WO1997016622A1

WO1997016622A1 - A drilling assembly, a coupling device for such drilling assembly, and a method of drilling holes in the underground using the drilling assembly

Info

Publication number: WO1997016622A1
Application number: PCT/DK1996/000450
Authority: WO
Inventors: John Myles Rigden; Henrik Michael Hansen
Original assignee: A/S DAMPSKIBSSELSKABET SVENDBORG & DAMPSKIBSSELSKABET AF 1912 A/S trading as MAERSK DIRECTIONAL DRILLING SERVICES
Priority date: 1995-10-30
Filing date: 1996-10-30
Publication date: 1997-05-09
Also published as: AU7280496A; DK121295A

Abstract

The invention relates to a drilling assembly for coiled tubing drilling. The invention further relates to a coupling device for such drilling assembly and a method of using the drilling assembly. The coupling device comprises a drill tube (1) and a drill pipe (2) and is characterised in comprising an upper rotating motor (6), which is able to rotate the drill pipe (2), thus permitting firstly the drilling of rectilinear bore holes by coiled tubing drilling, secondly the reduction of friction between the drill pipe and an inner side of a bore hole, and thirdly the use of stabilisers to increase directional stability of the drill pipe. In this manner it is possible to drill faster and to drill longer holes, especially in a horizontal direction.

Description

A DRILLING ASSEMBLY, A COUPLING DEVICE FOR SUCH DRILLING ASSEMBLY, AND A METHOD OF DRILLING HOLES IN THE UNDERGROUND USING THE DRILLING ASSEMBLY

Introduction

The present invention relates to a drilling assembly for drilling holes in the under¬ ground, said drilling assembly comprising a drill tube of the "coiled tubing" type, which is situated in an upper part of the drilling assembly and which is non-rotatable, and a drill pipe, which is situated in a lower part of the drilling assembly, said drill tube and said drill pipe being rotatably interconnected.

Drilling with a drilling assembly of the type initially mentioned is done in the follow¬ ing way. Drilling vertical bore holes by coiled tubing drilling involves no major prob- lems. Friction between the drill pipe and an inner side of the drill hole has no signifi¬ cant effect as gravity affects the direction of drilling assembly. Drilling horizontal bore holes means that initially a change of direction of the drilling assembly must be made from vertical to horizontal. This change of direction takes place as usually done in coiled tubing drilling, such as described in the following.

The drill head is bent so that a longitudinal axis of the drill head and the mud motor extend in an angle α in relation to a central longitudinal axis of the rest of the drill string. A centre of the drill head is thus displaced in relation to a central longitudinal axis of the drill string seen in a direction parallel to the longitudinal axis. When the drill head is driven by the mud motor, the outer jacket of which does not rotate, the drill head will drill in a direction a relative to a direction of the longitudinal axis ofthe drill pipe. However, the direction of the drill pipe will successively follow the direc¬ tion of the drill head and thus follow a curved path, which depends on and is deter¬ mined by the size ofthe angle α, among other things.

When the direction ofthe bore string has reached the desired direction, the direction of the drill head should now follow the same direction as the drill pipe. In principle, this is not possible since the angle α, in which the longitudinal axes of the drill head and the mud motor are bent in relation to the longitudinal axis of the rest of the drill pipe, cannot be changed. In conventional drilling, i.e. drilling with drill tubes rotating from the ground surface or from a drilling rig, the drilling of the drill head is raised in a direction that differs from the direction ofthe drill pipe by rotating the entire drill pipe so that the longitudinal axis of the drill head and the mud motor performs a slightly conical orbital movement around the longitudinal axis of the rest of the drill pipe while the drill head is simultaneously performing drilling. This technique is well known.

This, however, is not possible in coiled tubing drilling as the drilling tube is not ro¬ tated from the ground surface or from the drilling rig. Therefore, drilling of rectilinear holes is performed in a different manner in coiled tubing drilling. The relative dis¬ placement of the drill head centre relative to the central longitudinal axis of the drill pipe seen in a direction parallel to the longitudinal axis of the drill pipe is fixed and cannot be changed. Instead, a change of the absolute displacement of the drill head centre relative to the central longitudinal axis of the drill pipe is performed in such a manner that the drill head centre is displaced orbitally in steps of e.g. 30°. Thus, the drill head performs a first drilling in a first direction, then the drill head centre is dis- placed by 30° whereupon the drill head performs a drilling in a second direction dis¬ placed by 30° relative to the first direction. Then the drill head centre is displaced by further 30° whereupon drilling takes place in a third direction displaced by 30° relative to the second direction and by 60° relative to the first direction. This method is fol¬ lowed successively and the step-by-step displacement takes place relatively fre- quently. The drilling head may also be displaced freely. Thus, a drilling hole is estab¬ lished that zigzags through the underground but wherein the general and average di¬ rection is parallel to the longitudinal axis of the drill pipe. This method of drilling rectilinear holes by means of coiled tubing drilling and the disadvantages associated with it are overcome by a drilling assembly described in WO 93/10326.

WO 93/10326 describes a drilling assembly comprising the elements mentioned in the initial paragraph. The assembly comprises a drill pipe with a mud motor driving a drill head. A driving device with driving wheels is provided to make it possible to drive the drill pipe through the bore hole. Mounted between the driving device and the mud motor is a pivot joint, which makes it possible to couple the mud motor to the rest of the drill pipe so that the rotation of the mud motor does not only drive the drill head but also rotates the rest of the drill pipe. In this manner it is possible to reduce the friction between the drill pipe and the inner side of the bore hole while the longitudi¬ nal axis ofthe drill head performs a conical orbital movement. Engagement and disen¬ gagement ofthe pivot joint is accomplished by adjusting a flow of drilling mud that is conducted down to the drill pipe from the ground surface or from the drilling rig. This drilling assembly, however, possesses a few disadvantages making it less suited for use in connection with the drilling of horizontal holes in the underground. The number of movable and mutually displaceable elements is very comprehensive, both in the driving device and in the pivot joint. This increases to a considerable extent the risk of operation stoppages, which means that the drilling assembly has to be pulled up from the bore hole in order to remedy the cause of the operation stoppages. Furthermore, the manner in which the pivot joint is engaged and disengaged is complicated since it requires a high degree of certainty that the individual parts that are to go into mesh with each other have a possibility of going into mesh with each other without distur¬ bances from impurities in the drilling mud or mechanical defects in the pivot joint. However, the most important disadvantage is that it is necessary to use three different coupling devices in order to operate the drilling assembly. A first coupling device in order to establish a conical orbital movement of the drill head, a second coupling de¬ vice in order to establish a rotation ofthe entire drill pipe, and finally a third coupling device, which is not specifically described, and which is intended for engaging and disengaging the driving wheels. Furthermore, it is the mud motor that has to drive both the drive head and also the entire drive pipe when the second coupling device is engaged.

Thus, it is the object of the present invention to provide a drilling assembly that does not possess the above-mentioned disadvantages, and which makes it possible to pro¬ vide an operationally reliable and uncomplicated coupling of both the conical orbital movement ofthe drill head and the rotation ofthe drill pipe. This object is obtained by a drilling assembly that is characterised in that the drill pipe comprises an upper rotating motor designed to rotate a part of the drill pipe, that said part of the drill pipe is situated between the drill head and the drill tube, that a cou- pling device is provided between the drill tube and the upper rotating motor, and that a coupling device is designed for engaging and disengaging the upper rotating motor and the drill tube.

A drilling assembly provided with an upper rotating motor possesses the advantage that the mud motor only has to drive the drill head whereas the upper rotating motor only drives the drill pipe when this is to take place. The drill pipe is only to rotate when drilling is to take place in a straight line. When drilling is to take place in a curve, the rotation of the drill pipe is brought to an end. Engagement and disengage¬ ment ofthe upper rotating motor takes place by means ofthe coupling device.

A preferred embodiment of a coupling device is characterised in that the coupling de¬ vice comprises an outer tube having an inner diameter D and an inner tube having an outer diameter d, which is smaller than the diameter D of the outer tube, and that be¬ tween the inner tube and the outer tube a first passage is formed, that a first opening is provided in the inner tube to permit access from the inner tube to a first passage be¬ tween the inner tube and the outer tube, that a jacket with a hole is slidably aπanged around an outer side of the inner tube, that in a first position the jacket covers the first opening in the inner tube, and that in a second position the hole in the jacket coincides with the first opening in the inner tube, thus permitting access from the inner tube to the first passage between the inner tube and the outer tube.

The upper rotating motor may be incoφorated in conventional drive pipes for coiled tubing drilling without their having to be modified very much. Drill pipes are often constructed of assembled modules and, therefore, the upper rotating motor and the associated coupling device may be coupled into an existing drill pipe as additional modules. When rotating the entire drill pipe, such as with the drilling assembly according to the present invention, it is not necessary to use a separate driving device with driving wheels for driving the drill pipe through the bore hole. Besides the possibility of rotat¬ ing the bore head and the mud motor in a conical orbital movement, the rotation ofthe bore pipe also causes the friction ofthe drill pipe and the inner side ofthe bore hole to be reduced. Thus, the upper rotating motor gives the drilling assembly three important functions simultaneously, firstly the possibility of drilling rectilinear holes without having to zigzag through the underground, secondly the possibility of drilling longer and deeper holes, particularly horizontal holes, due to the reduced friction between the inner side of the bore hole and the drill pipe, and thirdly the possibility of placing stabilisers along the drill pipe so that the direction of the drill pipe may be maintained even if the drill head hits hard objects in the underground with a subsequent tendency to a change of direction.

The upper rotating motor will preferably be engaged and disengaged by a coupling device having characteristics such as mentioned above. By providing a possibility for the drilling mud to be conducted either directly or exclusively to the mud motor or to be conducted via an upper rotating motor to the mud motor, an operationally reliable manner of driving the drill pipe by means ofthe upper rotating motor is obtained.

The outer tube and the inner tube of the coupling device as well as the sleeve com¬ prised by the preferred coupling device provide the possibility of a simple, but effi¬ cient, manner of conducting the drilling mud either directly through the inner tube to the mud motor when the sleeve is in the first position, or through the first passage between the inner tube and the outer tube when the sleeve is in the second position.

The part of the drilling mud that is conducted through the passage is conducted to the upper rotating motor and from the upper rotating motor to the mud motor.

The sleeve is brought from the first position to the second position by increasing the flow of drilling mud. The sleeve, which is in a first position maintained in this posi¬ tion by a resilient element contacting a rear side of the flange of the sleeve, will be displaced to the second position when the flow of drilling mud is increased. The resii- ient element will then yield to a power that the flow of drilling mud exerts on the front of the flange. In the second position of the sleeve, access to the first passage between the inner tube and the outer tube is established so that the drilling mud is conducted through the passage and to the upper rotating motor.

The drill pipe may be provided with stabilisers, which constitute a local increase ofthe drill pipe diameter. The stabilisers stabilise the extent ofthe drill pipe through the bore hole.

Description of the drawing

The invention will now be explained in detail with reference to the accompanying drawing, wherein

fig. 1 is a schematic view of a drilling assembly according to the invention, fig. 2 illustrates the principle of drilling rectilinear holes with the drilling assembly according to the invention, fig. 3 is a schematic view of a cross-section through an embodiment of a coupling device according to the invention with a jacket in a first position, fig. 4 is a schematic view of the embodiment of a coupling device according to the invention with the jacket in a second position, fig. 5 is a schematic view of a cross-section through an embodiment of a drill pipe forming part of a drilling assembly according to the invention with a sleeve in a first position, fig. 6 is a schematic view of an embodiment of a drill string with the sleeve in a second position.

Fig. 1 illustrates a drilling assembly according to the invention. The drilling assembly comprises a flexible drill tube 1 and a drill pipe 2. The drill pipe 2 consists of different modules that are coupled together. The drill tube 1 may have different dimensions, but in the embodiment shown the drill tube has a diameter of 2 inches. The drill pipe 2 is fastened to the drill tube by means of a so-called dimple connector 3. A non-rotatable transition joint 4 forms a transition between the dimple connector 3 and the following part ofthe drill pipe 2.

An orientation tool 5 is provided between the non-rotatable joint and a coupling de- vice 6. The orientation tool 5 is designed to perform a change of an absolute angular position α of a longitudinal axis 1 ofthe drill head and the mud motor in relation to a longitudinal axis L ofthe rest ofthe drill pipe. This is described in further detail in fig.

2.

The coupling device 6 and an upper rotating motor 7 are provided in immediate ex¬ tension of the orientation tool 5. The coupling device is designed to engage and disen¬ gage the upper rotating motor 7. The coupling device 6 will be described in further detail in fig. 3 and fig. 4. The upper rotating motor 7 may be a motor known per se for use in combination with coiled tubing drilling such as a turbodrill motor.

A first stabiliser 8 is mounted between the upper rotating motor 7 and a transition joint 9. The stabiliser 8 is designed to contact an inner side of the bore hole (not shown) in order to maintain the drill pipe 2 in a substantially central position in the bore hole.

The transition joint 9 is mounted between the first stabiliser 8 and the two so-called NMDCs (Non Magnetic Drill Collars) 10, which comprise so-called MWD probes (Measuring While Drilling) (not shown). Mounted between the two NMDCs 10 and a mud motor 1 1 is a so-called UBHO (Universal Bottom Hole Orientator) 12. A UBHO is designed to calibrate the orientation of the individual modules in relation to each other before drilling is initiated.

The mud motor 11, which is designed to drive a drill head 13, is arranged in conjunc¬ tion with the drill head 13. However, positioned between the mud motor 1 1 and the drill head 13 is a second stabiliser 14, which preferably forms part of the mud motor. The drill head 13 is positioned at the end of the drill pipe 2 opposite the dimple con¬ nector 3. The longitudinal axis 1 ofthe drill head 13 and mud motor 11 is displaced by an angle of 0.5° in relation to the longitudinal axis L ofthe rest of the drill pipe. Fig. 2 illustrates how the drill head 13 is guided by the orientation tool 5 in order to permit a change or a maintenance of direction in a given drilling direction. A centre C of the drill head 13 is relatively displaced in relation to the longitudinal axis L of the rest ofthe drill pipe 2. The relative displacement ofthe centre C of the drill head 13 in relation to the longitudinal axis is constant R. However, it is possible by means of the orientation tool 5 to change the absolute position ofthe centre C ofthe drill head 13 in relation to the longitudinal axis L. A change of the position of the centre C of the drill head takes places stepwise, and in the embodiment shown by rotating the centre of the drill in steps of 30° around the longitudinal axis L.

By performing a change ofthe absolute position of the centre C of the drill head 13 in relation to the longitudinal axis L it is possible to change the drilling direction of the drilling assembly. This manner of changing the drilling direction is known. If a change of the drilling direction in relation to the direction of the longitudinal axis is desired, the centre C of the drill head 13 is rotated so that the absolute position of the centre C in relation to the longitudinal axis L is directed in the new desired drilling direction. If maintenance ofthe drilling direction in relation to the direction of the longitudinal axis L is desired, the upper rotating motor is engaged, whereby the longitudinal axis 1 (see fig. 1) ofthe drill head 13 and the mud motor 11 will perform a slightly conical orbital movement around the longitudinal axis L of the rest of the drill pipe 2. The drilling direction will then be rectilinear having a diameter ofthe bore hole that is larger than a diameter of the drill pipe 2 due to the conical orbital movement of the drill head 13 and the mud motor 1 1.

Fig. 3 illustrates an embodiment of the coupling device 6, which constitutes a module in the drill pipe 2 and which is designed for use in combination with the drilling as¬ sembly according to the invention. The coupling device 6 illustrated in fig. 1 is posi¬ tioned between the orientation tool 5 and the upper rotating motor 7. The coupling device 6 comprises an outer tube 14 having an inner diameter D and an inner tube 15 having an outer diameter d. A flow F of drilling mud is intended to be conducted into the coupling device 5 at an inlet 16 of the outer tube 14 and inlet 17 of the inner tube 15 and out of the coupling device through an outlet 18 of the outer tube 14 and a first outlet 19 of the inner tube 15. Between an inner side 20 of the outer tube 14 and an outer side 21 of the inner tube 15 a first passage 22 is formed. A first opening 23 is provided in the inner tube 15 so that access is established from the inner tube to the passage 22.

A jacket 24 is arranged around the inner tube 15. The jacket 24 is provided with a hole 25. In fig. 3 the jacket 24 is in a first position. In the first position of the jacket 24 the jacket covers the opening 23 in the inner tube 15. Access from the inner tube 15 to the passage 22 is thereby prevented. The jacket 24 is provided with a flange 26 extending perpendicularly relative to the jacket 24. A front 27 of the flange 26 faces the flow F of drilling mud. A rear side 28 of the flange 26 contacts a first end 29 of an elastic element 30. In the embodiment shown the elastic element 30 is constituted by a coil spring. A second end 31 of the coil spring contacts a stop 32 formed on the inner side 20 of the outer tube 14. A pawl 33 extends from the outer side 21 of the inner tube 15 outwardly through a groove (not shown) with which the jacket 24 is provided. The pawl 33 and the groove ensure that the jacket is not able to rotate around the inner tube 15. A pin 41 extends from an inner side 42 of the outer tube 14 in forward direc¬ tion toward a recess (not shown) in the front 27 of the flange 26. The pin 41 ensures that in the first position the jacket 24 is always in the same position relative to the outer tube 14. If this is not ensured, a displacement of the jacket 24 relative to the outer tube 14 will take place when using the orientation tool 5.

In fig. 4 the jacket 24 is in a second position. In the second position the hole 25 in the jacket 24 is situated at the first opening 23 in the inner tube 15. Access from the inner tube 15 to the passage 22 is thus established. The jacket 24 is displaced from the first position to the second position by adjusting the flow F of drilling mud, which is con¬ ducted from the drilling rig through the drill tube 1 and through the drill pipe 2. When the jacket 24 is in the first position, the flow F of drilling mud is moderate so that a power which the drilling mud exerts on the front 27 ofthe flange 26 of the jacket 14 is insufficient to overcome a resilient power exerted by the elastic element 30 on the rear side 28 of the flange 26. If the flow F of drilling mud is increased, the power exerted by the drilling mud on the front 27 ofthe flange 26 is also increased. At some moment the power will grow stronger than the power exerted by the elastic element 30 on the rear side 29 ofthe flange 26, and the jacket 24 will be displaced from the first position to the second position.

When the jacket 24 is in the first position, the drilling mud is only conducted from the inlet 16 of the outer tube 14 to the inlet 17 of the inner tube 15 and through the inner tube 15. When the jacket 24 is in the second position, the drilling mud is also con¬ ducted from the inlet 16 of the outer tube 14 to the inlet 17 of the inner tube 15. Then some of the drilling mud is conducted through the inner tube 15 and through the first opening 23 in the inner tube, the hole 25 in the jacket 24 and through the passage 22. The part of the drilling mud that is conducted through the passage 22, is initially con¬ ducted to the upper rotating motor 6 (see fig. 1 ) and subsequently to the mud motor 11. Thus, engagement and disengagement of the upper rotating motor 7 takes place by adjusting the flow F of drilling mud in order to change the position 24 of the jacket either to the first position or the second position.

Fig. 5 illustrates part of a drilling assembly according to the invention. The drilling assembly comprises the coupling device 6, the upper rotating motor 7 and the mud motor 1 1. The jacket 24 in the coupling device 6 is in the first position so that access from the inner tube 15 through the first opening 23 to the first passage 22 is prevented. In an opposite end 34 of the inner tube 15 a second passage 35 is formed between the inner tube 15 and the outer tube 14. In the opposite end 34 the inner tube 15 is pro¬ vided with a second opening 36. The inner tube 15 is provided with a sleeve 37. The sleeve 37 may be displaced axially in relation to the inner tube 15, and the sleeve 37 is provided with a hole 38. In the situation illustrated, the sleeve 37 is in a first position in relation to the inner tube 15 so that the hole 38 in the sleeve 37 is situated at the opening 36 in the inner tube 15, thus creating access from the inner tube 15 to the sec¬ ond passage 35. The second passage 35 is situated at a side of the upper rotating motor 7 opposite the first passage 22. A resilient element 39 is situated between the sleeve 37 and a rear end 40 ofthe inner tube 15. When the flow of drilling mud is moderate and the jacket 24 covers the first opening 23 in the inner tube, such as illustrated, the flow of drilling mud is only conducted through the inner tube 15 and down towards the rear end 40 of the inner tube 15. Due to the moderate flow of drilling mud the sleeve 37 will remain in its first position as the resilient power from the resilient element 39 is sufficient to resist the power ex¬ erted by the flow of drilling mud on the sleeve 37. The hole 38 in the sleeve 37 will thus be positioned at the second opening 36 in the inner tube 15. The drilling mud will then be conducted out into the second passage 35 and through the drilling pipe to the mud motor 1 1.

In fig. 6 the flow of drilling mud has been increased so that the jacket 24 around the inner tube 15 is displaced, whereby the hole 25 in the jacket 24 is positioned at the first opening 23 in the inner tube 15. Thus, part of the drilling mud is conducted to the upper rotating motor 7. The remaining part of the drilling mud is conducted through the inner tube 15 to the rear end 40 of the inner tube 15. Due to the increased flow of drilling mud, the power exerted by the drilling mud on the sleeve 37 will increase. The power exerted by the resilient element 39 on the sleeve 37 is no longer sufficient to retain the sleeve 37 in the first position and, therefore, the sleeve 37 will be displaced to a second position. The hole 38 in the sleeve 37 will then be displaced away from the second opening 36 in the inner tube 15, and there is no longer access from the inner tube 15 to the second passage 35. Now all the drilling mud will be conducted through the hole 25 in the jacket 24 and through the first opening 23 in the inner tube 15 to the first passage 22 and to the upper rotating motor 7. After the drilling mud has passed the upper rotating motor 7, the drilling mud is conducted to the mud motor 11.

The invention has been described above with reference to a specific embodiment of the coupling device. However, it will be possible to use different kinds of coupling devices that are able to adjust the feeding of drilling mud to the upper rotating motor. The upper rotating motor is preferably a positive displacement motor such as a motor of the spiral motor type, alternatively of the turbo drill type, but other types of motors may also be used. Thus, the coupling device and the upper rotating motor may be provided in such a manner that the regulation of the jacket position, respectively the driving force of the upper rotating motor, is not provided by the drilling mud, but other adjustment means, respectively driving forces, may be applied.

Claims

1. A drilling assembly for drilling holes in the underground, said drilling assembly comprising a drill tube of the "coiled tubing" type, which is situated in an upper part of the drilling assembly and which is non-rotatable, and a drill pipe, which is situated in a lower part of the drilling assembly, said drill tube and said drill pipe being rotata¬ bly interconnected, and said drill pipe comprising a mud motor driven by drilling mud, which is conducted through the drill tube and a drill pipe, and said drill pipe being provided at an outer end with a drill head driven by the mud motor and extending in an angle α in relation to the drill pipe so that a longitudinal axis of the drill head and the mud motor is positioned displaced in a fixed relative distance in relation to a longi¬ tudinal axis of the rest of the drill pipe seen parallel to the longitudinal axis, and said drill pipe comprising an orientation tool designed to change an absolute position ofthe drill head centre in relation to the drill pipe centre, c h a r a c t e r i s e d in that the drill pipe comprises an upper rotating motor designed to rotate a part of the drill pipe, that said part ofthe drill pipe is situated between the drill head and the drill tube, that a coupling device is provided between the drill tube and the upper rotating motor, and that a coupling device is designed for engaging and disengaging the upper rotating motor and the drill tube.

2. A drill pipe for use in combination with a drilling assembly according to claim 1, c h a r a c t e r i s e d in that the drill pipe comprises a second stabiliser, and that the second stabiliser is provided between the mud motor and the drill head.

3. A drill pipe for use in combination with a drilling assembly according to claim 1 or according to claim 2, c h a r a c t e r i s e d in that the drill pipe comprises a first stabiliser, and that the first stabiliser is provided after the upper rotating motor so that the first stabiliser is situated in the part ofthe drill pipe that the upper rotating motor is designed to rotate.

4. A drill pipe according to any one ofthe preceding claims, characterised in that the drill pipe comprises a coupling device designed to engage and disengage rota¬ tion of the upper rotating motor, and that the coupling device is situated between the drill pipe and the upper rotating motor.

5. A coupling device for use in combination with a drilling assembly according to claim 1, characterised in that the coupling device comprises an outer tube having an inner diameter D and an inner tube having an outer diameter d, which is smaller than the diameter D of the outer tube, and that between the inner tube and the outer tube a first passage is formed, that a first opening is provided in the inner tube to permit access from the inner tube to a first passage between the inner tube and the outer tube, that a jacket with a hole is slidably arranged around an outer side of the inner tube, that in a first position the jacket covers the first opening in the inner tube, and that in a second position the hole in the jacket coincides with the first opening in the inner tube, thus permitting access from the inner tube to the first passage between the inner tube and the outer tube.

6. A coupling device according to claim 6, characterised in that the jacket is provided with a flange having a front and a rear side, that the front faces a flow direc- tion ofthe drilling mud through the drill pipe, that the rear side contacts a front end of a resilient element, and that a rear end of the resilient element contacts a stop provided on the inner side ofthe outer tube.

7. A coupling device according to claim 6 or claim 7, characterised in that the outer side of the inner tube is provided with a pawl extending into a groove in the jacket.

8. A drill pipe according to any one of claims 6-8, characterised in that the inner side of the outer tube is provided with a pawl extending into a recess in the flange of the jacket.

9. A coupling device according to any one of claims 1-4, c h a r a c t e r i s e d in that in a rear end the inner tube is provided with a sleeve, and that between the inner tube and the outer tube a second passage is formed, that a second opening is provided in the inner tube to permit access from the inner tube to the second passage between the in¬ ner tube and the outer tube, that a sleeve with a hole is slidably arranged around an inner side ofthe inner tube, that in a first position the hole in the sleeve coincides with the second opening in the inner tube, thus permitting access from the inner tube to the second passage between the inner tube and the second tube, and that in a second posi¬ tion the hole in the sleeve covers the second opening in the inner tube.

10. A method of drilling holes in the underground by using a drilling assembly accord¬ ing to any one of claims 6-9, c h a r a c t e r i s e d in that a first small flow F of drilling mud is initially conducted through the drilling tube, to the drill pipe and to the inner tube and through the latter to the mud motor, whereby the drill head is driven, that a second large flow F is subsequently conducted through the drill tube, to the drill pipe both to the inner pipe and against the front of the flange of the jacket so that the jacket is slid from the first position to the second position, that part ofthe drilling mud is hereby conducted through the opening in the inner tube, through the opening in the jacket, through the passage between the inner tube and the second tube and to the up- per rotating motor, whereby the upper rotating motor is driven by the part of the drill¬ ing mud that is conducted through the passage, and that this part of the drilling mud is subsequently conducted to the mud motor.