GB2463250A

GB2463250A - A wind turbine blade formed from welded thermoplastic sections

Info

Publication number: GB2463250A
Application number: GB0816132A
Authority: GB
Inventors: Erwin Merijn Wouterson; Yoshiki Haraguchi
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2008-09-04
Filing date: 2008-09-04
Publication date: 2010-03-10
Also published as: WO2010025830A2; WO2010025830A3; GB0816132D0

Abstract

A wind turbine blade 10 comprises at least one component formed of two or more connected component sections 22, 24, each component section 22, 24 comprising at least one thermoplastic end region. The thermoplastic end regions of adjacent component sections are welded together at a weld interface 16, 18. Preferably the weld interface is a resistance weld interface. A wire mesh may be embedded in at least one of the end regions. Alternative welding techniques for example ultrasonic, infra red, ultra violet or friction stir welding may be used. One of the components may include a longitudinal beam 14.

Description

WIND TURBINE BLADE FORMED OF CONNECTED SECTJONS

The present invention relates to a blade for a wind turbine. In particular, the invention relates to a blade for a wind turbine which comprises at least one component formed of two or more connected sections and to a method of connecting adjacent component sections of such a blade using a thermoplastic welding technique.

With the development of increasingly large wind turbines, there is a requirement for wind turbine blades of greater length. However, as the size of wind turbine blades increases, it become less feasible to produce and transport one-piece blades, since the size of production io facilities and transport means must be increased, with a corresponding rise in associated costs.

It has previously been suggested to prepare a wind turbine blade in smaller sections which may be connected together to form the final blade structure at a later stage and/or facility.

For example, DE 297 04 151 and EP-A-1 761 702 both disclose a wind turbine blade formed of multiple connected sections, in which the sections are connected together by means of is mechanical joints. The sections may be connected together to form the assembled blade using conventional joining techniques, such as adhesion bonding or mechanical fastening; however, the use of such techniques is not always desirable. Both adhesion and mechanical fastening can be time consuming and costly. In addition, mechanical fastening requires holes to be drilled into the blade structure, which can result in stress concentrations when the blade is in use, whilst adhesion bonding often requires relatively complicated surface pre-treatments and curing steps. In addition, the overall weight of the blade is inevitably increased through the use of large numbers of fasteners, which in turn increases the stress on the load bearing components of the blade.

The use of connecting members at the interface between two blade sections has also been proposed, for example in WO-A-20041078462, in order to increase mechanical strength of the joint.

It would be desirable to provide a wind turbine blade formed of two or more sections that may be simply and efficiently connected together during assembly of the blade, without the need for the incorporation of complex components, or a large number of fasteners. It would be particularly desirable if the sections could be connected such that the joint, or interface between them has a high mechanical strength and does not incorporate components that significantly increase the overall weight of the blade or the load to which the blade is subjected. It would also be desirable to provide an improved, cost-effective method for connecting two or more sections of a wind turbine blade.

According to the present invention there is provided a wind turbine blade comprising at least one component formed of two or more connected component sections, each component section comprising at least one thermoplastic end region including a thermoplastic material, wherein the thermoplastic end regions of adjacent component sections are welded together at a weld interface.

According to the present invention there is also provided a component section for forming a component of the wind turbine blade described above, wherein the component io section comprises at least one thermoplastic end region including a thermoplastic material. The wind turbine blade component is formed by connecting two or more component sections according to the invention at a thermoplastic weld interface.

The term "thermoplastic" refers to a polymer material that softens upon heating and hardens again upon cooling. Such materials may be repeatedly softened and hardened so that they can be reversibly moulded to shape. Thermoplastic articles may be welded, or bonded together at a "weld interface" by heating the articles in order to melt the thermoplastic material at the interface and then applying pressure in order to fuse the thermoplastic materials across the interface. Thermoplastic materials offer a number of advantages which make them especially suitable for use in wind turbine blades. In particular, they have high strength and stiffness to weight ratios and provide good fatigue and corrosion resistance.

The thermoplastic material used at the weld interface in the wind turbine blades of the present invention should have a glass transition and melting temperature above the operating temperature of the blade, so that there is no risk of the material softening during normal use. It is also preferable if the performance characteristics of the thermoplastic material are not moisture dependent. It is of course important to bear in mind that the wind turbine may be installed in a wide varieties of environments, such as a low temperature region, a high temperature region or in an off shore location and the environmental conditions to which it may be subjected during normal use could therefore vary significantly. Suitable thermoplastic materials for use in the present invention include but are not limited to polybutylene terephthalate, polyethylene terephthalate, polypropylene, and polyethylene.

In the wind turbine blades of the present invention, a thermoplastic material is incorporated in at least one of the ends of each of the connected component sections, to provide a thermoplastic end region. Preferably, a thermoplastic end region is provided at the ends of the component section which are to be connected to an adjacent section. In order to connect adjacent component sections together, the thermoplastic end regions are welded together by heating the end regions at the interface in order to melt the thermoplastic material at the surfaces, bringing the melted surfaces into contact with each other and then applying pressure in order to mix the thermoplastic materials and fuse the surfaces of the end regions together. The surfaces are held in contact with each other until the thermoplastic material has cooled below its softening temperature.

Advantageously, the component sections can be welded together at high speed with minimum labour and cost and with little, if any, pre-treatment of the surfaces at the weld interface. In addition, the weld interface can be rapidly and easily repaired if necessary, by reheating the thermoplastic material at the weld interface in order to melt the thermoplastic material and separate the component sections from each other.

The resultant fusion bond between the surfaces of the two thermoplastic end regions at the weld interface provides a sufficiently strong connection between the adjacent component sections that no other means of connection are required. Advantageously, the provision of a weld interface therefore eliminates the need for conventional mechanical fasteners or adhesives at the interface, although they may be incorporated if it is desired to do so for any reason.

The component sections of wind turbine blades according to the invention are likely to be significantly shorter than a complete blade and can therefore be produced and transported more easily and more cost effectively. In particular, the component sections may be transported unassembled and subsequently assembled at, or close to the site where the turbine is to be used. This significantly reduces the difficulty and cost associated with transporting the blades and means that blades of increased size can be provided without significant changes in existing production or transport facilities.

The weld interface may be formed using any suitable technique to apply heat in order to melt the thermoplastic material at the surfaces of the adjacent thermoplastic end regions forming the interface. Suitable welding techniques include but are not limited to ultrasonic welding, induction welding, friction stir welding, fibre optics infra-red, ultraviolet heating and resistance welding.

Preferably, the weld interface is a resistance weld interface, which has been formed using a resistance welding technique. In resistance welding, a layer or strip of high resistance material is placed adjacent to or between the surfaces of the thermoplastic end regions at the interface and an electrical current is applied in order to heat the layer and melt the thermoplastic material at the weld interface. At the same time, or immediately after heating, pressure is applied to fuse the thermoplastic material across the interface such that when the material is cooled, the sections are welded together. The resistance welding equipment is simple, portable and inexpensive and the techniques are both fast and reliable. Resistance welding techniques are suitable for automation, if desired, and advantageously require a relatively low level of power.

In one embodiment of the present invention in which a resistance weld interface is provided, high resistance wire is embedded in at least one of the thermoplastic end regions, adjacent the weld interface. The resistance wire may be incorporated in the end region during moulding of the section. During assembly of the wind turbine blade, current is passed through the high resistance wire in order to melt the thermoplastic material at the interface between the adjacent thermoplastic end regions.

In an alternative embodiment in which a resistance weld interface is provided, a high resistance metallic mesh or gauze is provided between the thermoplastic end regions at the interface, in contact with the thermoplastic material. During assembly of the wind turbine blade component, current is passed through the mesh or gauze in order to weld the sections together.

The resistance wire or mesh remains embedded within the weld interface. This means that, if desired, the thermoplastic material at the weld interface can be heated again in order to separate the connected sections from each other. This may be useful, for example if it is desired to dissemble the blade, or repair or replace one of the component sections.

Wind turbine blades according to the invention may be of a first known design, in which the blade comprises an outer shell forming the airfoil of the blade and one or more central, inner beams which extends longitudinally through the inner cavity of the blade and are connected to the outer shell. In this type of blade, the inner beam is the main load-bearing component and it is therefore important to optimise its stiffness, particularly at the tip end of the blade. The outer shell may or may not contribute significantly to the overall stiffness of the blades.

Alternatively, wind turbine blades according to the invention may be a of second known design comprising two or more connected, reinforced outer shell portions and a pair of central, inner webs which connect the outer shell portions and extend longitudinally through the interior of the blade. Each web may take the form of, for example, a C-beam. Typically, the reinforced outer shell portions are the main load-bearing components and it is therefore important to optimise their stiffness, particularly at the tip end of the blade. Due to the increased stiffness of the outer shell portions, the inner webs of this type of blade contribute significantly less to the overall stiffness of the blades than the inner beam of the first type of blade described above.

In certain embodiments of the present invention, the outer shell of the wind turbine blade is formed of two or more component sections, connected together at a thermoplastic weld interface, as described above.

The outer shell may be formed of two or more component sections which are connected together in the longitudinal direction of the blade. For example, the outer shell may be formed of one or more body sections, providing the majority of the length of the outer shell, and a tip section, connected to the body section at the outer end of the blade. The provision of a separate tip section may be advantageous since it allows tip sections of differing designs or having different functions to be incorporated, as desired. For example, it may be desirable in certain environments to include a tip with higher stiffness or erosion resistance, or a tip that incorporates measures for reducing the impact of lightning, or bird impact. Alternatively or in addition, the tip may include lighting or may be of a specifically desired colour.

Preferably, the outer shell includes two or more body sections in addition to a tip section.

The use of multiple outer shell sections along the length of the blade advantageously allows the blade to be more specifically tailored to suit the environment in which the turbine is to be located or the requirements of the consumer. Outer shell sections having different shapes and sizes can be mixed and matched in order to optimise the aerodynamics of the blade, or alter other properties of the blade as desired. For example, by combining different sections, it is possible to vary the overall length of the blade, or its curvature, or the chord at different positions along the blade.

In addition to the thermoplastic material in the thermoplastic end regions of the blade sections, the outer shell sections may incorporate one or more other regions of thermoplastic material. For example, an outer shell section may include one or more additional thermoplastic regions extending in a lengthwise direction between the thermoplastic end regions of the section. In this way, the weld interface may be extended over a larger area of the section, which further strengthens the connection of the adjacent sections and enhances the fusion of the thermoplastic material to the other portions of the blade section. This also enables each blade section to be formed of two or more multiple connected portions connected at thermoplastic weld interfaces.

Conventionally, the outer shell of wind turbine blades are formed of two shell halves, a windward shell halve and a leeward shell halve. The present invention also includes a wind turbine blade of this construction in which the windward and leeward shell halves are connected together at a thermoplastic weld interface. The outer shell is preferably also formed of two or more outer shell sections connected in the longitudinal direction of the blade, each of which may be formed of windward and leeward shell halves, in the same way as a conventional blade. The shell halves of each section may be connected together using thermoplastic welding, but may alternatively be connected together by conventional means, such as adhesive or mechanical fasteners.

Typically, the inner beam of wind turbine blades of the first design described above does not extend along the full length of the blade and is therefore not as long as the outer shell. In some cases, it may therefore be feasible to produce and transport the inner beam in one piece.

However, if desired, the inner beam of wind turbine blades according to the invention (where present) may be formed of two or more inner beam sections connected at a thermoplastic weld interface. Preferably, the two or more inner beam sections are connected in the longitudinal direction of the inner beam.

The inner beam is preferably quadrangular in cross-section but other cross-sections may also be suitable, such as a circular, I-shaped or C-shaped cross-section. Inner beams of I-shaped or C-shaped cross-section are sometimes referred to as webs. The cross-section of the inner beam may be adapted in order to optimise the contact between the inner beam and the outer shell. For example, the surfaces of the beam to which the outer shell is connected (known as the "spar caps") may be shaped such that the contact area between the beam and the outer shell is maximised. Typically, to account for the decreasing size of the cross-section of the blade towards the tip end, there will be a corresponding decrease in the cross-section of the inner beam towards the tip end.

The spar caps of the inner beam may be formed from a single layer material but are preferably formed from a laminar material comprising two or more layers of the same, or differing materials. Some, or all of the layers of the laminar material may comprise reinforcing fibres. The fibres in each layer may be oriented in the same or a different direction to the fibres in the adjoining layer or layers. In certain embodiments, the orientation of the layers may be varied in order to alter the mechanical properties of the material. An example of a suitable laminar material is described in WO 2004/078465.

The spar caps may be completely covered by the outer shell portions, or may be at least partially exposed, so that their surfaces form a part of the exterior surface of the blade.

Preferably, the area of the thermoplastic end region of the component sections of wind turbine blades according to the invention is small compared to the overall area of the component section. Preferably, the thickness of the thermoplastic end regions at the weld interface is between about 0.1 mm and about 5 mm, wherein the "thickness" is measured substantially normally to the plane of the weld interface.

Preferably, where resistance wire or mesh is incorporated at the weld interface, the weld interface is covered by one or more layers of a non-conductive, insulating material in order to isolate the weld interface and protect it from lightening strikes. In addition, in such cases the weld interface should not be in contact with any lightening conductors which may be provided along the length of the blade in order to protect the blade from damage due to lightening.

The portions of the blade component other than the thermoplastic end regions may be formed of any suitable material. For example, the blade component will typically be formed of a fibrous composite material comprising a thermosetting resin through which reinforcing fibres have been distributed. In this case, the main structure of the blade component is therefore a thermoset structure, into or onto which a thermoplastic end region has been incorporated.

Alternatively, or in addition, it may be desired to incorporate one or more components in the blade that are formed entirely of the thermoplastic material. For example, it may be desired to have a tip portion formed of thermoplastic material, wherein the thermoplastic end region is integral with the rest of the structure.

By "thermosetting" or "thermoset" is meant a polymer material that becomes permanently hard and rigid when heated or cured. Once moulded, the thermosetting material cannot therefore be re-moulded or recycled. The thermosetting resin used in the blade component sections may be based on, for example, unsaturated polyester, polyurethane, polyvinyl ester, epoxy or combinations thereof. Most preferably, the resin is an epoxy resin.

Resin formations are well known in the art.

The resin may be provided as liquid, semisolid or solid resin. It may comprise two or more resin systems which may or may not be based on the same type of resin, such as two or more epoxy-based systems. Through the use of two or more resin systems, it may be possible to optimise the properties of the resin for the subsequent steps of processing, for example with respect to viscosity and timing/controlling of the curing process.

The reinforcing fibres in the thermosetting resin may include one or more of: carbon fibres; glass fibres, aramid fibres, synthetic fibres (e.g. acrylic, polyester, PAN, PET, PE, PP or PBO fibres), bio fibres (e.g. hemp, jute, cellulose fibres), mineral fibres (e.g. Rockwool�), metal fibres (e.g. steel, aluminium, brass, copper fibres) and boron fibres. The fibres provide strength and stiffness to the blade and may also be incorporated to improve particular properties of the component, such as shear strength or thermal properties.

The reinforcing fibres may be provided in any suitable form including but not limited to: prepregs, semi-pregs, woven or non-woven fabrics, mats, pre-forms, individual or groups of fibres, tows and tow-pregs.

The term "prepreg" refers to a substantially or fully impregnated collection of fibres, fibre tows, woven or non-woven fabric. Woven and non-woven fabrics are collections of individual fibres or fibre tows that are substantially dry, that is, not impregnated by a resin. Fibre tows are io bundles of large numbers of individual fibres.

The term "semi-preg" refers to a partially impregnated collection of fibres or fibre tows.

The partial impregnation provides for enhanced removal of gas through or along the dry fibres during consolidation and/or curing.

The term "tow-preg" refers to an at least partially impregnated fibre tow.

The term upreformn refers to a composite material comprising fibres and cured or uncured resin. The fibres are preferably provided in layers of oriented fibres. Examples of pie-forms and methods of preparing pre-forms are described in WO-A-2004/078442. In order to reduce waste, the pre-forms may be provided as a pre-formed slab, which has been produced with the desired shape and size so that it can be incorporated directly into the blade.

The inner beam and/or outer shell sections may be unconsolidated or at least partially consolidated. The term uconsolidatedn means that most, if not all of the gas has been removed from inside the fibrous composite material, giving a lower porosity. Pre-consolidated pre-forms are particularly suitable for use in the inner beam of wind turbine blades, since they provide good reproducibility, high strength and high homogeneity, and can be connected to other pre-forms or structures.

The fibrous composite material forming the inner beam and outer shell portions of the blade may be uncured, partially cured or fully cured. Typically, the curing of the material increases the stiffness.

The thermoplastic end regions may be incorporated into the component sections during moulding of the sections, or alternatively the thermoplastic material may be incorporated in the material forming the remainder of the section prior to the moulding step. For example, the thermoplastic may be co-cured onto the thermoset structure of the remainder of the component section. This is achieved by including a thermoplastic layer on top of the layup of thermoset fibre prepregs and consolidating the layer together with the thermosetting resin material. During curing, an interpenetrating network is formed between the thermosetting and thermoplastic materials, leaving the thermoplastic material on the outside of the therrnoset structure. As an alternative to the co-curing process, the thermoplastic may be co-bonded onto the thermoset structure. The thermoset structure is cured separately and once the structure has been cured, the thermoplastic layer is bonded on top of the thermoset structure.

According to the present invention there is also provided a method of connecting adjacent component sections to form a wind turbine blade as described above, wherein the method comprises the steps of: (a) arranging the two component sections with the thermoplastic end regions adjoining or overlapping at an interface; (b) applying heat in order to melt the thermoplastic material at the interface; (c) applying pressure at the interface in order to weld the thermoplastic end regions together; and (d) applying a cooling cycle to consolidate the joint region.

Step (a) may be carried out before step (b) or alternatively, step (b) may be carried out immediately before step (a) provided that step (a) is carried out whilst the thermoplastic materials are in a softened state. Alternatively, it may be possible to carry out steps (a) and (b) substantially simultaneously, so that the thermoplastic end regions are heated whilst the sections are aligned and brought together. Pressure is applied in order to fuse the melted thermoplastic material at the interface and should preferably be applied until the thermoplastic materials have cooled and hardened, in order to ensure the integrity of the weld interface. This may be achieved, for example, by clamping the component sections in position during welding and until the thermoplastic material at the weld interface has sufficiently hardened.

As described above, the necessary heat for step (b) may be applied using any suitable, known thermoplastic welding technique, such as ultrasonic welding, induction welding, friction stir welding or resistance welding. Similarly, the necessary cooling for step (d) may be applied using any suitable, known technique.

Preferably, the adjacent outer shell sections are connected using a resistance welding technique, as described above. This may be achieved by, for example, incorporating high resistance wire in at least one of the thermoplastic end regions at the interface or by incorporating a wire mesh or gauze between the thermoplastic end regions at the interface. A current may then be applied to the wire by means of electrodes in order to melt the thermoplastic material at the interface.

The invention will be further described, by way of example only, with reference to the following figures in which: Figure 1 shows a wind turbine with three blades; Figure 2 shows a cross section through part of a blade according to the invention; and Figure 3 shows a schematic, exploded view of the weld interface of the blade of Figures 2 and 3; and Figure 4 shows a cross section through part of an alternative blade according to the io invention.

Figure 1 illustrates a wind turbine I comprising a wind turbine tower 2 on which a wind turbine nacelle 3 is mounted. A wind turbine rotor 4 comprising at least one wind turbine blade is mounted on a hub 6. The hub 6 is connected to the nacelle 3 through a low speed shaft (not shown) extending from the nacelle front. The wind turbine illustrated in Figure 1 may be a small model intended from domestic or light utility usage, or may be a large model, such as those that are suitable for use in large scale electricity generation on a wind farm for example.

In the latter case, the diameter of the blades could be as large as 100 metres or more.

A cross section of a blade 10 according to the present invention is shown in Figure 2.

The blade 10 is suitable for mounting on a wind turbine of the type shown in Figure 1 and described above. The general design of the blade 10 is similar to that of well known, existing blades and comprises an outer shell 12 and a central, inner beam 14, which extends longitudinally through the interior of the blade 10.

The beam 14 is of a genera'ly quadrangular cross section and is connected to the outer shell 12 along upper 16 and lower 18 spar caps. The beam 14 is formed from an epoxy resin composite material including carbon reinforcement fibres, which are aligned in the longitudinal direction of the beam. The majority of the carbon fibres are incorporated in the spar caps 16,18 of the beam.

As can be seen from Figure 2, the outer shell 12 is formed of two connected outer shell sections, a first outer shell section 22 at the root end of the blade and a second outer shell section 24 at the tip end of the blade. Each of the outer shell sections 22, 24 includes a thermoplastic end region 26 (shown in Figure 3), formed of a thermoplastic material which has been incorporated in the outer shell section during the moulding process, such that the thermoplastic end region is integral with the remainder of the section. Each thermoplastic end region extends approximately 5 mm from the edge of the shell section. The remainder of each outer shell section is formed of a thermosetting epoxy resin.

The thermoplastic end regions 26 of the first 22 and second 24 outer shell sections are welded together at a weld interface 28. At the weld interface 28, the thermoplastic material of the thermoplastic end region of the first outer shell section 22 is fused with the thermoplastic end legion 26 of the second outer shell section 24 to provide a joint having sufficiently high io mechanical strength that no other connection means are required.

As shown more clearly in Figure 3, a high resistance wire mesh 30 has been incorporated between the thermoplastic end regions at the weld interface. The first 22 and second 24 outer shell sections have been connected together using a resistance welding technique, in which a current has been passed through the wire mesh 30, as described above.

A cross section of an alternative blade 40 according to the invention is shown in Figure 4. The blade 40 is suitable for mounting on a wind turbine of the type shown in Figure 1 and described above. The general design of the blade 40 is similar to that of the blade 10 shown in Figure 2 and described above. However, the blade 40 additionally includes a series of weld interfaces 42 extending in a lengthwise direction along each blade section, between the thermoplastic end regions. These longitudinal weld interfaces 42 increase the strength of the connection between the blade sections and improve the bonding of the thermoplastic material to the thermoset structure.

It will be appreciated that although the invention has been described with reference to wind turbine blades of a specific design, it includes wind turbines blades of any design which include a component that may feasibly be formed of two or more connected sections.

Similarly, although the invention has been described with reference to the outer shell and inner beam of a wind turbine blade, it will be clear to the skilled person that the "at least one component' of the present invention may include any component of the blade, or part thereof, which may feasibly be formed of two or more connected sections.

Claims

CLAIMS1. A wind turbine blade comprising at least one component formed of two or more connected component sections, each component section comprising at least one thermoplastic end region including a thermoplastic material, wherein the thermoplastic end regions of adjacent component sections are welded together at a weld interface.
2. A wind turbine blade according to claim 1 wherein the weld interface is a resistance weld interface.
3. A wind turbine blade according to claim 2 further comprising high resistance wire embedded in at least one of the thermoplastic end regions, adjacent the weld interface.
4. A wind turbine blade according to claim 3 further comprising a high resistance wire mesh between the thermoplastic end regions at the weld interface.
5. A wind turbine blade according to any preceding claim wherein the component sections are formed of a thermoset structure incorporating the at least one thermoplastic end region.
6. A wind turbine blade according to any preceding claim wherein the component sections are connected in the longitudinal direction of the blade.
7. A wind turbine blade according to any preceding claim wherein the at least one component includes the outer shell of the wind turbine blade, or a part thereof.
8. A wind turbine blade according to claim 7 wherein the outer shell is formed of at least one body section and a tip section.
9. A wind turbine blade according to any preceding claim wherein one or more of the component sections comprises at least one additional thermoplastic region extending from the thermoplastic end region.
10. A wind turbine blade according to any preceding claim wherein the at least one component includes a longitudinal inner beam, or a part thereof.
11. A wind turbine blade according to any preceding claim, wherein the weld interface is between about 0.1 and about 5 mm thick.
12. A component section for forming a component of a wind turbine blade according to any preceding claim, wherein the component section comprises at least one thermoplastic end region including a thermoplastic material.
13. A method of connecting two adjacent component sections of the wind turbine blade of any of claims I to 11, the method comprising: io (a) arranging the two component sections with the thermoplastic end regions adjoining or over'apping at an interface; (b) applying heat in order to melt the thermoplastic material at the interface; (c) applying pressure at the interface in order to weld the thermoplastic end regions together; and IS (d) applying a cooling cycle to consolidate the joint region.
14. A method according to claim 13 wherein the thermoplastic end regions of adjacent component sections are welded together using a resistance welding technique.
15. A method according to claim 14 wherein a high resistance wire mesh is incorporated at the interface between the thermoplastic end regions of adjacent outer shell sections and wherein step (b) is carried out by passing a current through the wire mesh.
16. A method according to claim 14 wherein at least one of the outer shell sections to be connected includes high resistance wire in the thermoplastic end region and wherein step (b) is carried out by passing a current through the resistance wire.
17. A method according to claim 13 wherein the thermoplastic end regions of adjacent component sections are welded together using ultrasonic welding, induction welding, friction stir welding, fibre optics infra-red or ultraviolet heating.