WO1991008374A2

WO1991008374A2 - Apparatus and method for centralising a casing

Info

Publication number: WO1991008374A2
Application number: PCT/GB1990/001891
Authority: WO
Inventors: Doug Morrison
Original assignee: Lasalle Engineering Limited
Priority date: 1989-12-05
Filing date: 1990-12-05
Publication date: 1991-06-13
Also published as: WO1991008374A3; GB8927502D0; AU7896491A

Abstract

Apparatus for centralising a casing comprises a centraliser (10), for example, a bow centraliser, having a plurality of spring bows (24) connecting to spaced end collars (20). A stop collar (12) is secured on to the casing (2). The stop collar (12) is arranged on the casing (2) between the two end collars (20) of the centraliser (10) within the spring bows (24) so that the stop collar (12) selectively abuts one of the end collars (20) and retains the centraliser on the casing. The stop collar (12) is arranged to be adhered to the casing (2) and for added security, grips the adhesive bond.

Description

IMPROVEMENTS IN OR RELATING TO AN APPARATUS AND METHOD FOR CENTRALISING A CASING

The present invention relates to an apparatus and method for centralising a casing.

In drilling operations, it is common to centralise a casing within a well bore or an outer casing to ensure the integrity of cement inserted between the inner casing and the outer casing or well bore. It has been found that it is helpful to reciprocate or rotate the casing during cementing to ensure a successful cementing operation.

Currently, casings are centralised using bow or rigid centralisers which are located on the casing by grub screw or wedge pin fastenings. Such fastenings have a number of problems. They protrude beyond the bows of the centraliser and are therefore unsuitable for use in well bores or outer casings with tight annular clearances. Furthermore, the existing fastenings do not usually give sufficient grip to the casing to prevent slippage. This is a particular problem where the casings have an integral connection with no external or internal upsets or protrusions, and hence where the centralisers can slip along the whole length of the casing.

When the spring bows of existing bow centralisers are compressed transversely, they no longer present a smooth profile and thereby prevent smooth rotation of the casing within the well bore or outer casing. This is also a problem.

The present invention seeks to reduce the problems identified with existing apparatus for centralising casings. According to a first aspect of the present invention there is provided an apparatus for centralising a casing, said apparatus comprising a centraliser comprising a pair of longitudinally spaced end collars connected by a plurality of elongate members, and a stop collar arranged to be secured onto a casing for locating said centraliser on the casing, said stop collar being arranged substantially coaxially with said centraliser intermediate said end collars and within said elongate members, and wherein said stop collar is arranged to selectively abut one of said end collars.

Preferably, said elongate members comprise a plurality of elongate ribs connecting said pair of end collars, and wherein said end collars are substantially coaxially arranged.

In an embodiment, said ribs are each arranged to extend over part of a spiral extending around a longitudinal axis of said centraliser.

In an embodiment, said centraliser comprises a pair of longitudinally spaced, substantially coaxially arranged end collars connected by a plurality of elongate spring bows, wherein each spring bow is arched relative to its longitudinal extent, and wherein each spring bow is transversely curved to be outwardly convex.

Preferably, the transverse curvature of each spring bow changes progressively along the length of each said spring bow. In a preferred embodiment, this progressive change in the transverse curvature along the length of each bow is from a minimum curvature adjacent each end portion of each spring bow to a maximum transverse curvature at the centre of its longitudinal extent. This progressive curvature ensures that as the arched spring bows are subject to substantially transverse compressive loads, they are able to maintain a smooth profile along their full length.

In a preferred embodiment, said stop collar comprises a surrounding band.

For example, said band forming said stop collar is interrupted whereby the circumferential dimension of the collar may be varied.

Preferably, the stop collar is an interrupted band of a stressed material arranged to grip the casing.

In an embodiment said stop collar is formed to include a plurality of slots.

Preferably, said stop collar comprises a surrounding band having co-operating means movable relative to one another whereby the circumferential dimension of said collar may be varied.

In a preferred embodiment, said co-operating means comprises an adjustment section integrally formed with said stop collar which extends substantially circumferentially of said stop collar and is received within a complementary cut-out formed in a co-operating portion of said collar. For example, the adjustment section may comprise a T-shaped section having a shank extending substantially circumferentially and a head attached to and extending transversely of said shank. Preferably, the complementary cut-out is defined between two generally circumferentially extending spaced legs. Each said leg preferably has a cranked shape to co-operate with the T-shaped section. In a particular embodiment, a respective bridging piece extends circumferentially and is arched along its circumferential extent. Each said bridging piece connects the co-operating means. For example, each said bridging piece may connect part of each said leg of the co-operating portion to the head of said T-shaped section. The circumferential extent of the stop collar may be reduced by flattening the arched bridging pieces and then bending the flattened bridging pieces over to overlay the stop collar.

The stop collar is arranged to be fixed to the casing without the use of screws, or other generally radially extending protrusions. In this respect, an adhesive or other bonding means is provided on the inner surface of said stop collar for fixing the stop collar to a casing.

Preferably, the ends of each elongate member are received within respective slots provided in the end collars of the centraliser.

The present invention also extends to a centralised casing structure comprising a casing on which is affixed an apparatus for centralising a casing as defined above.

According to a still further aspect of the present invention there is provided a centralised casing structure comprising a casing on which a centralising apparatus is fixed, said centralising apparatus comprising a centraliser comprising a pair of longitudinally spaced end collars connected by a plurality of elongate members, and a stop collar secured onto said casing, said stop collar being arranged substantially coaxially with said centraliser intermediate said end collars and within said, elongate members, wherein said stop collar is secured onto said casing by fixing means which do not have any generally transversely extending protrusions.

Preferably, said stop collar is adhered to said casing. Additionally and/or alternatively, said stop collar is arranged to have a circumferential dimension which can be varied, and is secured to said casing by reducing the circumferential dimension of the collar to grip the casing and apply pressure to the bond.

In an embodiment, said stop collar is an interrupted band of stressed material arranged to grip the casing and apply pressure to the bond.

The present invention also extends to a method of securing a centraliser on a casing, said centraliser comprising a pair of longitudinally spaced end collars connected by a plurality of elongate members, said method comprising the steps of affixing a stop collar to said casing by fixing means which do not have any generally transversely extending protrusions, said stop collar being secured on said casing to be substantially coaxially relative to said centraliser intermediate said end collars of said centraliser and within said elongate members.

According to a further aspect of the present invention there is provided a centraliser comprising a pair of longitudinally spaced, substantially coaxially arranged end collars connected by a plurality of elongate spring bows, wherein each spring bow is arched relative to its longitudinal extent, and wherein each spring bow is transversely curved to be outwardly convex.

The invention also extends to a centraliser comprising a pair of longitudinally spaced, substantially coaxially arranged end collars connected by a plurality of elongate members, wherein each elongate member comprises a rib arranged to extend over part of a spiral extending around a longitudinal axis of said centraliser.

According to a further aspect of the present invention there is provided a stop collar for a centraliser, said stop collar comprising a surrounding band whose circumferential dimension is variable whereby the stop collar is arranged to grip a casing.

In a preferred embodiment, said stop collar has co- operating means which comprise an adjustment section integrally formed with said stop collar which extends substantially circumferentially of said stop collar and is received within a complementary cut-out formed in a co¬ operating portion of said collar. For example, the adjustment section may comprise a T-shaped section have a shank extending substantially circumferentially and a head attached to and extending transversely of said shank. Preferably, the complementary cut-out is defined between two generally circumferentially extending spaced legs. Each said leg preferably has a cranked shape to co-operate with the T-shaped section.

In a particular embodiment, a respective bridging piece extends circumferentially and is arched along its circumferential extent. Each said bridging piece connects the co-operating means. For example, each said bridging piece may connect pert of each said leg of the co-operating portion to the head of said T-shaped section. The circumferential extent of the stop collar may be reduced by flattening the arched bridging pieces and then bending the flattened bridging pieces over to overlay the stop collar.

The present invention also extends to an apparatus for centralising a casing comprising a centraliser and a stop collar as defined above.

An embodiment of the present invention will be described below, by way of example, with reference to the accompanying drawings; in which:

Figure 1 shows a side elevational view of part of a casing on which is fitted apparatus of the invention having a stop collar and a centraliser,

Figure 2 shows a side elevational view of the centraliser of the apparatus of Figure 1, Figure 3 shows an end view of the centraliser of Figure 2,

Figure 4 shows a portion D of an end collar of the centraliser of Figure 2,

Figure 5A shows a cross-section of a spring bow taken along the line A-A of the centraliser of Figure 2,

Figure 5B shows a cross-section of a spring bow taken along the line B-B of the centraliser of Figure 2,

Figure 6 shows a side elevational view of the stop collar of the apparatus of Figure 1, Figure 7 shows an end view of the stop collar of Figure 6,

Figure 8 shows an enlargement of part C of the stop collar of Figure 7,

Figure 9 shows a perspective view of an alternative embodiment of a centraliser,

Figure 10 shows an alternative embodiment of a stop collar for use with a centraliser, and

Figures 11A and 11B show alternative shapes of slots for incorporating in a stop collar as shown in Figure 10.

A well bore or outer casing can be lined with a casing formed of a string of lengths of hollow tubing connected to one another. Cement may then be pumped into the annular space between the casing and the sides of the well bore or outer casing to protect the casing. Currently, it is usual to employ centralisers to centre each length of a casing within a well bore or outer casing so that an annulus of cement having a substantially uniform radial thickness is formed. This ensures the integrity of the cementing.

Figure 1 shows a side elevational view of part of the length of casing 2 on which an apparatus 1 for centralising the casing 2 within a well bore or outer casing has been provided. The apparatus 1 comprises a centraliser 10 and a stop collar 12 fitted within the centraliser.

The centraliser 10 comprises two longitudinally spaced, cylindrical, substantially coaxially arranged, end collars 20 connected together by a plurality of longitudinally extending elongate members 24 in the form of spring bows. Each end collar 20 is arranged to surround the casing 2 and has an inner diameter which is equal to or slightly larger than the outer diameter of the casing 2 such that each end collar 20 is movable longitudinally of the casing 2 and is rotatable around the casing 2. Each end collar 20 is also provided with a plurality of slots 26 in its outer surface. In the embodiment shown, six slots 26 are provided in each end collar 20, equally spaced around the circumference of the end collar. It will be seen, for example in Figure 4, that each slot 26 is substantially rectangular and has a longitudinal extend which is less than that of the end collar 20 such that each slot 26 as longitudinally spaced ends, each of which is spaced from a respective axial end of a said end collar 20.

Each spring bow 24 is a thin elongate element made of resilient material and shaped in an arch. Each spring bow 24 has two longitudinally spaced end portions 40 each of which is received within one of the slots 26 in a respective one of the end collars 20.

As can be seen from Figures 1 and 2, each spring bow 24 is arched relative to its longitudinal extent such that along the longitudinal axis thereof, said spaced end portions 40 are substantially planar and are connected by an arched portion which curves outwardly relative to the end collars 20. As is shown in Figures 5A and 5B, each spring bow 24 is also transversely curved to be outwardly convex. However, the transverse curvature (i.e. the reciprocal of the radius of curvature) of each spring bow 24 changes progressively from a minimum at the end portions 40, as indicated in Figure 5A, to a maximum at the centre of its longitudinal extent, as shown in Figure 5B. It will be appreciated that this progressive change in the transverse curvature along the length of each bow 24 means that increasing transverse curvatures are associated with increasing diameters of the centraliser as defined by the outward arching of each spring bow. As the spring bows 24 are subjected to substantially transverse compressive loads the transverse curvatures are flattened thereby ensuring that the compressed spring bows still have a smooth profile along their full length.

It has been seen that the end portions 40 of the spring bows 24 are received within the slots 26. These end portions 40 are arranged so as to present a smooth transition between the end collar 20 and the spring bows 24. Figure 4 shows a cross-sectional view of one of the slots 26 within which a sufficient portion of a respective end portion 40 is received to ensure that a strong connection between end portion 40 and the end collars 20 can be formed. In a preferred embodiment, the end portions 40 of the spring bows 24 are welded in the slots 26.

Figure 3 shows an end view of the centraliser 10 of Figure 2 and illustrates the spring bows 24 arching outwardly of the end collars 20.

Figures 6 to 8 show a stop collar 12 for use with the centraliser 10. The stop collar 12 is a metal band arranged to form a substantially endless collar having a substantially circular cross-section. The stop collar 12 is to surround the casing 2 in use, substantially coaxially with said end collars 20, and is provided with co-operating means 30, 35 to enable adjustment of the circumference of the stop collar so that it can be made to fit tightly around the casing 2.

As can be seen in Figure 6, the co-operating means includes an integral adjustment section 30 which comprises a T-shaped section 32 which has a shank 42 extending substantially circumferentially of the stop collar 12. A head 44 is attached to, and extends transversely of, said shank 42 to form the T-shape, and is received within a complementary cut-out 38 formed in a co-operating portion 35. The adjustment section 30 follows the curvature of the stop collar 12 as is clear from Figure 7 which shows an end view of the stop collar. Two circumferentially extending shoulders 34 extend on either side of, and spaced from, the shank 42 of the T-shaped section 32 so as to form guides 50.

The co-operating portion 35 of the adjustment section comprises a pair of spaced legs 36 between which the shank 42 of the T-shaped section 32 is arranged to extend. Each of these legs 35 has a cranked shape defined by two transversely offset, substantially parallel, circumferentially extending elongate portions 46 connected by way of an intermediate portion 48. The transversely outer elongate portion 46 of each leg 36 extends circumferentially of the stop collar 12, and these outer elongate portions 46 are joined whereby a head portion 39 of the cut-out 38 is defined. In addition, the facing edges of the two outer elongate portions 46 define guide edges along which transversely spaced, outer edges of the head 44 of the T-shaped section 32 can be guided. Similarly, facing edges of the two other, transversely inner elongate portions 46, provide guides for transversely spaced, outer edges of the shank 42. It will be appreciated that the free ends of the legs 36 are guided by the guides 50.

The adjustment section 30 is arranged to be slid relative to the co-operating portion 35, guided along the guides provided, such that the circumference of the stop collar 12 can be reduced. To enable this relative movement, the intermediate portion 48 of each leg 36 is joined to the head 44 by way of an outwardly arched bridging piece 100. As can be seen from Figures 7 and 8, the two bridging pieces are arched along the circumferential extent of the stop collar 12, and they can be flattened, for example, by hammering.

It will be appreciated that as each bridging piece 100 is deformed such that its rch is flattened,, the intermediate portion 48 o± each leg 36 is brought near to the head 44 whereby the collar circumference is reduced. Preferably, the flattened bridging pieces 100 are also bent over, for example, by hammering to overlie either the respective intermediate portion 48 or the head 44. This enables the stop collar 12 in use to be positioned around a casing 2 and then to be fitted tightly onto the casing simply by hammering the bridging pieces 100 into their flattened condition. Screws and the like, which would protrude outwardly of the casing are not required. Preferably, the stop collar 12 is also adhered to the casing 2 to achieve a strong and reliable grip on the casing. Any suitable adhesive or bonding means may be employed.

In use, an apparatus 1 is fitted onto a casing 2 which is to be centred within a well bore or outer casing. For example, one or more apparatus 1 could be provided for each length of the casing 2. The apparatus 1 is assembled with the stop collar 12 loosely within the centraliser 10 between the two end collars 20, and is then slipped onto, and slid along, the length to the desired position. A suitable adhesive is provided between the stop collar 12 and the casing 2 to adhere the stop collar in position. The circumference of the stop collar 12 is reduced by the movement of the integral adjustment section 30 to grip the casing 2 and also to apply pressure to the adhesive bond. It will be appreciated that, once the stop collar 12 has been fixed onto the casing 2, it acts to locate the centraliser 10 on the casing by abutment of the stop collar 12 with one of the end collars 20 of the centraliser. However, although movement of the centraliser 10 along the casing 2 is limited by the stop collar 12, rotation of the centraliser 10 around the casing is possible.

In use, once an apparatus 1 has been attached to each length of the casing, the lengths are connected together into a string which is introduced into the well bore or outer casing. The apparatus 1 prevents differential sticking as the casing is run into the bore or outer casing or well bore. Once the casing has been located and centralised, primary cementing is undertaken in known manner to introduce cement between the casing and the well bore or an outer casing. It is known that rotation and reciprocation of the centralisers 10 during primary cementing is advantageous in ensuring a successful cement job. The apparatus 1 described and illustrated herein is enabled for smooth rotation during cementing, has sufficient grip to allow reciprocation, and is an effective centraliser. In addition, the apparatus of the invention described above is particularly advantageous for use where there are tight annular clearances in the well bore or outer casing.

Thus, and in known manner, the casing is centralised by way of the arched spring bows 24 which define a diameter for the centraliser 10 which is greater than the inner diameter of the well bore. The spring bows 24 of each centraliser 10 are compressed transversely as they are passed into the well bore thereby acting to centralise the length of casing within the well bore or outer casing.

As explained above, due to the progressive transverse curvature of each spring bow 24, as the spring bows are transversely compressed they are able to maintain a smooth profile along their full length through a flattening of the transverse curvature. This is most important as it enables the centraliser to be rotated within the casing even under high compressive loads.

The structure of the spring bows also enables the centraliser apparatus 1 to present a smooth outer periphery at all times, and this is important in ensuring the integrity of the cementing. It is also noted that, although the grip of the stop collar on the casing is secure, it is not necessary for the stop collar, or indeed the centraliser, to have any protruding screws or other fixing means. It will be appreciated that it is important to ensure that the stop collar 12 has a secure grip on the casing 2 to prevent slippage of the centraliser. Heretofore it has not been possible to prevent slippage without the use of protrusions, for example, such as grub screws, wedge pin fasteners, and the like, on the centraliser. Even so, a reliable grip has been difficult to obtain especially where the casings themselves have an integral connection with no external or internal upsets or protrusions. The absence of protrusions on the stop collar 12 also prevents the possibility of snagging of the spring bows 24 by such protrusions.

It will be appreciated that as a casing 2 is moved longitudinally within the well bore or outer casing, the casing will initially move relative to the centraliser 10, although the stop collar 12 will move with the casing. Of course, continued movement of the casing 2 will cause the stop collar 12 to abut one of the end collars 20 and thus pull the centraliser 10 with the casing 2. It is advantageous that the centraliser 10 is always pulled, irrespective of whether the casing is being run into or run out of the well bore or outer casing^

In the embodiments described and illustrated above, the stop collar 12 has been described as an annular endless collar formed of a metal band which is slipped around the casing 2. Alternatively, the metal band could be provided and formed around the casing 2 on site. In this embodiment it would be particularly convenient for the ends of the band to be formed to define the adjustment section 30 and the co-operating portion 35. The bridging pieces 100 could be integrally formed with the intermediate portions 48 of the legs 36 and welded, or otherwise secured, to the head 44, as the band encircles the casing 2. An alternative embodiment of a centraliser 10' is shown in Figure 9. As can be seen, the centraliser 10' comprises two longitudinally spaced, cylindrical, substantially coaxially arranged end collars 20' connected together by a plurality of longitudinally extending elongate members 24' in the form of substantially rigid ribs. Each of the ribs is arranged to extend over part of a spiral extending around a longitudinal axis of the centraliser. It will be seen that in this embodiment, which provides a rigid centraliser 10' , each end of each rib 24' is integrally formed with the respective end collar 20.

The centraliser shown in Figure 9 may be used with the stop collar 12 as shown in Figures 6 to 8, and clearly is used as described above.

Figure 10 shows an alternative embodiment of a stop collar 12' for use either with the rigid centraliser of Figure 9 or the spring centraliser of Figure 2. The stop collar 12' is a metal band arranged to form a collar having a substantially circular cross-section. It will be seen that the collar 12' is not endless, but is interrupted at 30' to enable it to be fitted onto a casing 2 in use. In this respect, the collar 12' is preferably made of a sprung metal prestressed to take up the circular shape shown in Figure 10. However, it will be appreciated that the ends 30' thereof may be moved apart, against the bias of the prestressing to enable the stop collar 12' to be fitted around the casing 2. It has been found that the stressing of the material is sufficient to urge the stop collar, when in position, to return to its circular shape. Furthermore, it has been found that the stressing of the stop collar 12' is able to grip the casing 2 and to apply pressure to adhesive provided between the stop collar 12' and the casing 2 to secure the adhesive bond. During tests on adhesives to be used between a stop collar as 12' and the casing 2, it was discovered that the shear strength with increasing surface area is not linear and that most of the load is borne by leading and trailing edges of a bond rather than at the central area.

Accordingly, a plurality of slots 112 are provided to extend through the stop collar 12'. It will be appreciated that two adjacent slots provide leading and trailing edges on the stop collar 12' for bonding by the adhesive to the casing 2.

In Figure 10, a plurality of substantially linear slots are arranged in rows around the circumferential extent of the stop collar 12'. However, the slots may be arranged in any pattern in the stop collar and this pattern may be regular or random.

Figures 11A and 11B show alternative shapes for individual ones of the slots 112. The shape and distribution of these slots is chosen as required to give the best adhesion characteristics.

It will be appreciated that other modifications in, and variations to, the invention as described above may be made within the scope of this application.

Claims

1. An apparatus for centralising a casing, said apparatus comprising a centraliser comprising a pair of longitudinally spaced end collars connected by a plurality of elongate members, and a stop collar arranged to be secured onto a casing for locating said centraliser on the casing, said stop collar being arranged substantially coaxially with said centraliser intermediate said end collars and within said elongate members, and wherein said stop collar is arranged to selectively abut one of said end collars.

2. Apparatus as claimed in Claim 1, wherein said elongate members comprise a plurality of elongate ribs connecting said pair of end collars, and wherein said end collars are substantially coaxially arranged.

3. Apparatus as claimed in Claim 2, wherein said ribs are each arranged to extend over part of a spiral extending around a longitudinal axis of said centraliser.

4. Apparatus as claimed in Claim 1, wherein said centraliser comprises a pair of longitudinally spaced, substantially coaxially arranged end collars connected by a plurality of elongate spring bows, wherein each spring bow is arched relative to its longitudinal extent, and wherein each spring bow is transversely curved to be outwardly conve .

5. Apparatus as claimed in Claim 4, wherein the transverse curvature of each spring bow changes progressively along the length of each said spring bow.

6. Apparatus as claimed in Claim 5, wherein said progressive change in the transverse curvature along the progressive change in the transverse curvature along the length of each bow is from a minimum curvature adjacent each end portion of each spring bow to a maximum transverse curvature at the centre of its longitudinal extent.

7. Apparatus as claimed in any preceding claim, wherein said stop collar comprises a surrounding band.

8. Apparatus as claimed in Claim 7, wherein said band forming said stop collar is interrupted whereby the circumferential dimension of the collar may be varied.

9. Apparatus as claimed in any preceding claim, wherein said stop collar is formed to include a plurality of slots.

10. Apparatus as claimed in any preceding claim, wherein said stop collar comprises a surrounding band having co¬ operating means movable relative to one another whereby the circumferential dimension of said collar may be varied.

11. Apparatus as claimed in Claim 10, wherein said co¬ operating means comprises an adjustment section integrally formed with said stop collar which extends substantially circumferentially of said stop collar and is received within a complementary cut-out formed in a co-operating portion of said collar.

12. Apparatus as claimed in Claim 11, wherein said adjustment section comprises a T-shaped section having a shank extending substantially circumferentially and a head attached to and extending transversely of said shank, said complementary cut-out is defined between two generally circumferentially extending spaced legs, and wherein each said leg has a cranked shape to co-operate with said T- shaped section.

13. Apparatus as claimed in Claim 10, 11 or 12, wherein a respective bridging piece extends circumferentially and is arched along its circumferential extent, and wherein each said bridging piece connects the co-operating means.

14. Apparatus as claimed in any preceding claim, wherein said stop collar is arranged to be fixed to the casing without the use of screws, or other generally radially extending protrusions.

15. Apparatus as claimed in Claim 14, wherein an adhesive or other bonding means is provided on the inner surface of said stop collar for fixing the stop collar to a casing.

16. Apparatus as claimed in any preceding claim, wherein the ends of each elongate member are received within respective slots provided in the end collars of the centraliser.

17. A centralised casing structure comprising a casing on which is affixed an apparatus for centralising a casing as claimed in any of Claims 1 to 16.

18. A centralised casing structure comprising a casing on which a centralising apparatus is fixed, said centralising apparatus comprising a centraliser comprising a pair of longitudinally spaced end collars connected by a plurality of elongate members, and a stop collar secured onto said casing, said stop collar being arranged substantially coaxially with said centraliser intermediate said end collars and within said elongate members, wherein said stop collar is fixed onto said casing by fixing means which do not have any generally transversely extending protrusions.

19. A casing structure as claimed in Claim 18, wherein said stop collar is adhered to said casing.

20. A casing structure as claimed in Claim 18 or 19, wherein said stop collar is arranged to have a circumferential dimension which can be varied, and is secured to said casing by reducing the circumferential dimension of the collar to grip the casing and apply pressure to the bond.

21. A casing structure as claimed in any of Claims 18 to 20, wherein said stop collar is an interrupted band of stressed material arranged to grip the casing and apply pressure to the bond.

22. A method of securing a centraliser on a casing, said centraliser comprising a pair of longitudinally spaced end collars connected by a plurality of elongate members, said method comprising the steps of affixing a stop collar to said casing by fixing means which do not have any generally transversely extending protrusions, said stop collar being secured on said casing to be substantially coaxially relative to said centraliser intermediate said end collars of said centraliser and within said elongate members.

23. A centraliser comprising a pair of longitudinally spaced, substantially coaxially arranged end collars connected by a plurality of elongate spring bows, wherein each spring bow is arched relative to its longitudinal extent, and wherein each spring bow is transversely curved to be outwardly convex.

24. A centraliser as claimed in Claim 23, wherein the transverse curvature of each spring bow changes progressively along the length of each said spring bow.

25. A centraliser as claimed in Claim 24, wherein this progressive change in the transverse curvature along the length of each bow is from a minimum curvature adjacent each end portion of each spring bow to a maximum transverse curvature at the centre of its longitudinal extent.

26. A centraliser comprising a pair of longitudinally spaced, substantially coaxially arranged end collars connected by a plurality of elongate members, wherein each elongate member comprises a rib arranged to extend over part of a spiral extending around a longitudinal axis of said centraliser.

27. A stop collar for a centraliser, said stop collar comprising a surrounding band whose circumferential dimension is variable whereby the stop collar is arranged to grip a casing.

28. A stop collar as claimed in Claim 27, having co¬ operating means comprising an adjustment section integrally formed with said stop collar which extends substantially circumferentially of said stop collar and is received within a complementary cut-out formed in a co-operating portion of said collar.

29. A stop collar as claimed in Claim 28, wherein said complementary cut-out is defined* between two generally circumferentially extending spaced legs, and wherein each said leg preferably has a cranked shape to co-operate with the adjustment section which is T-shaped.

30. An apparatus for centralising a casing comprising a centraliser as claimed in any of Claims 23 to 26 and a stop collar as claimed in any of Claims 27 to 29.

31. A centraliser substantially as hereinbefore described with reference to the accompanying drawings.

32. A stop collar substantially as hereinbefore described with reference to the accompanying drawings.

33. An apparatus for centralising a casing substantially as hereinbefore described with reference to the accompanying drawings.

34. A centralised casing structure substantially as hereinbefore described with reference to the accompanying drawings.