FIELD OF THE INVENTION

The present invention relates to a method of manufacturing a carcass for a flexible pipe, and in particular such a method resulting in the carcass having a final pitch which is within a predetermined pitch value range.

BACKGROUND OF THE INVENTION

Flexible offshore pipes, hereafter referred to as flexible pipes, are for example used in the oil and gas industry for raising or transporting hydrocarbons from a subsea well head to a platform or floating equipment such as a Floating Production and Storage Offloading unit known by the abbreviation FPSO. Such steel armored flexible pipes for offshore applications are generally known from the standard "Recommended Practice for Flexible Pipe", ANSI/API 17B, fifth Edition, May 2014 (hereafter API17B), and the standard "Specification for Unbonded Flexible Pipe", ANSI/API 17J, Fourth edition, May 2014 (hereafter API 17J), published by the American Petroleum Institute.

Such a flexible pipe comprises an internal pressure sheath or inner liner with a hollow bore through which the hydrocarbons are transported. The internal pressure sheath is in most cases produced from polymer material, which is extruded to form a tubular internal pressure sheath, which has a high degree of impermeability in respect of the hydrocarbons to be transported. The internal pressure sheath may be supported and reinforced by several other layers, such as pressure armors and tensile armors. These layers are normally applied on the outer surface of internal pressure sheath. However, due to external pressure, it may sometimes be necessary to support the internal pressure sheath on the inner surface against collapse due to a pressure drop in the pipe bore. Normally, such a support is a metal structure known as a carcass.

The carcass of a flexible pipe is typically made from a metal strip which is shaped and helically wound to form an interlocked layer with the main objective to provide resistance against the external pressure applied to flexible pipes. When the flexible pipe is used at large sea depths, e.g. within the oil and gas industry, such an external pressure will be due to high pressure in the surrounding sea.

The pitch of such a carcass is the distance between two subsequent windings in the direction of the longitudinal axis of the carcass. The pitch is a parameter of interest, because it influences a number of characteristics of the carcass, such as the mechanical strength, the weight, and the minimum possible bending radius that can be applied to the final pipe structure. The pitch also influences the properties in relation to flow-induced pulsations in the flexible pipes. Furthermore, if the pitch is not close to constant after the winding process, the carcass will not be straight which can cause problems with the handling during the further manufacturing. This effect is named "kneeling", because it looks as if the carcass bends and deforms in a series of "knees". The effect is three-dimensional and caused by the carcass manufacturing process, such as due to torsional build-up of deformations in the longitudinal direction of the carcass.

A flexible pipe to be used within the oil and gas industry comprises a number of layers. As a next step after the manufacturing of the carcass, it typically has a pressure sheath extruded thereon. Thus, in addition to the potential difficulties related to the handling of the carcass itself, a non-straight carcass will result in an un-even application of the pressure sheath layer. If the pressure sheath is applied to a not straight carcass, it will be locked in an irregular shape which becomes a problem for the remaining manufacturing processes and for the structure of the flexible pipe in operation. E.g., waviness of the pressure sheath layer is critical to fatigue performance of the flexible pipe. Furthermore, if the pitch is not within prescribed limits, it may result in the flexible pipe not meeting requirements with respect to e.g. possible bending and stretching so that larger safety margins are to be applied which potentially causes disadvantageous design requirements.

JP 2003285360 A discloses a method for manufacturing a flexible composite pipe having stable bending characteristics with reduced pitch variation.

WO 2018115787 A1 discloses a method for regulating the pitch of the turns of a metal carcass of a flexible pipe. An improved method of manufacturing a carcass for a flexible pipe would be advantageous.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a method of manufacturing a carcass for a flexible pipe with which the straightness of the carcass is improved compared to known methods.

It is another object of the present invention to provide a method of manufacturing a carcass for a flexible pipe with which the pitch of the windings of the carcass can be ensured to be within predetermined limits.

It is a further object of the present invention to provide an alternative to the prior art.

In particular, it may be seen as an object of the present invention to provide a method of manufacturing a carcass for a flexible pipe that solves the above mentioned problems of the prior art.

SUMMARY OF THE INVENTION

Thus, the above-described object and several other objects are intended to be obtained in a first aspect of the invention by providing a method of manufacturing a carcass for a flexible pipe, the method comprising:

- providing a tubular carcass extending along a carcass axis,

- applying a first force to at least a first region of the carcass, the first force having a first force component parallel to the carcass axis, so that the pitch of the windings is decreased to be within a first pitch value range, and

- subsequently applying a second force to at least a second region of the carcass where the pitch of the windings is within the first pitch value range, the second force having a second force component parallel to the carcass axis, so that the pitch of the windings is increased to be within a second pitch value range, the pitch within the second pitch value range being larger than the pitch within the first pitch value range. The flexible pipe is preferably an unbonded flexible pipe.

A method of the invention relates to the manufacturing of the carcass itself irrespective of the remaining production steps leading to the final flexible pipe. It may e.g. be a method that is used to make a carcass which is temporarily stored on reels for later use in the manufacturing of a flexible pipe.

The first and second pitch value ranges may vary along the length of a flexible pipe. It may e.g. be advantageous to use different second pitch values and different sizes of the second pitch value ranges depending on a local strength requirement for a given flexible pipe.

In the following description, the pitch values are shown between two neighbouring windings. In practice - e.g. in relation to quality control - the pitch values may be taken as an average over a larger number of windings.

In some embodiments of the invention, the tubular carcass has been manufactured by: - providing at least one metal strip having opposite first and second surfaces and opposite first and second longitudinal edges,

- shaping the metal strip to have a first fold along the first longitudinal edge and facing towards the first surface, and a second fold along the second longitudinal edge and facing towards the second surface, - helically winding the metal strip while providing that the first fold engages and interlocks with the second fold so that coherent windings of metal strip form the tubular carcass.

The step of shaping the metal strip provides a shape which ensures that the step of winding results in establishment of frictional forces between subsequent windings. These frictional forces are large enough to ensure that the pitch of the carcass remains within the second pitch value range after the finishing of the manufacturing and until a pressure sheath as well as the additional layers been applied. The pitch values set by the extension step may differ from the pitch values of the final flexible pipe. The two steps of applying a first force and applying a second force can be referred to as a compression step and an extension step, respectively. The purpose of the compression step is that only extension of the carcass is needed in the second step of the process. The development work leading to the present invention has shown that it is hereby possible to simplify the design of the tooling used compared to other tested methods, such as attempting to apply the desired pitch in a single step. A further advantage of the compression step is that if the carcass is not straight, compressing it will make it straight and remove all "kneeling". By subsequently applying a controlled and uniform extension, the straightness is maintained. This is advantageous both with respect to the handling of the carcass itself and also in relation to the subsequent steps, including the application of a pressure sheath. The compression and extension steps can e.g. be controlled by performing the compression or extension until predetermined threshold pressures are measured. These threshold pressures are predetermined to be the ones resulting in the pitch of the windings being within the first pitch value range and the second pitch value range, respectively. Such predetermination can e.g. be performed by a combination of computer simulations and experimental work. The underlying mechanical mechanisms will depend on a number of parameters including the thickness of the metal strip, the cross-sectional shape of the windings of the carcass, and the metal material used. The final second target pitch value should preferably be such that the flexible pipe can be bend to a minimum bending radius without the carcass being geometrically locked. The actual choice of the second pitch value range will depend on the application for which the flexible pipe is to be used, and it will therefore be determined as part of the design process.

The method can be a continuous process where the carcass advances at a substantially constant speed. It can also be a step-wise process, e.g. wherein the advancement of the carcass is temporarily stopped during the application of the first and/or the second forces.

In some embodiments of the invention, the first force is applied by at least one pair of mutually collaborating movable grippers arranged around the carcass at a distance from each other in a direction parallel to the carcass axis, the distance between the grippers forming the first region of the carcass, the method comprising the following steps:

- letting each gripper apply a gripping force in the radial direction to at least a part of a circumference of the carcass,

- with the gripping forces maintained, moving the grippers towards each other so that the windings are compressed,

- releasing the gripping force of at least one of the grippers, and

- moving at least one of the grippers, so that the distance between the grippers is increased, and

- repeating the steps for a subsequent first region of the carcass.

In embodiments of the invention having such grippers, the grippers may be movably arranged, such as on rails, for movement parallel to the carcass axis, and there may always be at least one of the grippers applying gripping force to the carcass simultaneously with moving the grippers parallel to the carcass axis and thereby applying a force in the longitudinal direction for advancement of the carcass. An example of such an embodiment will be described below in relation to the figures.

In alternative embodiments of the invention, the first force is applied by moving the carcass in the direction of the carcass axis through a rotating compression tool arranged circumferentially around the carcass, the compression tool having at least one radially inwardly extending protrusion following at least a part of a helical path with a pitch corresponding to a first target pitch value, wherein the at least one protrusion is dimensioned to engage with the windings of the carcass and thereby push them together. Alternatively, the pitch of the compression tool may be non-constant so that the desired pitch value is gradually provided.

Such a compression tool may enclose the full circumference of the carcass in a manner resembling a nut engaging with a threaded element which in this relation is the carcass with the windings forming a helical path to engage with the compression tool. However, the compression tool may also comprise a plurality of compression tool elements arranged spaced apart, i.e. with gaps between them in the circumferential direction. In other alternative embodiments of the invention, the first force is applied by a series of rollers arranged on a roller holder rotating around the carcass with the axes of rotation of the rollers being parallel to, or substantially parallel to, and offset from the carcass axis, wherein the rollers are arranged and shaped to engage with the windings of the carcass when the carcass moves in the direction of the carcass axis, the arrangement and shape of the rollers being so that the windings of the carcass are pushed together by the engagement with the rollers. The arrangement of the rollers also include that they are offset from each other in the direction parallel to the carcass axis to allow for the engagement with the helical windings of the carcass.

In other alternative embodiments of the invention, the first force is applied by an upstream tensioner and a co-operating downstream tensioner, each tensioner comprising at least two movable tracks arranged to extend parallel to the carcass axis and with engagement surfaces of the tracks being in engagement with a part of the circumference of the carcass, the tensioners being arranged spaced from each other in a direction of advancement of the carcass so that a longitudinal distance between the tracks of the upstream tensioner and the tracks of the downstream tensioner forms the first region of the carcass, wherein the tracks of the upstream tensioner operate at a higher speed than the tracks of the downstream tensioner so that their corresponding engagements with the carcass result in the application of the first force. The word "tensioner" is used to designate some embodiments of the tools that can be used to apply the first force resulting in a compression of the windings of the carcass. This wording is chosen, because it will be well known to a person working within this technical field. By "upstream" and "downstream", reference is made to the direction of advancement of the carcass during the manufacturing process. This means that a specific region of the carcass passes the upstream tensioner before it passes the downstream tensioner. It also means that the upstream tensioner is located before the first region of the carcass, where the first force is applied, and the downstream tensioner is located after the first region of the carcass. The movable tracks of a tensioner are typically arranged equidistantly around the circumference of the carcass. I.e. when there are two tracks, they are typically offset 180 degrees from each other, and when there are three tracks, they are typically offset 120 degrees from each other. The scope of protection covers any number of tracks per tensioner, but typically there will be two or three. The number of tracks does not need to be the same for the upstream and the downstream tensioners. When the number of tracks are the same, the tracks of the two tensioners may be arranged in continuation of each other, or they may be mutually circumferentially offset.

Each of the tracks may be in the form of an endless belt, such as a belt in the form of a flexible band made from e.g. rubber. It may also be in the form of coherent pads, such as an endless chain having the links provided with pads. A person skilled in the art will know the different types of tensioners and corresponding tracks and will know which to choose for a given application.

By "longitudinal distance" is meant a distance along an imaginary line parallel to the carcass axis.

In some embodiments of the invention, the downstream tensioner is controlled to have a desired speed of its tracks, and the upstream tensioner is controlled to have a speed of its tracks resulting in the first force being applied. By "desired speed" is meant a speed matching the subsequent step of applying the second force in order to extend the carcass after it has been compressed by the first force. Which speed to use will be determined as part of the design of the whole process. It will e.g. depend on which methods that are used for the application of the first and second forces, the dimensions and materials of the carcass, and the first and second pitch value ranges. It will typically be determined by a combination of simulations and experimentation as well as experience from previously performed processes.

In the above-mentioned embodiments of the invention, the control of the speed of the tracks of the upstream tensioner will depend on the actual type of tensioner used. Depending on the selected actuator technology, different operational parameters like electrical current or hydraulic pressure makes it possible to base the control of a torque from which the resulting first force can be estimated.

In other alternative embodiments of the invention, the first force is applied by a brake and an upstream tensioner comprising at least two tracks, the upstream tensioner and the brake being arranged to extend parallel to the carcass axis and with engagement surfaces of the tracks and the brake in engagement with a part of the circumference of the carcass, the tensioner and the brake being arranged spaced from each other in a direction of advancement of the carcass so that a longitudinal distance between the tracks of the upstream tensioner and the brake forms the first region of the carcass, wherein the upstream tensioner is controlled based on feedback from at least one sensor arranged at or near the location where the pitch of the windings is increased to be within the second pitch value range, the sensor providing signals representative of the process, and wherein the brake is controlled to apply a desired braking force radially to the carcass so that the engagements with the carcass by the tracks and the brake result in the application of the first force.

A sensor as just described may be used also in the other embodiments to ensure that the carcass leaves the manufacturing system at a desired speed.

In some embodiments having a brake, the brake is in the form of one or more blocks of material that is pressed towards the outer surface of the carcass and thereby provides a braking force due to the frictional engagement. In other embodiments, the brake may be in the form of a tensioner having a track of a type as described above. Then the braking action may be provided e.g. by applying torque constraints in the control of the movement of the tracks.

In a manner corresponding to what was described above in relation to the application of the first force for some embodiments of the invention, the second force may applied by moving the carcass in the direction of the carcass axis through a rotating extension tool arranged circumferentially around the carcass, the extension tool having at least one radially inwardly extending protrusion following at least a part of a helical path with a pitch corresponding to a second target pitch value, wherein the helical protrusion is dimensioned to engage with the windings of the carcass and thereby push them apart. As also explained in relation to the compression tool, the extension tool may also comprise plurality of extension tool elements arranged spaced apart, i.e. with gaps between them in the circumferential direction.

In alternative embodiments of the invention, the second force may be applied by a series of rollers arranged on a roller holder rotating around the carcass with the axes of rotation of the rollers being parallel to, or substantially parallel to, and offset from the carcass axis, wherein the rollers are arranged and shaped to engage with the windings of the carcass when the carcass moves in the direction of the carcass axis, the arrangement and shape of the rollers being so that the windings of the carcass are pushed apart by the engagement with the rollers. As for the compression tool, also for the extension tool, the arrangement of the rollers include that they are offset from each other in the direction parallel to, or substantially parallel to, the carcass axis to allow for the engagement with the helical windings of the carcass.

An advantage of using rotating rollers is that this requires much lower forces than the corresponding frictional forces caused by a tool working by sliding.

In some embodiments of the invention wherein the second force is applied by rollers,

- the rollers may each have a circumferentially arranged protrusion,

- for each roller, the cross-sectional shape of the protrusion, in a plane containing the axis of rotation, at the circumference may have a radius of curvature,

- the rollers may have different radii of curvature, and

- the rollers may be arranged in order of increasing radius of curvature with respect to when they are brought into engagement with the advancing carcass.

Design work and experiments performed during the development of the present invention have shown that an advantageous effect of the different radii of the rollers is that the carcass advances along a straight path after leaving the rollers. The roller holder may further be provided with guide rollers supporting the carcass centred in the correct position with respect to the machine. Hereby it is easier to ensure that the manufacturing runs smoothly and results in a carcass with the desired final pitch.

For all embodiments of the invention, the manner of applying the second force may be chosen independently of how the first force is applied. I.e. the first and second forces may be applied by tools based on the same or different working principles. Furthermore, alternative to or in combination with moving the carcass trough the compression and extension tools, the tools could be moved in the direction of the carcass axis instead of moving the carcass.

As described above, the first pitch value range and the second pitch value range will be determined as part of the design process. As an example, in some embodiments, the first pitch value range is determined as a first target pitch value ± 10%, such as a first target pitch value ± 5%, such as a first target pitch value ± 1%. By "target" is preferably meant an optimal value e.g. determined by computer simulations. The pitch being in the first pitch value range does not necessarily mean that the windings are compressed until subsequent windings cannot be further compressed without deforming the metal strip material. It is sufficient that the windings are sufficiently compressed to ensure that the final pitch is within the second pitch value range.

Correspondingly, the second pitch value range may be determined as a second target pitch value ± 10%, such as a second target pitch value ± 5%, such as a second target pitch value ± 1%.

In any of the embodiments as described above, a pressure sheath may be applied to the carcass while the pitch of the windings is within the second pitch value range.

The above-described object and several other objects may also be obtained in a second aspect of the invention by providing a system for manufacturing a carcass for a flexible pipe, the system comprising:

- a feeder for feeding a tubular carcass extending along a carcass axis into the system, - a first force application tool for applying a first force to at least a first region of the carcass, the first force having a first force component parallel to the carcass axis, so that the pitch of the windings is decreased to be within a first pitch value range, and

- a second force application tool for subsequently applying a second force to at least a second region of the carcass where the pitch of the windings is within the first pitch value range, the second force having a second force component parallel to the carcass axis, so that the pitch of the windings is increased to be within a second pitch value range, the pitch within the second pitch value range being larger than the pitch within the first pitch value range.

In some embodiments of the system, the system further comprises:

- a transporter for feeding at least one metal strip having opposite first and second surfaces and opposite first and second longitudinal edges into the system,

- a shaping tool for shaping the metal strip to have a first fold along the first longitudinal edge and facing towards the first surface, and a second fold along the second longitudinal edge and facing towards the second surface,

- a winding tool for helically winding the metal strip while providing that the first fold engages and interlocks with the second fold so that coherent windings of metal strip form the tubular carcass.

In the following, different embodiments of such a system are summarized without any further comments. However, the above comments on methods according to the first aspect of the invention also apply to systems according to the second aspect of the invention.

In some embodiments of a system according to the invention, the first force application tool comprises:

- at least one pair of mutually collaborating movable grippers arranged around the carcass at a distance from each other in a direction parallel to the carcass axis, the distance between the grippers forming the first region of the carcass, each gripper being adapted to apply a gripping force in the radial direction to at least a part of a circumference of the carcass,

- a gripper mover for moving the grippers back and forth with respect to the carcass axis, and - a controller for controlling the gripping force of the grippers and the movement of the grippers.

In such systems comprising grippers, the grippers and the control thereof may be designed so that there is always at least one of the grippers applying gripping force to the carcass simultaneously with moving the at least one gripper thereby applying a force in the longitudinal direction for advancement of the carcass.

In alternative embodiments, the first force application tool comprises a rotatable compression tool arranged circumferentially around the carcass, the compression tool having at least one radially inwardly extending protrusion following at least a part of a helical path with a pitch corresponding to a first target pitch value, wherein the at least one protrusion is dimensioned to engage with the windings of the carcass and thereby push them together when the first force is applied by moving the carcass in the direction of the carcass axis through the compression tool.

In other alternative embodiments, the first force application tool comprises a series of rollers arranged on a roller holder rotatably arranged around the carcass with the axes of rotation of the rollers being parallel to, or substantially parallel to, and offset from the carcass axis of the carcass being manufactured by use of the system, wherein the rollers are arranged and shaped to engage with the windings of the carcass when the carcass moves in the direction of the carcass axis, the arrangement and shape of the rollers being so that the windings of the carcass are pushed together by the engagement with the rollers.

In other alternative embodiments, the first force application tool comprises:

- an upstream tensioner and

- a co-operating downstream tensioner, wherein each tensioner comprises at least two movable tracks arranged to extend parallel to the carcass axis and with engagement surfaces of the tracks being in engagement with a part of the circumference of the carcass, the tensioners being arranged spaced from each other in a direction of advancement of the carcass, so that a longitudinal distance between the tracks of the upstream tensioner and the tracks of the downstream tensioner forms the first region of the carcass, and wherein the tracks of the upstream tensioner operate at a higher speed than the tracks of the downstream tensioner during use of the system, so that their corresponding engagements with the carcass result in the application of the first force (Fi).

The downstream tensioner may be configured to be controlled to have a desired speed of its tracks, and the upstream tensioner may be configured to be controlled to have a speed of its tracks resulting in the first force being applied.

In other alternative embodiments, the first force application tool comprises:

- a brake and

- an upstream tensioner comprising at least two tracks, wherein the upstream tensioner and the brake are arranged to extend parallel to the longitudinal axis of the carcass and with engagement surfaces of the tracks and the brake in engagement with a part of the circumference of the carcass, the tensioner and the brake being arranged spaced from each other in a direction of advancement of the carcass, so that a longitudinal distance between the tracks of the upstream tensioner and the brake forms the first region of the carcass, wherein the upstream tensioner is configured to be controlled based on feedback from at least one sensor arranged at or near the location where the pitch of the windings is increased to be within the second pitch value range, the sensor providing signals representative of the process, and wherein the brake is configured to be controlled to apply a desired braking force radially to the carcass so that the engagements with the carcass by the tracks and the brake result in the application of the first force.

In any of the embodiments of a system as described above, the second force application tool may comprise a rotating extension tool arranged circumferentially around the carcass during use of the system, the extension tool having at least one radially inwardly extending protrusion following at least a part of a helical path with a pitch corresponding to a second target pitch value, wherein the helical protrusion is dimensioned to engage with the windings of the carcass and thereby push them apart when the carcass is moved in the direction of the carcass axis.

In alternative embodiments of a system according to the invention, the second force application tool comprises series of rollers arranged on a roller holder rotatable around the carcass during use of the system with the axes of rotation of the rollers being parallel to, or substantially parallel to, and offset from the carcass axis, wherein the rollers are arranged and shaped to engage with the windings of the carcass when the carcass moves in the direction of the carcass axis, the arrangement and shape of the rollers being so that the windings of the carcass are pushed apart by the engagement with the rollers.

In such embodiment wherein the second force application tool comprises series of rollers,

- the rollers may each have a circumferentially arranged protrusion,

- for each roller, the cross-sectional shape of the protrusion, in a plane containing the axis of rotation, at the circumference may have a radius of curvature,

- the rollers may have different radii of curvature, and

- the rollers may be arranged in order of increasing radius of curvature with respect to when they are brought into engagement with the advancing carcass.

The first pitch value range may be determined as a first target pitch value ± 10%, such as a first target pitch value ± 5%, such as a first target pitch value ± 1%.

The second pitch value range may be determined as a second target pitch value ± 10%, such as a first target pitch value ± 5%, such as a second target pitch value ± 1%.

In any of the systems as described above, the system may further comprise an extrusion tool for applying a pressure sheath to the carcass while the pitch of the windings is within the second pitch value range.

The first and second aspects of the present invention may each be combined with any of the other aspects. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE FIGURES

The method of manufacturing a carcass as well as a corresponding system according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Figure 1 shows schematically the forming of a carcass from a metal strip. The part of the invention shown in this figure resembles a known method.

Figure 2 shows schematically the overall idea of first applying a compression step and then applying an extension step to the carcass of figure 1.

Figure 3 shows schematically an example how the first force can be applied to the windings of the carcass.

Figure 4 shows schematically an embodiment of a system according to the invention.

Figures 5. a to 5.g show schematically how the grippers of the system in figure 4 apply the first force to the carcass and advance the carcass.

Figure 6 shows schematically other examples of how the first force can be applied to the windings of the carcass.

Figure 7 shows schematically and in partial views the rollers of the embodiment in figure 4, the rollers having circumferential protrusions with different radii of curvature.

Figures 8 to 10 show schematically others examples of how the first force can be applied to the windings of the carcass.

Figure 11 show schematically a carcass with a pressure sheath. DETAILED DESCRIPTION OF AN EMBODIMENT

Figure 1 shows an example of forming a carcass 1 for a flexible pipe; the different parts of the figures are not drawn to the same scale. This part of the process resembles a known method, and the tools used are therefore shown as circles. It will be known to a person skilled in the art how to perform this part of the invention. In the embodiment illustrated in figure 1, only one metal strip 2 is used for forming the carcass 1, but the invention also covers embodiments, wherein the carcass 1 is formed from a plurality of metal strips. The system comprises a transporter 3 for feeding the metal strip 2 into the system. The metal strip 2 has opposite first and second surfaces 4a, 4b and opposite first and second longitudinal edges 5a, 5b. A shaping tool 6 is used for shaping the metal strip 2 to have a first fold 6a along the first longitudinal edge 5a and facing towards the first surface 4a, and a second fold 6b along the second longitudinal edge 5b and facing towards the second surface 4b. Then a winding tool 7 is used for helically winding the shaped metal strip 2 while providing that the first fold 6a engages and interlocks with the second fold 6b so that coherent windings of metal strip form a tubular carcass 1 extending along a carcass axis 8. The carcass 1 has a pitch which is shown in figure 1 as P. As seen, the pitch P is the longitudinal distance between corresponding points of neighbouring windings of the carcass 1. Even though the method of forming the carcass 1 is described as two separate steps, the scope of the invention also covers embodiments, wherein the shaping and the winding as well as the resulting interlocking is performed in one tool integrating these steps into one combined process step. A method according to the invention also covers the manufacturing of other types of carcasses than the one shown in figure 1.

When the carcass 1 has been formed e.g. as described in relation to figure 1, it is subjected to first a compression step and then an extension step as shown schematically in figure 2. Figure 2. a shows the compression step, wherein a first force Fi is applied to a first region of the carcass 1, and the first force having a first force component parallel to the carcass axis 8. Hereby the pitch P of the windings is decreased to be within a first pitch value range. In figure 2. a only one first pitch value Pi is shown and not a range. Subsequently, a second force F2 is applied to at least a second region 10 of the carcass 1 where the pitch of the windings is within the first pitch value range. This second region 10 does not need to be identical to the first region 9, but it should be a part of the carcass 1 that has been compressed. It could e.g. be a smaller part thereof at a time. The second force F2 has a second force component parallel to the carcass axis as shown in figure 2.b. Hereby the pitch of the windings is increased to be within a second pitch value range, the pitch within the second pitch value range being larger than the pitch within the first pitch value range. In figure 2.b only one second pitch value P2 is shown and not a range. The first and second forces FI,F2 can be applied in different ways as will be described in the following. The forces may also have force components in other directions than parallel to the carcass axis.

Figure 3 shows schematically and in cross-sectional view an embodiment of the invention, wherein the first force is applied by a pair of mutually collaborating movable grippers 11. The grippers 11 are arranged around the carcass 1 at a distance from each other in a direction parallel to the carcass axis 8. As shown in figure 3. a, the distance between the grippers 11 forms the first region 9 of the carcass 1, i.e. the region to which the first force Fi is applied in order to compress the windings and thereby decrease the pitch P. Each gripper 11 applies a gripping force in the radial direction to at least a part of a circumference of the carcass; these gripping forces are shown as arrows in the figure. The force is preferably applied along almost the whole circumference so that a uniform loading is ensured. As shown in figure 3.b, with the gripping forces maintained, the grippers 11 move towards each other so that the windings are compressed. The compression can e.g. be controlled by moving the grippers 11 towards each other until a predetermined threshold pressure is measured. This threshold pressure is predetermined to be the one resulting in the pitch Pi of the windings being within the first pitch value range. Then the gripping force of at least one of the grippers 11 is released as shown in figure 3.c, and one of the grippers 11 is moved so that the distance between the grippers 11 is increased as shown in figure 3.d. These steps then repeated for a subsequent first region of the carcass 1. This can become possible either by moving the pair of grippers 11 along a stationary carcass 1 or by advancing the carcass 1 with respect to the location of the grippers 11. A possible design of a system for the manufacturing a carcass 1 according to the present invention is shown schematically in figure 4. The system applies the first force Fi to the carcass 1 in correspondence with the description of figure 3. The rollers 18 at the end of the system are for the application of the second force as will be described below.

The grippers 11 of the embodiment in figure 4 are movably arranged on rails 12 for movement parallel to the carcass axis. There is always at least one of the grippers 11 applying gripping force to the carcass 1 simultaneously with moving the grippers 11 parallel to the carcass axis. Hereby a force is applied in the longitudinal direction for advancement of the carcass 11 through the system. The radial gripping force is in this embodiment applied by moving the elements forming the grippers 11 along rails 13 oriented perpendicular to the direction of the carcass axis. In the embodiment in figure 4, there are two rails 12 in the longitudinal direction and one rail 13 per pair of grippers in the perpendicular direction. However, other numbers of rails will also be possible. The rails 12,13 are shown as elongated grooves in which matching protrusions (not shown) on the moving plates can move. Other kinds of rails will also be possible as will be well known for a person skilled in the art. The system has a controller (not shown) for controlling the gripping force of the grippers 11 and the movement of the grippers. There may be a separate controller for the grippers 11, or it may be the part of a controlling unit used to control the whole system.

Figure 5 shows schematically a sequence of how the grippers of the system in figure 4 apply the first force to the carcass and advance the carcass. In figure 5, reference numbers are left out to increase the clarity of the figure. The movements of the different parts of the system are indicated with individual arrows. The compressing forces Fi as in figure 4 are also indicated with arrows.

Figure 6 shows schematically an alternative way of applying the first force. Here the compression step where the first force Fi is applied is performed by moving the carcass 1 in the direction of the carcass axis 8 through a rotating compression tool 14 arranged circumferentially around the carcass 1. Such an embodiment resembles a nut being screwed onto a threaded rod. In the embodiment in figure 6. a, the compression tool 14 extends along the whole circumference, and in the embodiment in figure 6.b, the compression tool 14 is made from more parts with gaps 16 there between. In both embodiments, the compression tool 14 has at least one radially inwardly extending protrusion 17 following at least a part of a helical path with a pitch Pi corresponding to a first target pitch value; see figure 6.c showing a cross sectional view of the compression tool 14 in figure 6. a. The protrusion 17 is dimensioned to engage with the windings of the carcass 1 and thereby push them together.

In the same manner as described for the compression tool 14, the second force F2 could also be applied by moving the carcass 1 in the direction of the carcass axis 8 through a rotating extension tool arranged circumferentially around the carcass 1. Such an extension tool has at least one radially inwardly extending protrusion following at least a part of a helical path with a pitch corresponding to a second target pitch value. This helical protrusion is dimensioned to engage with the windings of the carcass and thereby push them apart. Thus, such an extension tool would look as the compression tool of figure 6.

As mentioned above, the system schematically shown in figure 4 comprises a series of rollers 18 arranged on a roller holder 19 for the application of the second force in order to provide the windings with a pitch P2 being within a second pitch value range. The part of the system enabling the rotational movement of the roller holder 19 and thereby also the rollers 18 is not illustrated in these figures. However, it will be well known to a person skilled in the art how to provide for rotational movement of parts of such a system as well as the control thereof. During the manufacturing, the roller holder 19 rotates around the carcass 1 with the axes of rotation of the rollers 18 being parallel to, or substantially parallel to, and offset from the carcass axis 8. The rollers 18 are arranged and shaped to engage with the windings of the carcass 1, when the carcass 1 moves in the direction of the carcass axis 8. As described in relation to figure 5, this continuous advancement is effected by the moving grippers 11 also used to apply the first force Fi. If relevant, further means of enabling the advancement may also be used, such as an additional tool member pulling or pushing the carcass forwards. The arrangement and shape of the rollers 18 are so that the windings of the carcass 1 are pushed apart by the engagement with the rollers 18. The rollers 18 in the embodiment of figures 4 and 5 each has a circumferentially arranged protrusion 20 for engaging with the windings of the carcass 1. The roller holder 19 is also provided with guide rollers 21 supporting the carcass 1 centred in the correct position with respect to the machine. Hereby it is easier to ensure that the manufacturing runs smoothly and results in a carcass 1 with the desired final pitch P2. Figure 7. a shows schematically the region at the periphery of three different rollers 18 used to increase the pitch P gradually as shown in figure 7.b. Figure 7. a shows the cross section in a plane containing the axis of rotation of the rollers 18. The rollers 18 have different radii of curvature and are arranged in order of increasing radius of curvature with respect to when they are brought into engagement with the advancing carcass 1. By letting the windings engage gradually with rollers 18 having increasing radii, the loading of each roller 18 as well as the local stresses on the carcass 1 will be lower than if only one shape of roller 18 had been used. It will also be lower than for the embodiment with the extension tool as described above in relation to figure 6.

By letting the carcass 1 have the second force F2 applied to the windings, the final pitch P2 is within a second pitch value range, the pitch within the second pitch value range being larger than the pitch Pi within the first pitch value range. The carcass 1 is then advanced towards the next processing step which may e.g. be to have a polymer pressure sheath applied to the outer surface by extrusion. The carcass may be moved directly from the system as described above and into the extruder for the application of the pressure sheath. However, it may also be wound onto large spools for temporary storage between the production steps.

Figures 8 to 10 show schematically others examples of how the first force Fi can be applied to the windings of the carcass 1 by use of one or more tensioners. These examples can be used in combination with any the described ways of applying the second force.

In the embodiment in figure 8, the first force Fi is applied by an upstream tensioner 22 and a co-operating downstream tensioner 23. In the illustrated embodiment, each tensioner comprises two movable tracks 24,25 arranged to extend parallel to the carcass axis 8 and with engagement surfaces of the tracks 24,25 being in engagement with a part of the circumference of the carcass 1. The tensioners 22,23 are arranged spaced from each other in a direction of advancement of the carcass, the direction being from left to right in the figure.

The tensioners 22,23 are arranged so that a longitudinal distance between the tracks 24 of the upstream tensioner 22 and the tracks 25 of the downstream tensioner 25 forms the first region 9 of the carcass 1. As described above, the tracks 24 of the upstream tensioner 22 operate at a higher speed than the tracks 25 of the downstream tensioner23 so that their corresponding engagements with the carcass 1 result in the application of the first force Fi. In this embodiment, the downstream tensioner 23 is controlled to have a desired speed of its tracks 25, and the upstream tensioner 22 is controlled to have a speed of its tracks 24 resulting in the first force Fi being applied.

Figure 9 shows an alternative embodiment in which the first force Fi is applied by a brake 26 and an upstream tensioner 22 comprising two tracks 24. The upstream tensioner 22 and the brake 26 are arranged to extend parallel to the carcass axis 8 and with engagement surfaces of the tracks 24 and the brake 26 in engagement with a part of the circumference of the carcass 1. The upstream tensioner 22 and the brake 26 are arranged spaced from each other in a direction of advancement of the carcass 1 so that a longitudinal distance between the tracks 24 of the upstream tensioner 22 and the brake 26 forms the first region 9 of the carcass 1. The brake 26 of the illustrated embodiment is in the form of two blocks of material that is pressed towards the outer surface of the carcass 1 and thereby provides a braking force F _n due to the frictional engagement. The upstream tensioner 22 can be controlled based on feedback from at least one sensor (not shown) arranged at or near the location where the pitch P of the windings is increased to be within the second pitch value range, the sensor providing signals representative of the process. The brake 26 is controlled to apply a desired braking force Fn radially to the carcass 1 so that the engagements with the carcass 1 by the tracks 24 and the brake 26 result in the application of the first force Fi.

Figure 10 shows an alternative to the embodiment in figure 9. Here the brake is in the form of a downstream tensioner 23 having tracks 25 as described above. Then the braking action may be provided e.g. by applying torque constraints in the control of the movement of the tracks 25. Figure 11 show schematically a carcass with a pressure sheath 27. Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Furthermore, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.