Bore hole underreamer

An underreamer, comprising;

• an elongate, generally cylindrical body defining a longitudinal bore at least partially therethrough, and at least first and second peripheral bores extending generally from the longitudinal bore to an external surface of the body;
• at least first and second cutting arms being disposed at least partially within the first and second peripheral bores, respectively;
• each cutting arm being extendible from a first position in which the cutting arm is generally flush or recessed with respect to the external surface, to a second position in which the cutting arm is extended with respect to the external surface;
• a locking member disposed within the hollow body, the locking member having a locked position in which the locking member maintains the cutting arms in the first position, and an unlocked position in which the locking member allows the cutting arm to extend to the second position;
• and
• wherein fluid pressure in the longitudinal bore, greater than a threshold pressure, acting upon at least one of the cutting arms causes the at least one of the cutting arms to force the locking member from the locked position to the unlocked position.

An underreamer for a bore hole in a terrestrial formation, the underreamer comprising:

• a housing defining a hollow body and at least first and second bores in fluid communication with the hollow body, wherein the hollow body is operable to receive a pressurized drilling fluid;
• first and second cutting arms disposed in the first and second bores, respectively, and operable to slide in their respective bore from an inactive position to an active position when actuated by the pressurized drilling fluid, wherein the first and second cutting arms are sized relative to an inside diameter of the hollow body so that the first and second cutting arms can be installed in their respective bores from the interior of the hollow body;
• upstream and downstream ends of the hollow body, wherein the upstream end has external threads of a first diameter, and the downstream end has a threaded hole with a second diameter greater than the first diameter;
• and
• an adaptation piece screwed into the threaded hole, wherein the adaptation piece includes a male thread fitting the threaded hole and a female thread fitting the external threads.

An underreamer for a bore hole in a terrestrial formation, the underreamer comprising:

• a housing defining a hollow body and at least first and second bores in fluid communication with the hollow body, wherein the hollow body is operable to receive a pressurized drilling fluid;
• first and second cutting arms disposed in the first and second bores, respectively, each cutting arm having a protrusion arranged at an internal end accessible to the fluid;
• a locking member operable to engage the first and second cutting arms wherein the locking member maintains the first and second cutting arms in the inactive position until the locking member is released;
• and
• wherein the first and second cutting arms are operable to slide in their respective bore from an inactive position to an active position when actuated by the fluid, and wherein the protrusion of each cutting arm is arranged to inhibit a flow of the fluid in the hollow body when the cutting arms are in the inactive position and to not inhibit the flow of the fluid when the first and second cutting arms are in the active position, so that a detectable difference in fluid pressure is produced between the active and inactive positions.