Background of the Invention Fiber reinforced composite shafts exhibit advantages over metallic shafts in being lighter in weight, more resistant to corrosion, stronger and more inert. This is a particular advantage in many applications, such as riser pipes, drill pipes, motor casings and shafts.

Tubular fiber reinforced composite tubes or shafts are formed from a resinous material which is reinforced by glass fibers. In particular, filaments bearing a non- hardened resinous material (i. e. an uncured thermosetting resin) are wound around a mandrel until the desired thickness has been established. It is common practice to wind each layer at a different angle to the length axis, for example the first layer is wound at +30° and the next at-30°. The composite material comprises mutually- parallel fibers, such as glass fibers, carbon fibers or aramid fibers, embedded in a matrix, such as a thermosetting matrix, for example an exposy resin, joined, at its two ends, to metallic end coupling members, the connection being ensured by different techniques. Thereafter the resinous material is solidified (i. e. cured). The premolded threaded end portions can be mounted at the ends of the tubular composite, such as by winding of filaments directly around the end portion during the winding process.

However, direct welding or bonding of a resin shaft to metal does not normally create a sufficiently strong and durable connection on a consistent and reliable basis. The problem of joining composite cylinders or other tubular bodies is a recurring one in the design and manufacture of missile bodies, pressure vessels and other load bearing structures.

The properties of composites are such that they can find particular use in offshore oil and gas exploration, especially in risers. A riser is a long column established to connect the subsea installation with a surface platform. With exploration moving to even larger ocean depths a focus is on reducing the weight of the riser. In addition, composite materials can offer high resistance against corrosion, high thermal isolation and high damping combined with good fatigue properties. The high thermal isolation is especially important to avoid the formation of hydrate plugs in the riser. A riser has typical an OD of 20"and must withstand an overpressure of

500 bar (50 MPa). The riser will normally be in tension, which creates high tensile forces in the pipes.

The main problem when coupling a number of pipes is to ensure that the bond between the composite tube and the metallic coupling parts is strong enough to withstand high shear forces generated by tension, for example when deployed in a several thousand meters long riser column.

In US Patent no. 5,443, 099 there is shown a joint where the composite is wound around an inner member, which is a metallic end coupling member, comprising a wedge-shaped part and a separate outer part placed so as to grip the composite tube end in a sandwich, the securing between the composite tube and metallic end coupling member being ensured by metallic pins. The solution employing pins is very complicated, requiring precision drilling and tedious hand work to remove burrs and sharp edges. In automated assembly lines these operations require extra machine steps. The forces will be concentrated around the pins, thereby introducing load points which can lead to failure.

A standard prior art design employs a metallic coupling end with three grooves. A combination of helical and hoop layers are used, the hoop layers being used to"lock down"the helix layers because of the radial stiffness of the hoop layer. The axial load, either as a result of the pressure in a closed vessel (motor casing) or tension (risers, shafts), will result in shear forces between the composite and the metal.

These shear forces must be taken up in the transition area in such a way that metal do not lose contact with the composite, at the same time avoiding breaking the fibers in the composite. It has been very difficult to distribute the shear loads evenly over the whole length of the metallic end coupling and engage all grooves approximately the same. Stiff transitions that may result in load concentrations and bending moments will also be a challenge for the optimizing of the design. The grooves should be filled with a filler. A glass fiber is especially suited to this because it has higher shear strength than carbon. The high loads (torque or tensile) which are to be transmitted by a composite shaft require that an extremely strong and durable connection be established between the sleeves and the shaft body.

Previous proposals for mounting sleeves by employing adhesives or by wrapping the filament bundles around circumferential grooves on the sleeve periphery cannot be relied upon to provide a connection of the requisite strength and durability.

In US Patent no. 5,227, 208 there is shown a joint of this kind where a number of circumferential grooves are cut into the outer surface of the metal end coupling part.

The cut circumferential grooves are wound with a filament overlay consisting of helix and hoop windings which employ low and high tension, respectively. The

helical windings of the tubular composite structure is applied at low tension, so that higher tension hoop layers is used to pull the helix layers down into the grooves.

However, analyses of failed joints have shown that the composite filaments are not able to evenly distribute the tensile loads evenly along the length of the end coupling. Tests have shown that the first contact point (i. e. the first wrapping) carries the main load. The load carrying forces tend thus to be concentrated at the transition between composite and metal with the forces transmitted through only a few strands of filament, i. e. the inner layer. This sets up a concentration that is a potential shear zone that may cause the joint to break.

Summary and Objects of the invention An object of the present invention is to provide an improved method for joining of a metal part to a composite part and also an improved joint of a metal part and a composite part. It is a special object of the invention to provide novel methods and apparatus for securing metal end connector sleeves to the ends of fiber reinforced resin shafts to enable the shafts to transmit high torsional loads or tensional loads. A particular object of the invention is to provide the joining of a tubular composite structure to another metal structure in such a way as to form an assembly with adequate strength in tension, internal pressurization, bending, compression, and shear as to be useful in a wide variety of applications.

According to the invention the method for joining a metal part with a t least two circumferential outward facing recesses, to a composite part is improved by inserting at least one layer of elastic material between two fibrous resinous layers, which layers cover different recesses in the metal part in a joint, made up according to prior art. The elastic layer is applied in an area at least extending to the recess where a first of the two layers terminates.

When talking about a layer extending to, one talk about a layer constituting a part of the composite part that should be joined with the metal part, i. e. the composite tube, shaft or riser. With extending to this means runs from the whole of the composite part to a point over the metal part. The elastic layer according to the invention does not run all the way down in the composite part, but starts a bit before the metal part.

By this one achieves a division of the transmission of stress forces in the joint between at least two areas in the joint in stead of one major transferring point which is the case with prior art. The stress forces are divided between to areas, which areas are the edges of the recesses faced towards the main composite of the joint, e. g. the composite shaft or tube. The important area of the recess is the edge of the recess towards the main composite part where the transmission of forces takes

place. It is the change in outer dimension of the metal part from a larger to a smaller dimension of a for instance circular sleeve and because of this the change in the longitudinal direction of a fibrous resinous layer that forms the basis of transmission of forces and it is this area that is important to include in the elastic layer.

This change in the longitudinal direction of the fibrous layer may also be achieved by having outwardly protruding ribs on the metal sleeve. To have circumferential recesses or ribs are more or less the same solution they both provide edges over which a fibrous layer runs from a mainly longitudinal direction and then in a direction more towards the metal part for a short distance.

The elastic layer in accordance with the invention is applied between two fibrous layers which each terminate in different recesses, in other words where one layer runs over one edge and changes its direction and the other layer keeps its longitudinal direction over the mentioned first edge and changes its direction over another edge. There may according to the invention be one or several other layers between these two layers. The elastic layer may be applied between to layers terminating in the same recess, i. e. there is at least one extra fibrous layer between the elastic layer and the layer terminating in the other recess. In this embodiment the elastic layer will also change its direction.

However, the layer may also be located between a fibrous layer and a filler. In this embodiment the elastic layer will run only in a mainly longitudinal direction, and not bend down in the recess.

The fibrous layers may be wound in a lot of different pattern, and normally there will also be another outer fibrous layer, normally wound in a different pattern, to keep all layers and filler material in place. There are several ways the different layers may be configured, both with regards to winding pattern and also amount of and composition of the different layers depending on the use of the joint and the necessary forces the joint should undertake.

The use of the elastic layers, "stockings", will provide a small amount of slippage between the layers. These will"stretch"the fibers and thus enable them to have more points of anchoring. This will distribute the loads across the different recesses more evenly. The elastomer can be any known substance that exhibits the correct properties, such as a rubber or a polymer.

Calculations have shown that the use of elastomer results in a better distribution of shear forces across the joint. Each edge of a recess will carry approximately the same load. Previously the high loads on or near the first recess have forced

designers to use more material to enable the joint to withstand the applied forces.

The invention will result in a saving of material and at the same time avoid breaking of fibers.

The integrally wound joint structure as further discussed below provide a solution to the recurring problem of joining composite structures to other structures in the manufacture of pipes, shafts, pressure vessels and other load bearing structures.

The following description of the invention relates to a convenient and effective means for joining a tubular filament wound composite structure, with circular, square, or any other cross section to another body of any shape. The joining is achieved by filament winding over a joint tube with appropriate sized and spaced groove (s) running circumferentially on the outer surface. The joint tube must be equipped at the end to be joined with threads or some other means of attachment.

The joint tube will supply an effective union with the filament wound composite at one end while the other end will provide, by threads or some other conventional means, for joining the composite tube to another structure.

The helical windings of the tubular composite structure should preferably be applied at low tension, so that the helix layers will follow the contours of the grooves when laid down.

After conventional composite curing and separating of the assembly from the mandrel the composite tube may be joined to another body by means of the threads or other conventional coupling mechanism.

Brief description of the Drawings The invention will now be described with reference to the accompanying drawing where Fig. 1 shows a joint according to prior art, Fig. 2 shows a first embodiment of the invention, and Fig. 3 shows a second embodiment of the invention.

Description of the preferred Embodiments In Fig. 1 is shown by way of an example a prior art composite joint showing a cylindrical end coupling with circumferential grooves and threads for joining the pipe with other pipes in an end to end relationship. Directional numerals 11,12 and 13 show the direction of forces acting upon the joint and causing shear stresses in the joint.

The method for forming the integrally wound joint comprises completing the following steps: The pipe 1 has a metallic end coupling 2. The end coupling has a number of circumferential grooves 3,4, 5 cut into its outer surface. Threads are provided for connecting the end coupling with a corresponding end coupling in an end-to-end relationship, the threads being cut either on the inside of the pipe or on the outside.

In the embodiment shown, three grooves are cut into the metal outer surface, but any number can be used, as appropriate for the calculated loads. The grooves may also be of different shape and configurations. For example, it has been found advantageous to provide the first groove with a smaller slope than the other grooves. The end is prepared in the conventional manner to receive filament winding. The pipe is placed on or abutted against a conventional mandrel (not shown) and held in place in such a way as to be integrated into the filament wound structure. The mandrel is prepared by given a coating of release substance to resist the adherence thereto of resin or adhesives.

Firstly one or more helical layers 21,22 of carbon fiber (30°) is wetted with resin and wrapped around the joint cylinder. The layer (s) extend into the first groove 3.

The next step is to employ a filler 23 to the first groove, the groove being filled until it is flush with the top of first layer. A glass/exposy filler (30°) is preferably used.

Now a one or more helical layers 24,25 of carbon fiber (30°) is wrapped around the pipe, including the filler, extending into the second groove.

The second groove is in the same manner as previously described filled with a glass/exposy filler 26 until it is flush with the outer surface.

The same procedure is again employed for the third groove, first laying down one or more carbon fiber layers 27,28 (30°), and the groove being filled with a glass/exposy filler 29.

This procedure can continue as long as necessary, according to the number of grooves cut into the metal. However, it is envisaged that three grooves will be sufficient to provide the required strength.

Lastly hoop layers 30 are wound onto the end coupling to provide additional radial stiffness, in the conventional manner. The layers extend from the cylindrical (pipe)

area till the end of the coupling. The hoop layers are wound over the filler layers in the groove areas, pulling down the helix firmly into the grooves.

Alternatively, hoop layers may be substituted for the filler in the last groove 5.

If so desired, one or more layers of glass cloth (or some similar orthotropic cloth which is not too stiff to conform to the outer contour of the end coupling) can be wrapped around the joint cylinder to protect the carbon against both corrosion and the contact forces against the end coupling.

When the filament winding phase of fabrication is completed the assembly may be placed in a conventional oven and allowed to cure. The composite and joint cylinder may then be removed from the mandrel in the normal fashion.

In fig. 2 is shown a first embodiment of the invention. After the first layers of carbon have been completed, and the first groove filled, a layer 31 of an elastomer material is applied to the outer surface of the carbon and filler. In a preferred embodiment of the invention the elastomer layer is only a thin sheet that extends only partway onto the composite tube and partway into the second groove 3.

Thereafter the next helical layer (s) 24,25 of carbon fibers is wrapped around the pipe, extending into the end of the second groove.

After the second layer (s) of carbon have been laid down, a second elastomer layer is applied to the outer surface of the carbon and filler. The elastomer extends partway into the second groove 4.

Thereafter the third carbon layer and filler is laid down and hoop layer (s) are wound around the end coupling as earlier described.

In Fig. 3 there is shown an alternate embodiment of the invention, the main difference between this and the Fig. 2 embodiment is that the elastomer layer is located between two carbon layers with different orientation, between 25,26 and 28,29 respectively, .

Because of the elastomer it is preferred to use a self-curing resin instead of using high-temperature curing.