Downhole stabiliser

Abstract

There is described a downhole stabilizer (10) for use in a drill string adjacent the drill bit. The stabilizer (10) includes a hollow and generally cylindrical casing (12) the outer surface of which defines three angularly spaced protrusions (24, 26, 28), the radially outer surfaces of which define a diameter which is marginally less than the diameter of the bore to be drilled. Two of the protrusions (24, 26) carry radially extensible and retractable spacer members (42-52) which are interconnected so that radially outward movement of one member (42-46) results in radially inward movement of the other member (48-52) and vice versa, effectively to vary the radial length of the respective protrusions. A control mandrel (56) is mounted within the casing (12) and is selectively movable between first and second positions. The control mandrel (56) further defines abutments (58, 60) for limiting the inward movement of the space members (42-52) and in the first position the mandrel permitting a greater degree of travel of one spacer member (42-46) than the other (48-52), and in a second position providing the opposite. The arrangement is such that, in use, the stabilizer (10) with the mandrel (56) in its first position causes the drill to veer laterally about a generally vertical axis in a first direction and with the mandrel in its second position the drill bit is caused to veer laterally in the opposite direction.

Dependencies are inferred from the claim text.