Patent ID: 8251161

Claim:
A downhole tool, comprising: a generally tubular body having an axial cavity extending therethrough; a hollow piston configured to slide along the axial cavity; an activation device for activating the hollow piston, the activation device comprising: a trigger element integral with the hollow piston, the trigger element being movable from a first position in which the trigger element is held in abutment against a support on the tubular body and the piston is fixed, towards a retracted second position in which the trigger element is not in abutment against the support and the hollow piston is released; a generally tubular relief sleeve for the trigger element, wherein the sleeve is coupled with the hollow piston using at least one shear pin when the trigger element is in the first position to prevent movement of the trigger element from the first position to the second position; and wherein the sleeve is disposed between the trigger element and an extension of the hollow piston, to allow the tubular relief sleeve to slide between the trigger element and the extension after shearing of the at least one shear pin; and a temporary closure for at least temporarily closing the tubular relief sleeve to cause a rise in hydraulic pressure on the tubular sleeve to shear the at least one shear pin, and to cause a sliding of the tubular relief sleeve into a release position allowing movement of the trigger element from the first position to the second position.