System and method for the production of composite tubular members and a tubular member made thereof.

Introduction The present invention discloses a system and method for the production of long composite tubular members and a tubular member made thereof. The production of composite tubular members presents particular difficulties when a filament winding technology is used, especially if the tubular members are of some length. A significant proportion of the cost of production of composite tubulars is associated with the transition between the composite and the flange. The present invention allows for the production of very long such composite tubulars by filament winding, thus reducing the overall cost of production due to a reduced number of flanges.

Filament winding requires the fibre tows used during the winding process to be tightly wound about a mandrel or liner. The fiber must be wound using a very high fiber tension. If the mandrel or liner is of any significant length and / or is flexible, this will imply that the mandrel or liner is bent by the fibre tension. If the filament winding is performed while the mandrel or liner is placed in a horisontal position, the mandrel may exhibit significant gravitationally induced sag. This renders the accurate winding of the fibre tows around the mandrel or liner difficult. When the length of the liner or mandrel increases, the natural frequency will be low and the system may become vulnerable to low frequency resonance induced by the force from fiber tension or unbalanced body loads during rotation of the axis. This further complicates the filament winding of the fibers about the mandrel or liner.

The objective of the present invention is to overcome the aforementioned obstacles and comprises a method for production of long flexible composite tubular members and a tubular member made thereof.

Composite tubulars members are of major importance when applied to situations where there is need of a strong light-weight tubular member, exhibiting a strong resistance to breakage, such as for example in oil and gas exploration applications or wind energy production. Examples may include so-called risers, struts and even drilling pipes, or the load carrying beam in windmill blades.

Background art

The basic principle of filament winding is well known from the art. The filament winding process was initially invented and applied to the production of missile casings, nose cones and fuselage structures, technical fields wherein strong but lightweight structures are required. However the field of use has greatly expanded, and filament winding is a technology that today is applied to various technical fields. utvidet, og filamentvikling er en teknologis som ikke anvendes i forskjellige tekniske omrader.

Filament winding may be defined as constituting the winding of a resin impregnated fibre tow or tape onto and around a mandrel or liner surface, the fibre positioned in a precise geometric pattern. This is usually accomplished by rotating the mandrel while a delivery head precisely positions the impregnated fibre tows onto the mandrel or liner surface. Filament winding machines operate on the principles of controlling machine motion through various axes of motion. Evidently, spindle and mandrel movement must be controlled as well as the horisontal carriage motion axis and the cross or radial carriage motion axis. For more complicated winding patterns, additional axes may be added.

A major problem when filament winding flexible structures is to compensate for the gravitationally induced sagging of the structure itself. As the mandrel or liner is rotated during the fibre tow placement, the gravitational force will induce sagging on the mandrel or liner, thus inducing a cyclical force variation onto the liner. This force variation may fatally weaken the mandrel or liner, and thus the composite tubular member, and may eventually incur the failure of the tubular. This renders the accurate winding of the fiber tows difficult, and may cause unacceptable loads on the tubular structure. Loads from the fiber tension during winding may also contribute to this problem, or will be a limiting mechanism when the liner or mandrel is placed in a vertical position. A further problem to be addressed to the low natural frequencies which will be present in long and thin tubular structures with low bending stiffness, an issue that applies to both horizontally and vertically arranged liners/mandrels. The natural frequency of the system effectively limits the rotation speed of the mandrel or liner, and thus the production speed. If the natural frequency is very low, it will increase the winding time and thus the cost of production. Resonance may arise in such systems due to inevitable production errors, imperfections and imbalances in the tubular itself. Such imperfections may displace the centre of gravity somewhat out of the central axis of the mandrel or liner, and may thus provide a system unbalance. Also a small unbalance in the system may excite large amplitude vibrations when this imposed excitation has a frequency close to the system's natural frequency. Thus the highest safe allowable rotation speed must be much lower than the natural frequency of the system.

GB344356 "Improvements in or relating to supports for tubular mandrels" discloses a system for the formation of flexible tubing said method comprising the use of an internal support extending lengthwise within a mandrel, said internal support arranged for providing support to the mandrel. No mention is made of this system being utilised for filament winding. Further, GB344356 describes a horisontally arranged rotating mandrel.

US2006188342 discloses a system in which bending moments are applied at both ends of a tubular in order to counteract sagging. However this will result in unwanted stress cycling on the welds in the liner during filament winding and may result in the tubular being fatally weakened.

Short summary of the invention

The system and method according to the invention overcomes at least some of the problems described above and comprises a method for production of a composite tubular member comprising a mandrel or liner by filament winding of one or more placed fibre tows. The new and characterising steps of the method according to the invention comprise:

*providing said tension member with means for centering said tension member in a mandrel or liner,

^* inserting said tension member into said mandrel or liner,

^* tensioning said tension member by means of a tensioning device, * performing filament winding said of one or more fibre tows about said mandrel or liner,

* loosening said tension member for the formation of said composite tubular member, and

* removing said tension member from said composite tubular member.

The invention further discloses a system for production of a composite tubular member, said composite tubular member comprising a mandrel or liner and an outer composite material laminate, said system comprising the following features: a tension member for being arranged within said mandrel or liner, said tension member being provided with centering means for centering said mandrel or liner.said rotation means arranged for rotating said mandrel or liner, winding means arranged for winding said fibres about said mandrel for the formation of a composite tubular member, wherein

- said centring means being provided with bearings arranged for allowing the rotational movement of said tension member with respect to said mandrel or liner,

- said tension member being further provided with one or more fixed end terminations arranged for being tensioned by means of a tensioning device.

The use of a tubular member formed according to the method described above in oil exploration is claimed.

The use of a composite tubular member produced according to the method of claim 1 , for usage as load carrying beam in wind turbine blades is further claimed. Further advantageous embodiments of the invention are described in the attached dependent claims.

Figure captions

The attached figures are for illustration purposes only and should not be construed to in any way to limit the scope of the invention, which shall only be limited by the attached claims.

Fig. 1 illustrates the background art which shows a horizontal filament winding apparatus. The mandrel is rotated around the axis shown as C, the winding apparatus is

shown having movement axes z and x, and a delivery head is shown having a rotational movement about an axis a.

Fig. 2a illustrates a major shortcoming of the background art, in which is shown a typical sag profile of a tubular of some length in which the end portions of the tubular (21,22) have no degrees of freedom. The tubular (2) may be seen bending downwards due to the gravitational force.

Fig. 2b illustrates the same situation as in Fig. 2a but wherein a tension member (4) is arranged within said mandrel or liner (2), said tension member (4) being tensioned.

Fig. 3a illustrates a first embodiment of system according to the invention in which is shown a sectional view of the mandrel or liner (2), the tension member (4), and one of the end portions (21,22) of the mandrel or liner (2). The end piece (7) is shown within the mandrel or liner (2).

Fig. 3b illustrates the same embodiment of the invention as shown in Fig. 3a, but wherein a portion of the tension member (4) and the end piece (7) is shown extending out through the end portion (21) of the mandrel or liner (2).

Fig. 3c illustrates a sectional view of a detail of the bearing element of the invention, in which is shown the tension member (4), a first fixed element (41) on said tension member (4), the bearing (42), and a second fixed element (43) fitting closely against the internal surface of mandrel or liner (2). Fig. 4a-Fig. 4h is a series of figures schematically illustrating a method according to the invention.

Fig. 4a shows the liner or mandrel (2) with said tension member (4) and said end piece (7) extending out of the mandrel or liner (2) before entering an aperture in the rotation means (9). The tensioner (8) comprising tension means (82) is shown in a lowered position. Fig. 4b illustrates the same situation as in Fig. 4a, but wherein the end piece (7) is shown inserted in to an aperture (94) of a rotation means (9), and the tension means (82) is shown in a higher position.

Fig. 4c illustrates the same situation as in Fig. 4b but wherein the end portion (21) is shown abutting the rotation means (9), and in which the tensioning means (82) is shown in a raised position.

Fig. 4d illustrates the arrangement of fastening means (93) fixing said end portion (21) to said rotation means (9).

Fig. 4e illustrates the arrangement of a sleeve (81) about said end piece (7).

Fig. 4f illustrates the tensioning means (82) being lowered to abut said sleeve (81) and end piece arrangement (7, 81).

Fig. 4g illustrates the arrangement of fastening means (83) fastening said sleeve (81) and end piece arrangement (7, 81) to said tensioning means (82).

Fig. 4h illustrates the tensioning means (82) in an elevated position, thus tensioning said tension member (4), and providing support to said mandrel or liner (2).

Fig. 5 is a perspective sectional view of the system in which may be seen additional details of the system such as bearings (92) and a more detailed view of an example of the rotation means (91) according to the present invention.

Fig. 6 is a perspective sectional view of the mechanical details of the system.

It is to be understood that the illustrations may equally portray any side of the system and although the terms "lowered" and "raised" are used to illustrate the positioning of component parts of the system, these expressions are only meant to illustrate the respective relative positioning of said components. It is further to be understood that the system as illustrated in the figures may be arranged at one or both of the ends of the mandrel or liner (2). Various elements may further be omitted at each end of the mandrel or liner (2), such that for example only a first end portion (21) be provided with a tensioner (8), whereas only the second end portion (22) is provided with rotation means (9). Alternatively both end portions (21,22) may be provided with rotation means (9) whereas only a first end portion (21) may be provided with tension means and so forth.

Preferred embodiments of the invention

The system and method according to the invention will in the hereinafter be described with reference to the attached figures. The present invention discloses a method and system for production of a composite tubular member (1) as well as a tubular member formed thereof. The method according to the invention comprises providing a mandrel or liner (2) about which fibre tows (3) impregnated with resin (6) are to be placed in predetermined geometric patterns according to the method known as filament winding. Filament winding is particularly well adapted to the production of light-weight flexible structures, and examples of use of filament winding are well known. However due to the particular problems which arise during the production of long tubular members (1), it has been very difficult to produce sufficiently long tubulars to be of interest in for instance oil exploitation applications. The problems associated with filament winding of hollow tubulars have been discussed in the section "Background art". The method according to the invention resolves these problems and comprises the following steps:

^* providing a tension member (4), said tension member (4) extending further than said mandrel or liner (2), ^* providing said tension member (4) with means (41) for centering said tension member (4) in a mandrel or liner (2),

* inserting said tension member (4) into said mandrel or liner (2),

^* tensioning said tension member (4) by means of a tensioner (8),

* performing filament winding said one or more fibre tows (3) about said mandrel or liner (2),

* curing of the resin (6) of said fibre tows (3), and

* loosening said tension member (4) for the formation of said composite tubular member (1).

The tension member (4) should in a preferred embodiment of the invention be longer than the mandrel or liner (2) in order for facilitating the tensioning of said tension member (4). It is clear that tension systems more easily will be able to engage with the tension member (4) when at least a portion of the tension member (4) is situated at the exterior of the mandrel or liner (2). However, tension systems may be found in which the tension member (4) is shorter than the tube or mandrel (2), and in which the tensioner (8) engages with the tension member (4) in the interior of the mandrel or liner (2). Such systems shall also be considered to be within the scope of the invention. An illustration of a possible configuration of the system according to the invention is shown in Figs. 3a and 3b.

The tension member (4) may be made of any material susceptible of withstanding high tensions in the longitudinal direction. The tension member (4) may for instance be formed by a lightweight high strength material such as carbon fibre composites or even high strength steel. As the natural frequency of a system is related to system mass it is advantageous to use an as light as possible material providing the necessary resistance to the tension applied. However, any material having the necessary material properties may be used in the method according to the invention as will be evident to a person skilled in the art.

Said means (41) for centering the tension member (4) within the tube or mandrel (2) are important as to the proper functioning of the system as a whole. Said means (41) provide support to the mandrel or liner (2) when it is subject to axial forces such as provided by the filament winding process itself. Said centering means (41)should thus in an embodiment of the method according to the invention be arranged for tightly fitting against the interior wall of the mandrel or liner (2). This will provide added stability to the system during the filament winding process as described below.

The number of centering means (41) arranged along the tension member (4) the mandrel or liner (2) length may additionally be adjusted according to the length and material properties of the mandrel or liner (2) itself. In an embodiment of the invention, said centering means (41) comprise an outer portion (43) arranged for supporting the inner surface of the mandrel or liner (2), an inner portion (41) fixedly attached to the tension member (4), and bearings (42) between said inner and outer portions (41, 43) allowing for the rotational movement of the tension member (4) with respect to said mandrel or liner (2). The bearings (42) may be of any kind adapted to withstand the loads exerted upon it, and may advantageously be roller or sliding bearings. An illustration of details of said centering means (41) is shown in Fig. 3c.

In an embodiment of the invention end pieces (7) are arranged on one or more of the end portions (21 ,22) of the tension member (4). Such end pieces (7) may advantageously be arranged in conical shape for facilitating the tensioning of the tension member (4) by the

tensioner (8). Other shapes for the end pieces (7) are obviously possible and should be considered to be within the scope of the invention. The end pieces (7) may be welded or otherwise attached to the tension member (4) and may be constituted of any suitable material. In an embodiment of the invention, one or more sleeves (81) is arranged for tightly fitting about said one or more end pieces (7) and are further arranged for being disengagebly attached to the tensioner (8) or tension means (82). This allows for the rapid connection of the tension member (4) to the tensioner (8) or tension means (82) and by adequate adaption of the sleeve (82) will render the system adapted for various tension member (4) diameters. The tensioner (8) of the method according to the invention may be of any kind susceptible of providing sufficient tension to the tension member (4). Although in an embodiment of the invention the tension means (82) are one or more hydraulic pistons and cylinders, other configurations may be provided.

After tension has been applied to the tension member (4), the mandrel or liner (2) is provided with sufficient stiffness for the filament winding process to take place at an increased speed compared to when the system is untensioned. The filament winding may be performed by any normal filament winding system as is evident.

When the fibres tows (3) have been placed on the mandrel or liner (3) the resin (6) of the fibres tows (3) must be given time to cure. The curing process should be carefully monitored so as for limiting resin (6) agglomerations at any point of the mandrel or liner (2) and fibre (3) arrangement. In an alternative embodiment of the invention, additional resin (6) may be applied during curing so as for strengthening the tubular member (1). Alternatively, if there is an excess of resin (6), the excess may be removed from the tubular member (1). This will ensure that the finished tubular member (1) is with a relatively smooth and even outer surface. This is a problem especially during the performance of vertical filament winding according to the invention.

In an embodiment of the invention, the fibre tows (3) are impregnated with a thermosetting resin (6), in which the thermosetting resin (6) is cured prior to the release of the tension member (4). In an embodiment of the invention, the fibre tows (3) comprise a thermoplastic matrix material (31). While filament winding the fibre tows (3) into place onto and about said mandrel or liner (2), a simultaneous heat fusing of said fiber tows (3) onto the mandrel or liner (2) is performed. This is the equivalent of welding the fibre tows(3) into place, and may be performed for instance by means of laser welding or the like. In this embodiment of the invention, all issues regarding the curing of the resin (6) of the fiber tows (3) are avoided.

The tension member (4) may be left in place in the finished tubular member (1) is so desired after the production of said tubular (1). However in an advantageous embodiment of

the invention, the tension member (4) is removed from the tubular (1) so as for maximising the available internal volume in the tubular (1).

As discussed above, during the production of tubulars according to the method disclosed in the invention the natural frequency of the system is a limiting factor. The tensioning of the tension member (4) greatly increases the stiffness of the system, thus increasing the natural frequency of the system allowing for higher rotation speeds and therefore production speeds.

In an alternative preferred embodiment of the invention, the mandrel or liner (2) is rotated by means of rotation means (9) during the filament winding. The rotation means (9) may comprise any means being able to rotate the mandrel or liner (2) during the filament winding, and may in a preferred embodiment of the invention comprise means (93) for locking one or more of said end portions (21, 22) of the mandrel or liner (2) to said rotation means (9) and in which said rotation means (9) is rotated by an external power source. As is evident to a person skilled in the art, other configurations are possible such as the fibre delivery system being able to rotate about the mandrel or liner (2), and in which the liner or mandrel (2) is kept stationary. In an alternative embodiment of the invention, both mandrel and liner (2) as well as the fiber delivery system is rotated during the filament winding process.

Figs. 4a-4h schematically illustrates a method for the tensioning of tension member (4) according to the present invention. Other methods are of course possible as will be evident to a person skilled in the art. The tension member (4) comprising an end piece (7) and the desired number of centering devices (41) is positioned below the tensioner (8) and rotation device (9). The end piece (7) and at least an upper portion of the tension member (4) is then brought through an aperture (94) in the rotation device (9). The end portion (21) of the liner or mandrel (2) is then brought into contact with the rotation device (91), and fastened thereto by means of fastening devices (93) such as for instance bolts or the like (Fig. 4d). A sleeve (81) is then securely arranged about the end piece (7), and is brought into contact with the tension means (82). Fastening means (83) are arranged between said sleeve (81) and said tension means (82), and the tension means (82) are the raised so as for tensioning the tension member (4). Although the centering means (41) fit tightly against the inner surface of mandrel or liner (2), they may be moved along the longitudinal axis of the mandrel or liner (2) thus allowing the tensioning of the tension member (4).

Please refer to Fig. 6 for the description of the mechanical structure. The apparatus comprises one portion mounted on a first, fixed plate (100), and another portion mounted on second plates (104a, 104b), here arranged on rods (102) extending from the first plate (100). In the preferred embodiment, tension means (82) in the form of an actuator, e.g. a hydraulic piston, an electromagnetic actuator, or a mechanical tensioner device, is fixed to the plate (100) and may be extended through an aperture in said plate (100) to be fastened to a split sleeve (81) arranged for receiving and holding an end piece (7) of the tension member (4),

and arranged for exerting a desired pulling force on said end piece (7) when said tension means (82) is actuated. A rotation device (91) comprises a cogged wheel with a centered aperture (94) for the tension member (4) and with an inner rim for attachment to the end portion (21) of the liner or mandrel (2). A motor is arranged for rotating the rotation device (91). Said rotation device (91) is arranged in roller bearings (92a, 92b) between said second plates (104a, 104b), and is arranged coaxially with the axis of the tensioner apparatus (8). A similar lower arrangement may be arranged for receiving, holding and tensioning the opposite, lower end of the mandrel (2) and tension member (4). In a simplified embodiment of the invention, the lower arrangement may be arranged without a rotation motor. Optionally, the second plates (104) may be arranged movable on said rods (102) for adjusting the elevation of the rotation device (91) and for adjusting the tension of the liner or mandrel (2), if required.

The mandrel or liner (2) may now be rotated with respect to tension member (4). The tension member (4) is tensioned and provides support during the filament winding process. The tension member (4) may be tensioned by a force which utilizes a large portion of the tension member failure load. For tubes with internal diameter of 4", a tension member with force capacity of 1000 kN or more can easily be designed.

In an embodiment of the method according to the invention, the filament winding process is performed vertically, that is the mandrel or liner (2) is arranged vertically within the winding and tensioning system prior to the filament winding process. The vertical arrangement of the mandrel or liner (2) presents several advantages, the most important is that this arrangement eliminates the problems of sagging. A vertically positioned mandrel or liner (2) will not be subjected to gravitational force displacing said mandrel or liner (2) with respect to the longitudinal axis of the mandrel or liner (2). The problem of sagging may be somewhat alleviated by tensioning the mandrel or liner (2), but may not be eliminated when performing horisontal filament winding. Fig. 2a is an illustration of the results of a mathematical simulation of a tubular in which the end portions of said tubular have no degrees of freedom. The filament winding of a structure presenting a sagged profile such as this presents challenges to which there are no known solutions. The vertical arrangement of the mandrel or liner (2) introduces some challenges to the process such as the uneven cross-linking of the resin (6) during curing. Resin (6) may flow in a downwards direction due to gravitational force, and may thus set unequally. However these problems are easily solved for a person skilled in the art, by for instance removing the excess resin (6) from the bottom portion of the mandrel or liner (2) during curing, and applying additional resin (6) at the top portion of the mandrel or liner (2) if needed.

A first approach towards reducing the problems of sagging is the arrangement of the tension member (4) within the mandrel or liner (2) according to the method of the present invention. Experimentation has shown that sagging may be considerably reduced by the

arrangement and tensioning of said tension member (4) within the mandrel or liner (2). This is clearly shown in Figs 2b and 2a comparing the sag of a mandrel or liner (2) with and without a tensioned tension member (4) respectively. The problem of sagging may be somewhat alleviated by tensioning the mandrel or liner (2) , however such tensioning induces new problems, in particular related to the curing of the resin (6) and creation of possible residual stresses within the tubular (1) . If the mandrel or liner (2) is released before adequate curing of the fiber-resin matrix, this may induce the disadvantageous compressive strains and at worst wrinkling of fibers (3) having a near longitudinal orientation. If the mandrel or liner (2) is released after the curing of the composite (3), large internal residual stresses within the tubular (1) will be created, which may during later use result in premature failure also by delamination between the line and the composite laminate. Tensioning the mandrel or liner (2) further presents particular difficulties relating to machinery as the mandrel or liner (2) must in addition to be rotated also be tensioned. The rotating system must thus resist both axial and rotational loads. Lastly it will often be undesirable to tension the mandrel or liner (2), as the mandrel or liner (2) might not be able to resist the necessary tension to adequately reduce sagging.

In the embodiment of the invention in which the mandrel or liner (2) is arranged vertically there will not be a problem of gravitationally induced sag, but rather a problem of bulging. The problem with low natural frequency will be present as well. This presents particular problems, but by means of tensioning the tension member (4), these problems are able to be solved as the physical characteristics of the problem of bulging are similar to the problems due to sagging.

In an embodiment of the invention the mandrel or liner (2) itself is tensioned so as for providing additional stiffness to the system. This will allow for even higher production speeds as the natural frequency of the system will increase given the increased stiffness of the system. Means for implementing such mandrel or liner (2) tensioning can advantageously be arranged on one or both ends (21 , 22) of the mandrel or liner (2) to be tensioned. An arrangement in which both the mandrel or liner (2) and the tension member (4) within the mandrel or liner (2) is tensioned may prove beneficent if very long tubulars (1) are to be produced, such as tubulars (1) being in excess of 60 meters long.