VORRICHTUNG ZUM RICHTEN DES VORDERTEILS EINES BOHRROHRES

PCT No. PCT/NL90/00173 Sec. 371 Date May 26, 1992 Sec. 102(e) Date May 26, 1992 PCT Filed Nov. 20, 1990 PCT Pub. No. WO91/08370 PCT Pub. Date Jun. 13, 1991.An apparatus for steering a forward end of a drill string having a motor. The apparatus includes a shaft connected to the motor. The shaft has a thickened portion. Drilling fluid having an operating mass flow rate, and a housing are provided. A piston is enclosed within the housing, and is displaceable by a mass flow of the drilling fluid. The piston has a central opening for the passage of the drilling fluid and has the shaft extending therethrough. The piston includes a displaceable sleeve within the housing, a first spring urging against the sleeve and the housing, and a displaceable piston part coaxial with the sleeve within the housing. The piston part has an inwardly directed collar movable towards the thickened portion of the shaft by an increase of the mass flow rate of the drilling fluid, whereby the collar and the thickened portion cooperate to reduce a size of the central opening, thus causing the drilling fluid to exert a thrust pressure on the piston part so that the piston part and the sleeve are moved abuttingly together against the urging of the first spring to an adjusting position. The piston is provided with a second spring exerting a spring force less than the first spring and having a first end urging against the sleeve, and a second end urging against the piston part. The apparatus further includes an adjusting organ connected to the sleeve and being displaceable when the piston is in the adjusting position.