Steering system for drill string

1. A system for forming a directional borehole, comprising:

a rotary steerable system having a plurality of actuator pistons slidably mounted in a mechanical structure, the mechanical structure comprising:

a radially inward portion containing ports to deliver actuating mud to the plurality of actuator pistons;

a radially outward portion; and

a piston chamber portion extending radially between the radially inward portion and the radially outward portion, the piston chamber portion having piston passages for receiving the plurality of actuator pistons, the mechanical structure defining a main flow passage extending longitudinally through the rotary steerable system between the radially inward portion and the radially outward portion for directing a flow of actuating mud to a drill bit, the piston passages extending radially inward of the main flow passage and into the radially inward portion in a radial plane perpendicular to a centerline of the mechanical structure intersecting the piston passages and the main flow passage.

2. The system as recited in claim 1 , further comprising a valve system positioned radially inward of the main flow passage.

3. The system as recited in claim 1 , wherein the actuator pistons act against corresponding steering pads.

4. The system as recited in claim 3 , wherein the rotary steerable system further comprises cam followers located between the actuator pistons and the corresponding steering pads.

5. The system as recited in claim 1 , wherein the actuator pistons have circular cross-sections.

6. The system as recited in claim 1 , wherein the actuator pistons have non-circular cross-sections.

7. The system as recited in claim 1 , wherein the rotary steerable system is connected into a drill string.

8. The system as recited in claim 1 , wherein the plurality of actuator pistons comprises three actuator pistons.

9. The system as recited in claim 1 , wherein at least a portion of each actuator piston reciprocates to a position radially inward of the main flow passage.

10. A method, comprising:

providing a rotary steerable system with a mechanical structure having a main flow passage for delivering drilling mud to a drill bit, the main flow passage being located radially outward of a radially inward portion of the mechanical structure;

positioning the rotary steerable system in a drill string;

routing drilling mud through the drill string including through the main flow passage;

using the drilling mud to operate actuator pistons of the rotary steerable system via valve controlled ports located at a radially inward position relative to the main flow passage and within the radially inward portion of the mechanical structure; and

directing a remainder of the drilling mud past the rotary steerable system via the main flow passage, wherein in a radial plane perpendicular to a centerline of the mechanical structure intersecting the main flow passage and the valve controlled ports, the valve controlled ports are located radially inward of the main flow passage.

11. The method as recited in claim 10 , further comprising locating a corresponding valve system in the radially inward portion of the mechanical structure.

12. The method as recited in claim 11 , further comprising mounting the actuator pistons in corresponding piston passages oriented radially in the mechanical structure.

13. The method as recited in claim 12 , further comprising actuating the actuator pistons to selectively move a plurality of corresponding steering pads.

14. The method as recited in claim 13 , further comprising using cam followers between the actuator pistons and the corresponding steering pads.

15. The method as recited in claim 13 , further comprising pivotably coupling each corresponding steering pad to the mechanical structure of the rotary steerable system.

16. The method as recited in claim 10 , wherein using comprises reciprocating the actuator pistons such that a portion of each actuator piston moves to a position radially inward of the main flow passage.

17. A system for steering during drilling, comprising:

a drill string having a rotary steerable system, the drill string being constructed to enable a flow of drilling mud therethrough, the rotary steerable system comprising:

a plurality of actuator pistons operated within corresponding piston passages by the flow of drilling mud;

a plurality of ports located at a radially inward position within the rotary steerable system, the ports of the plurality of ports being positioned to direct flow of the drilling mud to corresponding actuator pistons of the plurality of actuator pistons; and

a mechanical structure having, in a radial plane perpendicular to a centerline of the rotary steerable system, a main flow passage located radially outward of the plurality of ports and radially outward of radially inward portions of the corresponding piston passages, the main flow passage directing a remainder of the drilling mud past the rotary steerable system.

18. The system as recited in claim 17 , wherein the rotary steerable system further comprises a corresponding valve system to control flow of the drilling mud to selected ports of the plurality of ports, the plurality of ports and the corresponding valve system being located in a radially inward portion of the mechanical structure.

19. The system as recited in claim 18 , wherein the actuator pistons move within corresponding piston passages oriented radially in the mechanical structure.

20. The system as recited in claim 19 , wherein the rotary steerable system further comprises a plurality of corresponding steering pads, the actuator pistons being actuated to move selected, corresponding steering pads.