Downhole motor lock-up tool

1. Downhole apparatus for limiting rotation of a rotor for transmitting torque to a downhole tool relative to a stator associated with the said rotor, the downhole apparatus comprising a body within a bore of which a locking member is located so as to be movable between a first axial position relative to the body, in which the locking member is disengaged from a rotor so as to allow rotation of said rotor relative to said locking member, and a second axial position relative to the body, in which the locking member is engaged with said rotor so as to limit rotation of said rotor relative to said locking member, the apparatus further comprising means for limiting rotational movement of the locking member relative to the body when said locking member is located in said second axial position, wherein said locking member is movable from said first axial position to said second axial position by the application of a static fluid pressure to a differential area of said locking member, and means for selectively applying static fluid pressure to said differential area of said locking member.

2. Downhole apparatus as claimed in claim 1 , wherein said selective means for applying static fluid pressure comprises means for permitting intermediate movement of said locking member from said first axial position to an intermediate position relative to the body wherein the area of locking member exposed to said static fluid pressure is different to that exposed to said static fluid pressure when said locking member is located in said first axial position.

3. Downhole apparatus as claimed in claim 2 , wherein said body comprises two portions movable relative to one another so as to permit said intermediate movement of said locking member relative to one of said body portions.

4. Downhole apparatus as claimed in claim 3 , wherein said body portions are axially movable relative to one another.

5. Downhole apparatus as claimed in claim 3 , wherein said locking member is selectively retained in said first axial position relative to said one body portion by frangible means.

6. Downhole apparatus as claimed in claim 3 , wherein said locking member is selectively retained in said first axial position and said intermediate position relative to the other one of said body portions.

7. Downhole apparatus as claimed in claim 3 , wherein said two body portions are screw threadedly connected to one another.

8. Downhole apparatus as claimed in claim 7 , wherein the screw threaded connection between said two body portions is sufficiently loose to permit said relative intermediate movement between said locking member and said one body portion.

9. Downhole apparatus as claimed in claim 8 , wherein said screw threaded connection comprises a two-start thread with one of the two threads thereof removed.

10. Downhole apparatus as claimed in claim 9 , wherein said screw threaded connection comprises a thread collar spanning said two body portion.

11. Downhole apparatus for limiting rotation of a rotor for transmitting torque to a downhole tool relative to a stator associated with the said rotor, the downhole apparatus comprising a body within a bore of which a locking member is located so as to be movable between a first axial position relative to the body, in which the locking member is disengaged from a rotor so as to allow rotation of said rotor relative to said locking member, and a second axial position relative to the body, in which the locking member is engaged with said rotor so as to limit rotation of said rotor relative to said locking member, said locking member being engageable with said rotor by a plurality of complementary castellations respectively provided adjacent lowermost and uppermost ends of said rotor and said locking member, the apparatus further comprising means for limiting rotational movement of the locking member relative to the body when said locking member is located in said second axial position, wherein said locking member is movable from said first axial position to said second axial position by the application of a static fluid pressure to a differential area of said locking member, and means for selectively applying static fluid pressure to said differential area of said locking member.

12. Downhole apparatus as claimed in claim 11 , wherein said selective means for applying static fluid pressure comprises means for permitting intermediate movement of said locking member from said first axial position to an intermediate position relative to the body wherein the area of locking member exposed to said static fluid pressure is different to that exposed to said static fluid pressure when said locking member is located in said first axial position.

13. Downhole apparatus as claimed in claim 12 , wherein said body comprises two portions movable relative to one another so as to permit said intermediate movement of said locking member relative to one of said body portions.

14. Downhole apparatus as claimed in claim 13 , wherein said body portions are axially movable relative to one another.

15. Downhole apparatus as claimed in claim 13 , wherein said locking member is selectively retained in said first axial position relative to said one body portion by frangible means.

16. Downhole apparatus as claimed in claim 13 , wherein said locking member is selectively retained in said first axial position and said intermediate position relative to the other one of said body portions.

17. Downhole apparatus as claimed in claim 13 , wherein said two body portions are screw threadedly connected to one another.

18. Downhole apparatus as claimed in claim 17 , wherein the screw threaded connection between said two body portions is sufficiently loose to permit said relative intermediate movement between said locking member and said one body portion.

19. Downhole apparatus as claimed in claim 18 , wherein said screw threaded connection comprises a two-start thread with one of the two threads thereof removed.

20. Downhole apparatus as claimed in claim 19 , wherein said screw threaded connection comprises a thread collar spanning said two body portion.