GB2446380A

GB2446380A - Back Support Pad

Info

Publication number: GB2446380A
Application number: GB0702450A
Authority: GB
Inventors: Christian Cocca; Andrea Provesi; Francesco Negrisolo
Original assignee: Saipem SpA
Current assignee: Saipem SpA
Priority date: 2007-02-08
Filing date: 2007-02-08
Publication date: 2008-08-13
Also published as: GB0702450D0

Abstract

A back support pad 1 comprising a deformable cushion 11 preferably mounted on a copper shoe 13, can be used when welding two pipes 7a, 7b together. The cushion 11 conforms to the back surface of the pipe joint and supports the weld pool 5 created during welding of the pipe joint. The cushion 11 may be designed to conduct heat away from the joint. The deformable surface of the cushion 11 facilitates good conductive contact between the pipes 7a, 7b and the copper shoe 13.

Description

* 2446380 Welding support pad

Background of the Invention

The present invention relates to back support pads for welding, and especially back support pads for pipe welding.

During welding, especially during the first weld pass, it is usually necessary to use a back support pad to support the weld pool.

For pipe welding, it is desirable for the back support pad to fit to different pipe diameters or degrees of ovaliation of the pipe1in. In dditJon, it is desirable for the back support pad to dissipate some of the heat generated during welding thereby assisting in cooling of the weld.

US 4,363,954 describes a back support device for pipe welding. The back support device comprises a set of trapezoidal solid copper pads disposed to form a ring around the internal diameter of a pipe. The copper pads are constantly pressed against the interior surface of the pipe by radial thrust springs.

The back support device in US 4,363,954 enables, to some extent, the back support pads to be moved to accommodate the ovalisation of a pipe or small variation in pipe diameter.

However the contact between the solid copper pads and the interior of the pipeline can be affected by gaps/steps in the pipe joint caused by local variations in the alignment, fit and/or tolerance of the two pipes to be welded. As a result, certain areas of the outer support surface of the pads may not be in direct contact with the inner surface of the pipe, possibly leading to a reduction in the quality of the weld and also an increase in local pad wearing. In addition, a relatively small contact footprint may limit the efficiency by which the pad dissipates heat from the weld.

Also, known pads may be eroded during welding, causing inclusion of some pad material in the weld pool. This can reduce the strength of the weld. For example, pads having a high electrical conductivity (such as copper pads) allow the welding arc current to flow into the pad. This causes pocking and may create pollution of the weld. Pocking may also cause further gaps between the pad and the pipeline.

Summary of the Invention

The present invention seeks to mitigate or remove at least one of the above-mentioned problems.

The present invention provides a back support pad for supporting, from the back surface of a pipe joint, a weld pool created during the welding of a pipe joint, wherein the back support pad comprises a deformable cushion for conforming to the back surface. The deformable cushion may enable a relatively large contact area to be formed between the back support pad and the pipe joint surface during use. This may reduce localised pad wearing. The present invention may also reduce the likelihood that parts of the pad overheat (as may occur if there are small, local contact areas) . This may reduce the likelihood of inclusion of pad material in the weld, and may therefore improved weld quality. It will of course be appreciated that the support pads disclosed in US 4,363,954, being made of solid copper, are not deformable.

The cushion of the present invention may in contrast allow the back support pad to conform to the shape of the pipes in the region of the weld and to accommodate and cornpenst for local variations in the shape. The cushion is preferably repeatedly deformable, so that it may deform to accommodate variances in the back surface of the pipe joints shape of the interior.

The cushion may be elastically deformable.

The cushion is preferably heat-conductive and preferably has relatively good thermal conductivity to allow heat from the weld to be readily dissipated. The large contact area coupled with the heat-conductivity means that the pad may be relatively efficient at dissipating heat from the weld. The thermal conductivity of the cushion may be greater than lOWm K', and may be greater than 50Wm'K'. The thermal conductivity of the cushion is preferably more than lOOWm'K', more preferably more than 200Wm'K' and may even be more than 300Wm'K1. The thermal conductivity of the cushion may be greater than that of an equivalent cushion made from pure lead. The lead cushion to be used as the reference in this context may be a real cushion or a mathematical model, from which an overall thermal conductivity is calculated. The lead cushion (the reference cushion) may be equivalent in the sense that the reference cushion has the same shape and structure as that of the cushion of the invention, but is of course likely to differ in other respects, such as mass, colour, resiliency and the like.

The thermal conductance of the cushion from the exterior surface of the cushion, that in use is in contact with the weld pool, to the exterior surface on the opposite side of the cushion, may be greater than 0.1WK', is preferably greater than 1WK', and may be greater than 1OWK'. The thermal conductance of the cushion may be greater than 100WK'.

The thermal conductivity/conductance may be measured by any convenient means. The thermal conductivity/conductance should be ascertained by measuring the thermal conductivity from an exterior surface of the cushion that in use is in contact with the weld pool to the exterior surface on the opposite side of the cushion. The thermal conductivity/conductance should be measured whilst the mean temperature of the cushion is at ambient room temperature (i.e. at about 25 C), and not operational temperature, with a temperature gradient of about 10K. Such temperatures and temperature gradients are readily ascertained, allowing easy measurement of thermal conductivity. Such room-temperature conductivity measurements provide a good enough indication of the thermal properties of materials at operational temperatures.

The cushion is preferably a relatively poor electrical conductor. The cushion is preferably a poor electrical conductor in at least the radial direction (i.e. the radial direction during use of the back support pad) . The poor electrical conductivity tends to reduce chances of the welding arc current flowing into th heck suppc)rt pad. This is thought to reduce the chances of inclusion of pad material in the weld. Reducing the current flowing into the pad may also reduce pocking of the pad. A minority, by mass, of the material of the cushion may have a low electrical conductivity. For example, the cushion may comprise, at least in part, an electrical insulator, for example an electrically insulating layer, such that the overall electrical conductivity of the cushion is low in the radial direction.

The overall electrical conductivity of the cushion in the radial direction is preferably less than i05 Srri', more preferably less than io Srr(1, and yet more preferably less than lO Sm'. The overall electrical conductivity of the cushion in the radial direction may be less than 100 Srn'. The electrical conductance of the cushion in the radial direction is preferably less than laOS, more preferably less than is, and yet more preferably less than 102S.

The electrical conductivity/conductance may be measured by any convenient means. The electrical conductivity/conductance should be ascertained by measuring the current that flows through the cushion for a potential difference of 5 Volts applied across the cushion at two points, one point being on the exterior surface of the cushion that in use is in contact with the weld pool and the other point being on the exterior surface on the opposite side of the cushion. The electrical conductivity/conductance should be measured whilst the mean temperature of the cushion is at room temperature (i.e. at about 25 C) The cushion may be so shaped to support a surface of a pipe joint, wherein the surface so supported extends only part way around the circumference of the pipe. The length of the cushion in the circumferential direction may be less than 500mm, and is preferably less than 300mm. The length of the cushion may be greater than 100mm. The width of the cushion (measured in use in the direction of the axis of the pipe) may be less than 150mm, and is preferably less than 70mm. The width of the cushion may be greater than 10mm. The cushion, being deformable, has a variable thickness, in that the thickness may change depending on the shape adopted by the

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cushion. The thickness is preferably such that in use the average thickness of the cushion, as measured in the direction from the side of the cushion that in use is in contact with the weld pool to the opposite side and averaged over the region of contact, is greater than 4mm and more preferably greater than 6mm. The unloaded thickness of the cushion may be greater than 8mm. The volume encompassed by the outer surface of the cushion may be between 10-6 m3 and i0 m3. The volume encompassed by the outer surface of the cushion may be between i0 m3 and iO m Preferably, the cushion comprises a skin. The skin is prefrh1v n cmtr skin which: during use. contacts the pipe joint. The cushion may for example comprise an outer skin and an interior, the skin being made of material different from that of the interior. The skin is preferably a relatively poor electrical conductor. The skin is preferably arranged such that it acts as a poor electrical conductor (for example being an electrical insulator), whilst not having a great impact on the thermal conductivity of the cushion. For example, the ratio of the electrical conductance to thermal conductance may be relatively low (for example less than that of pure Copper, or less than 150, 000KV2 or even less than 1KV 2) such that a relatively thin skin may have a large effect on the electrical conductivity of the cushion, whilst not greatly affecting the thermal conductivity. The skin may comprise diamond, which can have a very high thermal conductivity, and a low electrical conductivity. The skin may comprise a ceramic material, which can have a relatively high thermal conductivity for its electrical conductivity.

The electrical conductivity of the material forming the skin is preferably less than that of Copper, and preferably less than that of Aluminium. The overall electrical conductivity of the cushion may be less than 95% of (and possibly 90% of, or even 80% of) the overall electrical conductivity of an equivalent cushion made from pure copper.

The electrical conductivity of the material forming the skin is preferably less than 30 x 106 Srn1 (when measured at 300K) The electrical conductivity of the material forming the skin may be less than iO Sm1. The electrical conductivity of the material that forms the skin divided by the mean thickness of the skin (in the case of a mesh defining the skin, mentioned below, the skin will have areas where its thickness is zero), when not compressed, may be less than 5 x 10"S m2, is preferably less than 10"S m2, and may even be less than 5 x 1010s m2.

The thermal conductivity of the material forming the skin is preferably greater than that of pure Lead. The thermal conductivity of the material forming the skin is preferably greater than 0.lWm'K' , and more preferably greater than lWm 1K' and yet more preferably greater than lOWm'K' The thermal conductivity of the material that forms the skin divided by the mean thickness of the skin (in the case of a mesh defining the skin, mentioned below, the skin will have areas where its thickness is zero), when not compressed, may be greater than 50 x W m2K', is preferably greater than 100 x l0 W m2K', and may be greater than 200 x W m2K1.

The skin may comprise a mesh, or netting. The skin may be metallic. The skin may comprise tungsten, or alloys thereof. The skin may comprise titanium, or alloys thereof.

The skin may comprise copper, or alloys thereof. The skin may comprise a metallic tissue. The tissue may comprise meshed rniteria1. Thc skin may comprise a plurality of layers of metallic material or tissue. The skin may comprise a plurality of layers of different metallic tissues. For example, the skin may comprise one metallic tissue of copper and another metallic tissue of tungsten. The skin may be multi-layered. The skin may be in the form of a membrane.

The skin is preferably in of a material that substantially prevents the passage of the material forming the interior of the cushion to the exterior of the cushion through the skin.

The skin may be resiliently deforrnable in at least one direction. The skin may be formed from a mesh able to he stretched in at least one direction parallel to the skin surface. The amount of reversible stretching permitted by the mesh may be greater than an increase of 10% in one direction.

The skin may have a thickness of less than 2mm, and may have a thickness less than 1mm.

The skin preferably encloses a filler. Of course, the interior of the cushion need not be in the form of a material wholly enclosed by the skin. The material defining the interior of the cushion will be referred to herein as the filler. The filler is preferably heat-conductive. The bulk thermal conductivity of the material defining the filler is preferably more than 50 Wrri'K', more preferably more than 200 Wm'K' and yet more preferably more than 300 Wrn1K' (when measured in respect of the material of the filler when in solid form) . The filler may also be electrically conductive (especially in the embodiment of the present invention in which the skin is a poor electrical conductor and the filler is therefore electrically insulated to a large degree) The filler may be in any number of forms, depending on the material and configuration of the skin. For example, the filler may comprise metallic wool. The filler may comprise powder. The filler may comprise gel. The filler may comprise liquid. The filler may comprise granules. The filler may comprise two or more of the aforementioned types of material.

The forms of filler described above are malleable and are particularly advantageous since they tend to enable the cushion to be easily deformable. It will of course be appreciated that certain forms of filler are incompatible with certain forms of skin. For example, a mesh skin would not be used, by itself, to contain liquid.

It will be appreciated that other forms of filler are also possible. For example, the filler may comprise a polymer/metal matrix. Whilst the filler is preferably deformable, it will be appreciated that it need not necessarily be so. For example, the skin only may be deformable.

Above it is stated that the cushion may be so shaped to support a surface of a pipe joint, wherein the surface so supported extends only part way around the circumference of the pipe. Thij. the heck support pad may be suitable for supporting the weld pool on only a circumferential portion of the pipe joint. Alternatively, the back support pad may be suitable for supporting the weld pool around substantially all the circumference of the pipe joint. For example, the cushion may be torus-shaped for contact with the circumference of the weld joint. Such a torus-shaped cushion may be supported during use by means of a plurality of radially extending support members. In use, the torus-shaped cushion may be urged into contact with the back surface by means of the radially extending support members acting to support the cushion. Alternatively, or additionally, the torus-shaped cushion may be urged into contact with the back surface by means of the pressure of the material inside the cushion. The interior of the cushion may be pressurised, at least during use. Such a torus-shaped cushion may include two ends that meet to form a join in the torus-shape.

The back support pad may be configured and arranged so as to support the weld pool from outside the pipe. In such a case, it will be appreciated that the support pad may be arranged to allow welding from inside the pipe, the back of the weld thus being at the outer circumference of the pipe.

Alternatively, the back support pad may be configured and arranged so as to support the weld pool from inside the pipe.

The back support pad may comprise a support member for supporting the cushion and for urging, in use, the cushion towards the pipe joint. The support member may for example be in the form of a base plate. The cushion is preferably 0 deformable on opposite sides such that the shape of the cushion may be deformed to adopt and accommodate the local variations in shape at both the region of the weld joint and at the opposite side of the cushion, where for example the cushion may abut against a solid support member as described immediately above. The support member may assist in transferring heat from the weld joint via the cushion. The majority by weight of the support member may be metallic.

The back support pad may comprise a base plate, wherein the cushion is mounted on the base plate. The base plate may be metallic, for example copper. The base plate may be trapezoidal in shape. The base plate may be substantially the same as the trapezoidal copper pads disclosed in US 4,363,954 or in the corresponding UK patent application (GB 2 067 945A) The contents of those publications are fully incorporated herein by reference. The back support pad of the present invention may incorporate any of the features disclosed in those patent publications. In particular, the claims of the present application may be amended to include the feature of the back support pad comprising a base plate that is the same as one of the pads as disclosed or claimed in either of those patent publications.

The beck 3upport pad may include a spring to assist the conforming of the cushion against the surface of the pipes.

There may be provided a plurality of such springs for one cushion.

The back support pad may further comprise cooling means for cooling the back support pad. The cushion may comprise the cooling means. Alternatively, in an embodiment of the invention in which the back support pad comprises a base plate, the base plate may comprise the cooling means.

The cooling means may comprise at least one channel through which, in use, coolant may be passed. The cooling means may comprise a multiplicity of channels through which, in use, coolant may pass. The coolant may be a gaseous coolant. The coolant may be a liquid coolant. The cooling means may comprise a coolant inlet and a coolant outlet, so that in use coolant may be passed into the back support pad via the inlet to cool the pad for coolant, thereby heating the coolant, and so that coolant so heated may be passed out of the back support pad via the outlet. Coolant so passed out of the outlet may then be cooled by separate means ready to be circulated back into the pad.

According to another aspect of the invention there is provided a pipe welding apparatus for use on a pipe joint, wherein the pipe welding apparatus comprises a back support pad as described herein, the back support pad being arranged for application to the back surface of the pipe joint. The pipe welding apparatus may comprise a multiplicity of back support pads as described herein, the back support pads being arranged for application to the back surface of the pipe joint. During use, the back support pad(s) may be arranged circurnferentially around the joint. The pipe welding apparatus may comprise an internal line-up clamp. Those skilled in the art will appreciate that a line-up clamp is arranged so as to be able, in use, to line-up and clamp two pipes to be welded to each other end-to-end.

According to yet another aspect of the invention there is provided a kit of parts comprising a deformable cushion for converting a back support pad into a back support pad as described herein.

There is also provided a method of welding two pipes together, the method including the following steps: arranging two pipe sections in coaxial alignment thereby defining a circumferential joint to be welded, providing at least one welding torch for welding, from outside the pipe, the circumferential joint, providing a deformable cushion inside the pipe, urging the cushion against the joint so that the cushion conforms to the local shape of the interior surface of the pipe sections, welding the pipes together with said at least one welding torc'h therhy fnrmna weld pool.

supporting the weld pool from the interior of the pipe with the deformable cushion. The cushion may thus be urged against the joint so that the shape of the cushion changes to correspond with local variations in the shape of the interior surface of the pipe sections.

The method may be performed when laying a pipeline, for example an underwater pipeline. The method may be performed when laying a deepsea pipeline, for example at depths of greater than l,000m. The pipeline may be a gas and/or oil pipeline. The method may include bevelling the ends of the pipe sections to be welded. One pipe section may be defined by the end section of a pipeline. The method may utilise a plurality or torches, which are operated simultaneously.

During performance of the method the temperature of the part of the cushion supporting the weld pool may reach a temperature greater than 250 degrees Celsius. The cushion may be actively cooled during performance of the method, for example by a cooling means as described herein.

It will be appreciated that features described herein with reference to the first aspect of the invention are equally applicable to other aspects of the invention. For example, the kit of parts may comprise a cushion, the cushion comprising a skin and a filler. back support pad according to any aspect of the invention may be used in performing the method according to the invention. The cushion as used in the method of the invention may thus incorporate any of the features described with reference to the back support pad of the invention.

Description of the Drawings

Various embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings of which: Figure la is a sectional view of a back support pad according to a first embodiment of the invention during use on a pipe joint weld; Figure lb is an enlarged sectional view of the back support pad shown in Figure la; Figure 2 is a sectional view of a back support pad according to a second embodiment of the invention during use on a pipe joint weld; Figure 3a is a sectional view of a copper shoe for supporting a back support pad according to a third embodiment of the invention; Figure 3b is a partial plan view of the copper shoe shown in Figure 3a, looking on arrow D; Figure 3c is a partial plan view of the copper shoe shown in Figure lnnking cm arrow E: Figure 3d is a sectional view of the part of the copper shoe shown in Figure 3b, taken along B-B; and Figure 3e is a sectional view of the part of the copper shoe shown in Figure 3c, taken along A-A.

Detailed Description

Referring to Figure la, a back support pad 1 according to a first embodiment of the invention is located, during use, on the interior surface of a pipe joint 3. The pipe joint is formed between the bevelled ends of the two 24" diameter pipes 7a and 7b.

The back support pad 1 is pressed against the interior of the pipe joint by radial thrust springs (not shown) and is part of a larger welding apparatus (not shown) comprising ten identical back support pads circumferentially arranged around the interior of the pipe. For the sake of clarity, only one back support pad 1 is shown in Figures la and lb and only one pad is described in detail below. The back support pad 1 is arranged to support the weld pool 5 created during the first weld pass of the welding torch 9 along the circumferential pipe joint 3.

According to the first embodiment of the invention, the back support pad 1 comprises an elastically deformable cushion 11 mounted on a copper base plate 13. The copper base plate 13 is shown in highly schematic form in Figure la and a less schematic representation is shown in Figure lb. The copper base plate and the radial thrust springs are similar to the copper pads and radial pistons described and illustrated in US 4,363,954. One significant difference however is that the copper pad 1 includes a recess 2 in which the cushion 11 is accommodated. Other than that the copper base plate and the radial thrust springs are substantially the same as those of US 4,363,954, and so are not described in further detail here.

The deformable cushion comprises an outer skin 15 enclosing a filler 17. The skin 15 is made of a thin flexible multi-layer tungsten mesh. The mesh has a weave that allows the mesh to be stretched uni-directionally (without permanent -10 -deformation) . The filler 17 is made up of bundles of copper wool (obtained from Palmer Engineered Products, Inc., Ohio).

The wool is sufficiently fine and tightly bundled that it exhibits resiliency; the cushion will deform under compressive load by means of the wool being compressed, but when the load is removed the wool will expand so that the cushion expands and decompresses. The skin is itself fixed to the base plate via copper fasteners (not shown) The flexibility of the skin 15 and copper wool 17 means that the cushion 11 is sufficiently deformable to conform to the back surface of the pipe joint 3. Thus, the cushion 11 maintains a relativaly large contact footprint on the interior surface of the pipe joint 3, regardless (within limits of course) of variations in the pipe diameter/joint imperfections and/or ovalisation of the pipe.

The cushion is trapezoidal in shape when viewed from above and has a width of 25mm (horizontally across the Figure la), a length (measured along its centre) of about 200mm (into the page of Figure la), and a thickness of llmmi (measured in the vertical direction in Figure la) . The pipe internal circumference of the pipe is just under 2m and so the ten cushions laid end to end extend round the entire internal circumference of the pipe. As can be seen in Figure lb, the cushion 11 in its decompressed state projects about 3mm above the base plate top surface 13a surrounding the recess 2. The thickness of the base plate (from bottom surface 13b to top surface 13a) is about 24mm and the recess 2 has a depth of about 8mm. The thickness of the cushion in its decompressed state is therefore at least 20% larger than the depth of the recess 2.

The skin construction is such that its electrical conductivity is low compared to the copper filler. The thickness of the tungsten wire that forms the mesh of the skin is about 0.04 mm and the thickness of the skin at its thickest is about 0.12mm. The skin has an electrical conductance less than that of a equivalent skin made from a sheet of tungsten having a thickness 0.12mm as a result of the mesh construction. Thus, the overall electrical conductivity of the cushion is less than that which would be provided by the copper wool alone, and may for example be less than about 106 Sm1 and possibly less than 0.05 x 106 Sm'. The electrical conductance of the cushion across its thickness in the radial direction may be less than about lOS. This reduces the risk of the welding arc current flowing into the pad. Thus, material from the cushion tends not to fuse and pollute the weld pool.

The thickness of the skin and its thermal conductivity are however such that the skin is able to thermally conduct relatively efficiently. Thus, whilst the electrical conductivity of the cushion, is not very high, the thermal -11 -conductivity is not unduly low. The materials used in the cushion are such that the cushion has a relatively high thermal conductivity. The overall thermal conductivity of the cushion (skin and filler together) is greater than about lOWrn K'. This, coupled with the large contact footprint, enables the back support pad 1 to effectively dissipate heat generated during welding.

During use the weld pool may reach temperatures of around 900 C or greater. The surface of the cushion next to the weld pool may therefore have a similarly high temperature, for example around 800 to 850 C, which is close to the melting temperature of coppcr (typically just over 1000 C) . The opposite side of the cushion may have a much lower temperature, for example, of the order of 300 C (which may require active cooling of the cushion) . The mean temperature of the pad, during operation, maytherefore reach temperatures in the range of 400 to 500 C, and possibly exceed them.

In addition, in consequence of the deforrnability of the cushion 11, areas of locally high temperature arid wear tend not to occur on the back support pad during use.

Figure 2 shows a sectional-view of a back support pad 101 according to a second embodiment of the invention during welding of a pipe joint 103 between pipes 107. The back support pad 101 is as described with reference to the first embodiment of the invention except for the differences described below.

The skin 115 has a mean thickness of about 0.2mm and is formed from copper netting tissue (obtained from Palmer Engineered Products, Inc., Ohio) and encloses a filler 117 of copper grains. The copper grains are sufficiently large to be enclosed within the netting 115, but sufficiently small to move relative to one another such that the cushion 111 is, as a whole, deformable. The grains include grains of different diameters to increase the packing efficiency and so as to reduce the air gaps in the cushion.

The skin 117 is itself welded to the base plate 113 along a circumferentially-extending (i.e. into the page in figure 2) side seam 119.

The electrical conductivity of the filler is greater than that of the copper wool embodiment as a result of the density of copper in the cushion being higher. The electrical conductivity of the skin is also higher as a result of the fact that it is made of copper. Whilst the skin is a relatively good electrical insulator, and may therefore not provide as good protection against pocking as the first embodiment, the second embodiment of the invention still provides advantages over the prior art arrangements described in US 4,363,954. The cushion, being made from copper, is an excellent thermal conductor and as a result of its flexible/deformable construction is able to provide good -12 -thermal contact between the base plate 113 and the pipe being welded. Even if there is significant pocking of the cushion during use, any resulting physical defects need not significantly adversely affect the cushion from being able to sustain good thermal contact during subsequent use, because any indents caused by the pocking may effectively be reduced or eliminated by means of locally deforming the cushion (as a result of the cushion's flexibility/deformability) According to the second embodiment of the invention, the welding apparatus comprises cooling means 121 for cooling the back support pad 101. The cooling means 121 comprises a channel 123 and chamber 125 arrangement located within the base plate 113 of each back support pad 101.

During use, coolant is pumped through the channel 123 and chambers 125 in the base plate 113 thereby removing heat generated during welding, from the back support pad 101 and the weld pool 105.

A third embodiment of the invention utilises a back support pad comprising a deformable cushion mounted on a copper base plate, the copper base plate being different in shape from those of the first and second embodiments. Figure 3a shows a side view of the copper base plate 213. The base plate 213 is trapezoidal in shape when viewed in plan. The ends of the base plate thus taper, as can be seen in Figures 3b and 3c, which show views along arrows D and E, respectively. Figures 3d arid 3e show cross-sectional views of the ends of the base plate taken along planes BB and A-A, respectively. It can be seen that the cross-sectional shape includes a 5 degree chamfer. The base plate does not, in this embodiment, include a recess for the cushion (not shown), and instead the cushion is attached directly to the top 213a of the base plate 213.

Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. For example, the cushion could comprise any material with appropriate thermal properties. The cushion may comprise a single, uniform structure such as a deformable block of conductive matrix.

The cushion may be torus-shaped for fitting the entire circumferential surface of the interior of the pipe joint.

Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described

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-13 -as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims.

Claims

-14 -Claims 1. A back support pad for supporting, from the back surface

of a pipe joint, a weld pool created during the welding of a pipe joint, wherein the back support pad comprises a deformable cushion for conforming to the back surface.
2. A back support pad according to claim 1 wherein the thermal conductivity of the cushion is more than 10Wm1K1.
3. A back support pad according to claim 1 or claim 2 wherein the thprm1 rnndiictivity of the cushion is greater than that of an equivalent cushion made from pure lead.
4. A back support pad according to any preceding claim wherein the overall electrical conductivity of the cushion is less than 90% of the overall electrical conductivity of an equivalent cushion made entirely from pure copper.
5. A back support pad according to any preceding claim, wherein the cushion comprises an outer skin and an interior, the skin being made of material different from that of the interior.
6. A back support pad according to claim 5, wherein the interior of the cushion is defined by filler, which is substantially wholly enclosed by the outer skin.
7. A back support pad according to claim 5 or claim 6, wherein the skin has an electrical conductivity of less than that of an equivalent skin made entirely from Aluminium.
8. A back support pad according to any of claims 5 to 7, wherein (a) the thermal conductivity of the skin divided by (b) the mean thickness of the skin when not compressed is greater than 50kw rn2K'.
9. A back support pad according to any preceding claim, wherein the interior of the cushion comprises metallic wool.
10. A back support pad according to any preceding claim wherein the interior of the cushion comprises metallic granules.
11. A back support pad according to any preceding claim, further comprising a solid base plate, wherein the cushion is mounted on the base plate.

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12. A back support pad according to any preceding claim further comprising cooling means for cooling the back support pad.
13. A back support pad according to claim 12, when dependent on claim 11, wherein the base plate comprises the cooling means.
14. A back support pad according to claim 12 or claim 13 wherein the cooling means comprises at least one channel through which, in use, coolant may be passed.
15. A pipe welding apparatus for use on a pipe joint, wherein the pipe welding apparatus comprises at least one back support pad according to any of claims 1 to 14, the back support pad being arranged for application to the back surface of the pipe joint.
16. A pipe welding apparatus for use on a pipe joint, wherein the pipe welding apparatus comprises a multiplicity of back support pads, each being in accordance with any of claims 1 to 14, the back support pads each being arranged for application to the back surface of the pipe joint.
17. A kit of parts comprising a deformable cushion for converting a back support pad into a back support pad according to any of claims 1 to 14.
18. A method of welding two pipes together, the method including the following steps: arranging two pipe sections in coaxial alignment thereby defining a circumferential joint to be welded, providing at Ieast one welding torch for welding, from outside the pipe, the circumferential joint, providing a deformable cushion inside the pipe, urging the cushion against the joint so that the cushion conforms to the local shape of the interior surface of the pipe sections, welding the pipes together with said at least one welding torch thereby forming a weld pool, supporting the weld pool from the interior of the pipe with the deformable cushion.