The present invention relates to a composite pipe, at least comprising an inner pipe and one or more layers of strip material wound around the inner pipe.

Such a composite pipe is known from European Patent Specification No. 060,856, which describes a method for the reinforcement of a hollow body such as a pipe or a container for holding a medium under pressure. For this, a flexible strip comprising fibres embedded in a synthetic resin is wound around the hollow body. After winding, the various turns of the strip are fixed to the hollow body and to each other by gluing or heating. A steel pipe is mentioned as an example of a hollow body.

Such a method has a major disadvantage. The pipe formed by the method is particularly rigid and therefore cannot be wound with great length onto, for example, a reel, in order to permit easy transportation and posi¬ tioning of such a great length of pipe. Besides, the above- mentioned limited flexibility is a disadvantage for laying of the pipes if, for example, an uneven base is present, or the pipe has to be laid with different bends in it. A coupling piece then has to be used for each bend.

The object of the present invention is to overcome the above-mentioned disadvantage, and to this end the invention is characterised in that the inner pipe is made of thermoplastic material and the one or more layers of strip material wound around the inner pipe ,ire connected to each other and/or to the inner pipe only over a part of the length of the inner pipe. According to the invention a pipe which is particularly flexible and can be wound up with a relatively small radius of curvature is provided. This means that great lengths of pipe can be transported easily, and great lengths of pipe can thus also be laid without

connecting pieces. The latter can be advantageous, for example, in the case of applications such as the laying of pipes over the seabed or the like. The pipe according to the invention can also be bent easily around all kinds of obstacles. The invention provides a pipe which is extremel flexible and yet can withstand a high load, i.e. both internal pressure load and tensile load.

The one or more layers of strip material wound around the inner, pipe are preferably connected at the ends of the inner pipe to each other, and/or to the inner pipe. This simplifies the production of the pipe and increases the flexibility, due to the fact that the layers, with the exception of the ends, lie completely free.

In order to improve further the flexibility of the pipe according to the invention, the strip material of eac layer is advantageously wound in such a way that there is a space measuring 2 - 20% of the width of the strip material, and preferably 5 - 10% of the width of the strip material, between the turns of the strip material. The. flexibility of the pipe is improved by providing a space between the turns of the strip material of each layer. Winding with a space between turns is also simpler than winding without a space, while the turns of the strip material of each layer must lie closely against each other. In particular, the strip material of the second layer is wound in such a way that it covers the space between the turns of the first layer. It can in fact happen that when the pipe is being used for conveying a medium under high pressure the material of the inner pipe is pressed into the space between the turns of the first layer. Winding the second layer in such a way that it covers the space between the turns of the first layer means that the material of the inner pipe is prevented from penetrating further, and possibly even leakage is prevented from occurring. It will be clear that the winding angle of the wound strip material must be selected in such a way as is advantageous for the application in question. Thus, a small

winding angle will be desirable in the case of high tensile loads and a large winding angle in the case of low tensile load and high internal pressure load. Combinations of different winding angles are also possible. The width of the strip material of the first two layers is advantageously smaller than the width of the strip material of the other layers. The space between the turns of the strip material of the first layer is smaller then, which means that less penetration of the material of the inner pipe can occur in the case of, for example, a high internal pressure load, something which benefits the durability of the pipe according to the invention.

The strip material can be of many types of materials, but it is preferable to use strip material made of a fibre strip which may or may not be impregnated with plastic, and preferably a polypropylene-impregnated glassfibre strip. Fibre strip with or without plastic impregnation is extremely strong and durable. In the description, plastic strip reinforced with fibre strip, woven strip, metal gauze strip, metal woven strip etc. are also included.

In order to be able to protect the wound layers from the external environment such as, for example, soil, water, contaminations and damage, depending on the application, an outer jacket is preferably provided around the inner pipe provided with one or more layers of loosely wound strip material, the outer jacket advantageously being extruded round the inner pipe provided with one or more layers of loosely wound strip material. By means of extrusion the outer pipe in the not yet hardened state is provided around the inner pipe provided with one or more layers of loosely wound strip material, thus producing a close-fitting contact between the outer jacket and the outer layers of wound strip material. The outer jacket also ensures cohesion of the layers of wound strip material. The invention also relates to a composite pipe, at least comprising an inner pipe and one or more layers of strip material wound around the inner pipe, the turns of

the one or more layers being connected to each other, whic is characterised in that the inner pipe is made of ther¬ moplastic material, and in that the turns are connected only locally to each other, and the or each layer is displaceable relative to the inner pipe. The local connection can be achieved by, for example, techniques which are known to the person skilled in the art, such as welding, gluing etc. The pipe thus produced has good flexibility and can withstand an extremely high internal pressure load and tensile load. The winding angle can in fact be selected in such a way that it can withstand a maximum internal pressure load, while the necessary tensil load resistance can be adjusted by varying the distance between successive connections between the turns. Due to the fact that the or each layer is not connected to the inner pipe, it is possible to wind the or each layer separately onto a mandrel, and connect the turns locally and then slide them over the inner pipe. In this way it is ensured that during the local connection by welding or the like the inner pipe is not accidentally damaged or the strip material fixed to the inner pipe.

It is preferable to use reinforced polypropylene stri as the strip material, since it has very good strength and durability properties. Moreover, the turns of the abovementioned material are very easily connected by welding, gluing or the like.

Due to the fact that the turns rest against each other, it is also ensured that in the case of high internal pressure loads material of the inner pipe is not pressed between the turns.

Finally, the invention relates to a reel which is provided with a pipe according to the invention wound thereon.

The invention will be explained in greater detail below with reference to the appended drawing, in which: Fig. 1 shows a partial cross-section of a cutaway composite pipe according to one embodiment of the

invention; and

Fig. 2 shows a partial cross-section of a cutaway composite pipe according to another embodiment of the invention. In Fig. l reference number l indicates a polyethylene inner pipe, which is provided with layers 2, 3, 4, 5 and 6 of loosely wound strip material 7. The spaces 8 between the turns of the strip material 7 are clearly visible in each layer. Layer 2 is wound with the same winding angle as layer 3, while layer 3 covers the spaces 8 of layer 2. The position of the spaces 8 of layer 2 is indicated by dotted lines in layer 3. In this way it is clear that the strip material 7 of layer 3 is wound over the space between the turns of the strip material of layer 2. Layers 2 and 3 are wound with a winding angle of 120° to the longitudinal axis. In the case of layer 4 the strip material 8 is wound with a winding angle of 60° to the longitudinal axis. Finally, two layers 5 ancT 6 are wound with a smaller and opposite winding angl 'over layer 4. Reference number 9 indicates an outer jacket which is extruded around layer 6. This outer jacket can be of any desired plastic, but in this case it is by way of example also of polyethylene. The outer jacket can, of course, be of any other usable material such as, for example, gauze- type or fabric material.

The outer jacket ensures the cohesion of the pipe according to the invention and protects it from damage and the penetration of substances from the environment, such as soil, water, contaminations and the like. The material of the inner pipe is in the present example polyethylene, therefore thermoplastic. Polyethy¬ lene, in general a thermoplastic material, makes it possible to connect the turns to the inner pipe by heating at the places where this is desired. Apart from polyethy- lene, many other thermoplastic materials, such as polypropylene, polyvinyl chloride, polyamide, polybutene, etc., can be used.

The strip material of the layers used can be the same or different. It preferably comprises glassfibre strip impregnated with polypropylene, comprising a width of for example 2.75 cm. In particular, the inner two layers are wound with strip material of a smaller width than the other layers, in order to make the space between the turns of the strip material of the first layer as small as possible. In this way when there is a high internal pressure load, less material of the inner pipe can penetrate through it. It will be clear that a pipe according to the invention which is provided with layers of wound strip material with winding angles corresponding approximately to those of layers 2, 3 and 4 is more suitable for applications in which a high internal pressure occurs in the pipe than a pipe which is wound with strip material at a winding angle which corresponds to that of layers 5 and 6. The layers 5 and 6, with the winding angle indicated, are more suitable for a pipe which is subject to a great tensile load. The desired winding angle of the turns of the layers can be set depending on the application. The pipe shown in the figure is suitable either for applications in which both an internal pressure load and a tensile load occur.

The wound strip material of layer 2 is welded to inner pipe 1 at the ends, layer 3 is welded to layer 2 at the ends, layer 4 is welded to layer 3 at the ends etc. It is, however, not absolutely necessary for the assembly of layers of strip material 7 wound round the inner pipe 1 and welded together to be connected also to the inner pipe 1. For the connection of several pipes according to this embodiment, standard connecting pieces can be used, but it must be borne in mind that when a pipe according to the invention is cut through, the ends of the strip material of the layers must be fixed to each other at the abovementioned cut ends and possibly to the inner pipe before it is connected to a connecting piece. This is to prevent unwinding.

Fig. 2 shows an embodiment in which only one layer is wound around the inner pipe 1, while the turns of the strip material 7 lie against each other and are locally connected to each other by welding. By selecting the winding angle close to 90°, such a pipe can resist a very high internal pressure load. The level of the resistance to tensile load can be set by varying the distance between two connecting points 10. Stretched polypropylene strip is preferably used as the strip material. The fact that the turns lie against one another prevents the material of the inner pipe from being pressed between the turns when there is a high internal pressure load.

It will be clear that several layers can be applied around the inner pipe with winding angles which are suitable for the particular application of the pipe. Layers according to Fig. 1 can also be combined with layers according to Fig. 2.