PIPE BENDING HEAD

Abstract

A bending apparatus for forming a helical strip of material. The bending apparatus comprises at least one bending head and is arranged to deform a strip of material into a helical form by applying a stress to a ridge running longitudinally along the strip of material to provide a plastic deformation of the ridge. The helical form of the helical strip of material is a permanent distortion due to the deformation of the ridge applied by the at least one bending head.

Description

TECHNICAL FIELD

The present disclosure relates to pipe machinery for the creation of pipes. Specifically, it relates to a pipe bending head apparatus, a helically wound pipe structure and to a method for forming a strip of material into a helically wound pipe structure.

BACKGROUND

Pipeline construction is generally performed using prefabricated pipe segments made in a conventional factory off-site, and not with mobile pipe manufacturing in the field.

Pipes can be created by winding a pre-formed strip of material onto a rotating mandrel. Alternatively, a mandrel is static and strips are wrapped around the mandrel. A strip of material may interlock with itself if self-overlapping, or interlock with an additional strip of material wrapped about the first. GB2496137B and GB2433453B, for example, discloses pipe manufactured from self-overlapping strips.

The wrapped material strip(s) may be retained in a helical shape forming a pipe structure. A first way to do this is to axially tension the elastically deformed material strips so that the material strips constrict a mandrel and thereby retain a pipe shape. A second way to do this is to radially compress the elastically deformed material strips so that the radial compressional force balances a force generated by elastic deformation of the strip material that is resisting being bent into the pipe shape. Alternatively, an adhesive is applied to the deformed material strips after they are formed into a pipe shape. A third way to do this is to plastically deform the material strips into a pipe shape, however without heat this requires the respective material strips to be deformed excessively with radius smaller than that of the desired pipe shape so that the material springs back into the desired pipe shape (in a process called springback). Springback must be compensated for by adding a springback factor (a percentage of the deformation) to the degree of deformation. The plastic deformation creates a large amount of stress in the material that can lead to stress fractures in the material, especially as the material must be deformed to a smaller radius compared to a desired radius to compensate for springback.

There are problems associated with pipes manufactured from wrapped material strips using known methods. First the requirement for an adhesive to hold the layers of the pipe together imposes limitations on the use of pipe manufacturing in the field with a mobile machine as clean room conditions are usually required for structure adhesives. For example, if wrapped pipe material is not properly interlocked then a burst strength of the pipe may be adversely affected. Second, forming material strips into helical shapes with a self-overlapping method results in an imbalance of radial stresses along the manufactured pipe length, which causes the manufactured pipe to tend to form a conical shape rather than a tubular shape unless countervailing rollers are used across the wall thickness of the pipe. This method is only possible for short lengths of pipe The second problem of an imbalance of radial stresses may be at least partially addressed by reacting the stresses using an internal roller to resist the stresses. However, the inclusion of one or more internal rollers introduces limits for a length that a manufactured pipe can extend to therefore this solution has severe limitations where moderate or longer lengths of manufactured pipe are desired.

SUMMARY

Examples of preferred aspects and embodiments of the invention are as set out in the accompanying independent and dependent claims.

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

A first aspect provides a bending apparatus for forming a helical strip of material, wherein the bending apparatus comprises at least two bending heads; each bending head is arranged to deform a strip of material into a helical form by applying a stress to at least one ridge running longitudinally along the strip of material to provide a plastic deformation of the at least one ridge, wherein the helical form of the helical strip of material is a permanent distortion due to the deformation of the ridge applied by the bending head. The resulting pair of strips each comprise an asymmetric interlock feature formed by the at least one ridge which are arranged to engage with one another when one strip is wrapped around the other. A non-ridge portion of each helical strip of material is deformed into a cylindrical shape by elastic deformation.

A second aspect provides a method for forming a strip of material into a helically wound pipe structure, the method comprising: at a bending head of a bending apparatus, receiving a length of strip material, wherein the strip material comprises at least one ridge along its length; at the bending head, plastically deforming the length of strip material into a helical form by applying a force to the ridge along the length of the ridge.

It will also be apparent to anyone of ordinary skill in the art, that some of the preferred features indicated above as preferable in the context of one of the aspects of the disclosed technology indicated may replace one or more preferred features of other ones of the preferred aspects of the disclosed technology. Such apparent combinations are not explicitly listed above under each such possible additional aspect for the sake of conciseness.

Other examples will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the disclosed technology.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an isometric drawing of a bending head forming a pipe.

FIG. 2 is a side view of a bending head.

FIG. 3 is an end view of a bending head.

FIG. 4 is a top view of a bending head.

FIG. 5 is a longitudinal sectional view of a pipe.

FIG. 6 is a face plate configuration comprising two bending heads.

FIG. 7 is a side view of a pipe formation machine.

FIG. 8 is an isometric an end section of an exemplary formed pipe.

The accompanying drawings illustrate various examples. The skilled person will appreciate that the illustrated element boundaries (e.g., boxes, groups of boxes, or other shapes) in the drawings represent one example of the boundaries. It may be that in some examples, one element may be designed as multiple elements or that multiple elements may be designed as one element. Common reference numerals are used throughout the figures, where appropriate, to indicate similar features.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating the general principles of the present technology and is not meant to limit the inventive concepts claimed herein. As will be apparent to anyone of ordinary skill in the art, one or more or all of the particular features described herein in the context of one embodiment or example are also present in some other embodiment(s) or example(s) and/or can be used in combination with other described features in various possible combinations and permutations in some other embodiment(s) or example(s).

Pipe manufacturing machines and components thereof discussed below are designed to manufacture pipe on a continuous basis on a mobile platform in the field. This means that the diameter of the pipe is formed in the field.

FIG. 1 is an isometric drawing of a bending head 15 for wrapping a length of material around a liner pipe 11. The bending head 15 is forming an inner material strip 12 into a helical form surrounding the liner pipe 11. The inner material strip 12 is preformed by means of cold roll forming stations and has at least two ridges running along the length of the strip which form an asymmetric interlock feature designed to interlock with an outer material strip 13. The bending head 15 has a number of rollers with some rollers having concave portions (female rollers) around their outer circumference to receive the ridges of the material strip, while other rollers having convex portions (male rollers) around their outer circumference to project into ridges of the material strip. Two rollers 1, 3 with concave portions are illustrated in the figure while other rollers are obscured from view. The rollers form the shape of the material strip around the liner pipe by exerting bending and sheer forces to the material strip passing through the bending head 15. Bending head 15 is formed by a scissor frame made comprising an outer scissor frame 10 and an inner scissor frame 9. The scissor frames 9, 10 rotate about a pivot thereby the radius of a curve applied to material strip 12 may be adjusted by altering the angle formed between the scissor frames 9, 10.

The rollers of the bending head 15 (both visible and hidden in FIG. 1) bend the material strip 12 into a suitable helical shape to wrap around the liner pipe 11. The bending head 12 bends the material strip 12 to a radius smaller than that of the liner pipe 12 so that the radius of the material strip 12 and liner pipe 11 match or are similar after springback of the material strip 12 when bending forces applied by the rollers of the bending head are removed.

The description and illustration use the term liner pipe, however this may instead be an inner pipe, but not an innermost pipe.

FIG. 2 is a side view of a bending head 15 with an inner material strip 12 passing through the bending head and onto a liner pipe 11. The scissor frame is illustrated as being formed by an outer scissor frame 10 and an inner scissor frame 9, which rotate about pivot 6 in order to alter the degree of bending of the material strip passing through the bending head. A closing of the scissor frame increases an amount of bending applied to the material strip 12 passing through the bending head. In the figure, the scissor frame is shown to be open in order to increase comprehension of the reader, however in operation the scissor frame 9, 10 would be more closed in order to impart increased bending to the material strip 12. Rollers are attached to the scissor frames and extend between sides of the inner and outer scissor frames. In the figure, a group of three rollers 1, 2, 3 together form a bending roller set and a group of two rollers 4, 5 form a forming roller set. The forming roller set 4, 5 comprises male and female rollers which impart a through axis compression or pitch so that a material strip emerges from the forming roller set with a changed shape across the material strip cross section. The illustrated bending roller set 1, 2, 3 includes male and female rollers to impart bending along a longitudinal direction of the material strip to form a radius of the material strip matching or similar to a radius of an outer surface of the liner pipe 11. Each roller set requires at least two rollers to form the set. There may be multiple sets of bending or forming rollers provided in a bending head in order to form or bend the strip in stages.

In FIG. 2 there are three female rollers 1, 3 and 5 and two male rollers 2, 4 shaped in order to form a material strip with asymmetric shaped ridge(s) projecting away from a centre of the liner pipe 2. Not illustrated is an optional prior bending head to form the material strip into a predetermined curvature prior to being formed and further bent by the illustrated bending head. Prior bending heads are discussed in relation to FIG. 6.

In FIG. 2 the bending head 15 includes an adjustable bolt connection 7 for controlling the angle between the inner 9 and outer 10 scissor frames. The adjustable bolt connection 7 control the radius applied to the material strip 12. Of the bending set, one female roller 1 moves with the outer scissor frame to increase or decrease a bending force on the material strip 12.

The bending head 15 also includes a rotation adjustment plate 8 for controlling an angle that material strip is deformed by in a plane corresponding to the flat sections of the material strip. Altering the rotation adjustment plane 8 alters a pitch of a helix that the material strip forms when wrapped around a liner pipe 11. Increasing the rotation adjustment plane 8 increases the pitch of the helix and the material strip will overlap with itself to a lesser degree or not at all; and decreasing the rotation adjustment plane 8 decreases the pitch of the helix and the material strip will overlap with itself to a greater degree.

FIG. 3 is an end view of a bending head whereby a material strip would exit the bending head towards a viewer of the figure (out of the page) and due to its radius, wrap material strip around a liner pipe 11. The pitch of the helical form of output material strip is determined by adjusting rotation adjustment plane 8 so an amount of overlapping of the interlocking layered material strips may be controlled.

In FIG. 3, the bending head is mounted to a rotating face plate 14 so that the output material strip is wrapped around the stationary liner pipe 11 as the bending head rotates around the liner pipe 11. An angled mounting plate connects the rotating face plate and the bending head. In the figure, the angle between the bending head plane and the plane that the liner pipe core is driven through a pipe forming machine is labelled a. Altering the angle changes residual stresses within strip material after winding the strip material onto the liner pipe 11. Specifically, if the strip material has two or more ridges running along its length, the ridges providing an interlock feature with one or more other interlock strips, the distinct ridges have distinct residual stresses after their shape is adjusted by the bending head, mainly out of plane to the shape of the ridge, prior to it being wound around the liner pipe 11. The differing residual stresses creates a neutral shape for the wrapped strip material when formed around the liner pipe 11 in a helical form.

Rollers 1, 2, 3 are illustrated as having a cylindrical form with fixed radius (except for their concave/convex female/male portions). One or more roller sets may have a generally cylindrical shape with a bulging centre or a mirror of that roller shape.

FIG. 4 is a top view of a bending head. The outer scissor frame 10 supports female roller 1 and is pivotally coupled to inner scissor frame 9 by pivot 6. Male rollers 2, 4 and female rollers 3, 5 extend between arms of the inner scissor frame 9.

FIG. 5 is a longitudinal sectional view of a pipe (not to scale) comprising a liner pipe 11, two sections of one inner material strip 12, and a section of an outer 13 material strip. Each strip has an asymmetrical interlock ridge feature that is formed from the respective strip and projecting away from the liner pipe 11. The figure illustrates an overlapping section where interlocks of the outer material strip 13 each engage with separate respective interlocks of the inner material strips 12 sections. These asymmetric ridge shapes are designed to interlock when the pipe experiences internal fluid pressure so that they continuously resist radial expansion of the liner pipe to maintain constant radius controlled by the interlocking layers of strip reinforcement and not by the softer liner pipe. The inner and outer material strips are helically wrapped around liner pipe 11 and the interlock ridge points form a helix along the liner pipe. Each material strip abuts itself when wound on to the liner pipe 11 of an inner layer of material strip 12. For example, the inner material strip is illustrated in the figure with two windings of the same strip being visible—each respective winding abutting an edge of another respective winding. This abutting of respective windings for each layer and using multiple concentric layers to increase pipe wall thickness addresses a problem of unbalanced stresses from overlapping windings of a single strip which may cause a manufactured pipe to otherwise take a conical form whereby the radius of the manufactured pipe is not constant. A manufactured pipe that may change in radius is not one suitable to manufacturing anything but a short length of pipe. A small gap between successive windings of the inner winding 12 of FIG. 5 is illustrated. Increasing the gap between successive windings of a single layer increases the flexibility of a manufactured pipe and a moderate gap is possible without adversely affecting a burst strength of the manufactured pipe.

The asymmetric interlock features of the inner and outer windings are illustrated in FIG. 5. Each winding is largely flat except for at least two interlock features (two interlock features in each winding layer are illustrated and described as this is the simplest implementation enabling interlocking winding layers). Each interlock feature in the figures is formed to have two distinct sections: a first section has winding layer material bent to a greater angle with respect to a liner pipe than a second section. On the inner layer, the material bent to a greater angle from the liner pipe provides a surface to be compressed by the outer layer to resist an expansion of the gap between successive winding from the inner layer. The outer layer interlocking features mirror the inner layer features but are greater in size in order to receive the inner layer features for a tighter fit. Alternatively shaped interlock features may be used other than those illustrated in the accompanying figures.

FIG. 6 is a face plate 22 configuration comprising two balanced bending heads: a first bending head 15A is for wrapping the inner material strip 12 around the liner pipe 11, and a second bending head 15B is for wrapping the outer material strip 13 around the inner material strip 12. The inner and outer material strips 12, 13 are fed to the first and second bending heads 15A, 15B with a prior bent radius greater than the radius of the liner pipe 11. This prior bending of the material strips 12, 13 is performed by prior bending heads: a first prior bending head 17A for the inner material strip 12, and a second prior bending head 17B for the outer material strip 13. In FIG. 6 the two bending heads are balanced so that strip tensions transmitted to the liner pipe 11 are balanced such that the liner pipe remains straight in its passage through the machine as shown in FIG. 7.

The inner material strip 12 and the outer material strip 13 have a radius larger than the liner pipe 11 before they have their radius reduced by the prior bending heads 17A, 17B; however, the material strips 12, 13 both comprise longitudinal ridges for interlocking prior to entering the prior bending heads 17A, 17B. The longitudinal ridges are formed by roll forming stations 19A, 19B, which receive material strips from respective feed cassettes 21A, 21B. A first roll forming station 19A forms ridge(s) along the material that will form the inner material strip 12, and second roll forming station 19B forms ridge(s) along the material that will form the outer material strip 13. The first feed cassette 21A stores coiled material strip to later form the inner material strip 12, and the second feed cassette 21B stores coiled material strip to later form the outer material strip 12. The machine components of FIG. 6, except for the liner pipe 11, are coupled to a face plate 22 that rotates about the liner pipe 11 thereby simultaneously wrapping the liner pipe 11 with an inner material strip 12 that is not self-overlapping, and an outer material strip 13 that is also not self-overlapping, but with the outer material strip overlapping the inner material strip so that the respective material strips overlap and ridges of the respective material strips engage with each other and interlock the inner and outer material strips 12, 13.

Each bending head in FIG. 6, including both bending heads 15A, 15B and both prior bending heads 17A, 17B, receive a material strip after it is formed by a roll forming station 19A, 19B. The interlocking portions, also described as ridges, in a material strip are present when the material strip passes through a bending head. As described in relation to FIG. 1, the male and female rollers of the bending heads are formed to engage with the interlocking features of a material strip, i.e., the ridges. A female roller is shaped to receive ridges at a first side of a material strip and a male roller is shaped to extend into the same ridges at a second, opposite side of the material strip. Otherwise stated, the bending heads comprise sets of rollers that have a mating profile matching that of the interlocking ridge section profiles of the material strips. In this manner, male and female rollers may engage with interlocking sections of material strips and thereby provide a force to the interlocking ridge sections without necessarily providing a force to the non-interlocking flat sections of the material strip (the non-interlocking flat sections of the material strip are at each edge of the strip and between the interlocking ridge sections of the strip). By applying force to the interlocking ridge sections to bend and form the material strips by plastic deformation, an amount of stress applied to non-interlocking flat sections of material strips is limited and therefore the likelihood of microcracks and weakening of non-interlocking flat sections of material strips is reduced. Further, material strips deformed by bending heads remain in their deformed shape almost entirely due to plastic deformation of the interlocking ridge sections, applied by shaped rollers, rather than plastic deformation of non-interlocking flat sections of material strips, which are predominately bent elastically but held in a deformed shape by the plastically deformed interlocking ridge sections. An advantage of plastically deforming interlocking ridge sections rather than non-interlocking flat sections of material strips is that material stresses in the material strips near edges of the strips are decreased, therefore the likelihood of microcracks at the edges of the material strips are decreased. A microcrack at an edge of a material strip greatly increases a likelihood of failure of the material as a larger crack can form and spread from the microcrack edge point into the material. A further advantage is that by deforming the material strips by plastically deforming the interlocking ridge sections, the overall deformation can be better controlled than by using only the non-interlocking flat sections for plastic deformation and issues surrounding springback of the non-interlocking flat sections can be avoided, which, in turn, means that longer sections of pipe can be manufactured due to better control of material strip shape. While forming and bending steps may be combined, the arrangement illustrated in FIG. 6 has roll forming stations 19A, 19B separate from prior bending heads 17A, 17B, which are separate from bending heads 15A, 15B. Some or all of the rollers may be driven in order to drive the material strips through the components.

In one example, a first prior bending head 17A receives material strip 12 from a first roll forming station 19A. The received material strip has an approximately infinite bend radius, i.e. flat, and the first prior bending head 17A bends the material strip 12 to a bend diameter of approximately 1 meter. A first bending head 15A received the material strip 12 with bend radius of 1 meter from the first prior bending head 17A and outputs the material strip with approximately a 0.3048 meter (1 foot) diameter that matches the radius of a liner pipe 11 around which it is wrapped. The same process can be applied to any diameter of pipe.

FIG. 3 illustrated an end view of a bending head including a mounting plate that controls an angle between the bending head plane and the plane that the liner pipe 11, labelled a. Altering angle α alters a relative residual stress between the interlock features of a material strip once wound onto the liner pipe 11. Differing residual stresses in interlocking sections of a material strip, one the material strip is formed onto a liner pipe in a helical pattern, provides a neutral shape adjusted to the final configuration of the material strip on the liner pipe 11. The bending head is angled in both the horizontal and vertical planes to adjust for the required pitch angle of the winding process. In one illustrative example, a two-layered manufactured pipe on a 0.3048 meter (1 foot) diameter liner pipe 11 has differential plastic strains of 7% and 8% within the interlocking sections of a material strip so the helically shaped material strip has a 10 degree pitch angle. In one example, load cells mounted inside the bending heads 15 measure the force the bending head applied to the strip and provides a quality control method to ensure the plastic strains applied remain consistent. An amount of plastic strain in a curved portion of an asymmetric interlocking ridge section profile should be significantly greater than an amount of plastic strain in a non-interlocking flat section of a material strip. The strain is controlled biaxially by the bending head so that the type of strain in the material predominantly is bending strain, which contributes to forming a new desired formed shape at a correct pipe diameter, rather than tensile strain which can contribute to microcracking at flat free edges.

A ratio for strain in non-interlocking flat section to strain in interlocking ridge section profile of approximately 1:10 performs well, while a maximum ratio is approximately 1:5. Ideally, the plastic strain in a non-interlocking flat section is on average below 1% across the flat section and approximately 8% to 10% in the curved root of the interlocking ridge section profile. The plastic strain values will depend on the material properties of the material. However, in one example using high strength steel, the linear elastic zone is up to approximately 1%, the allowable uniaxial plastic strain is approximately 5% and the biaxial allowable strain is approximately 25%. These ratios on a material property basis may be derived from steel manufacturer's information, such as from a form limit diagram. The principal is to deform plastically locally in the ridge where the deformation follows biaxial forming rules of behaviour and keep the flat regions as far as possible within elastic limits. It is an aim to minimise strain in a flat region while maximising the strain in a curved region as far as possible without creating microcracking in the curved region, which is made possible by the bending head. Although the described systems and methods are relevant to bending heads 15A, 15B forming part of a larger system, such as a mobile pipe factory, the described bending head may be more simplistic and only bend material in a single axis using two or more rollers by plastically deforming a ridge section of the material while a non-ridge section is substantially elastically deformed.

The ratios and percentages in the description are particularly relevant to material reinforcement strips formed from high strength steel.

FIG. 7 is a side view of a pipe formation machine through which a core liner pipe 11 progresses and is surrounded by additional layers and has additional features incorporated therein. A caterpillar drive 30 forces the core liner pipe 11 through the pipe formation machine. The caterpillar drive 30 is mounted on an alignment skid 29 which aligns all the machine modules required for mobile production of pipe in the field. The core liner pipe 11 is driven through a pillar and rotating drive mechanism 31 for the rotating face winding head face plate 22 of FIG. 6 that winds the inner and outer material strips around the inner liner pipe 11. A feed reel of fibre optic cable 23 may also be coupled to the face plate 22 and one or more lengths of fibre optic cable are optionally incorporated into the pipe, and may be wrapped around the pipe. A freewheeling caterpillar drive 24 forces pipe already reinforced by the interlocked material strips into a coating unit 25. A laser profiler 28 may measure the reinforced pipe prior to any application of coating materials. Coating units are arranged on a mounting pillar 25 for applying environmental coating tapes to the reinforced pipe.

The pipe formation machine may include a feed reel of inner adhesive coating tape 26 and a feed reel of outer coating tape 27. Adhesive coating tape 26 may be added to the reinforced pipe for additional corrosion protection prior to an optional layer of outer coating tape 27 for providing mechanical protection to the reinforced pipe.

The pipe formation machine is on a mobile platform that may be supported using lifting eyes 32.

The pipe formation machine of FIG. 7 is a mobile pipe making factory that manufactures flexible unbonded composite pipe continuously in long lengths in the field. During operation, the machine may produce a pipe with spiral reinforcement, a spiral fibre optic cable and spiral environmental coating tape windings in a single factory. The drive and tape winding components are configured on to a rigid alignment skid so that alignment between the drive and winding modules is maintained when operating in the field.

FIG. 8 is an isometric view of an end section of an exemplary formed pipe. The pipe comprises a hollow liner pipe 11 formed, for example, from a polymer material, surrounded by an internal material strip 12 formed, for example, from steel, surrounded by an external material strip 13 formed, for example, from steel, surrounded by a wraparound corrosion coating 71 formed, for example, from modified and reinforced visco-elastic adhesive applied onto a polyethylene carrier film, surrounded by a mechanical coating 72 formed, for example, from spray-on vinyl, surrounded by a filler 73 formed, for example, from belzona as the pipe is introduced into an end fitting assembly.

A sleeve 74 formed, for example, from steel proximal to an end of the pipe compresses inner layers and retains the arrangement of those inner layers. There may be additional or fewer layers depending upon requirements for the pipe. At the end of the pipe, a stem 75 may be included and formed, for example, from steel. The stem 75 facilitates a mechanical fit with an additional pipe or other structure that may seal the end of the pipe.

A fibre optic cable 76 may be placed during pipe construction to run between specific layers of the pipe. The fibre optic cable 76 may run parallel to a longitudinal axis or spiral around the longitudinal axis as it runs along the pipe. The fibre optic cable 76 may be part of a sensing arrangement that detects changes in stress or strain of the pipe and/or changes in chemical elements proximal to the pipe in order to detect a leak of fluid from within the pipe.

The illustrated examples have two balanced rotating bending heads 15A, 15B simultaneously wrapping material around a liner pipe 11. However, additional pairs of bending heads may be provided to wrap additional interlocking layers of material providing additional reinforcement to a manufactured pipe—all bending heads are coupled to the alignment skid so form part of the mobile pipe factory. It is not essential to have balanced bending head pairs, however including balanced pairs provides more consistent pipe. Between two and 12 bending heads provide good results, made from up to six balanced bending head pairs for 12 bending heads in total. Although balanced pairs provide a more consistent pipe in certain circumstances, a pipe may be constructed using three strips, or multiples thereof, whereby a third strip wraps a second strip a consistent manner without a requirement for a fourth strip.

Material reinforcement strips may be formed from steel and steel alloys. Alternatively, other metals or ductile materials may be used to form material strips.

Any reference to ‘an’ item refers to one or more of those items. The term ‘comprising’ is used herein to mean including the method blocks or elements identified, but that such blocks or elements do not comprise an exclusive list and an apparatus may contain additional blocks or elements and a method may contain additional operations or elements. Furthermore, the blocks, elements and operations are themselves not impliedly closed.

The steps of the methods described herein may be carried out in any suitable order, or simultaneously where appropriate. The arrows between boxes in the figures show one example sequence of method steps but are not intended to exclude other sequences or the performance of multiple steps in parallel. Additionally, individual blocks may be deleted from any of the methods without departing from the spirit and scope of the subject matter described herein. Aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples without losing the effect sought. Where elements of the figures are shown connected by arrows, it will be appreciated that these arrows show just one example flow of communications (including data and control messages) between elements. The flow between elements may be in either direction or in both directions.

Where the description has explicitly disclosed in isolation some individual features, any apparent combination of two or more such features is considered also to be disclosed, to the extent that such features or combinations are apparent and capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1. A bending apparatus for forming a helical strip of material, wherein

the bending apparatus comprises at least two balanced bending heads;

a first bending head being arranged to deform a strip of material into a helical form to form a first helical strip of material by applying a stress to at least one ridge running longitudinally along the strip of material to provide a controlled plastic deformation of the ridge, wherein the helical form of the first helical strip of material is a permanent distortion due to the deformation of the at least one ridge applied by the at least one bending head; and

a second bending head being arranged to deform a second strip of material into a helical form to form a second helical strip of material by applying a stress to at least one ridge running longitudinally along the second strip of material to provide controlled plastic deformation of the at least one ridge, wherein the first helical strip of material comprises a first asymmetric interlock feature formed by the at least one ridge running longitudinally along the first helical strip of material and the second helical strip of material comprises a second asymmetric interlock feature arranged to engage with the first interlock feature when the second helical strip of material is wrapped around the first helical strip of material, and wherein a non-ridge portion of each helical strip of material is deformed into a cylindrical shape by elastic deformation.

2. The bending apparatus of claim 1, wherein the ridges are formed by at least two roll forming stations and the bending heads are arranged to receive the helical strip of material after it is formed by the rolling form stations.

3. The bending apparatus of claim 1, wherein the at least one ridge running longitudinally along the strip of material is adjacent to the non-ridge portion of the strip of material.

4. The bending apparatus of claim 1, wherein the non-ridge portion forms a cylindrical shape with a central axis coaxial with the central axis of helical strip of material.

5. The bending apparatus of claim 4, wherein the at least one ridge is circumferential about the cylindrical shape.

6. The bending apparatus of claim 1, wherein the at least one ridge of the first strip of material and the at least one ridge of the second strip of material are physically engaged by the bending apparatus.

7. The bending apparatus of claim 1, wherein the helical strips of material are formed around a liner pipe.

8. The bending apparatus of claim 1, wherein each bending head is a roller set comprising male and female roller shapes arranged to apply a stress to inner and outer surfaces of each ridge in order to deform the respective strip of material by plastically deforming the at least one ridge of the strip of material.

9. The bending apparatus of claim 1, wherein a ratio of a maximum strain of the non-ridge portion to the at least one ridge of the helical strip material within the bending head is approximately 1:8, greater than or equal to approximately 1:2, less than or equal to approximately 1:20, or between approximately 1:2 and approximately 1:20.

10. A method for forming a helical strip of material, the method comprising:

at a first bending head of a bending apparatus, receiving a first length of strip material, wherein the first length of strip material comprises at least one ridge along its length and a non-ridge section adjacent to the at least one ridge along its length;

at the first bending head, plastically deforming the first length of strip material into a helical form by applying a stress to the at least one ridge along the length of the ridge;

at a second bending head of the bending apparatus, receiving a second length of strip material, wherein the second length of strip material comprises at least one ridge along its length and a non-ridge section adjacent to the at least one ridge along its length;

at the second bending head, plastically deforming the second length of strip material into a helical form by applying a stress to the at least one ridge along the length of the ridge;

wherein there is substantially no plastic deformation of the non-ridge sections by the bending head; and

wherein the first and second strips of material have a common central axis when formed into a helical form.

11. The method of claim 10, wherein the bending head is a roller set comprising male and female roller shapes that apply a stress along the length of inner and outer surfaces of the ridge.

12. The method of claim 10 further comprising:

at the bending head, receiving a liner pipe, wherein the helical form of strip material surrounds the liner pipe.