FIELD OF THE INVENTION

This invention pertains generally to the field of pipe cutting and pipe-coating trimming. It further relates to an apparatus and method for the accurate cutting and trimming of coatings on pipes.

BACKGROUND OF THE INVENTION

Pipes for pipelines, e.g. oil and gas pipelines, are typically manufactured with coatings for insulation and to prevent corrosion (especially if for use underwater). These coated pipes are manufactured in bulk and produced in standardised lengths. Typically, a pipeline for subsea oil and gas use will have multiple coating layers for protection and insulations, typically including an epoxy coating to enhance adhesion of subsequent layers to the steel pipe, an adhesive layer, a solid polypropylene layer, an insulating foam (e.g. polypropylene) layer and an outer polymer or composite shield layer. The coating process is a complex procedure.

For use in any particular pipeline, it is necessary to join the pipes and sometimes cut and join the pipes according to lengths required. It has been a particular problem during field jointing of pipes that cracking and delamination can occur due to weakened coating material close to the joint or poor bonding of the joint infill coating. This problem can be overcome by stripping back the outer coatings of the outer, layers and leaving the original coated resin bonding layer of each pipe to be joined such that new in-fill resin bonding material for the jointing area can adhere well to the existing bonding and infill coating material formed thereon. Thus the pipe-end to be jointed is typically stripped to the steel for a short distance along the pipe (so that new resin can bind to the pipe at the joint) and then stripped to the resin material for a short distance. This substantially improves the integrity of coating material infilled at the jointing area and reduces the risk of delamination. Accordingly, there is a need for a pipe-coating cutting device to remove the outer layers of the coatings whilst leaving a good pipe finish or leaving the resin bonding layer intact. Typically, this coating cutting step is carried out using a fixed cutting blade mounted adjacent a guide wheel, which runs against a rotating pipe, the blade positioned relative the wheel to give a desired depth of cut. However, any undulations in the surface of the pipe or if the pipe moves to cause it to be off-centre relative the cutting blade lead to a poor finish and partial removal of the resin bonding (and resulting risk of delamination of the coating at the joining once deployed).

PROBLEM TO BE SOLVED BY THE INVENTION

There is a need for improvements in apparatus and methods for controlled removal of coating materials from pipes to obtain a clean finish and or to leave a portion of coating material of prescribed depth on the pipe.

It is an object of the invention to provide a pipe-coating cutting apparatus that provides a consistent and accurate cut to allow a good finish and enable a chosen depth of coating material to remain on the pipe.

It is further an object of the invention to provide a pipe-coating cutting device that is capable of reliably cutting a pipe-coating to leave a good finish even if the pipe moves, e.g. off-centre.

It is a still further object of the invention to provide a pipe-coating cutting device that is capable of cutting a pipe-coating to consistently leave a desired depth of coating material on the pipe in its full circumference even when there are local undulations in the pipe surface.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, there is provided a self-centering cutting head for the removal of coating on a pipe, the cutting head comprising: a pivot mount defining a longitudinal axis; a self-centering element comprising two pipe-engaging means for abutting the surface of a pipe the coating of which is to be cut, said self-centering element being mounted for movement pivotally about the pivot mount; and a cutting element comprising a cutting blade mount for a cutting blade, the cutting element configured to be co-pivotal with the self-centering element about the said longitudinal axis.

In a second aspect of the invention, there is provided a method of removing a coating on a pipe to retain a pre-determined coating depth, the method comprising: providing a cutting head for coating removal, the cutting head having two pipe-engaging means forming a self-centering element mounted for movement pivotally about a pivot mount defining a longitudinal axis and having a cutting blade configured to be co-pivotal with the self-centering element about said longitudinal axis; arranging for a coated pipe having a coating to be cut to leave a pre-determined depth of coat to be placed in contact with the pipe- engaging means and to be rotated about its longitudinal axis relative the cutting blade such as to remove from a circumference of the pipe a coating to leave a desired depth; and moving the pipe in the direction of its longitudinal axis whereby the coating is removed to leave a desired depth about the circumference of the pipe along a desired length of the pipe.

ADVANTAGES OF THE INVENTION

The apparatus and method of the present invention enable the removal of the coating of a pipe to desired depth or to leave a desired depth of coating (e.g. a resin bonding undercoat on a pipe) in an accurate and reliable manner whereby variations in positioning of the pipe relative the cutter (e.g. off- centering) is naturally corrected. The coating removal exercise using this apparatus can thereby be conducted much more efficiently and quickly. By accurately and reliable removing only outer layers of a coating and leaving an inner layer, e.g. a resin bonding layer, improved joint infill integrity can be achieved resulting in reduced maintenance etc. The apparatus and method may be applied to pipes of varying sizes and adjusted to cut coatings to leave coatings of pre-determined but variable depths and is thus highly versatile. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates in perspective view a preferred embodiment of the apparatus of the invention;

Figure 2 illustrates a side aspect of the apparatus of the invention according to a preferred embodiment in use cutting the coating of a pipe;

Figure 3 illustrates a plan view of the apparatus as shown in Figure

2.

DETAILED DESCRIPTION OF THE INVENTION

The apparatus and method of the present invention are characterized by the provision of a self-centering element for engaging with a pipe surface and a blade or blade mount for a blade for removing layers of coating from the surface (or from above the surface) of the pipe, the self-centering element and blade or blade mount being configured to be co-pivotal about a longitudinal axis defined by a pivot mount. By co-pivotal about the pivot mount it is meant that both the self-centering element and blade or blade mount are configured to pivot about the pivot mount together but may not pivot about the pivot mount independently of one another.

Accordingly, when a blade is configured to cut from a rotating pipe a coating to a particular depth as defined by the relative positions of the pipe- engaging means and the blade on the self-centering cutting head, it is capable of . continuing to reliably cut to that depth even if the pipe moves off-centre since as the pipe moves from its originally defined position, the pipe engaging means will adjust by moving about the pivot such that the relative position of blade and pipe circumference and blade and coating depth will remain constant. Thus as a pipe and cutting head move relative to one another along the length of the pipe from which the coating is to be cut, the pre-set depth of cutting may remain constant and accurate.

As mentioned above, the apparatus of the invention comprises a self-centering element having two pipe-engaging means and a cutting element comprising a cutting blade mount for a cutting blade, the self-centering element and cutting element being co-pivotal about the longitudinal axis of a pivot mount.

The pipe-engaging means may take any suitable form, for example lubricated pads, in line or parallel wheel assemblies or rollers, wheels or bearings. Preferably, each pipe-engaging means comprises a roller, wheel or bearing preferably mounted on an axle that is parallel with the longitudinal axis of the pivot mount.

Each pipe-engaging means may be arranged in any configuration relative to the other provided that a pipe substantially parallel with the longitudinal axis of the pivot mount may be placed in engagement with both pipe-engaging means and the pipe-engaging means contact the pipe at different points about one or two circumferences of the pipe (i.e. they are circumferentially separated).

Preferably, the circumferential separation is sufficient to allow small variations in the position of the pipe to cause the self-centering element to adjust accordingly thereby maintaining the position and depth of the cutting blade against the pipe coating. The radii formed from the contact points of each of the pipe-engaging means against the pipe are preferably at an angle to one another (when viewed in the longitudinal direction of the pipe) of at least 20° and preferably no more than 120°, more preferably in the range 30° to 100°, still more preferably at least 45°, still more preferably at least 60° and most preferably about 90°.

Preferably, the pipe-engaging means are arranged such that they are substantially in a plane perpendicular to the longitudinal axis of the pivot mount. Thus, in use, they preferably contact two points of a pipe on a single

circumference of the pipe, which points define radii having relative angles of at least 20° and preferably no more than 120°, more preferably in the range 30° to 100°, still more preferably at least 45°, still more preferably at least 60° and most preferably about 90°.

The self-centering element preferably comprises a fork arm having thereon, on the distal end of each fork, the two pipe-engaging means, which are preferably bearings. More preferably the bearings are yoke-type bearings. Ideally, the pipe-engaging means are in a plane substantially perpendicular to the longitudinal axis defined by the pivot mount and are located substantially equidistant from the longitudinal axis of the pivot mount (or the pivot). Thereby, against any pipe engaged with the pipe-engaging means, the self-centering element provides a centering function.

The cutting element comprises a cutting blade mount or holder, capable of receiving a cutting blade and which is preferably provided with a cutting blade for cutting the coating on a pipe. It may be configured in any suitable arrangement that allows it to cut the coating of a pipe engaged against the pipe-engaging means to or leaving a desired depth of coating. For example, the cutting element may be so arranged that the cutting blade is in the same or a different plane to that defined by the two pipe-engaging means. Preferably, the cutting element is in a different plane parallel to that defined by the two-pipe- engaging means and longitudinally distanced therefrom (thus enabling the pipe- engaging means to abut a portion of the pipe that has had its coating removed whilst the cutting element cuts further coating from the pipe).

In a preferred embodiment where the pipe-engaging means are equidistant from the pivot mount and preferably substantially in the same plane perpendicular to the longitudinal axis of the pivot mount, the cutting element is configured such that the blade mount (and blade) is equidistant from each pipe- engaging means. Accordingly, the blade preferably engages the pipe-coating at a central (i.e. substantially perpendicular to the surface) position and is maintained in a central position by virtue of the self-centering effect of the self-centering means.

The cutting element, although configured to be in a pivotally fixed arrangement relative the self-centering element about the longitudinal axis of the pivot mount, is preferably moveable linearly in a direction substantially perpendicular to the longitudinal axis.

Optionally, the cutting element (or cutting blade mount) is moveable and adjustable between a plurality of fixed positions along a line substantially perpendicular to the longitudinal axis to correspond to differing depths of cut of pipe coating (or different curvature of pipes engaged with the pipe-engaging means).

In an alternative, and preferred, embodiment, the cutting element is moveable along a line substantially perpendicular to the longitudinal axis of the pivot mount and is configured to be resilient to movement (resiliently mounted) in the direction toward the pivot mount. In this embodiment, the cutting element is preferably provided with a cutting depth guide. Thus the resiliently mounted cutting element provides a cutting blade to be positioned in a rest position relative to the pipe-engagement means such that, in use, as a pipe is engaged against the pipe-engagement means it is met with the resilient pressure of the cutting element on its coating surface. The cutting depth guide should be configured to allow a depth of cut to be taken from the coating of the pipe or preferably a depth of coating to be left on the pipe. Any such configuration of the cutting depth guide may be adopted in order to achieve that. Preferably, the cutting depth guide is provided by a pipe-contacting depth guide adjustable to protrude distally beyond a mounted blade (relative the pivot mount). The pipe contacting depth guide may be, for example, a lubricated pad, an in-line or parallel wheel assembly or a roller, wheel or bearing, but is preferably a roller, wheel or bearing. The pipe-contacting depth guide, or preferably roller bearing, may be adjustable along a continual range or from a plurality of fixed positions to vary the extent to which it protrudes beyond the cutting blade toward the pipe (and thus the depth of coating on the pipe that will be left uncut by the blade). In use, the pipe-contacting depth guide or roller bearing will be fixed in position relative the cutting blade to ensure a consistent depth of cut about the pipe.

Preferably, according to this embodiment, the pipe-contacting depth guide or roller bearing is positioned substantially parallel with the blade and substantially in a longitudinal plane (i.e. in the plane of the longitudinal axis of the pivot mount) with the blade. Preferably, the pipe-contacting depth guide or roller bearing is as close as possible to the blade, but longitudinally separated by at least an amount corresponding to the desired length of coating to be left intact along the surface of the pipe to be cut. Preferably, the pipe-contacting depth guide or roller bearing is configured to contact the bear pipe in a region in which the coating is completely absent such that the depth of coating to be left by the cutter can be accurately and consistently determined.

A particular advantage of providing the cutting element as a resiliently mounted element having a pipe-contacting depth guide or roller bearing is that undulations or variations in the surface characteristics of the pipe about its circumference (e.g. dents) can be accounted for when cutting the coating and the depth of coating material remaining after cutting is accurate and consistent about the circumference and along the length of the cut pipe.

Preferably, the resilience is provided by a spring loading or hydraulic loading.

Optionally, the pipe-contacting depth guide or roller bearing is adjustable in a longitudinal direction whereby the longitudinal separation of the roller bearing and the cutting blade against the pipe may be increased or decreased according to the desired length of cut (along the pipe) at the desired depth.

The cutting blade mount (or holder) is capable of receiving a cutting blade for cutting the coating of a pipe. Preferably the cutting blade mount has a serrated internal surface capable of receiving a blade having a serrated surface whereby replacing a blade may be readily achieved to give an identical cutting depth as in operation with the previous blade. The cutting blade is typically held in place within the cutting blade mount by a clamping plate and/or two or more screws. The cutting blade itself is preferably provided with a straight cutting edge (running substantially parallel with the pipe and the longitudinal axis of the pivot mount) to provide an even cut of coating from the pipe. The cutting blade may be ground at its leading edge to provide a desired or required chamfer.

Preferably, the cutting head comprises a main block or main housing block, which is mounted for resilient pivotable movement about the pivot mount. The main block has mounted thereon the self-centering element and the cutting element in pivotally fixed arrangement. The cutting element may be adjustable, in and out (linearly) along a line perpendicular to the longitudinal axis of the pivot mount, relative to the main block or, in the embodiment in which the cutting element is resiliently mounted the cutting element may be resiliency mounted onto the main block. The cutting element may therefore comprise a cutting tool block resiliently mounted to the main block, e.g. by fitting into a slot or groove cut into the main block and provided with a resilient means such as a resilient foam material (air compression), hydraulic means (hydraulic load) or a spring. The cutting tool block may comprise the cutting blade mount and the pipe- contacting depth guide or roller bearing.

The pivot mount defines a longitudinal axis about which the self- centering element and the cutting element (and in a particular embodiment the main housing block) may pivot when the pivot mount is so arranged. The pivot mount may comprise any configuration that allows the cutting head to pivot or rotate about a longitudinal axis. Preferably, the pivot mount may comprise or be mounted to a pivot shaft. The pivot shaft may be fixedly mounted on a backplate which may then be affixed to any suitable structure or the pivot shaft may form part, or an adaptation, of an existing tool assembly (e.g. an existing pipe cutting assembly). Alternatively the pivot shaft may form a part of the pivot mount and be fixedly connected with the cutting head and itself mounted to a further external mount whereby the cutting head may rotate or pivot about the defined longitudinal axis which external mount may be formed on a backplate or form a part of an existing structure or tool assembly.

Preferably, the pivot mount arrangement is such that the cutting head of the invention is resiliently pivotal about the longitudinal axis and/or is provided with a stop to control the position and/or alignment of the cutting head in a rest position (thus, for example, maintaining its accessibility for engagement with a pipe to be cut). Preferably, the cutting head of the invention is resiliently pivotal about the longitudinal axis, said pivotal resilience being provided by any suitable pivotal resilience means.

In a preferred embodiment of the invention, the cutting head comprises a main housing block as defined above, which main housing block has a pivot shaft mounted on the rear. According to this preferred embodiment, the pivot shaft is mounted onto a back plate or other structure by two pillow block bearings which allow the housing and the self-centering element (including, e.g. the fork arm) to center itself onto a pipe which is engaged therewith. Preferably according to this embodiment pivotal resilience is provided by provided by any suitable pivotal resilience means, which may be, for example, a pair of springs attached to the main housing and to the backplate and passing either side of the pivot shaft. Preferably, the pivotal resilience means are loaded such as to balance the weight of the self-centering cutting head whereby the self-centering cutting head is held in a rest position substantially horizontal position and at least within about 30° of the horizontal whereby a pipe may be readily engaged with the pipe- engaging means.

Preferably the self-centering cutting head of the invention is provided with an air ram or other load-providing means, e.g. in association with the back plate or structure to which the self-centering cutting head is mounted, to push the self-centering element and preferably fork arm against the pipe thereby keeping the cutting blade centred on the pipe.

Preferably the cutting head is configured to allow the blade to work upside down with the pipe running in reverse. This stops the blade cutting into the pipe with the result of an excellent finish to the pipe.

According to a particularly preferred embodiment of the apparatus of the invention, the self-centering cutting head comprises pivotally resiliently mounted about a pivot axis via a pivot shaft a main housing block having mounted thereto a self-centering element comprising a fork arm having two bearings (preferably yoke type) for locating against the pipe of choice, and a cutting element comprising a cutting blade holder and roller (bearing) which is spring loaded in the main block to push against the pipe thereby allowing the head to cut back the outer coating to the desired depth. The fork arm is preferably configured to allow coating on pipes of 150 mm diameter and upwards to be cut. The cutting holder preferably has an orifice held by one screw in the main block to prevent the tool from dislodging. The cutter holder, as mentioned above, is preferably serrated and also takes a serrated blade allowing a replacing blade to be changed easily and will give the exact same cutting depth. The blades, preferably, are secured with two screws and a clamping plate and the blades may optionally be ground to suit the chamfer desired or required. The roller bearing (of the cutting element) is preferably configured so that it runs on the metal of the pipe and is fixed to the cutting blade holder. This roller bearing, as discussed above, is responsible for controlling the depth of the cut leaving, for example, an inner bonding coating of the pipe intact. The depth of the cut may be controlled by an adjusting screw also fixed to the cutting head. The roller bearing, being adjacent the cutting blade in use, can run over weld seems and any imperfections of the pipe with great ease and as the actual cutting blade will always follow the trajectory defined by the roller, will give an excellent result. Preferably, the main housing block holds the fork arm. The housing block preferably comprises a pivot mount having a pivot shaft which is mounted, e.g., via two pillow block bearings to a back plate which can be fixed to an existing or purpose built machine. This preferbaly has an air-ram to push the fork arm against the pipe thereby keeping the cutting blade centred on the pipe.

The cutting head according to the invention as described generally above and with reference to specific embodiments above and below provides particular advantages to the cutting of pipe coatings from pipes for use in the oil and gas industry, which pipe coatings typically include an undercoat or resin- bonding coat which is only a few millimeters thick and which it is beneficial to keep intact in order to enhance the jointing process in situ and the integrity of any infill coating at pipe joints. In particular, the cutting head of the invention enables the following advantages: precision cutting, broad range of pipe-sizes to be cut, choice of length of cut, self-centering cutting head, leaves bonding coat intact, reverse action, upside down blade option, excellent finish.

The cutting head of the invention may be manufactured from any suitable materials. The main housing block, blade mount, blade, fork arm and/or back plate may be manufactured from, for example, steel or stainless steel.

The cutting head of the invention and of particular embodiments described above may be sized as appropriate according to the range of pipe sizes it is intended to work, which is preferably at least 150 mm diameter pipes. In one preferred embodiment, the apparatus is configured such that the pipe-contacting means (e.g. roller bearings), which are preferably in a plane perpendicular to the longitudinal axis defined by the pivot and pivot mount are separated by at least 5 cm, preferably up to 1 m, more preferably in the range 7 to 50 cm, still more preferably in the range 10 to 20 cm (e.g. from 10 to 15 cm). A self-centering element having pipe-contacting means in the range 10 to 15 cm would be particularly useful in cutting coatings off pipes of a diameter of for example from 150 mm to 500 mm.

The apparatus is preferably configured for the cutting blade to be positioned relatively close to the pipe-contacting means. Typically, the shortest distance between the cutting blade and that circumference of the pipe contacted by the pipe-contacting means would be the length of the cut-back required (i.e. the length along a pipe of base coating or resin bonding to be exposed from beneath the outer coatings), e.g. at least 30 mm, preferably at least 50 mm and typically no more than 200 mm, preferably no more than 100 mm. In the preferred

embodiment in which the cutting element is provided with a pipe-contacting depth guide (e.g. a roller bearing), the pipe-contacting depth guide may be as close as possible to the pipe-contacting means (especially since both should typically be contacting the leading exposed part of the pipe), e.g. up to 25 mm. The pipe- contacting depth guide should preferably be separated from the lagging edge of the blade by at least the length of the cut-back required (i.e. the length along a pipe of base coating or resin bonding to be exposed from beneath the outer coatings), e.g. at least 30 mm, preferably at least 50 mm and typically no more than 200 mm, preferably no more than 100 mm and most preferably in the range 40-60 mm (allowing for the most desirable length of cutback whilst maintaining the accuracy in depth achieved by close location of the depth guide to the blade).

The pipe-contacting depth guide may be configured to protrude beyond the blade by an amount corresponding to the depth of material (or base layer, e.g. resin bonding material) to be left in the cut-back. It is imperative that this base layer is not cut through to the pipe, hence the benefit of the accuracy of cut achieved by the present invention. The depth guide may therefore be set to protrude by any suitable amount required for the base layer, e.g. from 0.25 mm to 10 mm for example, preferably for the provision of a base layer of resin bonding material for pipes of a size mentioned above by an amount of from 0.4 mm to 1.0 mm and most preferably about 0.5 mm. The arrangement of the present invention enables accurate and consistent cuts to leave an intact under coat about 0.5 mm thick, which is a particular advantage of embodiments of the present invention.

The device may be adjustable to vary the dimensions above or may be designed to the required size within and outside the aforementioned dimensions (e.g. a larger pipe and larger cut-back).

In one embodiment the cutting head of the invention may be mounted on a tool assembly having a further cutting tool for removing hard outer coatings of a pipe and/or removing rapidly with less accuracy significant quantities of the coating whereby the fine pipe-coating cutting requirement can be met by the self-centering cutting head of the present invention.

As stated above, according to another aspect, the invention provides a method of removing a coating on a pipe to retain a pre-determined coating depth, the method comprising: providing a cutting head for coating removal, the cutting head having two pipe-engaging means forming a self- centering element mounted for movement pivotally about a pivot mount defining a longitudinal axis and having a cutting blade configured to be co-pivotal with the self-centering element about said longitudinal axis; arranging for a coated pipe having a coating to be cut to leave a pre-determined depth of coat to be placed in contact with the pipe-engaging means and to be rotated about its longitudinal axis relative the cutting blade such as to remove from a circumference of the pipe a coating to leave a desired depth; and moving the pipe in the direction of its longitudinal axis whereby the coating is removed to leave a desired depth about the circumference of the pipe along a desired length of the pipe.

Preferably, the cutting head used according to the method is further defined as described above. The invention will now be described in more detail, without limitation, with reference to the accompanying Figures.

In Figure 1 , a self-centering cutting head 1 comprises a main housing block 3 comprising a pivot mount having a pivot shaft 5 defining a longitudinal axis 7 about which the main block 3 may pivot. The pivot shaft 5 is mounted on backplate 9 by two pillow block bearings 1 1. The pivot-mounted block 3 is resiliently pivotable about the longitudinal axis 7 by virtue of springs 13 as resilient means, which serve the dual purpose of being resilient to pivoting of the block 5 about the longitudinal axis and keeping the block 3 and tools formed thereon horizontal and ready for engagement with a pipe for which a coating is to be cut (not shown). Main block 3 is provided with a self-centering element consisting of a fork arm 15 having formed at the distal ends of the forks two roller bearings 17 (preferably yoke type bearings) as pipe-engaging means. Adjacent the self-centering element is cutting element 19 lying in a plane perpendicular to the longitudinal axis and parallel to that of the roller bearings 17. The cutting element 19 comprises a cutting blade mount 21, cutting blade 23 and pipe-contacting depth guide 25, which is in this case a roller bearing. The cutting element 19 is resiliently mounted on the main block 3 by insertion into a mounting slot 27 provided with, as resilient means, spring 29.

Roller bearings 17 are configured to engage with a pipe (not shown) to have its coating removed, preferably engaging with a metal portion having no coating adjacent the coating edge requiring removal. Two roller bearings 17 engage with the pipe. Should the pipe or the machine holding the pipe move off-centre slightly, the provision of two engaging means against the pipe will cause the main housing 3 to pivot about longitudinal axis 7 and similarly the cutting element 19 and its blade 23 which is fixedly mounted to the main block 3 in respect of pivotal movement and disposed equidistant to each roller bearing 17 will be caused to pivot to the same extend and thus remain centred on the pipe being cut (or at least at the same relative position to the centre of the pipe). The configured depth of cut being achieved will therefore be more consistent irrespective of slight movements or bends in the pipe. The pipe-contacting depth guide, roller bearing 25, is configured, typically, to roll on the surface of the pipe (e.g. a metal surface having not coating material thereon), but optionally on an internal coating. It is configured to define the depth of material to be left (i.e. not removed) by the blade 23 on the pipe by protruding that distance beyond the blade 23 from the main block 3. The pipe- contacting depth guide 25 is typically positioned between the cutting blade mount 21 and the fork arm 15 such that both it and the roller bearings 17 are contacting the surface of the pipe. The depth of material to remain may be adjusted by adjusting the protrusion of depth guide 25 using adjusting screw 31. The longitudinal separation of the depth guide 25 and cutting blade 23 may be determined by the length of coating material (e.g. resin-bonding material) to be left along the pipe. The blade 23 may be ground to provide a chamfer as required.

In Figure 2, a side view of the self-centering cutting head 1 looks along the longitudinal axis 7 in the direction of an approaching pipe. A sector of the pipe 33 to be cut is shown with the base layer 35 to be retained and the overlayer 37 to be removed. The pipe-contacting roller bearings 17 and pipe- contacting depth guide 25 run ahead of the coating to be cut on the bare pipe 33 as it rotates (in a clockwise direction as viewed). Cutting blade 23 acts in concert with the pipe-contacting depth guide on the adjacent coated pipe to remove the over coating 37 of the desired depth and leave the base layer 35 (e.g. of resin bonding material). Any undulations in the pipe 33 felt by the pipe-contacting depth guide 25 will be felt and accounted for also by the blade 23 and a constant depth of materia] (base layer 35) on the pipe will be retained.

In Figure 3, which is a plan view of the self-centering cutting head 1 of Figure 2, it is shown that the pipe-contacting means 17, one of two roller bearings, of the self-centering element and also the pipe-contacting depth guide 25 of the cutting element run along the surface of the rotating pipe 33 at an uncoated portion typically. Self-centering of the cutting blade 23, which is in a pivotally fixed position (about the longitudinal axis 7) relative the self-centering element, is achieved by the pivot of the cutting head 1 including the self-centering element and the cutting element about the longitudinal axis 7 when the pipe moves off- centre (or from its original position) due to contact of the pipe-contacting means 17 at two positions about the pipe's 33 circumference. This is ensured by applying pressure between the pipe 33 and the pipe-contacting means 17, e.g. by an air ram (not shown) associated with the cutting head backplate. Cutting element 19 is resiliently mounted on the main housing block 3 such that the resilient means 29 (not shown) is in a compressed state when the pipe is in contact with the pipe-contacting means 17, whereby the pipe-contacting depth guide 25 is forced to contact the surface of the pipe and thus (by virtue of the degree by which the depth guide 25 protrudes beyond blade 23) the depth of coating remaining is controlled.

The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention.