GB2201176A

GB2201176A - Pipe centraliser

Info

Publication number: GB2201176A
Application number: GB08803945A
Authority: GB
Inventors: Wilfried Dreyfus; Thomas E Remp; Kurt Muller
Original assignee: Mueller Kurt
Current assignee: Mueller Kurt
Priority date: 1987-02-19
Filing date: 1988-02-19
Publication date: 1988-08-24
Also published as: GB8803945D0

Abstract

A centering cage for a cylindrical part has externally spring elements 36 which are connected with at least one support ring 32, 34 surrounding the structural part 10 to be centred. In order, while maintaining required centering forces, to lessen the resistances acting against insertion into pipes, bore-hole walls or the like, to reduce the cross-section impeding the flow of media and to reduce the tendency to fatigue of the springs, the spring elements 36 are arranged in respective planes which are inclined at an angle to the planes in the radial and axial direction to the support ring. <IMAGE>

Description

A CENTERING CAGE FOR A CYLINDRICAL STRUCTURAL PART This invention relates to a centering cage for a cylindrical structural part.

In the deep-drilling art, centering cages are used for the central guiding of pipes in the borehole or in pipe liners, more especially with holes deviating from the vertical.

The centering cages, which have been known for more than twenty years, consist of an upper and a lower supporting ring, which are connected by a number of spring stirrups which are bent radially outwards cf. EP-A- 0 196 339.

The supporting rings have a loose seating on the pipe, so that their axial displacement and, thus, the centering effect of the spring stirrups is assured.

However, in order to limit the axial displacement of the centering cages arranged at certain intervals on the pipe, a so-called stop ring is fixed between the support rings on the pipe.

Set for the centering cages are requirements which are only satisfied to a limited extent by the known types. The most important requirements are: - high centering forces, in order that the pipe embraced by the centering cage is held centrally and as accurately as possible within the surrounding pipe or hole; - low frictional forces of the spring stirrups on the wall of the surrounding pipe or borehole with the introduction of the pipe to be centered and the upward and downward movements thereof with certain operations; - low space requirement, i.e. the centering cage is to occupy as little as possible of the annular cross-section in order not to impede the medium, bore flushing or cement passing therethrough and to keep the pump resistance as a low level; - maintaining of the centering cage at the fitted position, which has to be guaranteed by a stop ring having good adhesion.

With the known centering cages, it has been attempted to take account of the requirement for high centering forces by the selection of high spring stirrups and a spring cross-section of correspondingly large dimensions.

However, both measures have an unfavourable effect on the insertion force, even if a certain remedy is achieved by offset arrangement of the curved portions of some adjacent spring stirrups in the axial direction, such as shown in the aforementioned EP-A-O 196 339. In addition, a broadening of the usually rectangular spring cross-section causes -a reduction of the annular cross-section of the bore-hole. The increase in size of the spring cross-section certainly increases the stiffness or rigidity of the springs, but reduces the tensional force after loading or stressing.

The spring shows fatigue, because the spring steel is stressed beyond the elastic limit in the upper and lower regions of the spring cross-section.

Restricted by the pipe-hole ratios which are usual in practice and prescribed checking criteria, all known centering cages have a similar structure with springs which have a cross-section in the region of 4 mm x 40 mm. With all of them, the use of a stop ring is necessary.

An object of the present invention is to provide a centering cage which maintains the required centering forces, has a reduced resistance to the insertion into a pipe, bore hole or the like, and where both the cross-section obstructing the flow of media and spring fatigue is reduced, and it is possible to dispense with a stop ring.

According to the present invention, a centering cage -comprises a plurality of externally arranged spring elements connected to at least one support ring arranged to surround the structural part to be centered, said spring elements being arranged in respective planes which are inclined to planes which lie in a radial and axial direction from the support ring.

By solving the problem in accordance with the invention, each spring stirrup which is subjected to torsional stress with which the force between the clamped and fixed ends acts on a lever arm which depends on the length of the projection of the spring stirrup. Already at the time of installing the centering-cage, the support rings can be clamped on the circumference of the structural part, i.e. the pipe, which is to be centered. The stop ring which hitherto was necessary can be dispensed with.

Centering cages, with the spring stirrups constructed according to the invention and subjected to torsion satisfy conditions which go beyond the minimum requirements, can be produced more cheaply and, by utilising the shaping possibilities which are offered by the form of the springs, have the following additional advantageous properties. Due to the bending of the spring stirrups being effected in the peripheral direction, the long side of the rectangular cross-section of the spring stirrup no longer lies in the flow of media in the annular space of the borehole, but only still the short side. As a consequence, the necessary space requirement is considerably reduced by comparison with conventional spring stirrups.With the hitherto usual spring stirrups having a crosssection of 40 mm x 4 mm, each such stirrup, with a width of 40 mm, contributes to the constriction of the annular space of the bore-hole whereas, with the spring stirrup according to the invention, it is now no longer 40 mm but only still the 4 mm which is involved, and this corresponds to a reduction of the space which is required to 10% as compared with the prior art.

Consequently, free space is obtained for further development possibilities. It is possible to reduce the material thickness of the stirrups* perhaps from 4 mm to 3 mm, and instead to increase the number for each centering cage. By suitable choice of the effective torsion cross-section, the ratio of centering force to insertion force can be optimised. A reduced material thickness has further advantages. Springs having a smaller material thickness, under like conditions, experience fatigue less quickly than springs of greater material thickness. Moreover, a stamping out of the springs is facilitated and first made possible, without overloading the stamping tools.

As already mentioned above, the construction according to the invention also permits stop rings to be dispensed with. This is in accordance with the wishes of many drilling experts, which concern the limiting to a minimum the number of components to be introduced into the hole, since parts which become loose can endanger the success of the drilling. In addition, a reduction in costs is hereby achieved.

With the centering cages as previously employed, frequently the stop rings become loose. If in this case a centering cage is forced by an obstruction against the following coupling, because of a loose stop ring, it-has a tendency to become larger in crosssection, which can lead to a blocking of the pipe length to be pushed in. In practice, with centering cages of the conventional type, the pipe socket is used as stop ring, i.e. the support rings are placed on both sides of the pipe socket. This procedure is, however, disputed, because the sockets have a relatively large diameter and this, in conjunction with the spring stirrups disposed thereabove and with tight annular space conditions, can lead to difficulties.Centering cages with spring stirrups constructed according to the invention also have a cleaning effect on the wall of the bore-hole, whereby it is possible to remove the socalled filter cake, this operation not being possible or only possible to an insufficient degree with the conventional centering cages. The centering cage constructed according to the invention is particularly advantageous with small bore-holes, because the spring stirrups'in practice can be fully applied; it may even be possible therewith to make narrower drillings, which is advantageous for many purposes.

Advantageous and preferred further developments as regards achieving the object according to the invention are characterised in the subsidiary claims.

The bending of the spring stirrups sidewards advantageously takes place tangentially, as indicated in claim 3.

Instead of two separate support rings, it is also possible to use a one-piece support sleeve for fitting the spring stirrups, as indicated in claim 4.

Instead of making support rings and spring stirrups separately and fixing the said stirrups on the said rings, the spring stirrups may also be formed in one piece from the wall of a hollow cylinder between annular marginal strips, as indicated in claims 5 to 7 for different construction forms. In this way, the manufacture is substantially facilitated and made less expensive.

By the construction according to claims 9 to 11, the insertion of the centering cage into the pipe is facilitated.

In order to reduce the frictional effect on the inside wall of the tube, the construction according to claim 13 is provided.

For producing a better cleaning or scraping action, the construction according to claim 14 is advantageous.

If it is desired to secure the centering cage in a fixed position, a construction according to claim 15 is advantageous, since a stretching of the spring stirrups is no longer necessary with the centering cage constructed according to the invention.

It is also possible, in one centering cage, to use a combination of spring stirrups bent outwards radially in conventionai~manner and spring stirrups constructed according to the present invention, as indicated in claim 16.

In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a conventional centering cage in side elevation in a tube; Figure 2 shows the centering cage according to Figure 1 in plan view; Figures 3 and 4 show a spring stirrup of the centering cage according to Figures 1 and 2 in plan view and side elevation, respectively; Figure 5 is a first constructional form of a centering cage according to the present invention in side elevation inside a pipe; Figure 6 is the centering cage according to Figure 5 in plan view; Figures 7 and 8 are spring stirrups of the centering cage according to Figures 5 and 6 in plan view and side elevation, respectively; Figure 9 is another constructional form of a centering cage according to the present invention in side elevation;; Figure 10 shows the centering cage according to Figure 9 in plan view; Figure 11 shows the centering cage according to Figure 9 or Figure 10 in unwound form; and Figures 12 to 20 show different forms of spring stirrups.

Figures 1 to 4 show a conventional centering cage 2 which consists of two spaced supporting rings 4 and 6 and a plurality of spring stirrups 8 which are rectangular in cross-section and the ends of which are connected fast to the support rings. The centering cage 2 is pushed on to a pipe 10 which is to be centred within an external lining pipe 12. The seating of the centering cage can be fixed by a stop ring 14 disposed between the support rings 4, 6 or by two stop rings 16, 18 which are arranged externally of the support rings and spaced from the latter. Figures 1 and 2 clearly show that the spring stirrups 8 are curved radially outwards so that they lie with the long side 20 of the rectangular cross-section in the media flow of the annular space 22 of the bore-hole and thus cause a considerable constriction of this annular space.

Figures 3 and 4 show the spring stirrup 8 in plan and in side elevation. At the time of inserting the centering cage, the radially effective force of reaction from the wall of the pipe liner 12 causes a stretching of the spring stirrups and a corresponding increase in the spacing of the support rings 4, 6.

Figures 5 to 20 show centering members or their spring elements (spring stirrups) according to the present invention.

Figures 5 to 8 show a first constructional form of a centering cage 30 which consists of two spaced supporting rings 32 and 34 and a plurality of spring stirrups 36 which are rectangular in cross section and the ends of which are connected fast to the support rings, for example, by welding. However, the spring stirrups 36 are no longer curved radially outwards, as with the prior art, but are bent outwardly and, at the same time, sidewards, as can be seen particularly clearly in Figures 6 and 7. The spring stirrups 36 can comprise symmetrical curves 38, such as those shown in Figure 7, or asymmetrical curves 39, asrepresented in Figure 18.

Possible constructional forms of the spring stirrups themselves are represented in Figures 12 to 17. The spring stirrup according to Figure 12 is of uniform width throughout its length. As regards the constructional form according to Figure 13, the spring stirrup is broadened in the region of its bowed section 40 extending furthest outwards and, with the constructional form of Figure 14, this region is made reduced. In the constructional form of Figure 15, the spring stirrup has a continuous bar 42 and a central arcuate part 44. With the embodiments according to Figures 16 and 17, the ends of the spring stirrups 36 comprise short strips 46 and 48, respectively, which form an annular section of the support rings. The strips 46 are formed in one piece with the spring stirrup 36 and the strips 48 are separate parts which are welded to the spring stirrup 36.

The curved portions of the spring stirrups according to Figures 12 to 17, instead of being symmetrical as shown, may also be made asymmetrical, as with the constructional form of Figure 18.

Figures 9 to 11 show a second constructional form of a centering cage 50, which likewise comprises two spaced supporting rings 52 and 54 and several spring stirrups 56, which are rectangular in crosssection* but with which the support rings and the spring stirrups are formed of one piece by a rectangular spring steel sheet 58 being formed with arcuate separating cuts 60 between two marginal strips 62, 64, these latter being curved to circular form and made into supporting rings, while the strips between the separating cuts are bent to form springy bowed members. The curves of the separating cuts can extend symmetrically, as shown in Figure 12, or asymmetrically, as shown in Figure 19.

Instead of the separating cuts, there may also be provided separate slots 66, such as those shown in Figure 20, whereby the number and total surface of the springy bowed members can be reduced as desired.

The edges of the springy bowed members, as shown diagrammatically in Figures 6 and 10, may be offset or bent-over, at least in the region of their furthest projecting curved section 40, in order to reduce the friction with the pipe wall.

If it is desired intentionally to produce a scraping and/or cleaning action, the offsetting or bending operation will be dispensed with; in addition, in this latter case, the edge sliding on the pipe wall may also be toothed in order to further improve the cleaning action.

The lateral bending of the spring stirrups advantageously takes place in a tangential direction, as shown particularly clearly in Figures 6 and 10 but, in any case, at a sufficiently large angle to the radial direction, so that the spring stirrups are no longer stretched in practice when the centering cage is pushed into the pipe, but is twisted substantially in the circumferential direction in the elasticity region.

Because of the outwardly and laterally extending curves, the spring stirrups are torsionally stressed and thus act as a torsion rod.

Depending on the size of the angle of lateral bending of the spring stirrup to the radial direction, the amount of the torsion or of the stretching on the operative mechanism can predominate. All combinations between torsion and stretching can be achieved without transition; only stretching with a curve extending strictly radially and only torsion with a curve disposed tangentially.

The drawings show spring stirrups having rectangular cross-section. However, also other crosssectional forms are possible, more especially a circular cross-section.

If it is intended that the centering cage should be permanently fixed, the support rings or the one-piece supporting sleeve itself is used as stop ring.

As already mentioned above, the curve of the spring stirrup may be made asymmetrical, i.e. offset in the axial direction. For this purpose, the shallower curved path will be provided in the insertion direction in order hereby to achieve a reduction of the insertion forces.

It is also possible to provide the centering cage with a combination of radially curved spring stirrups and spring stirrups which are tangentially curved or bent at an angle relative to the radial direction.

Claims

Claims:

1. A centering cage for a cylindrical structural part, more especially for liner pipes and riser pipes, said cage comprising a plurality of externally arranged spring elements connected to at least one support ring arranged to surround the structural part to be centered, said spring elements being arranged in respective planes which are inclined to planes which lie in a radial and axial direction from the support ring.

2. A centering cage according to claim 1, wherein the spring elements are spring stirrups formed with an outward curvature.

3. A centering cage according to claim 1 or 2, wherein the spring elements or spring stirrups are arranged in planes extending tangentially away from the support ring.

4. A centering cage according to any one of claims 1 to 3, wherein the spring elements or spring stirrups are secured to spaced supporting rings or on a one-piece supporting sleeve.

5. A centering cage according to any one of the preceding claims, wherein the spring stirrups are formed lamella-like in one piece from the wall of a hollow cylinder between annular marginal strips.

6. A centering cage according to claim 1 or 5, wherein the centering cage is formed from a rectangular spring steel sheet which is formed with arcuate, spaced separation cuts between two marginal strips.

7. A centering cage according to claim 1 or 5, wherein the centering cage is formed from a rectangular sheet of spring steel having arcuate spaced slits between two marginal strips.

8. A centering cage according to any one of claims 1 to 4, wherein the ends of the spring stirrups comprise transversely arranged, part-annular, flanges as parts of the supporting ring.

9. A centering cage according to any one of the preceding claims, wherein the curves of the spring stirrups are asymmetrical.

10. A centering cage according to claim 9, wherein the curves of the spring stirrups are arranged offset in the axial direction opposite to the direction of insertion.

11. A centering cage according to claim 9 or 10, wherein the curves have a shallower curved portion facing in the direction of introduction.

12. A centering cage according to any one of the preceding claims, in which the spring stirrups have a round rectangular cross-section.

13. A centering cage according to any one of the preceding claims, wherein at least the central part of that rim of the spring stirrup which is away from the insertion direction and facing the bore-hole wall is rounded or offset.

14. A centering cage according to any one of the preceding claims, wherein at least the central part of that rim of the spring stirrup facing the insertion direction and pointing towards the bore-hole wall is serrated.

15. A centering cage according to any one of the preceding claims, comprising a combination of radially curved springy bowed members and other such members curved tangentially or at an angle to the radial direction.

16. A centering cage according to any one of the preceding claims, comprising a combination of radially curved spring stirrups and spring stirrups curved tangentially or at an angle to the radial direction.

17. A centering cage for a cylindrical structural part substantially as hereinbefore described with reference to Figures 5 to 20 of the accompanying drawings.