Determination of drill bit rate of penetration from surface measurements

Abstract

A method of determining, from surface measurements, the rate of penetration .DELTA.d of a drill bit attached to a drill string suspended from a floating drilling rig by means of a suspension system and a motion compensator, the method comprising: a) determining a displacement .DELTA.s of the drill string at the surface; b) determining from the measurements a function .LAMBDA.' of drill string compliance .LAMBDA. such that .LAMBDA.'=.LAMBDA.(1-.theta.) (wherein .theta. is a contribution factor of the motion compensator to an axial force applied to the drill string at the surface); c) determining an axial force .DELTA.T in the suspension system applied at the surface; d) determining a vertical displacement .DELTA.M of the motion compensator; e) determining from the measurements a function .mu. of the motion compensator compliance .lambda..sub.M such that ##EQU1## and f) calculating the bit displacement .DELTA.d from the relationship .DELTA.d=.DELTA.s-.LAMBDA.'.DELTA.T-.mu..DELTA.M.

Dependencies are inferred from the claim text.