WO2001000961A1

WO2001000961A1 - An underreamer and a method of using the underreamer

Info

Publication number: WO2001000961A1
Application number: PCT/NO2000/000209
Authority: WO
Inventors: Thor BJØRNSTAD
Original assignee: Bjoernstad Thor
Priority date: 1999-06-24
Filing date: 2000-06-16
Publication date: 2001-01-04
Also published as: NO993137D0; NO310311B1; NO993137L; AU5115800A

Abstract

An underreamer (10) is formed for mounting into a drill string with a downstream drill bit, and comprises two concentric sleeve-shaped parts (12, 14) axially displaceable relative to one another, with a bore extending therethrough and radially movable cutting means (28), which are influenced by a mechanical activating device, which is arranged to make use of said relative movement to push out the cutting means (28) radially into an active position from a retracted inactive position, the activating device making use of opposite relative movement in order to allow the cutting means (28) to be brought back into their retracted inactive position.

Description

AN UNDERREAMER AND A METHOD OF USING THE UNDERREAMER

This invention relates to an underreamer of the kind which is formed to be mounted in a drill string with a drill bit connected to its free extremity, comprising an essentially sleeve-shaped body and one or more cutting means radially displaceable relative to the sleeve-shaped body, and incorporated in the underreamer as the cutting elements thereof, and having a radially retracted position within the outer circumferential surface of the sleeve-shaped body, and a radially extended position with cutting portions positioned radially outside said outer circumferential surface.

The invention also relates to a method of using an underreamer of this kind, additionally exhibiting features as specified in the characterizing part of Claim 1.

Underreamers of this general kind comprise cutting means in the form of cutting wheels or rolling cutters, which are moved radially outwards from their radially retracted inactive position into their functional position with their axes of rotation directed transversally, possibly at an angle relative to the axial direction of said concentric sleeve- shaped bodies of the underreamer, with the cutting means in a position to widen the hole predrilled by the bit,

From Norwegian patent No. 302.536 is known an underreamer of the kind initially mentioned, in which there is arranged a manoeuvring device, biased by a return spring, for the activation of the cutting means, and arranged to be influenced directly or indirectly by the supply of drilling mud in the drill string, in order thereby to bring the cutting means into radially outward movement, so that it/they will finally adopt an extended active cutting position, and is/are ready to perform the task of an underreamer.

This known underreamer, based on the supply of drilling mud and thus hydraulic activation, distinguishes itself in that upstream it has a coaxially oriented control valve arranged thereto with an open main position and a closed main position. In the open position thereof, supply of drilling mud to the manoeuvring device is allowed, whereas this supply is shut off in the closed position of the valve. Moreover, in the outer sleeve-shaped body are formed by-pass channels, extending from the control valve outside the manoeuvring device to the area of the drill bit. When the control valve is in its open position, drilling mud is supplied through the drill string to the manoeuvring device for the activation thereof, so that it provides out-swinging of the radially retracted cutting means into an active position, in order to perform underreaming.

The working principle of this hydraulic underreamer appears to be very simple, but the constructional configuration of the underreamer in practice has necessitated several partly complicated solutions, and the flow of drilling mud, which forms the basis of the hydraulic activation of the manoeuvring device for the cutting means on the one hand, and the general drilling activity on the other hand, may result, by minor malfunction of the control valve or unsatisfactory precision, in undesired out-swinging of the cutting means, for example while the underreamer is inside a set and cemented casing of smaller diameter than the cutting diameter, so that the casing will be damaged or destroyed.

Therefore, the aim has been, according to the present invention, to provide a mechanical activatable underreamer of the kind initially mentioned, whereby is obtained, by simple and cheap means, a constructionally simple, functionally reliable and dependable underreamer, which is very well s secured against being released and extended radially into its active cutting position, before the predetermined time of activation corresponding to the positioning of the region of the underreamer/cutting device at a safe distance from the lower free end of the casing which was run and cemented last. o In the same way is ensured, in every situation, that the aggregate of cutting means of the underreamer is not caused to release and swing out in a radial plane until the aggregate of cutting means is outside and at a suitable axial distance from the outer free end of said casing.

5 The situation prior to the use of underreamer is often such that a lower casing has been run and is to be fixed through cementing to the surface of the formation wall defining the well bore. During the cementing of this casing, not only the annulus between casing and formation wall surface is filled, o but also the lowermost region of the well bore, in which the hardened cement mixture in the lower region of the well bore will have to be drilled through before the drilling can proceed, since not only the hardened plug of cement mixture at the bottom of the well bore drilled so far, should be drilled through, but also some extra metres down beyond the bottom of the well bore, so that the axial area of the underreamer for radial outward/inward movement of the cutting means should with certainty have been brought to a safe distance from the free lower end of the casing run and cemented last.

With current measuring technology it can be determined with accuracy how far downstream the underreamer is at any time, at a distance from the outer end of the casing run and cemented last, and with a reliable releasing and activating device for the cutting device of the underreamer, it will always be possible to bring the cutting device into an active position at a favourable moment, i.e. once the axial area occupied by the cutting device of the underreamer in its active position, has passed said lower free end of the casing run last.

Said object is realized, according to the invention, in forming the underreamer, which is formed in accordance with the introductory part of Claim 1, as specified in the characterizing part of Claim 1.

An underreamer formed so that it exhibits the features appearing from Claim 1, may be of at least two general embodiments, and combinations of the features of these embodiments are assumed to provide advantageous mechanical, constructional configurations of further underreamers. In a known manner, an underreamer according to the invention comprises an elongate tubular body, which can be mounted in a drill string, and which is called, according to the invention, the first sleeve-shaped element of two essentially concentric, sleeve-shaped relatively displaceable elements. Moreover, the underreamer comprises one or more cutting means generally known in themselves, movable between a retracted position, in which the respective cutting means is retracted radially relative to the outer circumferential surface of the first sleeve-shaped element, and an underreaming position radially pushed out/swung out, with its one or more cutting means in an active working position.

A mechanical activating device, which may be based on cam surfaces, which, when displaced in the circumferential/longitudinal direction in response to said relative movement of the two concentric sleeve-shaped elements, push/swing out the cutting means or each cutting means radially, so that it/they adopt(s) an active underreaming position.

When the cutting means is being pushed out radially (without a swinging movement), a radially inner portion of the cutting means rests on a cam surface of a rotatable cam disc, which cam surface has a constantly increasing distance to the axis of rotation counted in the direction of rotation. With the cutting means in its position radially retracted, and guided in a port extending through the outer sleeve-shaped element, the cam surface portion abutting same has the smallest distance to the axis of rotation of the cam disc. On rotation in said direction of rotation, the distance between the cam surface portion resting pressingly at any time on the radially inner portion of the cutting means, increases as mentioned, so that the cutting means is successively forced radially outwards by the cam disc, until a radially inner stop portion of the cutting means is brought to abut a cooperating stop portion at the guide port of the radially outer sleeve-shaped element incorporated in the elongate tubular body, which can be mounted in the drill string, and which is formed with threaded pipe end couplings at either end, like the other pipes (e.g. drill pipes, drill collars) of the string.

It is convenient to use three cutting means equally spaced apart around the circumference, and the cam disc will then have three cam surfaces, each with the course specified above.

Between the two essentially concentric sleeve-shaped elements arranged for relative movement, there my advantageously be formed, in the embodiment discussed (with cutting means that can be pushed radially and are non-pivotal), a movement- converting device converting an axially displacing movement of one of the concentric relatively moveable sleeve-shaped elements into a twisting movement (combined movement: axial displacement during rotation or vice versa, i.e. a helical course of movement) of the second sleeve-shaped element, including the cam-disc of the latter, which turns together with said second sleeve-shaped element. Such a conversion of movement may be achieved in that the radially outer sleeve- shaped element is formed with one or more helical grooves, extending generally in the longitudinal direction of the sleeve-shaped elements, said grooves or each of said grooves being glidingly engaged by a transversal guide pin projecting radially from the radially inner sleeve-shaped element. Between the two essentially concentric sleeve-shaped elements there may be arranged, as an extra precautionary measure, shear pins, which have the task of completely preventing release, activation and forcing out of the one or more cutting means of the underreamer, whereas their axial region for forcing out the underreamer construction is inside said set, cemented casing. As long as the shear pins are intact, the concentric sleeve-shaped elements are not, of course, allowed any relative movement. That takes place only when the shear pins break.

When drilling through the hardened plug of cement mixture at the bottom of a well bore, where the last casing has been positioned and cemented to the surrounding formation wall surface, only the drill bit should be in use (it is then assumed that the effective diameter of the underreamer exceeds said casing diameter) and drilling takes place at a moderate torque in order not to cause any breaking of the shear pins.

When, as earlier explained, one is sure that the drill bit has made a borehole so far beyond the lower free end of the casing that the area for the forcing out of the one or more cutting means is at a position at a appropriate distance from said free end of the casing, the shear pins are brought to break. This is done by increasing the torque on the drill string.

A rational method of use of underreamers according to the invention, distinguishes itself through the steps of procedure that appear from the first independent Claim concerning a method. When the shear pins have been brought to break, the relative movability of the two concentric sleeve-shaped elements is established.

By the lowering of the drill string into the bottom of the borehole drilled by the drill bit below the free end of the casing run and cemented last, so that the two concentric sleeve-shaped elements are brought to be displaced longitudinally relative to one another, the movement of the radially outer sleeve-shaped element is transferred through the helical groove ( s) /slot ( s ) thereof and the guide pin(s) engaged in a glidingly displaceable manner therein.

When it is desirable that the one or more cutting means of the underreamer should be brought back into its/their inactive position radially retracted within the outer mantle s surface of the under reamer, corresponding to the circumferential surface of the radially outer sleeve-shaped element, the drill string is lifted, so that again a basically axial displacing movement of the two concentric sleeve-shaped elements can be initiated, said displacing o movement being converted, as a consequence of the slidingly displaceable engagement of the radially directed pin(s), of the radially outer sleeve-shaped element, in the longitudinal helical guiding grooves of the radially inner sleeve-shaped element, into a rotational movement, by i.a. the cam disc, 5 about an axis of rotation, which coincides with the common longitudinal axis of the two concentric sleeve-shaped elements.

With the one or more cutting means arranged purely radially displaceable, the upper edges of said means should slope o radially outwards in the axial direction from the top/downwards, so that the cutting means is/are forced successively inwards, while said chamfer is passing the downward free end surface of the casing mentioned above. The use of return spring(s) is then avoided, and a more simple mechanical configuration is thereby obtained.

In another embodiment of an underreamer the cutting means or each cutting means is supported differently and has in part a different configuration.

Here the cutting means has a substantial extent in the longitudinal direction of the underreamer, and is supported pivotal in an elongate opening extending through the radially outer sleeve-shaped element. If there are several, for example three, cutting means equally spaced apart, each cutting means is individually pivotal about a transversal pin/bolt in the wall of the outer sleeve-shaped element.

Each elongate cutting means is pivotally supported like a two-armed lever with a short and a longer arm, the cutting edge being located on the radially outer portion of the long lever.

The radially inner sleeve-shaped element has a cam body, which acts as a mechanical activating device for the cutting means, and which extends in a curve of successively increasing/decreasing radial distance from the common longitudinal axis of the elements. Along their radially inner edges, the elongate pivotal cutting means each have a curved extent, essentially complementary to the mechanical activating device in the form of said axially extending cam body. By axial displacement of the radially inner element with the longitudinal activating cam body, possibly during rotation of same, a relative movement of the cam body and the adjacent pivotal cutting means takes place, to make the individual cutting means swing out radially about its axis, said axis being perpendicular to the side surfaces of the respective cutting means, between which side surfaces the surface of the cutting edge is positioned.

In the swung-out cutting position, only the long lever with the cutting edge surface/bit is swung out relative to the radially outer sleeve-shaped element, through the elongate longitudinal openings, extending therethrough; the short lever of the cutting means (with regard to its support as a two-armed lever) performs, on the other hand, a radially inward turn about its pivot. On the other hand, the entire radially innermost edge surface of this pivotal cutting means, or of each of these individually pivotal cutting means, takes a radially inward concave/convex course adapted to the curved cam surface portions of the longitudinally displaceable/rotatable cam body, so that the portion of the cam body exhibiting the largest effective diameter at the surface abutting the radially innermost surface of the adjacent cutting means, comes to bear and press outwards against said adjacent contact surface of the cutting means, especially over quite a long end portion at the free end thereof. When the cam body, which activates the cutting means, is brought back into its deactivated inner position, the cutting means are released in the cutting position, in which they exhibit radially outer surfaces, sloping outwards in the radial direction from the top/downwards, so that by the withdrawing movement of the underreamer, they will be influenced by the lower free end of the casing run last, which end will force the cutting means, with said sloping surfaces of the cutting means as a guiding edge, into an inactive position, radially retracted within the outer mantle surface of the inner sleeve-shaped element.

Further details of constructional features of the underreamer according to the invention, and favourable operational steps of the method according to the invention will appear from the following description of examples of preferred embodiments, which are adapted partly with a view to achieving a simple underreamer construction which is reasonable to buy/operate, and adapted partly to the favourable operational steps of a rational method.

Reference is made to the accompanying drawings, in which:

Fig. 1 illustrates, in a perspective view, an underreamer formed in accordance with the invention, wherein the radially outer one of two essentially concentric, sleeve-shaped relatively moveable elements has been cut axially, the three shear pins (only two are visible in the figure) have shorn, and the underreamer cutting device is in an active position with the cutting edge/bit of the cutting means positioned outside the outer mantle surface of the radially outer sleeve-shaped element, and in which can be seen the position of the radial inward pin (of three pins equally spaced apart along the circumference) of said outer mantle surface in a helical groove/slot (of three such helical grooves/slots, one for each pin) formed in a radially inner sleeve-shaped element of the two concentric sleeve-shaped elements;

Fig. 2 shows a side view, partly in a longitudinal section, of the underreamer according to Fig. 1, but here the three cutting means (only two are visible in the figure) are in an inactive position, retracted radially within the mantle surface of the radially outer sleeve-shaped element, said pin of this outer sleeve-shaped element now adopting another position in the helical groove/slot of the radially inner sleeve-shaped element, namely at the upper end of the groove, whereas the pin in Fig. 1 is at the lower end of the groove;

Fig. 3 is a cross-section along the plane of section identified by the arrows III-III in Fig. 2 in the area of the cutting means which can be pushed radially, and which are in an inactive position, and the activating rotational cam disc in its stand-by position, in which the three cam surface portions thereof with the shortest distance to its axis of rotation are those resting on the opposing contact/abutting end surface of the respective cutting means;

Fig. 4 is a cross-section along the plane of section identified by the arrows IV-IV in Fig. 2 in the area of intact shear pins, which prevent, before breaking, relative movement of the two concentric sleeve-shaped elements of the underreamer, and in the area of the three helical grooves formed in the sleeve wall of the radially inner sleeve-shaped element;

Fig. 5 corresponds to Fig. 2, but here the cutting means are in the active position, pushed radially outwards, with cutting edges/bits positioned outside the mantle surface of the radially outer sleeve-shaped element, where the radially inward guide pins are at the lower end (stop position) of the respective helical grooves of the concentric radially inner sleeve-shaped element of the underreamer; otherwise in accordance with Fig. 1; Fig. 3a corresponds to Fig. 3 (cfr. the plane of section identified by the arrows Illa-IIIa in Fig. 5), but shows the three cutting means and their activating means in the form of the rotational cam disc with three cam surfaces of the same extent relative to one another and following successively one behind the other in the direction of rotation, i.e. with the same division as the cutting means;

Fig. 6 - 11 illustrate another embodiment of the underreamer according to the invention, in which there is used, instead of cutting means (Figs. 1-5, 3a) which can be pushed radially, cutting means individually supported and individually pivotal, which are arranged to be pivoted by means of an activating means in the form of a cam body, in which the curvature of the cam surfaces are oriented in the longitudinal direction of the underreamer;

Fig. 6 showing in perspective this embodiment of an underreamer in the active position, with elongate knife-like cutting means which are swung out through their respectively associated openings in the sleeve wall of the radially outer element;

Fig. 7 being a side view, partially in an axial section of the sleeve wall of the radially outer element with the pivotal knife-like cutting means retracted into the inactive position within the mantle surface of the outer element;

Fig. 8 being a cross-section along the line VIII-VIII in Fig. 7;

Fig. 9 being a cross-section along the line IX-IX in Fig. 7; Fig. 10 corresponding to Fig. 7, but here the knife-like cutting means of the underreamer are swung out into their active positions;

Fig. 11 being a cross-section along the line XI-XI in Fig. 10.

Reference is made to the drawings, first to the embodiment shown in Figs. 1-5.

The reference numeral 10 generally identifies an underreamer in the form of an elongate body with a cylindrical mantle surface, comprising two concentric sleeve-shaped elements 12 and 14. The radially inner sleeve-shaped element 14 has a lower conically narrowing threaded pin end coupling 16, whereas the radially outer sleeve-shaped element 12 has an upper internally conical threaded box connection 18 at its upper end. by means of the end couplings 16 and 18 the underreamer 10 may be mounted into the drill string (not shown), which is provided with a drill bit downstream of the pin end 16.

The inner sleeve-shaped element 14 has a central bore 20 extending therethrough, through which drilling fluid and other fluid may flow. The outer sleeve-shaped element 12 similarly has a through bore, partly for receiving, relatively movable thereto, the radially inner sleeve-shaped element 14, partly for maintaining the uninterrupted continuous passage of fluid through the drill string.

The radially outer sleeve-shaped element 12, which has externally a smooth cylindrical mantle surface, has internally several longitudinal sections/portions in the axial extension of each other, differing from one another with respect to internal diameter. The radially inner sleeve- shaped element 14 correspondingly has a number of longitudinal sections/longitudinal portions differing from one another in outer diameter. Insofar as such adjacent opposite surface portion of respectively the radially outer and the radially inner sleeve-shaped element abut one another in a manner glidably displaceable/rotatably, they have, relative to the support of the elements 12,14, a limited movability relative to one another. For this relative movability it should be fully sufficient to establish that it involves a limited axial displacing movability and a limited rotational movability about the longitudinal axis of the underreamer.

In the embodiment shown in Figs. 1-3, Figs. 3a and 4-5, the distance of relative movement of the sleeve-shaped elements 12 and 14 is limited to the travelling distance of a radially inward pin 22, which is supported by the radially outer sleeve-shaped element 12, and in a glidingly displaceable manner engages a helical groove 24 in the form of a slot formed in the sleeve wall of the radially inner sleeve-shaped element 14, from the upper end of the helical groove 24, Fig. 2, to its lower end, Figs. 1 and 5. As appears best from Fig. 4, there are three such helical guiding grooves 24 formed in the radially inner sleeve-shaped element 14.

Three shear pins 26 equally spaced apart in the circumferential direction and cut in the position adopted by the cutting means according to Fig. 1, in which the shorn shear pins 26 are identified by 26a, each have a radial extent and engage by equally big parts the sleeve wall of the radially outer sleeve-shaped element 12 and of the radially inner sleeve-shaped element 14.

In order to shear the shear pins 26, the torque applied to the drill string is increased, said torque being kept at a relatively low to moderate level while the drill bit, with the underreamer in its retracted inactive stand-by position, is drilling through said plug of cement mixture in the lower portion of the well bore, and then is continuing to drill the hole to a position at such a great distance from the lower end of the casing run and cemented last, that it is completely safe to bring the cutting means 28 of the underreamer out into their active cutting position.

It appears from the cross-sections in Fig. 3 and Fig. 3a, that there are three cutting means 28 spaced apart by equal angular distance in the circumferential direction. These cutting means 28 have a guide each and are each kept in place, in the active position, Fig. 3a, as well as in the inactive position, Fig. 3, in a respective radial port 30 extending through the outer sleeve wall, each cutting means 28, which has the form of an elongate rectangular plate, having an inner stop means 32 directed in the circumferential direction, for which there is formed a complementary recess 34 in the sleeve wall of the radially outer element 12.

While each cutting means 28 is being pushed radially outwards, the radially inner stop means 32 thereof finally engages the recess 34 of the sleeve wall of the radially outer sleeve-shaped element 12, and the movement of the respective cutting means 28, which is being pushed outwards, is thereby brought to an end, while each cutting means 28 is in a highly stable and well supported position, with about half of its radial extent retained safely in the port 30 and fixed by the matching engagement of the stop means 32 in the recess 34.

Reference is made to Figs. 3 and 3a.

In Fig. 3 all three cutting means 28 are fully retracted, i.e. withdrawn radially, so that they are entirely within the mantle surface of the radially outer sleeve-shaped underreamer element 12, corresponding to the position of the guide pins 22 at an upper end of the helical grooves 24, Fig. 2, whereas Fig. 3a shows the cutting means 28 in their active cutting position, completely pushed out, corresponding to the active position of the underreamer, and corresponding to the position of the guide pins 22 at the lower end of the helical grooves 24, Figs. 1 and 5.

An activating means 36 for the cutting means 28, in the form of a body similar to a cam disc, is included as a longitudinal portion/section of the radially inner sleeve- shaped underreamer element 14.

In a transversal, sectional plane, Figs. 3 and 3a, the activating means 36, resembling a cam disc, has three identical cam surfaces 38 following one behind the other in the circumferential direction.

According to Fig. 3 the radially inner edge surface of each of the cutting means 28 is oriented with its long sides in an upright position. (The scale is larger in Figs. 3, 3a and 4 than in Figs. 1, 2 and 5). The rectangular plate-shaped cutting means 28 has an upper chamfer 28", i.e. an outer edge surface that slopes radially outwards from the top downwards .

Thus, when the radially outer underreamer element 12 is lifted from the active position, shown in Figs. 1 and 5, into the inactive position shown in Fig. 2, by an upward pull at the drill string (not shown), to which the upper end 18 of the underreamer element 12 is firmly screwed, the cutting means 28 in the active position shown in Fig 3a, but with the cam-disc-resembling activating body 36 turned back into its inactive position, according to Fig. 3, will sooner or later contact the lower free end of the casing (not shown) run and cemented last. By chamfered contact surfaces 28' on each cutting means 28, as indicated above, the chamfers will work as guides and cause the cutting means to be forced radially inwards as long as their chamfers touch said downward-facing free circular inner circumference of the casing end.

In another embodiment of the underreamer according to Figs. 6-11 the same or corresponding reference numerals are used for parts, which correspond constructionally and/or functionally to parts in the embodiment according to Figs. 1- 5.

The elongate knife-like cutting means 28A have radially outer guiding, sloping cutting edges 29A and are pivotally supported one by one like two-armed levers on respective transversal pins 40 supported in the wall of the outer sleeve-shaped element 12. In the position shown in Fig. 7 the cutting means 28A are in their inactive position retracted within the outer surface of the outer sleeve-shaped element 12, Fig. 9.

Opposite a radially outer sloping cutting and guiding edge 29A, each cutting means 28A has an actuation surface 31A with a radially inward concave curvature, formed to cooperate with a cam surface of a three-dimensional curvature on an axially elongate activating means 36A, which changes its activating position through axial displacement or by moving along a helical path of travelling.

As in the preceding embodiment, the underreamer is placed against the bottom of the well bore to initiate a relative axial displacing movement of the two sleeve-shaped elements 12 and 14. The inner sleeve-shaped element 14, according to Fig. 6, has moved down relative to the inactive position shown in Fig. 7 , by a distance corresponding approximately to the axial length of the helical grooves 24A.

The radially outward convex cam surface of the activating means 36A has a short longitudinal portion, where the radial extent is at its maximum. Fig. 10 shows that when this portion with the maximum diameter is bearing pressingly on the cutting means 28A at the free lower ends thereof, the cutting means 28A adopt their active positions, Fig. 11, in which their radially outer cutting edges also form sloping, guiding edges extending radially outwards, counted from above downwards .

When the cutting means 28A of the underreamer 10 are to be folded in and brought into an inactive position, the process is the same as by the previous embodiment, i.e. a pull is exerted at the upper end of the underreamer 10, whereby the cutting means 28A will adopt the position shown in Fig. 7.

The knife-like cutting means 28A are arranged to swing through opening slots 30A through the sleeve wall of the outer sleeve-shaped element 12.

Claims

c l a i m s

1. An underreamer formed to be mounted in a drill string with a drill bit downstream, comprising an elongate sleeve- shaped body (10) with an axial bore (20) extending therethrough, surrounding one or more radially movable cutting means (28; 28A), which are movable between a retracted inactive position and an active position, in which cutting portions/edges of said one or more cutting means (28; 28A) extend radially beyond the outer surface of the body ( 10 ) , c h a r a c t e r i z e d i n that said elongate sleeve-shaped body (10) forms the radially outer element (12) of two essentially concentric sleeve-shaped elements (12, 14), which are arranged to be displaced relatively in the axial direction, and whose relatively opposite ends (18, 16) are formed to be connected to pipe end couplings in the drill string/drill bit, and that said one or more cutting means (28; 28A) has/have a mechanical activating device (36; 36A) arranged thereto and arranged to make use of said possibility of relative movement of the two concentric sleeve-shaped elements (12, 14) incorporated in the elongate body (10) of the underreamer, and convert said relative movement into a movement displacing the one or more cutting means , essentially in the radial direction, between the inactive and the active position, or vice versa, of the cutting means (28; 28A).

2. An underreamer as claimed in claim 1, c har ac t e r i z ed i n that the two essentially concentric sleeve-shaped elements (12, 14) supported relatively displaceable and rotatable, have been assigned a movement- o converting device, in which a pin-like guide means (22) secured to one element (12), in a slidingly displaceable manner engages a helical groove (24), which extends generally in the longitudinal direction, and which is formed in the sleeve wall of the second element (14), so that a longitudinally displacing movement of one element is converted into a rotational movement of the other element.

3. An underreamer as claimed in claim 1 or 2 , c h ar ac t e r i z e d i n that shear pin means (26, 26a) engage adjacent portions of the sleeve walls of the elements (12, 14), preferably in an axial end portion of each sleeve wall.

4. An underreamer as claimed in claim 1, 2 or 3, c h a r ac t e r i z e d in that the cutting means or each cutting means (28) has a body formed essentially as a thick plate with a sloping outer edge portion (28'), of a radial outward slope in the direction downstream in the active position of the cutting means.

5. An underreamer as claimed in claim 4, c h a r ac te r i z e d i n that the cutting means or each cutting means (28) is mounted for radial displacement in a corresponding port-like recess (30) of the outer sleeve- shaped element (12), and that the cutting means (28) has a stop means (32) limiting its radial movement when it is being pushed out.

6. An underreamer as claimed in claims 2 and 5, c h ar ac t e r i z ed i n that the activating device of the one or more cutting means (28) consists of a rotatable cam disc (36), which is included coaxially in the radially inner sleeve-shaped element (14), following the movements thereof, and which has, for the cutting means or each cutting means (28), a cam surface (38) abutting the radially inner end surface thereof, and having a radial distance to the centre of rotation of the cam disc (36), which changes successively in the direction of rotation of the cam disc.

7. An underreamer as claimed in claim 1, c h a r a c - t e r i z e d i n that the cutting means or each cutting means (28A) has an elongate knife-like configuration with a radially outer sloping cutting edge, which in its active position, directed downstream, slopes radially outwards, and that the knife-like cutting means (28A) is pivotally supported like a two-armed lever with the possibility of swinging out/in through an opening (30A) through the sleeve wall of the radially outer element (12).

8. An underreamer as claimed in claims 1 and 7, c h a r ac te r i z e d i n that the activating device for the one or more cutting means (28A) comprises a displaceably movable cam body (36A) extending axially with three- dimensionally curved convex cam surfaces, and that the radially inner edge surface (31A) of the one or more cutting means abutting the cam body (36A) and cooperating therewith, o causes a pivoting of the cutting means about a transversal pin (40) when the cam body (36A) is brought into a displacing/rotational movement, with the aim of moving the cutting means (28A) from its idle position into its active position or vice versa.

5 9. A method of use of the underreamer according to claims 1- 6, c h ar ac t e r i z e d i n that the drill string - in which the underreamer is mounted, upstream of the drill bit, and is incorporated as an elongate body comprising two concentric sleeve-shaped elements (12, 14) with an axial 0 passage (20) therethrough, and positioned for relative axial displacement - is lowered to bear on the bottom of the well bore to initiate said axial displacing movement of the two concentric sleeve-shaped elements, which linear movement is possibly converted into a rotational movement by an activating device (36) for the cutting means (28), and that through the movement caused in it, the activating device (36; 36A) forces the cutting means (28; 28A) out into its active position, and that - when the underreaming is completed - the drill string is lifted from its bearing on the bottom of the well bore, with the purpose of causing another reverse, relative, linear movement of the two concentric sleeve-shaped elements (12, 14) to enable the cutting means (28; 28A) to be retracted/pushed in.

10. A method as claimed in claim 9, and in which the underreamer (10) is additionally provided with one or more shear pins (26, 26a) extending through the sleeve walls of the two concentric elements (12, 14), c h a r ac t e r i z ed i n that the drill string is subjected to an increase in the torque from a surface position, which torque increase brings about the shearing of the shear pin(s) (26; 26a) .