Steerable drilling system

Abstract

A bias unit comprising at least one bias pad 20 moveable, by a piston 36, between retracted and extended positions, wherein the piston 36 is of non-circular effective cross-sectional shape.

1 . A bias unit comprising at least one bias pad moveable, by a piston, between retracted and extended positions, wherein the piston is of non-circular effective cross-sectional shape.  
   
   
       2 . A unit according to  claim 1 , wherein the piston is of effective cross-sectional shape having a major axis and a minor axis.  
   
   
       3 . A unit according to  claim 2 , wherein the bias pad is pivotally moveable about a pivot axis, and the piston is arranged such that the major axis lies substantially parallel to the pivot axis.  
   
   
       4 . A unit according to any of  claim 1 , wherein the piston is of obround effective cross-sectional shape.  
   
   
       5 . A unit according to  claim 1 , wherein the piston is slidable within a bore of curved or part-toroidal form.  
   
   
       6 . A unit according to  claim 1 , wherein the piston is attached to or forms part of the bias pad.  
   
   
       7 . A steerable drilling system incorporating a bias unit as claimed in  claim 1 , the drilling system including a drill bit the position and/or orientation of which is adjustable by the bias unit in use.