WO2009083531A1

WO2009083531A1 - A tubular element, the related method and tools to produce it

Info

Publication number: WO2009083531A1
Application number: PCT/EP2008/068158
Authority: WO
Inventors: Kaj Morbech Halling; Anton Bech; Mark Hancock
Original assignee: Vestas Wind Systems A/S
Priority date: 2007-12-28
Filing date: 2008-12-22
Publication date: 2009-07-09

Abstract

The present invention provides a method of manufacturing a tubular element (2) for a wind turbine blade of a reinforced polymeric material. The method provides a female mould (1) with a shape defining inner surface (3), and a core (7) being adapted to be arranged in the tubular cavity (5) of the female mould (4). Layers of fabric (6) of a fibre material is arranged along the lower inner surface of the lower mould portion, followed by arranging the core in the tubular cavity, and overlapping of end portions of the layers of fabric t form a closed tubular structure of the tubular element. Subsequently, the female mould is closed, and the layers of fabric and a resin are processed to provide the tubular element of a reinforced polymeric material.

Description

A TUBULAR ELEMENT, THE RELATED METHOD AND TOOLS TO PRODUCE IT

Technical field

The present invention relates to a method of manufacturing a tubular element for a wind turbine blade by the use of a female mould and a core. Furthermore, the invention relates to the tubular element itself and a mould system comprising a female mould and a core.

Background of the invention

One commonly used tubular element in a wind turbine blade is a spar which acts as a reinforcing beam. The spar is located between two shell parts, one defining a wind side shell part and the other one defining a lee side shell part. The spar is located in the cavity between the two wind turbine shell parts and extends substantially throughout the shell cavity in order to increase the strength of the wind turbine blade.

In order to increase the strength of the tubular elements and in order to limit the weight hereof, composite materials are often used for tubular elements to be used in wind turbine blades, since such blades are exposed to varying loads with high peeks.

Traditionally, tubular elements for wind turbine blades are manufactured by the use of a male mould, e.g. by winding a suitable material around a mandrel or a similar core element.

When winding or by other means applying a material onto a mandrel or a core, the inner geometry of the final tubular element is defined by the geometry of the mandrel or core and the mandrel or core therefore provides a well-defined and reproducible inner geometry. On the contrary, the outer geometry of the final tubular element is less well-defined as the effect of even small variations on the mandrel or core and/or small variation on the innermost layers of the winded material are increased with the number of windings.

Alternatively, tubular elements are sometimes made from two separately moulded elements which subsequently are joined in order to define a tubular element. In order to achieve a tubular element of the right size, a height adjustment element can be applied to assure that the final tubular element fits in the cavity between the two shell parts defining the wind turbine blade.

Summary of the invention

It is an object of embodiments of the present invention to provide an improved tubular element, an improved method of manufacturing such a tubular element, and an improved mould system.

In a first aspect, the invention provides a method of manufacturing a tubular element for a wind turbine blade of a reinforced polymeric material, the method comprising :

- providing a female mould with a shape defining inner surface, the female mould being provided in two mould portions so that a lower one of the mould portions forms a lower section of the shape defining inner surface and an upper one of the mould portions forms an upper section of the shape defining inner surface, the mould portions being movable relative to each other between a closed configuration where the sections of the inner surface are joined along a right side and a left side dividing line and forming an uninterrupted inner surface about a tubular cavity, and an open configuration in which the inner surface is broken;

- providing a core being adapted to be arranged in the tubular cavity;

- arranging layers of fabric of a fibre material along the lower inner surface of the lower mould portion; - arranging the core in the tubular cavity;

- using the core as support for end portions of the layers of fabric which are arranged in overlapping configuration to form a closed tubular structure of the tubular element with end portions of the layers overlapping each other;

- subsequently moving the mould portions to the closed configuration; and

- completing processing of the layers of fabric and a resin in the closed configuration to provide the tubular element of a reinforced polymeric material.

It should be understood, that by "tubular element" is in this connection meant a hollow element with an elongated shape. The shape may be non-uniform. The outer geometry may be of a rectangular shape, a circular shape, an oval shape, or any other shape. The inner geometry may be different from the outer shape, thus defining a tubular element in the form of an elongated ring of an arbitrary shape.

The female mould may be of a substantially elongated shape to enable manufacturing of tubular elements of a length of up to e.g. 45 metres or even more. In some embodiments, the female mould may be shorter due to manufacturing of shorter blades or due to manufacturing of parts of tubular elements which may subsequently be connected to each other to form a finished tubular element for a wind turbine blade.

The moulding of the spar against the shape defining inner surface enables manufacturing of spars for wind turbine blades where the outer surface of the spar has an exact and therefore reproducible geometry. Such a spar with reproducible outer surface geometry improves blade manufacturing by providing for a better fitting of the spar between the outer shell parts, and it may facilitate that the outer surface of the spar forms a part of the outer surface of the blade as such. In the closed configuration, a cross section of the female mould may be substantially rectangular, e.g. with rounded corners. The area of the cross section may decrease from the root end to the tip end along the length of the female mould in order to be able to manufacture a tubular element which fits a wind turbine blade having a decreased size at the tip end compared to the root end. However, the width of the female mould may increase locally to increase strength and stiffness of the tubular element locally. In a preferred embodiment, the female mould may have a shape adapted to manufacture a tubular element being approximately conical, i.e. may have a base which is substantially circular transforming into an approximately rectangular shape with rounded corners and with sides which taper towards each other. As an alternative hereto, the root end being substantially circular may be manufactured in a separate process and assembled with a tubular element of approximately rectangular shape at a later stage.

The width/length ratio of the tubular element to be manufactured may in one embodiment be in the range of 1-10%. As an example, the tubular element may have a length of approximately 45 metres, a maximum width of approximately 1.0 metres, and a maximum height of approximately 0.8 metres. The wall thickness of the tubular element, which may be defined as the distance between the outer surface and the internal surface of the tubular element may be in the range of 10-50 millimetres, such as 20-40 millimetres. Compared hereto, the minimal width of the tubular element may be approximately 100 millimetres. It should be understood that this is one example of a tubular element. Other tubular elements being both smaller and larger may also be manufactured.

The lower mould portion may have a cross-section being substantially U-shaped and may thus form the lower section of the shape defining inner surface. The inner surface may extend between two upper edges at which the lower mould portion may be joined to two lower edges of the upper mould portion, when closing the female mould. The upper mould portion may thus be used as a lid. The upper mould portion may likewise be U-shaped or may e.g. have a more flat shape. When closing the female mould, e.g. by joining the two upper edges of the lower mould portion to the two lower edges of the upper mould portion, the lower inner surface is joined to the upper inner surface along the right and left side dividing line.

The lower mould portion may have a cross-sectional shape forming a U with two upright side portions and a horizontal mid-portion. At the upper termination of the two upright side portions, the lower mould is joined with the upper mould portion, or at least the inner surfaces of the mould portions join at the upper termination of the two upright side portions. The right side dividing line is where the right side upright portion joins the upper mould portion, and the left side dividing line is where the left side upright portion joins the upper mould portion.

The right and left side dividing lines extend adjacent to each other, but they are not necessarily parallel to each other since the blade normally taper down from a wide root end towards a narrow tip -end. The right and left side dividing lines may even join each other at the tip end of the blade.

In order to be able to fully close the female mould, the female mould may comprise an end piece at both ends. These end pieces may in one embodiment be removable. If the female mould comprises end pieces, the sections of the inner surface may e.g. be joined along four dividing lines. These lines may form a continuous line along the opening of the lower mould portion.

The core may have a shape which matches the shape of the tubular cavity of the female mould, but other shapes may also be applicable. A shape which matches the inner geometry of the tubular element to be manufactured may be preferred.

The size of the core may be sufficiently small to allow the core to be arranged in the cavity of the female mould after having arranged a number of layers of fabric along the inner surface of the lower mould portion. As an example, the core may be arranged on the layers of fabric on a bottom portion of the lower mould portion while leaving a clearance distance between the inner side walls of the lower mould portion and the external sides of the core of approximately 2-8 millimetres. Consequently, the difference between the external size of the core and the shape defining inner surface of the female mould may correspond to the thickness of the layers of fabric and the clearance distance.

The core may have a size which is variable. As an example, this may be possible by an expandable core. The core may be at least so expandable that it fills out the clearance distance. By being more expandable than allowed by the clearance distance, the core may be able to press the layer of fabric fully together.

In order to be expandable, the core may comprise one or more bladders which are arranged to reshape the external surface of the core e.g. by raising an air pressure in the bladders. The bladders may alternatively be expanded by a liquid, such as water.

Alternatively, the core may be hollow and may thus be reshaped e.g. by raising a pressure in the cavity in the core, e.g. by air, by another gas, or by a liquid, such as water or oil.

As a part of the layers of fabric may be placed on the core, the core may comprise an inner member with a sufficient strength to carry this part without the core collapsing. Alternatively, the core itself may have a corresponding strength.

The layers of fabric may be layers of fibre cloth, woven or braided fibre materials, fibre mats or other types of sheets comprising fibres.

Different types of fibres, such a glass fibres, carbon fibres, synthetic fibres, bio fibres, mineral fibres, and metal fibres can be used. Furthermore, pre-pregs which are sheets comprising fibres being pre-impregnated with a resin may be used as layers of fabric when manufacturing the tubular element. The resin may be an organic polymeric liquid which, when converted to its final state for use, becomes solid. As an example, the resin may be an epoxy-based resin or a polyester-based resin, though other resin types may also be applied.

After having arranged the core in the tubular cavity, end portions of the layers of fabric are arranged in overlapping configuration. For simplicity, this overlapping configuration is also referred to as a braiding of the end portions, or it is referred to as "the end portions being interleaved". By the overlapping configuration, the layers form a closed tubular structure of the tubular element where portions from opposite directions in the tubular structure overlap each other.

Some of the end portions are from layers of fabric at one side of the core, while other end portions are from layers of fabric at the other side of the core. The end portions may alternating be placed on top of the core, e.g. one end portion from a first side, followed by an end portion from the other side, and then again an end portion form the first side. However, braiding of the end portions may also be carried out by taking two or three end portions from each side. Though, same number of end portions from each side is not necessary.

During the braiding process, the core is used as a support for the layers of fabric. For this purpose, the core may have a rigidity which facilitates that the core carries the layers of fabric.

After having arranged the end portions in overlapping configuration, the female mould can be closed by moving the two mould portions to the closed configuration.

Finally, the method comprises a step of completing processing of the layers of fabric and a resin to provide the tubular element.

The final completing step may comprise a step of expanding the core to press the layers of fabric towards the shape defining inner surface of the female mould in order to have a well-defined outer surface of the final tubular element. The outer surface may thus be a replica of the inner shape defining surface of the female mould.

Furthermore, the final step may comprise a step of evacuating the female mould, e.g. by evacuating air form the mould. Then the core may be expanded and resin may be injected into the mould. Instead of injecting the resin, it may be infused into the mould, e.g. by performing vacuum assisted infusion moulding. As a further alternative, the resin may have been added to the layers of fabric before arranging them in the lower mould portion, e.g. by using pre- pregs.

The final step may also comprise a step of curing the resin, e.g. by heating of the mould.

As mentioned above, the method may comprise steps of wetting the fabric with a resin, and curing the resin. Depending on the use of e.g. pre-pregs or similar products or not, the layers of fabric may be wetted before being arranged in the mould or after having been arranged in the mould.

When arranging the layers of fabric in the lower mould portion, at least one of the layers of fabric may be arranged across at least one of the dividing lines. Consequently, at least one end portion of one of the layers may be long enough to extend across one of the upper edges of the lower mould portion, while the other end portion of this layer may be shorter and therefore not extend across the edge of the mould, or it may also extend across another one of the at least one dividing lines. By arranging a layer of fabric across at least one of the dividing lines, this part of the layer of fabric may be long enough to be put on top of the core in order to be arranged in overlapping configuration with an end portion from a layer of fabric from the other side of the mould.

The end portions may thus be braided by arranging end portions of fabrics extending across one of the dividing lines and end portions of fabrics which extend across another of the dividing lines by turns. I.e. the fabrics are braided with one or more layers from one side alternating one or more layers from the other side. Though, the same number of end portions from each side is not necessary.

Braiding of end portions may however not only take place on top of the core. The layers of fabric may be arranged so that an end portion of one layer of fabric overlaps an end portion of another layer of fabric inside the lower mould portion when arranging the layers of fabric in the lower mould portion.

Some of the layers of fabric may be sufficiently long for their end portions to overlap each other, and consequently at least one layer of fabric may be arranged across both the right and left side dividing lines.

To reinforce the tubular element at one or more positions, the method may comprise a step of arranging a pre-cured composite element in the mould. As an example, pre-cured elements may be positioned at areas where the tubular element may be exposed to heavy loads or stress. This may e.g. be around corners of the tubular element.

Depending on the shape of the tubular element and the core, it may be difficult or impossible to obtain a satisfactory pressure from the core onto the layers of fabric in certain areas, e.g if the shape of the core and tubular element comprises angled sections which would be the case if the core and tubular element have a rectangular shape. In such a shape, the corners may cause problems since the pressure from the core onto the layers will typically be low compared to the force of the core onto the layers on the sides between the corners. Even with rounded corners, problems may arise.

To improve the strength of the tubular element and further to reduce the necessity of large pressure onto layers of fabric in such difficult areas, the method may comprise a step of arranging a pre-cured composite element in the mould so that portions of the tubular element are pre-fabricated or partly prefabricated. The pre-cured composite element may have a structure which, after finishing of the tubular element, is identical to the remaining part of the tubular element. I.e. it may be made from the same composite material and with the same number of layers as the remaining part of the tubular element. To further increase the strength of the difficult areas, e.g. at corners of the tubular element, the pre-cured element could also be made from materials with larger tensile strength, bending stiffness, or hardness etc. or the pre-cured elements could be made with a different surface structure, e.g. with a lower surface friction than the remaining part of the tubular element.

As the core in some embodiments may be taken out of the tubular element after manufacturing hereof, the bottom part of the external surface of the core may slide along especially the bottom part of the internal surface of the tubular element during removal. It may therefore be an advantage to reinforce the bottom part of the tubular element in order to protect it.

The pre-cured composite element may be arranged in the mould after the step of arranging the layers of fabric in the mould. Alternatively, the pre-cured element may be arranged between some of the layers of fabric or it may be arranged in the mould prior to the arrangement of the layers of fibres. As more pre-cured elements may be used, one or more elements may be arranged prior to arranging the layers of fabric, whereas one or more other elements may be arranged after arranging the layers of fabric. This may further be combined with one or more pre-cured elements being arranged between the layers of fabric.

As mentioned above, the method may further comprise a step of expanding the core to a degree whereby the layers of fabric are pressed towards the inner surface of the female mould. By pressing the layers of fabric towards the shape defining inner surface, the outer surface of the tubular element may be well- defined.

The core may be variable expandable in order to be able to expand it to a size which may depend on the number of layers of fabric being arranged in the female mould.

Furthermore, the method may comprise a step of lowering an air pressure in a space between the inner surface of the female mould and an external surface of the core. As the core may not fully fill out the space in the female mould after having arranged the layers of fabric and the core inside the mould, lowering the air pressure in this space may facilitate expansion of the core.

In order to be able to expand the core, the core may comprise one or more bladders which may be arranged to reshape the external surface of the core upon expansion, and the method may thus comprise a step of raising a fluid pressure in a bladder. The bladder may in one embodiment be ring-shaped and may extend along the perimeter of the core. In another embodiment, more bladders may be arranged along the perimeter, and in a further alternative a single bladder may be arranged substantially central in the core. Other positions and alternative numbers of bladders may also be applicable.

In a further alternative, the one or more bladders may be replaced by a core cavity in the core, i.e. the core may be hollow. The core may thus be expanded by raising an air-pressure in the core cavity.

Furthermore, the one or more bladders or the core cavity may not only be expanded by raising an air-pressure, the bladders and/or the core cavity may also be expanded e.g. by another gas or by a liquid, such as water.

In order to be able to expand the core, the core may comprise at least one portion being elastically expandable. The elastically expandable core portion may as an example be made of an elastomer so that this at least one core portion may be stretched upon raising the pressure inside it.

In a second aspect, the invention provides a tubular element for a wind turbine blade formed by layers of fabric of a fibre material in a cured resin, the tubular element comprising an outer surface towards an ambient space and an internal surface towards an inner cavity, the outer surface having a shape which is defined by a shape defining inner surface of a female mould, and the layers of fabric comprising end portions which are braided. It should be understood, that the tubular element may be manufactured by a method according to the above-described first aspect of the invention. Thus, the features of the first aspect of the invention may be applicable in relation to the tubular element of the second aspect of the invention.

As the outer surface of the tubular element is defined by the shape defining inner surface of the female mould, it may be possible to obtain an outer surface without extensive irregularities. Depending on the number of layers of fabric and the accuracy applied when arranging them in the lower mould portion and when braiding the end portions, the internal surface may be approximately as smooth as the outer surface. However, as even small variations as faults in the surface may be increased with the number of layers, the internal surface may be more irregular than the outer surface, and consequently, the outer surface may be at least as smooth as the internal surface.

In one embodiment, all end portions may be braided. Some of the end portions may be braided on top of the core, whereas some of the end portions may be braided by overlapping one end portion with an end portion of another layer of fabric along the inner surface of the lower mould portion.

The layers of fabric may be of different length, and at least one layer of the fabric may extend entirely around the inner cavity. A layer of fabric extending entirely around the inner cavity of the female mould may have overlapping end portions which are thus braided.

To reinforce the tubular element, it may comprise at least one pre-cured composite element. As an example, the pre-cured element may comprise layers of semi-finished composite elements, such as pre-pregs which have been stacked. The pre-cured elements may be made from the same composite material and with the same number of layers as the remaining part of the tubular element. However, to further increase the strength of areas of the tubular element, the pre-cured elements could also be made from materials with larger tensile strength, bending stiffness, hardness, etc. The internal surface of the tubular element may be formed by an external surface of a core. This may e.g. be done by pressing the layers of fabric towards the shape defining inner surface of the female mould by expansion of the core.

In a third aspect, the invention provides a mould system for manufacturing a tubular element for a wind turbine blade from one or more layers of fabric of a fibre material, the mould system comprising a female mould with a shape defining inner surface having a tubular shape forming a tubular cavity, and a core adapted to be arranged in the tubular cavity, the female mould comprising two mould portions so that a lower one of the mould portions forms a lower section of the shape defining inner surface and an upper one of the mould portions forms an upper section of the shape defining inner surface, the core forming an external surface adapted to form an internal surface of the tubular element, and being interchangeable between an unexpanded state and an expanded state, wherein the external surface of the core has a shape which at least in the expanded state matches a shape of the shape defining inner surface of the female mould.

It should be understood, that the above-mentioned steps and features of the first and second aspects of the invention may also be applicable in relation to the mould system of the third aspect of the invention.

To press the layers of fabric towards the entire shape defining inner surface of the female mould, the shape of the external surface of the core in the expanded state may match the shape of the inner surface of the female mould.

The distance between the external surface of the unexpanded core and the layers of fabric being arranged in the female mould may thus be approximately equal along both sides of the core and in some embodiments also along the top of the core. The distance may correspond to the thickness of the layers of fabric and clearance distance.

The shape of the external surface of the unexpanded core may be independent on the shape of the inner surface of the female mould. Consequently, the flexibility of the core and not the shape hereof may ensure that the core can press the layers of fabric towards the inner surface of the female mould when expanding the core.

In the unexpanded state the core may have a rigidity facilitating support of at least a part of the layers of fabric. This may facilitate braiding of end portions of the layers of fabric on top of the unexpanded core, as the core may carry a part of these layers during braiding.

The core may comprise at least one bladder being adapted to receive a fluid medium. By adding e.g. air or water to the one or more bladders the external surface of the core may be reshaped, and the core may thus be expanded.

The core may comprise an inner part and an outer skin which is elastically expandable. The inner part may be more rigid than the outer skin in order to facilitate support of at least a part of the layers of fabric. On the contrary, the outer skin may be elastically expandable allowing for reshaping and expansion of the core.

In one embodiment, at least a portion of the outer skin may be elastically expandable, whereas the entire outer skin may be elastically expandable in another embodiment. The elastically skin portion may as an example be made of an elastomer so that this skin portion may be stretched upon raising the pressure inside it.

To ensure that the core is interchangeable between the unexpanded state and the expanded state, the at least one bladder may be arranged between the inner part and the outer skin.

In a fourth aspect, the invention provides a core for a mould system for manufacturing a tubular element for a wind turbine blade, the core comprising an outer skin and a cavity, wherein the outer skin comprises at least one elastically expandable skin portion enabling expansion of an external surface of the core by raising a pressure of a fluid in the cavity. The pressure may be raised by an air-pressure or another gas-pressure, or by a liquid such as water or oil.

It should be understood, that the above-mentioned steps and features of the first, second, and third aspects of the invention may also be applicable in relation to the core of the fourth aspect of the invention.

In one embodiment, the core may comprise an inner part, and the cavity may comprise at least one bladder being arranged between the inner part and the outer skin for enabling expansion of an external surface of the core. The outer skin may comprise at least one skin portion being elastically expandable. The elastically expandable skin portion may as an example be made of an elastomer so that this at least one skin portion may be stretched upon raising the pressure inside the core, thus enabling expansion hereof.

The inner part may have a strength being sufficient to carry at least a part of the layers of fabric which may be positioned on top of the core.

As the core in some embodiments may be taken out of the tubular element after manufacturing hereof, the bottom part of the core may slide along e.g. the bottom part of the internal surface of the tubular element during removal of the core. In order to facilitate removal, it may be an advantage if the external surface at least at some positions has a low friction. Thus, the core may comprise a first outer surface portion with a first surface friction and a second outer surface portion with a second surface friction, the second surface friction being lower than the first surface friction.

The low friction may e.g. be obtained by coating parts of the external surface of the core, or by application of a material having a low friction.

In a fifth aspect, the invention provides a wind turbine blade comprising a tubular element. The tubular element may be manufactured according to the above-described method. It should be understood, that the above-mentioned features of the previously described aspects may be applicable to the fifth aspect of the invention.

Consequently, the wind turbine may comprise a tubular element with a well- defined outer geometry, an outer geometry which is a replica of the shape defining inner surface of the female mould.

In a sixth aspect, the invention provides a wind turbine comprising at least one wind turbine blade comprising a tubular element. It should be understood, that the above-mentioned features of the previously described aspects may be applicable to the sixth aspect of the invention. Furthermore, the tubular element may be manufactured according to the above-described method.

Brief description of the drawings

Embodiment of the invention will now be further described with reference to the drawings, in which :

Fig. 1 illustrates a female mould comprising a lower mould portion and an upper mould portion;

Fig. 2a illustrates an embodiment of a tubular element;

Fig. 2b is an enlarged part of Fig. 2a illustrating braided end portions of the layers of fabric;

Fig. 3 illustrates an embodiment of a female mould, an embodiment of a core, and an embodiment of a tubular element;

Fig. 4 illustrates another embodiment of a tubular element; and

Figs. 5-8 illustrate different embodiments of a core. Detailed description of the drawings

Fig. 1 illustrates a female mould 1 which can be used for manufacturing of a tubular element 2 (see Fig. 2) for a wind turbine blade.

The female mould 1 is provided with a shape defining inner surface 3. The female mould 1 is provided in two mould portions so that a lower one Ia of the mould portions forms a lower section of the shape defining inner surface and an upper one Ib of the mould portions forms an upper section of the shape defining inner surface. The mould portions Ia, Ib are movable relative to each other between a closed configuration and an open configuration. Fig. 1 illustrates the female mould in the closed configuration.

In the closed configuration, the sections of the inner surface 3 are joined along two dividing lines 4a, 4b to form an uninterrupted inner surface forming a perimeter around a tubular cavity 5. In the open configuration the inner surface is broken.

The female mould 1 also comprises end sections which are not shown in the illustrated embodiment.

Fig. 2a illustrates an embodiment of a tubular element 2 which have been manufactured by the used of a female mould 1 (see Fig. 1). The tubular element 2 comprises a plurality of layers of fabric 6 of a fibre material (see Fig. 2b which is an enlarged part of Fig. 2a).

The layers of fabric 6 have been wetted with a resin which is cured to obtain the final tubular element 2.

As illustrated in Fig. 2b the end portions of the layers of fabric 6 are braided, i.e. in this embodiment by alternating putting an end portion of a layer of fabric 6 from one side of the tubular element 2 on top of an end portion of a layer of fabric 6 from the other side of the tubular element 2. The enlarged section in Fig. 2b only illustrates one braided portion. However, it should be understood that the tubular element 2 may comprise more braided portions.

In order to be able to manufacture the tubular element of Fig. 2, a core 7 being adapted to be arranged in the tubular cavity 5 is provided, as illustrated in Fig. 3.

Fig. 3 illustrates a lower portion of a female mould Ia comprising a tubular element 2 and a core 7. When manufacturing a tubular element 2, the female mould 1 is opened and layers of fabric 6 of a fibre material are arranged along the inner surface 3 of the lower mould portion Ia. Subsequently, the core 7 is arranged on the layers of fabric 6 and the end portions of the fabric 6 are braided to form a closed tubular structure of the tubular element 2. Then, the upper mould portion Ib is put onto of the lower mould portion Ia to close the female mould 1. Finally, processing of the layers of fabric 6 and the resin is completed in the closed configuration in order to provide a tubular element 2 of a reinforced polymeric material.

In the embodiment illustrated in Fig. 3, the core 7 comprises an outer skin 8 and an inner part 9. A bladder 10 is arranged between the outer skin 8 and the inner part 9 allowing for expansion of the core 7 when raising an air-pressure inside the bladder 10. The outer skin 8 is made of an elastically expandable material, such as an elastomer which can be stretched when raising the air-pressure.

The above described step of processing the layers of fabric 6 and the resin in the closed configuration comprises a step of evacuating air from the area between the inner surface 3 of the female mould 1 and the external surface of the core 7, and a step of expanding the core 7 in order to press the layers of fabric 6 towards the inner shape defining surface 3 of the female mould to obtain a well- defined outer surface of the tubular element 2.

Furthermore, the processing step comprises a step of vacuum assisted resin infusion by which resin is distributed in the layers of fabric 6 and between the layers of fabric 6 to wet the fibres. The final step further comprises a step of curing the resin by heating the female mould 1.

Fig. 4 illustrates another embodiment of a tubular element 11 comprising layers of fabric 6 and pre-cured composite elements 12. The pre-cured elements 12 are positioned at the corners of the tubular element 11 to reduce the necessity of large pressure onto layers of fabric 6 in these areas, where it is difficult or even impossible to obtain a satisfactory pressure from the core 7 onto the layers of fabric 6. Furthermore, the tubular element 11 comprises semi-finished composite elements 13 comprising fibres in a resin, and elements 14 comprising foam.

Figs. 5-8 illustrate different embodiments of an expandable core 7, 15, 19, 25 to be used when manufacturing a tubular element 2, 11, as e.g. illustrated in Fig. 2 and Fig. 4.

The core 7 illustrated in Fig. 5 is equal to the core 7 of Fig. 3. Thus, the core 7 comprises an outer skin 8 and a hollow inner part 9. A bladder 10 is arranged between the outer skin 8 and the inner part 9 allowing for expansion of the core 7 when raising an air-pressure in the bladder 10. The outer skin 8 is made of an elastically expandable material. The hollow inner part 9 is made of a material ensuring a sufficient strength of the inner part for the inner part 9 to be able to carry at least a part of the layers of fabric 6 which are positioned on top of the core 7 during manufacturing of the tubular element 2 without the core is collapsing.

Fig. 6 illustrates another embodiment of a core 15. The core 15 comprises an outer skin 16 and a cavity 17. The outer skin 16 comprises four elastically expandable skin portions 18 enabling expansion of the external surface of the core 15 by raising a pressure of a fluid in the cavity 17. In the present embodiment, the four elastically expandable skin portions 18 comprise an elastomer. Fig. 7 illustrates third embodiment of an expandable core 19. The core 19 comprises an outer skin 20 and a cavity 21. The outer skin 16 comprises four elastically expandable skin portions 22 enabling expansion of the external surface of the core 19 by raising a pressure of a fluid in the cavity 21. In the present embodiment, the four elastically expandable skin portions 22 each comprises a relatively thin, U-shaped metal plate 23 and a cover plate 24. When raising the pressure inside the cavity 21, e.g. by compressed air, the opening of the U-shaped plates 21 will be extended to allow for expansion of the core 19. The cover plates 24 ensure that the U-shaped plates 23 are not filled with resin when wetting the layers of fabric 6 with herewith.

Fig. 8 illustrates a fourth embodiment of an expandable core 25 being similar to the core 7 of Fig. 5. Thus, the core 25 comprises an outer skin 8 and a hollow inner part 9. A bladder 10 is arranged between the outer skin 8 and the inner part 9 allowing for expansion of the core 7 when raising an air-pressure in the bladder 10. The outer skin 8 is made of an elastically expandable material.

Furthermore, the core 25 comprises a first outer surface portion 26 with a first surface friction and a second outer surface portion 27 with a second surface friction, the second surface friction being lower than the first surface friction.

The second surface portion 27 covers smaller areas of the first surface portion 26 in order to facilitate removal of the core 25 after manufacturing of a tubular element 2 and after lowering of the air-pressure. The low friction of the second surface portion 27 is in the present embodiment obtained by coating smaller areas of the outer skin 8.

Claims

1. A method of manufacturing a tubular element for a wind turbine blade of a reinforced polymeric material, the method comprising :

- providing a female mould with a shape defining inner surface, the female mould being provided in two mould portions so that a lower one of the mould portions forms a lower section of the shape defining inner surface and an upper one of the mould portions forms an upper section of the shape defining inner surface, the mould portions being movable relative to each other between a closed configuration where the sections of the inner surface are joined along a right side and a left side dividing line and form an uninterrupted inner surface about a tubular cavity, and an open configuration in which the inner surface is broken;

- providing a core being adapted to be arranged in the tubular cavity;

- arranging layers of fabric of a fibre material along the lower inner surface of the lower mould portion;

- arranging the core in the tubular cavity;

- subsequently moving the mould portions to the closed configuration; and

2. A method according to claim 1, further comprising the steps of; - wetting the fabric with a resin; and

- curing the resin.

3. A method according to claim 1 or 2, wherein at least one of the layers of fabric is arranged across one of the dividing lines.

4. A method according to any of the preceding claims, wherein the end portions are arranged in the overlapping configuration by arranging an end portion of a fabric which extends across one of the dividing lines alternating an end portion of a fabric which extends across another of the dividing lines.

5. A method according to claim 4, wherein at least one layer of fabric is arranged over two dividing lines.

6. A method according to any of the preceding claims, comprising a step of arranging a pre-cured composite element in the mould.

7. A method according to claim 6, wherein the pre-cured composite element is arranged in the mould after the step of arranging the layers of fabric in the mould.

8. A method according to any of the preceding claims, further comprising a step of expanding the core to a degree whereby the layers of fabric are biased towards the inner surface.

9. A method according to any of the preceding claims, further comprising a step of lowering an air pressure in a space between the inner surface and an external surface of the core.

10. A method according to any of the preceding claims, further comprising a step of raising a fluid pressure in a bladder arranged to reshape the external surface of the core upon expansion.

11. A tubular element for a wind turbine blade formed by layers of fabric of a fibre material in a cured resin, the element comprising an outer surface towards an ambient space and an internal surface towards an inner cavity, the outer surface having a shape which is defined by a shape defining inner surface of a female mould, and the layers of fabric comprising end portions which are arranged in overlapping configuration so that the element comprises a part with end portions of the layers overlapping each other.

12. A tubular element according to claim 11, wherein the outer surface is at least as smooth as the internal surface.

13. A tubular element according to claim 11 or 12, wherein all end portions is arranged in the overlapping configuration.

14. A tubular element according to any of claims 11-13, wherein at least one layer of the fabric covers the inner surface entirely.

15. A tubular element according to claim 14, wherein the at least one layer has end portions which are arranged in overlapping configuration so that its end portions overlap each other.

16. A tubular element according to any of claims 11-15, further comprising at least one pre-cured composite element.

17. A tubular element according to any of claims 11-16, wherein the internal surface is formed by an external surface of a core.

18. A mould system for manufacturing a tubular element for a wind turbine blade from one or more layers of fabric of a fibre material, the mould system comprising a female mould with a shape defining inner surface having a tubular shape forming a tubular cavity, and a core adapted to be arranged in the tubular cavity, the female mould comprising two mould portions so that a lower one of the mould portions forms a lower section of the shape defining inner surface and an upper one of the mould portions forms an upper section of the shape defining inner surface, the core forming an external surface adapted to form an internal surface of the tubular element, and being interchangeable between an unexpanded state and an expanded state, wherein the external surface of the core has a shape which at least in the expanded state matches a shape of the shape defining inner surface of the female mould.

19. A mould system according to claim 18, wherein the shape of the external surface of the core in the expanded state further matches the shape of the inner surface of the female mould.

20. A mould system according to claim 18 or 19, wherein the shape of the external surface of the unexpanded core is independent on the shape of the inner surface of the female mould.

21. A mould system according to any of claims 18-20, wherein the core, in the unexpanded state has a rigidity facilitating support of at least a part of the layers of fabric.

22. A mould system according to any of claims 18-21, wherein the core comprises at least one bladder being adapted to receive a fluid medium.

23. A mould system according to any of claims 18-22, wherein the core comprises an inner part and an outer skin which is elastically expandable.

24. A mould system according to claim 22 and 23, wherein the at least one bladder is arranged between the inner part and the outer skin.

25. A core for a mould system for manufacturing a tubular element for a wind turbine blade, the core comprising an outer skin and a cavity, wherein the outer skin comprises at least one elastically expandable skin portion enabling expansion of an external surface of the core by raising a pressure of a fluid in the cavity.

26. A core according to claim 25, further comprising an inner part, wherein the cavity comprises at least one bladder arranged between the inner part and the outer skin for enabling expansion of an external surface of the core.

27. A core according to claim 25 or 26, comprising a first outer surface portion with a first surface friction and a second outer surface portion with a second surface friction, the second surface friction being lower than the first surface friction.

28. A wind turbine blade comprising a tubular element according to any of claims 11-17.

29. A wind turbine comprising at least one wind turbine blade according to claim 28.