Pressurized liquid driven downhole drilling machine with a device for soft start from a flush position and impact piston making part of such a downhole drilling machine

Abstract

The invention relates to a pressurized liquid driven downhole drilling machine (1) with a soft start from flush position, wherein a drive surface of an impact piston (20) is alternately pressurized and pressure relieved, depending on the position of the valve slide (6) in a valve housing (4). A slide control interface (17) includes a pressure relief port (E) so that a pressure relief channel (40) is opened in the flush position between a drive chamber (30) for the impact piston and a slide control channel (4c). This allows the impact piston (20) to be partly retracted from the flush position by diverting a pressure flow from the drive chamber (30) to a control pressure surface (A2) of the valve slide (6) thereby triggering the valve slide to move from a holding position until the impact piston has moved to a balance point at which the pressure relief port (E) is closed.

1 . A pressurized liquid driven downhole drilling machine with a device for soft start from flush position, which downhole drilling machine comprises:
 a machine housing with a differential pressure area balanced impact piston configured with a holding position in the machine housing,   a valve housing with a differential pressure area balanced valve slide configured with a holding position in the valve housing,   a slide control interface into which a slide tube protrudes into and has a sliding fit against a flush channel section in a rear end portion of the impact piston,   a plurality of slide control ports which are active in the slide control interface and with which the position of the valve slide in the valve housing is controlled by the pressurization respectively pressure relief of a control pressure surface of the valve slide, wherein said control pressure surface is pressurized and pressure relieved respectively via a slide control channel provided in the slide tube,   wherein a drive surface on the impact piston is alternately pressurized respectively pressure relieved, depending on the position of the valve slide in the valve housing,   wherein the pressurized liquid driven downhole drilling machine includes a pressure relief port which is provided, in the flush position, to open a pressure relief channel between a for the impact piston provided drive chamber and the slide control channel, which pressure relief port is further configured to, under a progressively increasing throttling effect, allow the impact piston to be returned to an activation position retracted at some distance from the flush position by diverting a pressure flow from the valve chamber of the impact piston to the control pressure surface of the valve slide via the slide control channel and thereby triggering the valve slide to move away from its holding position until the impact piston during its movement reaches a balance point at which the pressure relief port is closed.   
     
     
         2 . The pressurized liquid driven downhole drilling machine according to  claim 1 , wherein the pressure relief port is provided in the slide control interface. 
     
     
         3 . The pressurized liquid driven downhole drilling machine according to  claim 1 , wherein the impact piston has a rear holding position in the machine housing and the impact piston is returned to the retracted activation position in direction of said rear holding position. 
     
     
         4 . The pressurized liquid driven downhole drilling machine according to  claim 1 , wherein the distance over which the impact piston is returned from the flush position to the balance point is less than a nominal distance between the flush position and the start position of the impact piston along a central axis of the downhole drilling machine. 
     
     
         5 . The pressurized liquid driven downhole drilling machine according to  claim 1 , wherein a ratio between the activation distance for restarting the downhole drilling machine from the retracted activation position in which the balance point is reached is the highest and a nominal distance between the flush position and the start position is in a range of 1:2 to 1:10, wherein a most preferred ratio is in a range of 1:5 to 1:10. 
     
     
         6 . The pressurized liquid driven downhole drilling machine according to  claim 1 , wherein the pressure relief port is configured to open the pressure relief channel in pressure flow transmitting cooperation with a slide control port provided on the slide tube. 
     
     
         7 . The pressurized liquid driven downhole drilling machine according to  claim 6 , wherein the pressure relief port and the slide control port provided on the slide tube are configured to be placed opposite each other in the flush position when the pressure relief channel is open in an initial position and the impact piston is in its advanced position in the machine housing. 
     
     
         8 . The pressurized liquid driven downhole drilling machine according to  claim 1 , wherein the pressure relief port comprises a groove-shaped radially directed recess in an inner surface of the impact piston in the slide control interface whose defined port opening is cut by a slide part of an outer surface of the slide tube. 
     
     
         9 . The pressurized liquid driven downhole drilling machine according to  claim 1 , wherein the pressure relief channel comprises one or a plurality of axial cavities which may consist of one or a plurality of boreholes which open into a driving surface making part of the impact piston. 
     
     
         10 . An impact piston intended to be part of a downhole drilling machine according to  claim 1 , comprising a pressure relief port with associated pressure relief channel which, in flush position, makes it possible to transfer a pressure flow from a drive chamber provided for the impact piston to a control pressure surface of a valve slide which triggers the valve slide to move away from a holding position for the valve slide in the valve housing.