Downhole apparatus

1. A downhole apparatus for location in a fluid-filled bore, the apparatus comprising:

a tubular body comprising a plurality of cylindrical chambers for containing a compressible fluid, wherein the chambers initially contain the fluid at atmospheric pressure,

pistons mounted in the chambers, wherein the pistons are operatively associated within a common sleeve such that translation of the pistons results in translation of the sleeve, wherein the pistons are coupled to the common sleeve to permit movement of the sleeve without requiring movement of all the pistons; and

a lock arrangement having a locking configuration for retaining the pistons in the chambers and having an unlocked configuration in which bore fluid pressure translates the pistons through the chambers.

2. The apparatus of claim 1 , wherein the lock arrangement comprises a lock member which is translatable to configure the lock arrangement between the locking configuration and the unlocked configuration to permit movement of the pistons.

3. The apparatus of claim 2 , wherein translation of the lock member allows movement of a coupling to permit movement between the pistons and the body.

4. The apparatus of claim 2 , wherein the translation of the lock member is in response to the bore fluid pressure.

5. The apparatus of claim 4 , wherein lock member comprises a balance piston.

6. The apparatus of claim 4 , wherein translation of the lock member requires displacement of a control fluid, wherein displacement of the control fluid is restricted when the lock arrangement is in the locking configuration.

7. The apparatus of claim 6 , comprising a valve arrangement to control flow of the control fluid, and movement of the lock member.

8. The apparatus of claim 7 , wherein the valve arrangement is operable to permit flow of the control fluid into a lower pressure volume.

9. The apparatus of claim 8 , wherein the lower pressure volume comprises an atmospheric chamber.

10. The apparatus of claim 1 , wherein the chambers are provided by a plurality of tubes mounted to the body.

11. The apparatus of claim 1 , wherein the chambers extend axially of the body and parallel to a main axis of the body.

12. The apparatus of claim 1 , wherein the chambers are at least one of circumferentially spaced and axially spaced.

13. The apparatus of claim 1 , wherein the lock arrangement comprises a receiver configured to receive a signal to initiate the process of changing the lock arrangement from the locked configuration to the unlocked configuration.

14. A downhole method comprising:

providing a tubular tool body, including pistons associated with a common sleeve and located in cylindrical chambers for containing a compressible fluid, wherein the chambers initially contain the fluid at atmospheric pressure;

running the tool body into a fluid-containing bore such that ambient pressure within the chambers increases, wherein the chambers are in fluid communication with the bore; and

translating the pistons through the chambers under the influence of the increased ambient pressure,

wherein translation of the pistons results in translation of the sleeve, and

wherein the pistons are coupled to the common sleeve to permit movement of the sleeve without requiring movement of all the pistons.

15. The method of claim 14 , comprising initially restraining the pistons relative to the body.

16. The method of claim 14 , comprising releasing the pistons in response to an activation signal relayed from surface.

17. The method of claim 14 , comprising coupling the pistons to a common member and combining forces generated by the pistons.

18. The method of claim 14 , comprising translating the pistons to operate a tool or device.

19. The method of claim 14 , comprising determining an actuating force required to be provided by the pistons, determining the ambient pressure available in the bore at an operating depth to translate the pistons, and determining the number of chambers and pistons necessary to provide the actuating force.