Patent Description:
It is known that the life and performance of new and existing pipelines can be extended and optimised by lining lengths of metal pipe with polymer liners. For example, the Applicant's Swagelining® pipe lining service allows existing pipelines to be remediated and new pipelines to be provided with corrosion resistance by installing a polymer liner that remains in tight contact with the inside of a host pipe.

To join adjacent lined pipe lengths, it is known to provide an electrofusion fitting to connect the inner pipe linings prior to welding the metal pipe lengths together. The Applicant's earlier International Application Publication Number <CIT>, <FIG> of which is reproduced in part in <FIG> of the present application, discloses an electrofusion fitting <NUM> and a method of forming a pipe joint between two lined metal pipe lengths 103a,103b incorporating such a fitting <NUM>. The electrofusion fitting <NUM> is a sleeve largely comprised of a thermoplastic material and includes heating coils 107a, 107b disposed at either end of the fitting <NUM>. In use, the lining 105a is stripped back and the electrofusion fitting <NUM> inserted into the end of metal pipe length 103a. The heating coils 107a are then provided with electrical power which causes the fitting <NUM> and the lining 105a in the vicinity of the coils to melt and fuse together. The process is repeated to fuse the fitting <NUM> to the lining 105b of the other metal pipe length 103b, after which the metal pipe lengths themselves 103a, 103b are welded together at <NUM>.

While the effect is such that a very effective weld is formed between the fitting <NUM> and the pipe linings 107a, 107b, it is difficult to ensure that the fusing process can be consistently repeated.

Accordingly, it is an object of at least one aspect of the present invention to provide a method of joining sections of plastic pipe using an electrofusion fitting that provides repeatable results. Embodiments of aspects of the present invention are intended to realise this object and to obviate or mitigate one or more further disadvantages of existing electrofusion fitting methods.

Further aims and objects of the invention will become apparent from reading the following description.

According to a first aspect of the invention, there is provided a method of compensating for misalignment between two plastic pipes during a welding process. The method comprises inserting an electrofusion fitting comprising a thermoplastic material into corresponding recesses in the ends of the plastic pipes, aligning the plastic pipes relative to one another, and heating corresponding portions of the electrofusion fitting to a first temperature lower than a melting point of the material of the electrofusion fitting to cause the portions to expand to fill the recesses. The method may comprise holding the plastic pipes in place while heating the portions of the electrofusion fitting to the first temperature.

The plastic pipes may be standalone, or may be liners of, e.g. carbon steel host pipes, in which case the host pipes are aligned prior to heating the electrofusion fitting to fill the recesses. Expansion of the electrofusion fitting caused by the initial heating step can compensate for misalignment by filling any gaps between the electrofusion fitting and the plastic pipes, ensuring complete contact when the weld takes place.

The method may further comprise heating corresponding portions of the electrofusion fitting to a second temperature higher than the melting point of the material of the electrofusion fitting to weld the fitting to the plastic pipes.

For example, the first temperature may be on the order of <NUM>, and the second temperature may be on the order of <NUM>.

The invention provides several advantages. Firstly, the electrofusion fitting, by virtue of the initial heating step or pre-heat, can expand to improve or ensure contact between the surface of the fitting and the plastic pipes. This is particularly advantageous when two such pipes are aligned for welding but there is misalignment between the pipes and/or the electrofusion fitting resulting in gaps therebetween. Expansion of the electrofusion fitting caused by the initial heating step may compensate for this misalignment by filling any such gaps between the electrofusion fitting and the pipes, ensuring complete contact when the weld takes place. Secondly, the initial heating step enables the subsequent welding step to be performed from the same starting temperature (or at least a known starting temperature) of the fitting and/or the pipe every time - meaning that the reliability of the fusion process and the likelihood of making an acceptable weld is vastly increased. Thirdly, the Applicant has found that the initial heating step improves energy coupling into heating elements, such as copper coils, during the subsequent fusion step with a resulting drop in power consumption - particularly at the outset.

Preferably, heating to the first temperature comprises supplying electrical current to one or more heating elements integral to the electrofusion fitting. Alternatively, heating to the first temperature comprises supplying electrical current to one or more heating elements separate from the electrofusion fitting.

Preferably, heating to the second temperature comprises supplying electrical current to the same one or more heating elements as when heating to the first temperature.

Alternatively, heating to the second temperature comprises supplying electrical current to a different heating element as when heating to the first temperature.

Preferably, electrical current is supplied to one or more heating coils disposed on or near a surface of the electrofusion fitting proximal the pipe.

Optionally, the method comprises monitoring a temperature of at least one of the portions of the electrofusion fitting. Preferably, the method comprises controlling the supply of electrical current to one or more heating elements responsive to the temperature of the or each portion of the electrofusion fitting to control the temperature.

Preferably, the method comprises inserting the electrofusion fitting into the end of the pipe. Preferably, the method comprises receiving the electrofusion fitting in a recess in the end of the pipe. The recess is preferably formed in an inner surface of the pipe, but may be formed in an outer surface of the pipe. Optionally, the method comprises machining the electrofusion fitting to fit the recess.

Alternatively, the electrofusion fitting abuts the end of the pipe.

Optionally, the method further comprises inserting the electrofusion fitting into the end of a subsequent section of plastic pipe, heating a corresponding portion of the electrofusion fitting to a first temperature lower than a melting point of the material of the electrofusion fitting, and subsequently heating the corresponding portion of the electrofusion fitting to a second temperature higher than the melting point of the material of the electrofusion fitting to weld the fitting to the subsequent pipe.

Optionally, the method comprises determining the time taken to create a weld between the electrofusion fitting and one of the plastic pipes and heating the corresponding portion of the electrofusion fitting for a corresponding time to create a weld between the electrofusion fitting and the other of the plastic pipes.

The Applicant has developed a non-destructive way of testing the integrity of an electrofusion weld in which a channel extends at least partially through the electrofusion fitting or the pipe to provide fluid access to a test region between the electrofusion fitting and the pipe in order to leak test the weld. Accordingly, the method may further comprise performing a leak test between the electrofusion fitting and one or both plastic pipes through a channel extending at least partially through the electrofusion fitting or one or both plastic pipes.

This can be performed after the weld has taken place to test the integrity of the weld. However, there is a significant benefit if the leak test is performed before the weld is performed, to confirm contact between the electrofusion fitting and the pipe following the initial heating step. Therefore, the method may alternatively, or additionally, comprise performing the leak test before welding the fitting to the pipe.

Embodiments of each aspect of the invention may comprise features corresponding to the preferred or optional features of the other aspects of the invention as appropriate.

References above to plastic pipe and sections of plastic pipe shall apply equally to lined pipe and sections of lined pipe analogous to said plastic pipes. That is to say that the invention finds utility in joining sections of standalone plastic pipes and pipe sections, and also in joining lined pipe and sections of lined pipe.

Aspects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the following drawings (like reference numerals referring to like features) in which:.

As discussed in the background to the invention above, it is difficult to ensure that an effective and consistent fusion process is carried out when connecting lined pipes or the like. An embodiment of the present invention is described below, with reference to the electrofusion fitting illustrated in <FIG>, which overcomes this problem with the prior art.

An electrofusion fitting <NUM> is formed by a cylindrical sleeve of a thermoplastic material having heating coils 207a, 207b disposed toward either end of the fitting <NUM>. The electrofusion fitting <NUM> is shaped and sized to be inserted into the end of a lined pipe section 203a/203b and received in a recess 206a/206b in the end of the pipe lining 205a/205b. The heating coils 207a/207b are embedded in the outer surface of the fitting <NUM>, so as to be adjacent to and facing an inner surface of the pipe lining 205a/205b, such that when provided with sufficient electrical power the thermoplastic material of the fitting <NUM> and of the lining 205a/205b in the vicinity of the coils melts and fuses together, thus creating circumferential or annular welds between the fitting <NUM> and the pipe lining 205a/205b.

<FIG> shows the electrofusion fitting <NUM> at two distinct stages that occur prior to this electrofusion process taking place; note that the scales have been exaggerated for the purposes of illustration. On the left-hand side, the electrofusion fitting <NUM> has just been inserted into the recess 206a in the pipe lining 205a and abuts against a shoulder formed by said recess 206a. The outer diameter of the electrofusion fitting <NUM> is less than the inner diameter of the recessed pipe lining 205a and as a result there is a gap between the outer surface of the electrofusion fitting <NUM> and the pipe lining 205a in the region where the weld will be formed.

It will also be realised that a gap between the electrofusion fitting <NUM> and the pipe lining 205a may be due to a misalignment between the pipe linings 205a, 205b in adjacent pipe sections 203a, 203b, or specifically between the recesses 206a, 206b. The gap might not be continuous or circumferential - in fact the electrofusion fitting might be in partial contact with the pipe lining 205a in the recess 206a.

The existence of a gap means that while a weld might still be formed (although it is more likely that only a partial weld will be formed) the efficiency of the weld can be compromised, and there can be significant uncertainty about the quality of the weld particularly if the gap is relatively large. Accordingly, prior to the electrofusion process being carried out and in accordance with the invention, the heating coil 207a is employed to heat the electrofusion fitting <NUM> to a first temperature, which is lower than a melting point of the material of the electrofusion fitting <NUM>, at which the electrofusion fitting <NUM> expands and as a result reduces or removes the gap - as shown on the right-hand side of <FIG>. This preheating step can therefore ensure that the electrofusion fitting <NUM> fills the recess 206b and ensures the surfaces of the electrofusion fitting <NUM> and the pipe lining 205b are in contact before the material is melted to actually form the weld.

Note that it is possible to check that the electrofusion fitting has filled the recess and ensure that the surfaces are in contact before the weld takes place. By providing a channel (not shown) extending at least partially through the electrofusion fitting <NUM> or the pipe lining 205b, fluid access can be provided to a test region there between. If there is a tight fitting between the electrofusion fitting <NUM> and the pipe lining 205b, a leak test performed on the test region via the channel will confirm this and the welding step can be performed with confidence. If the leak test fails, the fit up can be remade (by further preheating or by reworking or replacing the electrofusion fitting) until a leak test is completed successfully. The test can also be performed, via the same channel, after the weld has taken place to test the integrity of the weld.

By way of example, the preheating step might involve heating the electrofusion fitting to <NUM> for two minutes, and subsequently the welding step might involve heating the electrofusion fitting to <NUM> for five minutes.

Using an electrofusion fitting to connect two sections of lined pipe is typically carried out prior to welding together the host pipes themselves - although the steps may be performed in any order. In practice, the host pipes can be aligned ready for welding and held in place, for example using clamps, prior to (and during) preheating the electrofusion fitting. In this way, any internal misalignment between the electrofusion fitting and/or the pipe linings can be compensated without losing external alignment of the host pipes. This provides for an improved external weld in addition to the improved internal weld. (As noted above, the steps may be performed in any order, e.g. the host pipes may be welded together even prior to preheating the electrofusion fitting).

Furthermore, by preheating the electrofusion fitting <NUM> in this way, the electrofusion process by which the fitting <NUM> is welded to the pipe lining 205a/205b can start from a pre-determined temperature which means that the electrofusion process can be predictably and reliably performed. Much of the concern surrounding reliability of electrofusion welds stems from uncertainty around various parameters than can vary greatly from one process to the next. By providing a consistent start point for all electrofusion welding processes, continuity between subsequent electrofusion processes can be ensured. To this end, the temperature may be monitored as part of the electrofusion process.

In addition, the preheating step avoids the need to provide internal physical support to the electrofusion fitting during the subsequent electrofusion process (for example, using a clamp or support frame) because the expanded fitting can fill the recess and therefore retain and support itself. The use of clamps or support frames is known to add complexity and extend the process cycle time, and as a result increase expense, without the increase in confidence that a satisfactory (and consistent) weld has been performed that the present invention provides. In short, the present invention reduces cost and complexity while increasing confidence in both the joining of the linings and of the host pipes themselves.

Typically, a fusion cycle will be developed for a known set of conditions. For example it may be determined that, at an ambient temperature of <NUM>, supplying a known electrical current to the heating coils for six minutes is sufficient for the thermoplastic material of the fitting <NUM> and of the lining 205a/205b in the vicinity of the coils to melt and fuse together effectively. However, in the field, the ambient temperature may for example be <NUM> in which case it is possible that the weld will not be formed effectively with only a six minute fusion cycle. One solution would be to have a calibration table or the like to refer to. However, the present invention provides a more elegant solution in which a predetermined starting temperature is set by preheating the electrofusion fitting (and optionally the pipe lining) which means that the actual parameters that have been approved or qualified for a particular operation can be reliably and repeatably delivered in the field, with predictable results. However, a calibration table or look-up chart might still be employed.

As described above, the Applicant has also discovered that preheating the electrofusion fitting prior to performing the electrofusion step results in improved energy coupling into the heating coils. While the energy requirements of a single electrofusion process might only be reduced by a modest amount, a pipelay operation for example might involve several hundred electrofusion fittings and so the energy saving benefits will be cumulative.

<FIG> illustrate the power consumption during a fusion cycle (a) without a preheating step and (b) with a preheating step. In this particular example, the initial power consumption can be seen to be approx. <NUM> W lower in the fusion cycle comprising the preheating step. Based on models the Applicant anticipates that in low temperature operations (where the starting temperature without preheat may be significantly lower than with preheat) and/or when employing larger electrofusion fittings, the difference in power consumption will be significantly magnified.

It will of course be understood that while the present invention has been illustrated with reference to electrofusion fittings for joining lined pipe sections, such an electrofusion fitting can be employed to join standalone plastic pipes, or indeed to provide a plug or to blank off the end of a lined pipe section or standalone plastic pipe. Furthermore, while heating elements are described as embedded in a surface of the electrofusion fitting, the heating elements may be disposed elsewhere in the electrofusion fitting or even applied externally.

The invention provides improvements to electrofusion fitting methods that allow for continuity and repeatability of welds between an electrofusion fitting and a pipe lining (or stand-alone pipe). An electrofusion fitting for joining sections of lined pipe has heating elements configured to create at least one weld between the electrofusion fitting and a pipe lining, however prior to the weld step taking place the electrofusion fitting is heated and expands accordingly to ensure contact with the pipe lining. Preheating the electrofusion fitting also provides a predetermined starting temperature for the fitting and the lining which results in improved fusion cycle reliability. Furthermore, the need for clamps or support frames to support the electrofusion fitting in situ is removed, with corresponding reductions in cycle times, complexity, and hence cost.

Throughout the specification, unless the context demands otherwise, the terms 'comprise' or 'include', or variations such as 'comprises' or 'comprising', 'includes' or 'including' will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

Claim 1:
A method of compensating for misalignment between two plastic pipes (205a, 205b) during a welding process, the method comprising inserting an electrofusion fitting (<NUM>) comprising a thermoplastic material into corresponding recesses (206a, 206b) in the ends of the plastic pipes (205a, 205b), aligning the plastic pipes (205a, 205b) relative to one another, and heating corresponding portions of the electrofusion fitting (<NUM>) to a first temperature lower than a melting point of the material of the electrofusion fitting (<NUM>) to cause the portions to expand to fill the recesses (206a, 206b).