Patent ID: 7090037

Claim:
A device for anchoring a drill string in a borehole formed in an earth formation, comprising: an anchoring member connected to the drill string and being movable between a retracted position in which the anchoring member is retracted from the borehole wall and an extended position in which the anchoring member is extended against the borehole wall so as to anchor the drill string to the borehole wall; an activating member operable to move the anchoring member from the extended position to the retracted position by the action of pressure of drilling fluid present in the borehole, wherein the activating member includes a first piston/cylinder assembly arranged to move the anchoring member from the extended position to the retracted position upon a relative axial movement between the first piston and the first cylinder by the action of said pressure of drilling fluid acting on the first piston; a pressure reduction means comprising a further piston/cylinder assembly in fluid communication with the first piston/cylinder assembly via a hydraulic fluid conduit, said further piston/cylinder assembly being arranged to reduce the hydraulic fluid pressure exerted to the piston of the first piston/cylinder assembly by a relative axial movement between the piston and cylinder of the further assembly, thereby inducing the action of the drilling fluid present in the borehole to move the anchoring member from the extended position to the retracted position.