Downhole stabilizers

Claim 1

A downhole stabiliser for use in a drill string, the stabiliser having an effective diameter which is selectively variable between a minimum diameter and a maximum diameter, the stabiliser comprising a hollow and generally cylindrical casing having a string coupling at one end thereof, the casing mounting an angularly distributed array of radially movable radial spacers whose radially outer ends define said effective diameter, the casing internally and co-axially mounting a spacer actuating mandrel, the mandrel having a string coupling at one end thereof, the string coupling on the casing and the string coupling on the mandrel being at opposite ends of the downhole stabiliser to allow the downhole stabiliser to be operatively coupled into the drill string in use, the stabiliser being capable of telescopic contraction and extension between said string couplings by means of axially limited insertion of said mandrel into said casing and withdrawal of said mandrel from said casing, said mandrel and said casing being coupled to prevent relative rotation thereof, the mandrel having at least one camming surface, the radial spacers each having a camming surface on the respective radially inner end thereof, the camming surfaces on the mandrel and on the radial spacers co-operating to force the radial spacers radially outwards upon telescopic contraction of the downhole stabiliser by increased insertion of the mandrel into the casing, a mechanical detent within said casing and linking the casing with the mandrel in an axially extended condition of the stabiliser to restrain said telescopic contraction of the stabiliser while axially compressive forces on the stabiliser remain below a predetermined critical force, said detent being operable by application to the downhole stabiliser of an axially compressive force exceeding said critical force to release the mandrel from the casing to allow said telescopic contraction of the downhole stabiliser and consequent radial extension of said radial spacers, said detent allowing the stabiliser to remain in the contracted condition to retain said radial spacers in their radially outward positions until axially compressive forces on the stabiliser fall below a predetermined minimum axially compressive force to release the mandrel relative to the casing thus to allow the stabiliser to return to the axially extended condition and consequently to allow radial retraction of the radial spacers.