Pressurized liquid driven downhole drilling rig with means for soft start from flushing position and percussion piston constituting part of such downhole drilling rig

Abstract

The invention relates to a pressurized liquid-driven downhole drilling machine (1) soft-started from a flushing position, in which the drive surfaces of the percussion piston (20) alternately pressurize and depressurize depending on the position of the valve slide (6) in the valve housing (4). The sliding control interface (17) comprises a pressure relief port (E) such that a pressure relief channel (40) between the drive chamber (30) for the percussion piston and the sliding control channel (4c) opens in the flushing position. This allows the impingement piston (20) to partially retract from the flushing position by diverting the pressure flow from the drive chamber (30) to a control pressure surface (A2) of the valve slide (6), thereby triggering movement of the valve slide from the holding position, until the impingement piston moves to a balance point in which the pressure relief port (E) is closed.

Description

Technical Field

[0001] The invention relates to a pressurized liquid driven downhole drilling machine with a device for soft starting from a flushing position according to the preamble of patent claim 1. The invention also relates to an impact piston according to claim 10 forming part of such a downhole drilling machine. Background Art

[0002] The invention relates to a pressurized liquid driven downhole drilling machine of the type described below: the downhole drilling machine of the type described below is provided with a sliding tube which forms part of a valve housing and which projects into and slidably cooperates with a flushing channel section of a rear end portion of an impact piston in a sliding control interface and in which a sliding control port of the sliding control interface is used to control a valve slide received in the valve housing. Such a downhole drilling machine is known, for example, from SE 526 252.

[0003] This downhole drilling machine includes an impact mechanism having a machine housing in which an impact piston is accommodated and arranged so as to impact the rear end or neck of a drill bit, which is movably attached to a drill bit attachment (drill chuck) at the front end of the downhole drilling machine. The reciprocating movement of the impact piston is triggered by a valve slider received in a valve housing. The valve slider reverses the force exerted by the pressurized liquid on the impact piston so that the impact piston alternately moves backward in the direction from the drill bit neck, or accelerates forward to impact the drill bit neck. The impact piston has a pressure differential area balanced rear holding position in the machine housing and is continuously affected by a small hydraulic force that drives the impact piston to move backward from the drill bit toward the valve housing after each stroke. The force on the impact piston is changed by alternately pressurizing and depressurizing. Conversely, the movement of the impact piston triggers the movement of the valve slider so that the valve slider alternately resets the pressure acting on the driving surface of the impact piston. Accordingly, the valve slider is pressure differential area balanced and is always driven toward the forward holding position in the valve body with a small force. The hydraulic pressure on the valve slider is varied by alternately pressurizing or depressurizing the control pressure surface on the valve slider through the port in the slider control interface between the impact piston and the slider tube.

[0004] For downhole drilling machines of this type, a so-called flushing position may occur, in which the drill bit of the rock drilling machine stops suddenly during the drilling operation. The drill bit is here in its most forward end position and no longer applies feed force to the rock face in front of the drill bit. In this case, the impact piston rests against the neck of the fixed drill bit with full hydraulic force forward, while the pressurized liquid (water) rushes out of the front face through the drill hole in the drill bit. In order to be able to resume drilling work again, the stroke mechanism of the downhole drilling machine must be restarted, which is achieved by so-called cutting-in, in which the drill bit is applied to the rock face with such a force that the impact piston moves slightly backwards in the rock drilling machine by a starting distance until the impact piston and the sliding control port are finally in the machine housing in the starting position. In the starting position, the impact piston is in a position in which the sliding control port in the sliding control interface communicates with the sliding control channel forming part of the sliding housing, wherein the drive area of ​​the valve slide is pressurized, the valve slide is reset, and the stroke mechanism of the downhole drilling machine is restarted.

[0005] The starting distance required to reach the starting position again from the flushing position and execute the restarting position of the stroke mechanism can be defined as the distance along the central axis or longitudinal axis (machine direction) of the downhole drilling rig, through which the drill bit (impact piston) must be pushed back from the flushing position into the machine so that the sliding control port in the sliding control interface opens and the resulting pressure flow resets the valve sliding member in the valve housing from the holding position to the starting position.

[0006] When restarting from flushing mode with the downhole drill rig described, the problem is not only that the starting distance to reach the restart position is relatively long (typically about 10 mm or more), but also that the drill bit must apply a relatively large force to the oncoming rock face. The above-mentioned cut-in for restarting from the flushing position can cause undesirable and troublesome interruptions when drilling with the downhole drill rig, which will reduce the drilling efficiency. In addition, the uncontrolled entry and exit of the flushing mode of the known downhole drill rig is inconvenient because during drilling operations in fractured or fractured rock, the drill bit will generally encounter intermittent drilling resistance when drilling in such rock. It will be appreciated that the constant occurrence of flushing positions / cut-ins and the resulting setup time reduce the productivity of the downhole drill rig when drilling.

[0007] Figure 1AA portion of the downhole drilling machine 1 in the starting position is schematically shown in a longitudinal cross-sectional view, wherein the first sliding port C and the second sliding port D are respectively in mutually open positions. The downhole drilling machine 1 comprises: a machine housing 2, in which an impact piston 20 with a pressure difference air balance having a rear holding position is received in the machine housing 2 in an axially reciprocating manner; a valve housing 4, in which a valve slider 6 with a pressure difference air balance having a front holding position is received in an axially reciprocating manner in an annular sliding chamber 4b; a sliding tube 4a, which is included in the valve housing and protrudes into and slides with the radially widened flushing channel section 15a of the rear end portion of the impact piston 20 in the sliding control interface 17, wherein the first sliding control port C and the second sliding control port D are operated here, and the axial end position of the valve slider 6 in the sliding chamber 5 is controlled by pressurizing or depressurizing the control pressure surface A2 of the valve slider 6.

[0008] As can be seen from the drawings, the control pressure surface A2 is located inside the annular slide control chamber 4b to which the control pressure flow is guided via the slide control channel 4c defined in the slide tube 4a. The slide control channel 4c is a reversing channel that alternately guides pressurized liquid to the annular slide control chamber 4b in one direction for pressurizing the control pressure surface A2 of the valve slide 6, thereby driving the valve slide backward from its front holding position in the valve housing 4, and vice versa, in the second direction via the slide control chamber 4b under pressure flow to be discharged to the central flushing channel 15 via the second slide control port D. The discharge of the pressure flow in the slide control chamber is carried out via the radial discharge groove 16 located in the inner surface of the flushing channel section 15 rearward of the impact piston 20.

[0009] exist Figure 1B In the longitudinal section, the flushing position is schematically shown. Figure 1A A portion of a downhole drilling rig in which the first sliding control port C is open, wherein the pressure flow flows into the flushing channel 15, and the second sliding control port D leading to the inlet of the sliding control channel 4c is closed. In this position, the drill bit (not shown) and the impact piston 20 have been pushed out from the starting position by about 30 mm (compare the driving chamber 30 in Figure 1AIn the flushing position, the impact piston 20 is extended at full pressure on the drive area A1 in the annular drive chamber 30. In order to return from the flushing position to the starting position, the drill bit and the impact piston 20 must be pushed into the starting distance Z of at least about 10 mm with full force so that the first sliding control port C and the second sliding control port D are in a mutually open position (see the starting position in Figure A1). This means that in order to restart, the impact piston 20 must be driven to the rear of the machine housing 2 so that the first control sliding port C and the second control sliding port D are opened to pressurize the sliding control channel 4c and thereby pressurize the control pressure surface A2 of the valve slide 6 so that the trigger valve slide 6 changes from its front holding position in the valve housing 4 to its rear position and thereby restarts the stroke mechanism of the downhole drilling rig. Summary of the invention

[0010] It is therefore a first object of the present invention to provide a downhole drilling rig which can be restarted in a smoother and more efficient manner than downhole drilling rigs known hitherto. A second object of the present invention is to provide a downhole drilling rig which exhibits improved operating characteristics when drilling in broken, fractured rock.

[0011] These objects of the invention are achieved by a downhole drilling rig having the characteristics and properties of claim 1. Other characteristics and technical advantages of the invention appear in the dependent claims.

[0012] According to another aspect, the invention also relates to an impact piston intended for use in a downhole drilling machine according to the invention.

[0013] Thanks to the invention, the start-up distance Z' of the impact piston backwards in the machine housing becomes considerably shorter (about 1 mm in practice), which contributes significantly to a softer and more effective cut-in when restarting the downhole drilling rig.

[0014] According to an embodiment, the sliding control interface of the downhole drilling rig includes a pressure relief port E, which is configured to open a pressure relief passage between a driving chamber provided to the impact piston and a sliding control passage in a flushing position, and the pressure relief port E is also configured to, under a gradually increasing throttling effect, divert the pressure flow from the driving chamber of the impact piston via the sliding control passage to a control pressure surface of the valve sliding member, thereby triggering the valve sliding member to move away from its holding position, thereby allowing the impact piston to return in the direction of its holding position in the machine housing until a balance point is reached, at which the pressure relief port E is closed. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention is described in more detail below with the aid of examples of embodiment shown in the accompanying drawings, in which:

[0016] Figure 1A A longitudinal sectional view of a stroke mechanism portion included in a downhole drilling rig in a known embodiment is schematically shown, wherein the stroke mechanism portion is in a starting position, wherein a first control sliding port C and a second control sliding port D are in a mutually open position for pressurizing the sliding control channel and switching the valve sliding member from a front holding position in the valve housing to a starting position.

[0017] Figure 1B Schematically shows Figure 1A A longitudinal sectional view of the stroke mechanism portion of the downhole drilling rig in the flushing position, wherein the first sliding control port C is open and the second control sliding port D is closed, and the impact piston is extended under the action of the full liquid pressure in its drive chamber.

[0018] Figure 2 A longitudinal section through a fluid-driven downhole drilling rig according to the present invention is schematically shown, wherein the portion of the downhole drilling rig where the present invention is located is outlined with a dotted outline.

[0019] Figure 3 A longitudinal sectional view of a downhole drilling rig in a flushing position is schematically shown, and according to the present invention, a drive chamber of the impact piston is arranged to be pressure-flow-connected to a slide control channel included in a slide control interface via a pressure relief port E, and to pressurize a control pressure surface of a valve slide, for switching the valve slide from a holding position to its starting position.

[0020] Figure 4 Schematically shows the Figure 3 A longitudinal sectional view of a stroke mechanism portion in the flushing position, but in this position, according to the present invention, the impact piston has returned under the influence of the main area pressure difference to a new retracted start position at a certain distance from the flushing position in the machine housing of the downhole drilling rig and is set to a predetermined equilibrium point achieved in association with closing the pressure relief port E. DETAILED DESCRIPTION

[0021] Reference Figure 2, a liquid-driven downhole drilling machine is generally indicated by 1 and comprises a machine housing 2 defined by a cylindrical tube, which accommodates a hammer housing 3 at the front and a valve housing 4 at the rear, which accommodates an axially movable valve slide 6 in a slide control chamber 4b. A drill bit 7 is fixed in a chuck 8 at the front end of the machine housing 2 in a rotation-proof manner. The drill bit 7 has a neck 9 at its rear end. The neck 9 refers to the rear shaft-shaped part of the drill bit that receives the impact energy. A rear component 10 is screwed into the rear end of the machine housing 2. The rear component 10 is intended to be connected to a drill support (not shown) in the form of a drill string. Therefore, the rear component 10 forms a so-called drill string adapter, which is screwed and attached to the housing 2 by means of a thread 11 operating between the components. 12 represents a filter or a filter screen.

[0022] For example, a pressurized driving liquid (pressurized liquid), for example in the form of water, at a pressure of, for example, 18 MPa, is intended to be led from a power source in the form of a drill string, which is connected to the other end of the drill string via a central channel in the drill string. The drill head 7 has a central flushing channel 15, which ends in front of it in a flushing outlet 15c leading to the front surface of the drill head 7. The spent pressurized liquid guided through the flushing channel 15 thus forms a flushing liquid, by means of which drill cuttings can be removed from the area in front of the drill head 7. The machine housing 2 accommodates a stroke mechanism, generally indicated by 16, which is located between the rear part 10 and the drill head 7 (see also the area circled by the dashed outline).

[0023] See also Figure 3 and Figure 4 , it is shown in more detail that the stroke mechanism 16 basically comprises two parts, more precisely an impact piston 20, which is located at the front of the machine housing 2, and the impact piston 20 is controlled in a sliding manner and is received in the hammer housing 3 to move between the front end position and the corresponding rear end position, and a valve, which is located at the rear of the machine housing 2, and includes the valve housing 4, the sliding control chamber 4b and the axially movable valve slider 6. The valve slider 6 is sleeve-shaped and reciprocates in the sliding control chamber 4b in a sliding controlled manner. The valve housing 4 includes an inlet port and an outlet port (not shown), which can be connected or disconnected with the annular drive chamber 30 of the impact piston 20. Since the function of the valve slider 6 like this is to control the pressure flow into and out of the drive chamber 30 of the impact piston 20 and the related drive area A1, this part of the downhole drilling machine will not be described in detail.

[0024] Generally, 17 represents a sliding control interface having a first sliding control port C and a second sliding control port D, which alternately control the valve sliding member 6 to move between a front holding position and a rear end position of the valve sliding member 6 in a sliding control chamber 5 of a valve housing 4.

[0025] The impact piston 20 is pressure differential area balanced and therefore normally strives to move towards a rear holding position very close to the valve housing 4 under the action of a predetermined main first force f1, which is Figure 2 In the slide control interface 17, the slide tube 4a included in the valve housing 4 protrudes into the inner surface of the flushing channel section 15a with a widened diameter at the rear of the flushing channel 15 of the impact piston 20, and with its outer surface abuts against the inner surface of the flushing channel section 15a with a liquid-proof sliding fit. The valve slide 6 is pressure differential area balanced and is dedicated to the front holding position in the valve housing 4 under the action of a predetermined main second force f2, which is Figure 2 When the control pressure surface A2 (see FIG. 1 ) included in the valve slide 6 and constituting a part of the annular sliding control chamber 4b is Figure 1A ) is pressurized, and when the sliding control ports C and D are opened, a secondary backward force is obtained on the valve slide 6, that is, a power supplement to overcome the main second force f2. Figure 2 The force is indicated by F2 in the figure, and this force acts through the main pressure difference area to drive the valve slide 6 to the front holding position of the valve slide 6 in the valve housing 4. The slide tube 4a thus forms a reversing channel 4c, which guides the pressurized liquid to the annular slide control chamber 4b in an alternating manner in one direction so as to drive the valve slide 6 backward from the holding position by affecting the slide control pressure surface A2, and conversely, in the second direction, the pressurized liquid is discharged from the annular slide control chamber 4b to the central flushing channel 15 via the radial drainage grooves 16 in the inner surface of the flushing channel section 15a located at the rear of the impact piston 20.

[0026] Since, when the control pressure surface A2 of the valve slide 6 is pressurized, the rearwardly directed secondary force F2 of the valve slide 6 is greater than the forward differential pressure area-based primary force f2 on the valve slide, i.e., F2>f2, the valve slide 6 will remain in its rearward position in the valve housing 4, wherein the inlet port of the impact piston 20 is open and pressurized liquid flows into and pressurizes the impact piston driver A1, wherein the impact piston is accelerated toward the drill bit 7, which Figure 2 Indicated by arrow F1.

[0027] All the contents described above basically constitute known technology.

[0028] Reference Figure 3 and Figure 4 According to the present invention, a pressure relief port E is provided in the sliding control interface 17, and the pressure relief port is controlled by a sliding part 4a:1 in the sliding control interface 17, and the sliding part 4a:1 includes the outer surface of the sliding tube 4a and the second sliding control port D, that is, in practice, the orifice opening of the sliding control channel 4c leading to the sliding tube 4a is opened and closed according to the position of the pressure relief port E relative to the other sliding control port D.

[0029] The pressure relief port E is configured so that when the downhole drilling rig 1 is in the flushing position (see Figure 3 ), the pressure relief passage generally indicated by 40 provided in the impact piston 20 (see the detailed enlarged view) has an effective flow transfer between the drive chamber 30 of the impact piston 20 and the slide control passage 4c, wherein, when the pressure relief port E is located in the middle of the second slide control port D, the pressure relief port E allows the impact piston 20 to return in the direction of its rear holding position in the machine housing 2 by the prevailing area pressure difference f1 under the effect of gradually increasing throttling, until the impact piston 20 reaches a new retracted start position at a distance from the flushing position by the pressure flow branched from the drive chamber 30 of the impact piston 20 to the slide control passage 4c. The new retracted start position also results in a significantly shorter new start distance Z' to reach the start position and restart the stroke mechanism of the rock drilling machine.

[0030] As a result, the prevailing pressure in the sliding control channel 4c exerts pressure on the control pressure surface A2 of the valve sliding member 6, wherein the valve sliding member 6 is triggered to move away from its front holding position in the sliding control chamber 4b of the valve housing 4 (move backward) and move in the direction of its rear end position in the sliding control chamber 4b, which causes the discharge valve (not shown) controlled by the valve sliding member to open and the annular drive chamber 30 having the drive area A1 of the impact piston 20 to be depressurized.

[0031] In the example of the embodiment described here, the pressure relief port E comprises a radially directed groove-shaped recess 41 in the inner surface of the widened flushing channel section 15a of the impact piston 20 in the sliding control interface 17 at the rear of the impact piston 20, and one or more elongated cavities 42 extending parallel to the central axis of the downhole drilling rig 1, which can be formed by one or more boreholes or circular arc-shaped subspaces uniformly distributed along the circumference, which open in the driving surface A1 of the impact piston 20 in the driving chamber 30. By selecting the axial position of the groove-shaped recess 41 of the inner surface of the impact piston 20 in the sliding control interface 17, the desired starting distance Z' can be easily configured relative to the nominal starting distance Z (the nominal distance between the flushing position and the starting position) to achieve an effective soft restart of the downhole drilling rig.

[0032] like Figure 3 and Figure 4 As best shown in the two detailed enlargements in FIG. 1 , the pressure relief port E is controlled by the sliding portion 4a:1 of the sliding tube 4a, which cuts into a groove-shaped recess 41 located in the inner surface of the impact piston 20 at the sliding control interface 17.

[0033] like Figure 4 As best shown, the pressure relief port E remains open until a balance point is reached, at which the impact piston 20 has moved so far back in the sliding control interface 17 under the influence of the pressure differential area effect that the pressure relief port E is closed, wherein the impact piston 20 is positioned in a new starting position retracted a certain distance from the flushing position. In the example of embodiment described here, the relative position of the pressure relief port E with respect to the second sliding control port D is selected so that the starting distance Z' of the drill bit 7 and the impact piston 20 moving back in the rock drilling machine until the impact piston 20 itself is positioned in its new retracted starting position in the machine housing only reaches about 1 mm, i.e. the starting distance Z'=1 mm. Due to the positioning of the impact piston 20 in the new retracted starting position, a reduced starting distance Z' is obtained, i.e. the starting distance Z' required to reset the impact piston to the starting position with the sliding control ports C, D open is significantly smaller than the starting distance of the previously known technology, which significantly facilitates restarting and minimizes the risk of unplanned downtime of the underground drilling machine.

[0034] Thus, in contrast to rock drilling machines known hitherto, the distance over which the impact piston 20 returns from the flushing position to the equilibrium point is smaller than the nominal distance of the impact piston along the central axis of the downhole drilling machine 1 between the flushing position and the starting position.

[0035] The ratio between the new reduced start distance Z' for restarting the downhole drilling rig from the retracted start position reaching the equilibrium point and the nominal start distance between the flushing position and the start position is suitably in the range of 1:2 to 1:10. Practical experiments have shown that the most preferred ratio is in the range of 1:5 to 1:10, wherein the latter relationship has been shown to give particularly good results.

[0036] The downhole drilling rig 1 according to the present invention works in the following manner:

[0037] Reference Figure 3 , the downhole drilling machine 1 is shown in the flushing position, wherein the pressure relief port E is in the open position. The slide control passage 4c is pressurized by the drive chamber 30 of the impact piston, which resets the valve slide 5 by pressurizing the pressure area A2 of the valve slide 6. Then, in the flushing position, the impact piston 20 cannot remain as far outward or forward in the machine housing 2, but moves inward (backward in the machine housing) until the pressure relief port E is closed.

[0038] Therefore, the impact piston 20 moves backward in the machine housing until the impact piston 20 reaches the new starting position, withdrawing from the flushing position by the pressure flow branched from the drive chamber 30 of the impact piston 20 to the slide control channel 4c. The downhole drilling machine thus achieves a new reduced starting distance Z' for restarting the downhole drilling machine, which is shorter than the nominal starting distance, which, according to known technology, includes a nominal distance between the flushing position and the starting position relative to the movement of the impact piston 20 along the central axis.

[0039] The equilibrium point of the impact piston 20 in the flushing position is determined by the axially selected position of the pressure relief port E in the slide control interface 17 relative to the central axis of the downhole drilling rig. Through the appropriately selected configuration, it has proven suitable that a starting distance Z' of only about 1 mm for the impact piston 20 can be used for restarting the stroke mechanism of the downhole drilling rig, which means that the downhole drilling rig can be started smoothly and easily from the flushing position.

[0040] The resulting softer flushing position provides for a simpler and more controllable cutting process and thus also offers the possibility of more efficient drilling work in fractured or fractured rock.

Claims

1. A downhole drilling rig (1) driven by pressurized fluid, the downhole drilling rig (1) having a device for soft starting from a flushing position, the downhole drilling rig comprising: A machine housing (2) having a differential pressure area-compensating impact piston (20), wherein the impact piston (20) is designed to have a holding position in the machine housing, A valve housing (4) having a valve slide (6) for differential pressure area balancing, wherein the valve slide (6) is configured to have a holding position in the valve housing, a sliding control interface (17) into which a sliding tube (4a) projects and in sliding engagement with a flushing channel section (15a) in a rear end portion of the impact piston (20), a plurality of sliding control ports (C, D), the plurality of sliding control ports (C, D) functioning in the sliding control interface (17), and utilizing the plurality of sliding control ports (C, D) to control the position of the valve sliding member (6) in the valve housing (4) by respectively pressurizing and depressurizing a control pressure surface (A2) of the valve sliding member (6), wherein the control pressure surface (A2) is respectively pressurized and depressurized via a sliding control channel (4c) provided in the sliding tube (4a), wherein the drive surface (A1) on the impact piston (20) is alternately pressurized or depressurized depending on the position of the valve slide in the valve housing, The invention is characterized in that the impact piston (20) includes a pressure relief port (E), which is arranged to open a pressure relief channel (40) in the flushing position, and the pressure relief channel (40) is located between a drive chamber (30) provided to the impact piston (20) and the sliding control channel (4c), and the pressure relief port (E) is also configured to allow the impact piston (20) to return to a starting position retracted a certain distance from the flushing position by diverting the pressure flow from the drive chamber (30) of the impact piston (20) via the sliding control channel (4c) to the control pressure surface (A2) of the valve sliding member under a gradually increasing throttling effect, and thereby triggering the valve sliding member (6) to move away from the holding position of the valve sliding member (6), until the impact piston (20) reaches a balance point during its movement, at which the pressure relief port (E) is closed.

2. The pressurized fluid driven downhole drilling rig according to claim 1, wherein: The pressure relief port (E) is arranged in the sliding control interface (17).

3. A pressurized fluid driven downhole drilling rig according to any one of claims 1 to 2, wherein: The impact piston (20) has a rear holding position in the machine housing (2), and the impact piston (20) returns in the direction of the rear holding position to a retracted start position.

4. A pressurized liquid driven downhole drilling rig according to any one of claims 1 to 3, wherein: The distance traveled by the impact piston (20) from the flushing position back to the balance point is smaller than the nominal distance of the impact piston between the flushing position and the starting position along the central axis of the downhole drilling rig (1).

5. A pressurized fluid driven downhole drilling rig according to any one of claims 1 to 4, wherein: The ratio between the starting distance (Z') for restarting the downhole drilling rig from the retracted starting position reaching the equilibrium point and the nominal distance between the flushing position and the starting position is at most in the range of 1:2 to 1:10, wherein the most preferred ratio is in the range of 1:5 to 1:

10.

6. A pressurized fluid driven downhole drilling rig according to any one of claims 1 to 5, wherein: The pressure relief port (E) is configured to cooperate with a sliding control port (D) provided on the sliding tube (4a) to open the pressure relief passage (40) in a pressure flow transmission manner.

7. The pressurized fluid driven downhole drilling rig according to claim 6, wherein: The pressure relief port (E) and the sliding control port (D) provided on the sliding tube (4a) are configured to be positioned opposite to each other in the flushing position when the pressure relief passage (40) is opened in the initial position and the impact piston (20) is in the advanced position of the impact piston (20) in the machine housing (2).

8. A pressurized fluid driven downhole drilling rig according to any one of claims 1 to 7, wherein: The pressure relief port (E) comprises a radially directed groove-shaped recess (41) in the inner surface (15a) of the impact piston (20) in the sliding control interface (17), the port opening defined by the recess (41) being cut by a sliding portion (4a:1) of the outer surface of the sliding tube (4a).

9. A pressurized fluid driven downhole drilling rig according to any one of claims 1 to 8, wherein: The pressure relief passage (40) comprises one or more axial cavities (42) which can comprise one or more bores leading to a drive surface (A1) forming part of the impact piston (20).

10. An impact piston (20) intended to be part of a downhole drilling rig (1) according to any one of claims 1 to 9, characterized in that The impact piston (20) comprises a pressure relief port (E) with an associated pressure relief channel (40) which, in the flushing position, enables a pressure flow to be transmitted from a drive chamber (30) provided to the impact piston (20) to a control pressure surface (A2) of a valve slide (6), which thereby triggers the valve slide (6) to move away from its holding position in the valve housing (4).

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