Furthermore metal pipes have been found to be susceptible to corrosion from some liquids and gases that are transported such as C02 and H, S found in oil and gas and so require periodic maintenance.

Much research work has been involved in the development of pipes for high pressure transport applications which are able to overcome the above problems of metal pipes.

Reinforced thermoplastic pipe (RTP), Figure 1, is a new class of pipe that offers the corrosion resistance benefits of- polyethylene (PE) at pressures up to 100 bar whereas PE is limited to pressures below 7 bar. RTP is manufactured by overwrapping a thermoplastic liner e. g. plain PE pipe with a high-strength fibre reinforced tape. An outer layer is extruded over the reinforcement surface to shield the fibres from damage. Typically, RTP is manufactured in diameters of 4 inch to 20 inch, with service pressures ranging from 20 to 100 bar.

Compared to carbon steel linepipe, RTP is a corrosion resistant, flexible and lightweight material, which is easy to install and can be tailored to suit specific applications.

Although RTP material costs are higher than that of carbon steel pipe, the total costs of installing the flexible and lightweight RTP pipes are estimated to be at least competitive to that of installing carbon steel pipes, and are particularly so if the pipe is carrying aggressive products since the RTP pipes will not corrode, unlike carbon steel pipes. The durability of the pipe is expected to reduce operational costs.

The potential of RTP for oil and gas transmission applications is far reaching. The material is particularly attractive in countries with oil or gas reserves in remote areas such as Canada and Russia. The light weight of the material significantly reduces material transportation costs and installation times, allowing pipelines to be laid in areas with difficult access, such as mountainous or soft-soil locations. The material minimises the environmental damage caused by pipeline installation as lengths of pipe can be easily manhandled and no heavy excavation or transport vehicles are required. This is particularly beneficial in sensitive ecosystems such as jungle and tundra. Further savings can be achieved as the chemical resistance of RTP allows the transport of highly corrosive gases and/or liquids, so treatment equipment is not necessarily required.

Furthermore, the chemical resistance means that, unlike steel pipelines, there is no need for coating or cathodic protection. The required inspection periods are minimised and the repair costs of the pipeline reduced, thereby significantly reducing the operational costs compared to steel pipelines.

Some commercial experience with this type of material has been gained with an oil and gas mixture at 20 bar and 60°C.

Due to the corrosive nature of the mixture, which contains high concentrations of CO2 and H2S, leaks in the carbon steel flowlines appeared after 3 to 6 months of operation. In a pilot study 70 kilometres of 6"carbon steel flowline were replaced with RTP. The RTP flowline has proven very successful, reducing total installation times by a factor of eight compared to the equivalent carbon steel pipeline.

Other noted advantages were the significantly higher flowrates due to the smooth inner surface and the flexibility of the RTP, making elbows and anchors obsolete.

In late 1998 the first high pressure gas transmission application was installed in Siberia. Due to the light weight of RTP no heavy machinery was required and 2 km of RTP was installed in just under two days. The system was hydro- tested to 80 bar and is designed to have a service pressure up to 50 bar.

Although RTP has generated a large amount of interest from industry over recent years, one of the main concerns regarding this type of pipe material is the availability of a cheap and reliable jointing method. The high service pressure of RTP (up to 100 bar) requires jointing methods which can cope with a high load bearing capability.

Furthermore, on joining lengths of RTP it is important to ensure that none of the contained pressurised medium (liquid or gas) contacts the reinforcement fibres. This may cause the mechanical strength of the fibres to degrade and can result in accumulation of the pressuring medium in voids in the reinforcement layer as illustrated in Figure 2. The latter is a problem if the pressuring medium is a gas, as on de-pressurisation this pressure may separate the surrounding pipe-wall material, which can cause blistering (A), partial collapse (B) or a combination of the two.

Methods of joining lengths of RTP with mechanical type couplings are disclosed in FR 2728049, FR 2728051 and GB 2287996. The sealing methods in these mechanical jointing systems rely on rubber sealing rings and/or clamping the surface of pipe with sufficient force to ensure a leak-tight seal. However, these systems are very expensive due to the large amount of machining required and can require large forces to assemble. Although these sealing methods have been shown to be feasible in the short term, there is some concern that in the long term the clamping loads on the reinforced thermoplastic pipe ends may relax allowing a relative movement of the pipe in the joint causing the joints to leak.

Compared to the above mechanical type fittings the present invention provides a RTP joint that is cheaper to manufacture, is easier to assemble and has a guaranteed leak- tight seal that prevents ingress of the pressuring medium in the reinforcement. This invention develops an invention disclosed in GB 2280145A which describes a method of butt joining of fibre reinforced thermoplastic pipes. Although this butt-weld ensures that there is a leak-tight seal between the two reinforced thermoplastic pipe ends, this method does not result in a joint that can cope with service pressure up to 100 bar. Such high pressures induce high axial and circumferential stresses in the joint, causing the- joint to fail.

According to a first aspect of the present invention there is provided an electrofusion coupler suitable for joining the end of one reinforced thermoplastic pipe to the substantially adjacent and substantially axially aligned end of another reinforced thermoplastic pipe, the electrofusion coupler comprising an annular body to surround the adjacent ends of the pipes to be joined and heating means to weld the inside surface of the annular body of the electrofusion coupler to the plastics material of the outside surfaces of the reinforced thermoplastic pipes, wherein the electrofusion coupler is provided with axial reinforcing means.

The reinforcing means prevents or reduces the likelihood of the coupler being split or cracked by axial and circumferential stresses along the joint. This reduces the likelihood of an environmentally damaging, expensive and inconvenient pipeline burst.

The reinforcing means preferably engages the coupler at at least two points spaced axially along the length of the coupler. The two points may be the axial ends of the coupler. The reinforcing means may, for example, be one or more reinforcing meshes along at least part of the axial length of the coupler and which may or may not be integral with the coupler, a sleeve surrounding the coupler and arranged to engage the axial ends of the coupler or any of the examples described in the following description.

The adjacent and substantially axially aligned ends of the reinforced thermoplastic pipes are preferably butt welded to each other before the electrofusion coupler surrounds the adjacent ends of the pipes. This prevents fluids from inside the pipe from penetrating fibres of the RTP thus degrading the fibres by diffusion along the fibre path possibly causing the pipe to fail.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows the structure of a typical reinforced thermoplastic pipe (RTP); Figure 2 shows the result of the contained pressurised medium contacting the reinforcement fibres of a reinforced thermoplastic pipe; Figures 3 to 6 show various stages of joining two reinforced thermoplastic pipes together; Figure 7 shows on a larger scale the zone of the joint shown in Figure 4; and Figures 8 and 9 show alternative embodiments of the reinforcing means. Figure 3 shows two reinforced thermoplastic pipes 10,12 which are to be joined by a butt fused welded joint. One pipe 12 has an electrofusion coupler 14 encircling the pipe 12.

The coupler 14 fits loosely around the pipe 12, with a diametric clearance 16 being shown. The coupler 14 carries a heating means which in this example is an electrical element embedded in the thermoplastic wall of the coupler comprising two groups 18,20 of windings surrounding the pipe 12. The heating element is connected to terminals (not shown) to supply electrical energy to the windings 18,20. The coupler 14 is provided with a reinforcing means along its axial length which in this case is a sleeve 22 having a flange 24 at each axial end arranged to engage the axial ends of the coupler 14. The reinforcing means in this example is made of steel but could be made of any suitable reinforcing material such as composite, plastic or ceramic. In the present example the reinforcing means is applied to the coupler 14 by first positioning the cylindrical sleeve 22 over the electrofusion coupler 14, positioning the annular flanges 24 on either side of the sleeve 22 and securing the flanges 24 to the axial ends of the sleeve 22 in this case with studs which are not shown.

Each pipe 10,12 in this example comprises an inner pipe 30 of thermoplastic material having an inside surface 32; a layer or layers of reinforcement fibres 34 wound around the inner pipe 30 or extending longitudinally down the pipe 30; and an external layer of thermoplastic material 36, which has been added to the inner pipe 30 following application of the layer or layers 34. The external layer 36 has an outside surface 38.

Each pipe 10,12 thus has a pipe wall between the inside surface 32 and the outside surface 38.

Before joining the end surfaces of the pipes 10,12 they are accurately trimmed so that clean surfaces 40,42 of the pipes are produced. This operation is performed while the pipes 10, 12 are held in a butt fusion machine (not shown). The pipes 10,12 are then forced by hydraulic cylinders against a heater plate (not shown) which is interposed between the pipe ends 40,42. Initial beads are formed on each pipe as the thermoplastic material becomes molten and is upset by the applied axial load.

The pipes 10,12 are held against the heater plate for a heat- soak period. Next, the pipes 10,12 are retracted from the plate, the plate is removed and the pipes 10,12 are brought together under an axial load. Molten material from the end portions of the pipes 10,12 is upset, involving inward and outward flow of thermoplastic material to form final internal and external beads 50,52 shown in Figures 4 and 7.

The pipes 10,12 are allowed to cool, the axial force being removed during the final stage of cooling although the pipes 10,12 are constrained against axial separating movement (Figure 4).

The or each reinforcement layer 34 of each pipe 10,12 is wholly or primarily displaced outwardly at 58 by the outward flow of thermoplastic material from its original position as shown in Figure 7. The result is that, at the welded joint, there is a layer 60 of thermoplastic material adjoining the common inside surface 32 which is free from reinforcement fibres. Next, the external bead 52 is removed (Figure 5) and the electrofusion coupler 14 is centralised over the butt fused welded joint by sliding it along the pipe 12 until it half covers the pipe 10 and half covers the pipe 12.

Finally, the terminals are connected to a source of electrical energy and current is passed through the heating element for a sufficient time for the coupler 14 to become welded by electrofusion to the outside surfaces of the end portions of the pipes 10,12 as shown in Figure 6. A small central zone, between the group 18 of windings and the group 20 of windings, is not fused and contains the stub of the external bead 52. Alternatively, before the end surfaces of the pipes 10,12 are joined, the external layer of thermoplastic material 36 and the layer or layers of reinforcement fibres 34 may be cut back from the ends of the pipes 10,12 to be joined leaving the inner pipe 30 to protrude. When the ends of the pipes 10,12 are then butt welded together there is then no or negligible flow of reinforcement fibre.

By way of example, joints have been made between pipes of polyethylene reinforced with aramid fibres. The pipes were 125 millimetres in outside diameter and had a wall thickness of 11.5 millimetres. The quotient found by dividing the outside diameter by the wall thickness, which is known as the Standard Dimension Ratio (SDR), was 11. The internal and external beads were 11 millimetres wide (ie measured parallel to the length of the pipe) and 5 millimetres high (ie measured radially with respect to the pipe). In other examples, the internal and external beads were very small, being 3 millimetres wide and 2 millimetres high.

In these examples, the inner pipe 30 and the external layer 36 were of the same grade of polyethylene. However, in general, the inner pipe 30 and the external layer 36 can be composed of the same or different grades of the same thermoplastic material or be composed of the same or different thermoplastic materials.

The larger beads are preferred where it is necessary to perform quality control checks on the performance and consistency of the butt weld, the bead being readily removed for inspection. On the other hand, very small beads are advantageous because the internal bead does not impede the flow of product as much as the larger bead. Also, it would not be necessary to remove the external bead in order to allow the electrofusion coupler to slide over the butt fused joint, the clearance between the coupler and the pipe being large enough to accommodate the external bead. In practice any size of bead could be produced depending on the engineering requirements. Also, the invention is applicable to a wide range of pipe sizes and SDRs, the examples quoted are given purely as examples.

In the completed joint, the flow of molten pipe material during the making of the butt fused welded joint ensures that there is a layer 60, which is free from reinforcement fibres, which is at approximately the same thickness as the thickness of the original inner tube 30. Consequently aggressive chemicals or fluids flowing in the pipe, which may be under a pressure of, for example, 25 bar are kept away from the highly stressed fibre reinforcement. However, as there are fibres in the outer part of the butt joint, the weld interface is weakened, resulting in the risk of brittle fracture from axial or bending loads on the pipe were it not for the use of the axially reinforced electrofusion coupler 14.

The electrofusion coupler 14 is fusion welded over the butt welded joint and the reinforcement means, in this case the sleeve 22 and flanges 24, provide considerable axial strength to the joint. The coupler 14 and reinforcement means 22,24 reduce the risk of the butt joint breaking in service and also protect the fibres protruding to or close to the external surface of the debeaded butt joint from the external environment. If access to the fibres was permitted, pressurised gases and liquids would permeate considerable distances along the fibres by capillary action. High pressure gas within the fibres could lead to disbonding along the reinforced layer. This could result in bursting the outer layer of the pipe or, in the event of pipeline depressurisation, collapse of the inner pipe 30.

If required, two or more, but preferably an odd number of axially aligned electrofusion couplers 14 could be butt- welded together in series with a suitable length reinforcing sleeve 22 applied to provide an even stronger joint.

Tests have been conducted to illustrate the mechanical integrity of the joint. A joint was made as described above by butt welding two adjacent axially aligned reinforced thermoplastic pipes, removing the external bead and positioning a series of three axially aligned electrofusion couplers that are butt welded together over the pipe joint.

The electrofusion couplers are then energised and fused to the pipes over the joint. A metal sleeve was then positioned over the series of electrofusion couplers, flanges positioned either side of the metal sleeves and the flanges secured to the metal sleeve by studs. The open end of each pipe was blocked with a closure means threaded onto the open end of each pipe to form a vessel. Water was then supplied to the inside of the vessel as a pressurising medium via a suitable inlet and the pressure of the vessel was gradually increased until the vessel failed. It was found that the vessel failed at a pressure of 286 bar (the reinforced thermoplastic pipe having a rated failure pressure of 290 bar) and that the failure was located in the pipe about 1.5 pipe diameters away from the fusion joint. This shows that the reinforced electrofusion coupler joint of the present invention is stronger than the pipe and is able to withstand considerable axial stress produced by high pressure within the pipes.

The reinforcing means 22 need not necessarily be a sleeve as in the above examples which is preferable because it can handle all axial and circumferential loading due to internal pressures, but could also be for example a mesh or a cage made from, for example, metal such as steel, plastic such as tape, composite or ceramic.

In the example shown in Figure 8 the electrofusion coupler 14 was reinforced with a reinforcing mesh 62 acting as the reinforcing means inside the plastics material forming the electrofusion coupler 14. Alternatively the mesh 62 could be secured to the outside of the electrofusion coupler at points separated along the axial length of the coupler.

In the example shown in Figure 9 a bracket, in this case in the form of a ring 64, was placed at each axial end of the electrofusion coupler 14 and a securing means, in this case a series of two or more circumferentially spaced rods 66 are provided to hold the brackets so that they engage the coupler 14 at its axial ends to prevent the coupler from splitting under axial stress. In this example the rods 66 are made from metal and have screw threads at each end which are arranged when in use to extend through correspondingly sized holes in the ring upon which nuts 68 are secured to keep the rings tight against the coupler 14.

In another example which is not illustrated, the electrofusion coupler 14 could be reinforced with one or more elongate wound thermoplastic filaments which could be arranged axially inside the coupling 14 when it is made or which could be secured to the outside of the coupling at two or more axially spaced positions. The present invention is not limited to the examples described above, but is applicable to any reinforcing means which prevents or reduces the liklihood of the electrofusion coupler from splitting under axial pressure.