GB2350386A - Traction apparatus - Google Patents

Abstract

A traction apparatus for propulsion along a bore has a front portion 13 preferably having a front brush section 18, and a rear portion 14 preferably having a rear brush section 19 for engaging an inner traction surface 10 at locations spaced apart the bore. Each portion 13, 14 is adapted to engage the traction surface 10 so that the traction apparatus is movable relatively freely in one direction along the bore, but substantially less freely in the opposite direction. In a first phase, one of the portions 13,14 is moved along the traction surface 10 in one direction, while the other portion 14,13 is prevented from substantially moving in the opposite direction. In a second phase, which alternates with the first phase, the other portion 14,13 is moved along the traction surface 10 in the one direction whilst the one portion 13,14 is prevented from substantially moving in the opposite direction.

Description

2350386 7raction Apparatus" This invention relates to traction apparatus,

and is concerned especially, but not exclusively, with traction apparatus for use in a downhole tool which is adapted for operation in horizontal wells or bores.

Reference is also made to British Patent Application No. 9903322.7 of which this application is a divisional.

Within the oil and petroleum industry there is a requirement to deploy and operate equipment along bores in open formation hole, steel eased hole and through tubular members such as marine risers and sub-sea pipelines. In predominately vertical sections of well bores and risers this is usually achieved by using smaller diameter tubular members such as drill pipe, jointed tubing or coiled tubing as a string on which to hang the equipment. In many cases the use of steel cable (wire line), with or without electric conductors installed within it, is also common. All of these approaches rely on gravity to provide a force which assists in deploying the equipment.

In the case of marine pipe lines which are generally horizontal, "pigs" which are basically pistons sealing against the pipe wall, are used to deploy and operate cleaning and inspection equipment, by hydraulically pumping them along the pipe, normally in one direction.

Within the oil and petroleum industry to date the requirement to deploy equipment has been fulfilled in these ways.

However, as oil and gas reserves become scarcer or depleted, methods for more efficient production are being developed.

In recent years horizontal drilling has proved to enhance greatly the rate of production from wells producing in tight or depleted forrnation. Tight formations 1 2 typically are hydrocarbon-bearing formations with poor permeability, such as the Austin Chalk in the United States and the Danian Chalk in the Danish Sector of the North Sea.

In these tight formations oil production rates have dropped rapidly when conventional wells have been drilled. This is due to the small section of producing formation open to the well bore.

However, when the well bore has been drilled horizontally through the oil producing zones, the producing section of the hole is greatly extended resulting in dramatic increases in production. This has also proved to be effective in depleted formations which have been produced for some years and have dropped in production output. - However, horizontal drilling has many inherent difficulties, a major one being that the forces of gravity are no longer working in favour of deploying and operating 0 equipment within these long horizontal bores.

This basic change in well geometry has led to operations which normally could have been carried on wireline in a cost effective way now being carried out by the use of stiff tubulars to deploy equipment, for example drill pipe and tubing conveyed logs which cost significantly more than wireline deployed logs.

Sub-sea and surface pipeline are also increasing in length and complexity and pig technology does not fully satisfy current and future needs. There is currently a need for a traction apparatus which can be used effectively in downhole applications including horizontal bores.

According to the present invention there is provided a traction apparatus for propulsion along a bore, the apparatus comprising a body having first and second traction members spaced along the body for engaging an inner traction surface of the bore at locations spaced apart along the bore, each traction member being adapted to engage the traction surface such that the traction member is movable relatively freely in one direction along the bore, but substantially less freely in the opposite direction along 3 the bore, the first traction member being fixedly mounted on an axial shaft and the second traction member being mounted on the shaft so as to pen-nit axial movement between the second traction member and the shaft, and propulsion means for forcing, the traction members alternately apart and together to move the body along the bore, the propulsion means acting, in a first phase, to move one of the traction members along the traction surface in said one direction while the other traction member is prevented from substantially moving in said opposite direction, and the propulsion means acting, in a second phase, which alternates with the first phase, to move said other traction member along the traction surface in said one direction while said one traction member is 10 prevented from substantially moving in said opposite direction.

The invention will now be described, by way of example, with reference to accompanying drawings, in which:

Figure 1 shows an embodiment of traction apparatus incorporated in a downhole tool; Figure 2a is a schematic cross-sectional view of traction apparatus in accordance with the present application which is hydraulically powered in use; Figure 2b is a graph showing hydraulic fluid pressure versus time for the apparatus of Figure 2a in use; Figure 3 is a schematic cross-sectional view of an embodiment of the parent application in use; Figure 4a is a schematic cross-sectional view of a detail of the embodiment of Figure 3 with a variation in configuration; Figure 4b is a schematic cross-sectional view of part of a farther variation of the embodiment of Figure 3; 0 3 4 Figure 4c is a cross-sectional view showing a detail of the embodiment of Figure 4b; Figures 5a, 6a and 7a are schematic illustrations showing side views of the sequential positions of elements of a further embodiment of the parent application in use; Figures 5b, 6b and 7b are schematic end views corresponding to Figures 5a, 6a and 7a, respectively; Figures 8a and 8b show schematically embodiments of brush sections suitable for use in embodiments of apparatus in accordance with the present invention; and Figures 9a and 9b show, respectively, a perspective view and a cross- sectional view of an embodiment of a pig which includes traction members.

Figure 1 shows an embodiment of traction apparatus incorporated into a downhole tool 1. The downhole tool comprises a body 2 which is elongate and which has a threaded front end portion 3 and a threaded rear end portion 4 to allow attachment within a tool string. (It should be appreciated that the terms "front end" and "rear end" are used for convenience only and should not be considered limiting. Terms such as "in front" and "rearward", which will be used hereafter, are to be understood accordingly.) The tool body is provided with brush portions of which three, designated 5a, 5b and 5c, are shown. Each brush portion 5a, 5b and 5c includes a number of brush sections and each brush section includes a large number of resilient bristles which, in this embodiment, comprise traction members, and which extend outwardly from the body 2. The bristles thus have inner ends attached to the body and outer ends distal from the body.

If the downhole tool I is inserted front end first into a bore with a diameter larger than the diameter of the body 2 but slightly smaller than the external diameter formed by the outer ends of the bristles, then the bristles will be bent back, by the contact with the inner wall of the bore, such that the outer ends of the bristles are axially behind the inner ends of the bristles. Under these circumstances the outer ends of the bristles will contact the inner wall of the bore and will offer more resistance to rearward motion of the tool than to forward motion of the tool. The bristles therefore move preferentially in the forward direction as opposed to the rearward direction. Preferred embodiments of the present invention employ the principle behind this phenomenon to allow propulsion of a tool by providing relative movement or oscillation between two or more brush sections (i.e. two or more groups of bristles constituting traction members).

Figure 2a shows schematically an embodiment of traction apparatus in accordance with the present invention. The apparatus comprises first to fifth sections 12a to 12e respectively.

The sections 12a to 12e are connected by a pipe 16 which carries hydraulic fluid.

First to fourth resilient members 17a to 17d are provided between the first to fifth sections 12a to l2e.

The apparatus, as illustrated in Figure 2a, is provided within a horizontal bore which has an inner wall 10 the surface of which constitutes an inner traction surface.

The second section 12b of the apparatus will now be described in detail. The other sections 12a, 12c, 12d, 12e are similar in structure and function and will not be separately described in detail.

The second section 12b inc-ludes a front portion 13 provided with a front brush section 18 and a rear portion 14 provided with a rear brush section 19. The brush portions 18, 19 are formed from resilient bristles which are, in use, deformed by contact with the inner wall 10 so that the outermost end of each bristle is to the rear of the innermost end of the bristle. The bristles thus constitute traction members which are adapted to move preferentially in one direction (to the right as shown in Figure 2). The rear portion 14 is fixed around the pipe 16, is co0axial with the pipe 16, and includes a larger diameter part 14a and a smaller diameter part 14b. The smaller diameter part l4b is forward of the larger diameter part 14a. Where the diameter changes between the 6 larger diameter part 14a and the smaller diameter part 14b an abutment shoulder 14c is formed.

The front portion 13 is able to move axially with respect to the pipe 16 and is sealed against the pipe 16 by a sliding seal 20. The front portion is cup shaped having a base part 13a which contacts the pipe 16 and a cylindrical hollow part 13b, extending rearward from the base part 13a, which is radially spaced apart from the pipe 16.

The inner diameter of the hollow part 13b of the front portion 13 is substantially the same as the outer diameter of the smaller diameter part 14b of the rear portion 14. The smaller diameter part 14b fits inside the hollow part 13b and a sliding seal 15 is provided therebetween. As the rear portion 14 is fixed with respect to the pipe 16 and the front portion 13 is able to move axially with respect to the pipe 16, the hollow part 13b is able to move axially with respect to the smaller diameter part 14b so as to cover more or less of the smaller diameter part 14b.

The hollow part 13b has a longer axial length than the smaller diameter part 14b so that, when the smaller diameter part 14b is completely covered by the hollow part 13b, the rearmost end of the hollow parts 13b abuts the abutment shoulder 14c but the forwardmost end of the smaller diameter part 14b does not reach the base part 13a of the front portion 13. A hydraulic fluid space 21 is formed between the base part 13a and the forwardmost end of the smaller diameter part 14b. A hydraulic fluid outlet 22 from the pipe 16 is provided to supply fluid to the hydraulic fluid space 21.

In use, the hydraulic fluid pressure in the pipe 16 is increased to force fluid into the hydraulic fluid space 2 1. This forces apart the front portion 13 and the rear portion 14. Since the front portion 13 is less resistant to forward motion than the rear portion 14 is to rearward motion (because of the action of the brush portions 18, 19) this results in the front portion 13 being forced forward while the rear portion 14 stays stationary.

This results in axial lengthening of the hydraulic fluid space 21 and compression of the second resilient member 17b.

7 The hydraulic fluid pressure in the pipe 16 is then reduced so that the front portion 13 and the rear portion 14 are forced together by the action of the resilient member l7b, forcing hydraulic fluid from the hydraulic fluid space 21 via the outlet 22 into the pipe 16. As the front portion 13 and the rear portion 14 are forced together, the considerable resistance of the front portion 13 to rearward motion ensures that the front portion remains substantially stationary with respect of the inner wall 10 of the bore, so the rear portion is forced for-wards with respect to the inner wall 10.

Each cycle of increase and decrease of fluid pressure in the pipe 16 therefore results in the apparatus taking a "step" in the desired direction along the bore. It should, of course, be appreciated that, although the above has been described with respect to only one section 12b of the apparatus of Figure 2a, the other sections 12a, 12c, 12d, l2e respond similarly to increases and decreases in fluid pressure. Figure 2b shows how fluid pressure may be varied with time in order to obtain movement of the apparatus at a rate of about two steps per second. (One PSI is equal to about 6.9 x 103 Pa.) Figure 3 shows an alternative embodiment of a downhole tool Ia including 0 traction apparatus according to the parent application suitable for use on an electric line.

Figure 4a schematically shows a detail of a variation of the embodiment of Figure 3. The downhole tool Ia is illustrated in Figures 3 and 4a as being within a horizontal bore with an inner wall I Oa. Referring to Figure 3, the tool I a has a front end portion 3a and a rear end portion 4a. Furthermore the tool I a includes an electric m otor which drives an axle 31 aligned axially along the centre of the tool Ia. The axle 31 extends axially from the motor and is journaled at its end distal from the motor 30 in a bearing 32.

Mounted on the axle 31, between the motor 30 and the bearing 32, are first to sixth collars 41 to 46 which are inclined, at an angle away from the normal, with respect to the axis of rotation of the axle 30. First to sixth annular brush sections 51 to 56 are mounted respectively on the first to sixth collars 41 to 46 via first to sixth annular bearings 61 to 66, as may be appreciated more particularly by referring to Figure 4a.

8 For conciseness only one the first of the collar-bearing-brush assemblies will be described in detail, but it will be appreciated that the other assemblies correspond.

Still referring to Figure 4a, the collar 41 is fixed to an annular inner race 61 a of C.

the bearing 61 which rotatably supports, via a plurality of rolling members 61b, an annular outer race 61c of the bearing 61. Upon the outer race 61c of the bearing 61 is fixed an annular base part 51a of the brush section 51, which supports a plurality of bristles 5 1 b of brush section 5 1.

When the axle 31 is rotated by the motor 30 the collar 41 rotates so that its leading edge rotates about the axis of the axle 31. Because it is supported on the bearing 61, the first brush section 51 is not caused to rotate by the rotation of the collar 4 1. However, as the collar 41 rotates, the base part 5 1 a of the brush section 5 1 is moved so that any given point on the base part 5 1 a is moved one cycle backwards and forwards relative to the axle for each rotation of the axle.

The bristles 51b of the first brush section 51 are thus forced forwards and backwards in contact with the inner wall 10a. The bristles move preferentially in the forward direction and thus provide little reaction force on the too] when moved forward against the inner wall 10a. By contrast, the bristles offer considerably more resistance when forced in the rearward direction and thus provide considerable reaction force on the tool. Rotation of the axle 31 thus provides a net forward force to propel the tool in the for-ward direction.

As illustrated in Figures 3 and 4a, a number of brush sections 51 to 56 are provided in order to provide greater traction than would be afforded by any one of the brush sections. It is preferable to have the brush sections out of phase in order to distribute the thrust circi4mferentially around the tool. In Figure 3 each of thq brush sections is shown as being 180 degrees out of phase with the adjacent brush sections, so that, as shown, the uppermost parts of the second, fourth and sixth brush sections 52, 54, 56 are forwardmost and the lowermost parts of the first, third and fifth brush sections 51, 53, 55 are forwardmost.

9 In the variation of Figure 4a a different phase distribution is illustrated. In particular the forwardmost part of the third brush section 53 is the part which would extend furthest out of the page (not shown), and the forwardmost part of the fourth brush section 54 is the part which extends furthest into the page. Thus, in Figure 4a, each of the brush sections 51 to 56 is 180 degrees out of phase with a first one of its neighbours, and each brush section which has two neighbours is also 90 degrees out of phase with the second of its neighbours. Such an arrangement can provide improved stability under traction. It should be noted that, in Figure 4a, because the planes of the third and fourth brush sections 53, 54 are not normal to the page, more of the base parts 53a, 54a and bristles 53b, 54b of the third and fourth brush sections 53, 54 can be seen than of the other brush sections.

Figure 4b illustrates a further variation of the embodiment of Figure 3. Figure 4c shows in detail part of the embodiment of Figure 4b. As shown in Figure 4b, first and second brush sections 57, 58 are mounted on an axle 131 which can be rotated by a motor 130. The brush sections 57, 58 each include a base section 57a, 58a and bristles 7b, 5 8b for engaging the inner wall 1 Oa.

Mounted on the axle 131 are first and second collars 47, 48 corresponding generally to the collars 41 to 46 of the embodiment of Figure 3. Attached to the collars 47, 48 are first and second annular bearings 67, 68, corresponding generally to bearings 61 to 66 of the embodiment of Figure 3 and each including an annular inner race 67a, 68a, rolling members 67b, 68b and an annular outer race 67c, 68c. Attached to the respective outer races 67c, 68c of the bearings 67, 68 are respective annular brush-base holders.67d, 68d, each adapted to receive one or more brush base sections. Thus the brush base sections 57a, 58a are not attached directly to the bearing outer races 67c, 68c, but are instead fitted into the brush base holders 67d, 68d facilitating replacement of the brush sections 57, 58.

Unlike the collars 41 to 46 of Figures 3 and 4a, the collars 47, 48 in the embodiment of Figures 4b and 4c are mounted on the axle 131 by fixing pins 47a, 48a which extend through respective holes 47b, 48b which pass through the collars 47, 48 in a direction perpendicular to the axle 13 1.

In this embodiment the brush sections 57, 58 are positioned forwardly of the centre lines of the bearings 67, 68 so that the bristles 57b, 58b are movable towards and 0 away from the inner wall 10a by rotation of the axle 131. This swashing action is particularly advantageous as it ensure that the bristles are urged into increasing contact with the inner wall as the bristles are moved backwards to propel the tool in the forward direction, whilst decreasing the contact pressure as the brist les are moved in the forward direction in which little reaction force is provided on the tool.

The embodiments of Figures 3 to 4c thus provide traction apparatus in which traction, and corresponding motion, is provided by moving different traction members (bristles in-this embodiment) which are rigidly connected to each other (via the brush base parts) at different velocities in the axial direction, at any given time.

Figures 5a, 5b, 6a, 6b, 7a and 7b illustrate the action of a traction device, constituting a further embodiment of the parent application, in which axial motion is provided by forcing traction members in a radial direction with respect to the downhole tool lb.

In this embodiment, the downhole tool lb is provided with first to eighth brush sections of which, for clarity in the drawings, the first and second brush sections 71, 72 are shown in each of Figures 5a to 7b, the third and fourth brush sections 73, 74 are shown in Figures 5b, 6b and 7b only, the first and sixth brush sections 75, 76 are shown in Figures 5a, 6a and 7a only, and the seventh and eighth brush sections are not shown at all.

Each of the brush sections 71 to 76 is attached to the main body of the downhole tool I b by a respective arm member 81 to 86 which is radially extendable away from the main body of the tool lb.

Figures 5a and 5b show the positions of the arm members 81 to 86 and brush sections 71 to 76 in an inactive position in which all of the arms 81 to 86 are in their respective retracted positions and the outermost ends of the brush sections 71 to 76 (that I I is the outermost ends of the bristles) are in light contact with an inner wall 10b of a horizontal bore.

Figures 6a and 6b show the positions of the arm members 81 to 86 and brush sections 71 to 76 at a first stage in a traction cycle. At this time the arms 81 to 84 of the first to fourth brush sections 71 to 74 are fully radially extended, forcing the bristles of the brush sections7l to 74 against the inner wall 10b. This radial extension causes the brush sections 71 to 74 to push against the inner wall l0b in the backward direction, which applies a reaction force in the forward direction (rightward as shown in Figures 5a, 6a, 7a) on the body of the tool lb. The force will tend to move the body of the tool in the forward direction. The broken lines in Figures 6a to 7a correspond to the positions of the brush sections 71 to 76 in Figures 5a and 5b so that the forward movement can be appreciated. As shown in Figure 6a, at this point of the traction cycle the fifth and sixth arms 85, 86 and seventh and eighth arms (not shown) remain in their retracted position.

Figures 7a and 7b show the positions of the arm members 81 to 86 ad brush sections 712 to 76 at a second stage in the traction cycle. At this time the fifth and sixth arms 85 and 86 and the seventh and eighth arms (not shown) are fully radially extended forcing the fifth and sixth brush sections 75, 76 and the seventh and eighth brush sections (not shown) against the inner wall 10b. As in the case of the first to fourth brush sections 71 to 74, described above, this applies a force and corresponding movement to the body of the tool Ia in the forward (rightward) direction. The first to fourth arms retract as the fifth to eighth arms extend so that, as shown in Figures 7a and 7b, the first to fourth arms are fully retracted when the fifth to eighth arms are ful.1y extended.

Continuous cycling between the position shown in Figures 6a, 6b and the position shown in Figures 7a, 7b will impart a continuous propulsive force to'the body of the tool Ia. Embodiments are envisaged in which traction members may be moved both axially and radially and either the axial movement or the radial movement might predominate.

12 One of the many driving mechanisms may be used to extend and retract the arms 81 to 86. For example, mechanical means, such as a rotating shaft with four-lobed cams, could be used. Alternatively, a hydraulic system could be employed. As a further alternative an electro -mechanical system could be used. It will also be appreciated that these and other driving mechanisms could be suitable for driving the motion of the traction members in the other embodiments of the invention.

It will be appreciated that, in certain embodiments, the traction members will, in equilibrium (that is when not contacting a traction surface), be substantially perpendicular to the axis of the traction apparatus. In such embodiments, it is the constriction of the traction members which effectively sets the preferential direction of motion. In such embodiments it may be possible to reverse the preferential direction of motion by overpulling the tool, i.e. by providing a sharp or jarring force. In other embodiments, it may be more appropriate to reverse the preferential direction by retracting and redeploying the traction members.

It will be appreciated that, although the preferred embodiments described herein are disclosed as including brushes in which the bristles constitute traction members, other types of traction member may be used provided that they are able to contact the traction surface and, when in contact, move preferentially in one direction as compared with the other direction. It is preferred that the traction members are resilient elongate members, such as leaf springs or bristles. In the case of bristles, it is preferred that the bristles be encapsulated in a block of resilient material in order to reduce wear.

Figures 8a and 8b show embodiments of first and second brush sections 180a, 180b, respectively. Figure 8a shows a round brush section 180a having a number of bristles 182a encapsulated in a matrix 184a of urethane or other suitably resilient material. The bristles 182a are supported in a brush base section 186a comprising a generally cylindrical metal casing for holding the bristle bases. A threaded connection portion 188a is provided facilitating easy Fitting and replacement. Other types of connection could, of course, be used. In this embodiment only the bristle tips are uncovered by the matrix 184a.

13 Figure 8b shows a rectangular brush section 180b having a number of bristles 182b encapsulated in a rubber matrix 184b. the bristles 182b are supported in a brush base 186b which consists of a block of foundation material. A connection portion 188b is provided. In this embodiment a predetermined length of the bristles 182b extends from the outer end of the rubber matrix 184b.

The contact of the traction members on the traction surface is important in order to obtain preferential movement in one direction. In preferred embodiments it is desirable that the ends or tips of the traction members engage the traction surface. The length of the traction members is therefore important, since if a traction member is too short it might not reach the traction surface, and if the traction member is too long it might be an axial surface of the traction member, rather than the tip of the traction members, which engages the traction surface. In practice, for many types of traction member, a range of lengths provide an acceptable result. Choice of length may be of particular importance in embodiments such as those of Figures 3 to 7b in which the distance between the innermost end of the traction member and the traction surface varies during operation of the apparatus. It is desirable that an effective length of traction member is maintained at all times.

It should be appreciated that the distribution of the traction members may be varied according to the circumstances. It is desirable, but not essential, to have traction members diametrically opposed on the apparatus in order to maintain good stability.

Traction members may (or groups of traction members) may be axially or circumferential ly spaced as desired. The number and properties of the traction members may also be varied according to the circumstances.

Figures 9a-and 9b show a pig 90 including bristles 92 encapsulated in a matrix 94. The bristles 92 are set into an annular bristle base 96 made of a foundation material, in an inclined manner. Outer tips 92a of the bristles 92 extend out of the matrix 94 for engaging the inner wall I Oa.

In use, the pig 90 can be moved to a desired position, for example on a drill string, by application of continuous fluid or gas pressure on the rearward side (the 14 leftward side as shown in Figure 9b. When the progress of the pig is impeded such that the continuous pressure is insufficient to move the pig in the desired direction, the pig can be oscillated in order to provide traction because of the preferential motion of the bristle tips 92a against the wall 1 Oa in the forward direction. 5 Modifications and improvements may be incorporated without departing from the scope of the invention.

Claims (11)

CLAIMS:

1. A traction apparatus for propulsion along a bore, the apparatus comprising a body having first and second traction members spaced along the body for engaging an inner traction surface of the bore at locations spaced apart alone, the bore, each traction member being adapted to engage the traction surface such that the traction member is movable relatively freely in one direction along the bore, but substantially less freely in the opposite direction along the bore, the first traction member being fixedly mounted on an axial shaft and the second traction member being mounted on the shaft so as to permit axial movement between the second traction member and the shaft, and propulsion means for forcing the traction members alternately apart and together to move the body along the bore, the propulsion means acting, in a first phase, to move one of the traction members along the traction surface in said one direction while the other traction member is prevented from substantially moving in said opposite direction, and the propulsion means acting, in a second phase, which alternates with the first phase, to move said other traction member along the traction surface in said one direction while said one traction member is prevented from substantially moving in said opposite direction.

2. A traction apparatus according to claim 1, wherein the propulsion means comprises hydraulic means for introducing hydraulic fluid into, and removing hydraulic fluid from, a chamber in order to force the traction members apart and together.

3. A traction apparatus according to claim 2, wherein the hydraulic means comprises a piston and cylinder mounte " d on the shaft and bearing the traction members, the piston and cylinder defining the chamber for hydraulic fluid therebetween.

4. A traction apparatus according to claim 2 or 3, wherein the hydraulic means includes an axial passage within the shaft for supplying hydraulic fluid to the chamber.

5. A traction apparatus according to claim 4, wherein the chamber is in fluid communication with the axial passage by way of at least one fluid orifice in an outer wall of the shaft.

16

6. A traction apparatus according to claim 3, 4 or 5, wherein the piston is fixed to the shaft and the cylinder surrounds the piston and is sealingly mounted on the shaft so as to be slidable along the shaft relative to the piston.

7. A traction apparatus according to claim 3, 4, 5 or 6, wherein the piston comprises a larger diameter part bearing one of the traction members and a smaller diameter part which fits within the cylinder, the cylinder bearing the other traction member at a position remote from the larger diameter part of the piston.

8. A traction apparatus according to any of claims 2 to 7, wherein the hydraulic means is adapted to supply pulsed hydraulic fluid to the chamber in order to move the traction members apart and together repetitively.

9. A traction apparatus according to any preceding claim, which comprises a plurality of bodies bearing first and second traction members and coupled together by being mounted on a common shaft so that movement apart and together of each pair of traction members contributes to movement of the apparatus along the bore.

10. A traction apparatus according to claim 9, wherein the bodies are coupled together by resilient members permitting movement of the traction members of one body relative to the or each adjacent body.

11. A traction apparatus substantially as hereinbefore described with reference to 25 Figures 2a and 2b of the accompanying drawings.