connection to and rotation with a drive string extending downward from the top drive, the pipe gripper having gripping elements that move radially into engagement with a string of pipe;

a pull-down mechanism adapted to be mounted to the rig and secured to a non-rotating portion of the pipe gripper for selectively exerting a downward force on the mandrel;

a sensor adapted to be coupled into the drive string to sense tensile forces in the drive string between the top drive and the pipe gripper; and

a controller linked to the sensor and the pull-down mechanism for controlling the pull-down mechanism, the controller being configured to cause the pull-down mechanism to exert a downward force to maintain a substantially constant tensile force in the drive string.

11. The apparatus according to claim 10 , wherein:

the pull-down mechanism comprises a pair of winches; and

a cable wrapped around each of the winches, the cables being in operative engagement with opposite sides of the non-rotating portion of the pipe gripper.

12. An apparatus for performing well construction operations with a drilling rig having a top drive, comprising:

a pipe gripper having a mandrel with an upper end for connection to and rotation with a drive string extending downward from the top drive, the pipe gripper having gripping elements that move radially into engagement with a string of pipe;

a pull-down mechanism adapted to be mounted to the rig and secured to a non-rotating portion of the pipe gripper for selectively exerting a downward force on the mandrel, wherein the non-rotating portion of the pipe gripper comprises a frame of the pipe gripper; and

wherein the pipe gripper further comprises:

a thrust bearing mounted between the mandrel and the frame; and

the pull-down mechanism is connected to the frame, such that the downward force imposed by the pull-down mechanism transfers through the thrust bearing to the mandrel and through the gripping elements to the string of pipe.

13. An apparatus for performing well construction operations with a drilling rig having a top drive, comprising:

a pipe gripper having a mandrel with an upper end for connection to and rotation with a drive string extending downward from the top drive, the pipe gripper having gripping elements that move radially into engagement with a string of pipe;

a pull-down mechanism adapted to be mounted to the rig and secured to a non-rotating portion of the pipe gripper for selectively exerting a downward force on the mandrel, wherein:

the non rotating portion of the pipe gripper comprises a bracket;

the pull-down mechanism has portions secured to opposite sides of the bracket; and

the apparatus further comprises:

a pair of links having upper ends pivotally secured to the opposite sides of the bracket; and

an elevator supported by lower ends of the links.

14. A method for performing well construction operations with a drilling rig having a top drive, comprising:

securing a mandrel of a pipe gripper to a drive string extending downward from the top drive;

moving gripping elements of the pipe gripper radially into engagement with a string of pipe;

lowering the string of pipe into the well;

with a pull-down mechanism, exerting a downward force on the mandrel;

sensing axial forces in the drive string between the top drive and the pipe gripper; and

causing the pull-down mechanism to exert a downward force to the non-rotating portion of the pipe gripper if the axial forces sensed become compressive while the string of pipe is being lowered into the well.

15. The method according to claim 14 , further comprising:

sensing weight being supported by the top drive; and

controlling the downward force exerted on the mandrel by the pull-down mechanism in response to the weight sensed.

16. The method according to claim 14 , further comprising:

rotating a drill bit attached to a lower end of the string of pipe as the string of pipe is being lowered.

17. The method according to claim 14 , wherein:

the pull-down mechanism is controlled so as to maintain a constant rate of descent of the string of pipe.

18. A method for performing well construction operations with a drilling rig having a top drive, comprising:

securing a mandrel of a pipe gripper to a drive string extending downward from the top drive;

moving gripping elements of the pipe gripper radially into engagement with a string of pipe;

lowering the string of pipe into the well;

with a pull-down mechanism, exerting a downward force on the mandrel;

sensing tensile forces in the drive string between the top drive and the pipe gripper; and

causing the pull-down mechanism to exert a downward force to maintain a substantially constant tensile force in the drive string.

19. The apparatus according to claim 18 , wherein causing the pull-down mechanism to exert the downward force comprises actuating a pair of winches.