Patent Publication Number: US-2023139461-A1

Title: Pipe coating removal apparatus

Description:
FIELD 
     The present invention relates to the preparation of pipe coatings for the subsequent application of joint coatings, for coating joints between pipe sections of pipelines. The invention has particular utility for oil or gas pipeline field joints. 
     BACKGROUND 
     Pipelines in the oil and gas industry are typically formed from multiple lengths of steel pipe sections that are welded together end-to-end as they are being laid. To prevent corrosion of the steel pipe sections and to reduce heat loss of fluids transported by the pipelines, the pipe sections are coated with one or more protective and/or insulative layers, typically a multi-layer coating comprising, for example, an epoxy bottom layer (especially fusion-bonded epoxy, FBE) followed by one or more polyethylene (e.g. polypropylene and/or modified polypropylene) outer layer(s). The pipe sections are usually coated at a factory remote from the location in which they are to be laid. This is often referred to as factory-applied coating and it is generally more cost effective than coating pipe sections on site where they are laid. (“On site”, generally known as “in the field”, may be on land, or at sea from a pipe-laying ship.) At the factory, the coating is applied to the outside of the pipe sections and a short length (known as a “cutback” region) is left uncoated at each end of the pipe section. The uncoated ends are necessary to enable the steel pipe sections to be welded together to form the pipeline in the field. The welded uncoated ends, known as field joints, must be coated in order to provide the necessary protection and/or insulation, and such coating is known as the field joint coating. 
     To prepare a field joint region of the pipeline for the field joint coating, each end region of the factory coating is typically machined by hand using power tools to remove a thin top layer, so that any dirt and grease is removed, and so that there will be good adhesion between the factory coating material and the field joint coating material. Additionally, unless already present, chamfers or bevels are hand machined into the polyethylene layer(s) to provide a gradual decrease in thickness of the factory coating material in a direction towards the uncoated lengths of pipe sections. Furthermore, a short length, known as a “toe”, of the FBE material extending from each end portion of the polyethylene material is typically abraded to clean the external surface of the FBE material and to ensure good adhesion to the field joint coating material. 
     Field joints are commonly coated by means of an injection-moulded polypropylene (“IMPP”) coating process. The exposed steel pipe section ends are heated, e.g. by induction heating. A layer of powdered fusion-bonded epoxy primer is then typically applied to the heated pipe section ends, and a thin layer of polypropylene is typically applied to the FBE primer during the FBE curing time. The end regions of the factory-applied coating, including the chamfers, are then heated, e.g. by means of radiant (infrared) heating. The field joint is then completely enclosed in a heavy-duty mould that defines a cavity around the welded pipe joint, the uncoated ends of the pipe sections, and the end regions of the factory-applied coating. Heated molten polypropylene (or modified polypropylene) is then injected into the mould cavity, to fill the cavity, and is allowed to cool. Once the injection-moulded polypropylene has cooled and solidified, the mould is removed from the field joint, leaving the solidified polypropylene field joint coating in place. Other types of field joint coating processes are also known. For example, the injection-moulded polyurethane (IMPU) coating process uses a chemically curable urethane material instead of injecting polypropylene as the mould-infill material around the field joint. 
     Australian Patent No. AU 2012204047 B2 discloses an underwater pipeline coating removal apparatus for enabling a section of an underwater pipeline to be repaired or replaced. The apparatus comprises a rotating cutting tool for removal of an outer reinforced-concrete layer of the underwater pipe, a rotating milling tool for removal of polymer coating from the pipe, and a rotating brushing tool for removing a residual polymer coating layer. All three tools are arranged to move radially towards and away from the pipe, and each of them has an axis of rotation that is parallel to the longitudinal axis of the pipe. 
     German Patent No. DE 102004031756 B4 discloses an apparatus for applying a bevel or chamfer to the polymer coating of a pipe in preparation for a field joint coating. In use, the apparatus is fixed to the exterior surface of a pipe by means of a link chain and a tensioning helical spring, and the apparatus also includes wheels which allow rotation of the apparatus around the pipe circumference. The apparatus may be guided around the pipe by hand or by means of a drive arrangement. The apparatus includes a drive with a working head to produce the chamfer, and the drive is supported by a holding means which enables its height and degree of inclination relative to the pipe to be adjusted, so that the desired inclination angle of the chamfer, within a range of approximately 15 to 25 degrees to the pipe axis, can be produced. 
     There is a need to improve the process of preparing a pipeline field joint for the field joint coating. 
     SUMMARY 
     In a first aspect, the present invention provides a pipe coating material removal apparatus according to claim  1  of the appended claims. 
     A second aspect of the invention provides a method of preparing a pipe coating in readiness for receiving a field-applied coating, according to claim  20  of the appended claims. 
     Preferred, and other optional, features of the invention are defined and described in the dependent claims. 
     Accordingly, a first aspect of the invention provides a pipe coating material removal apparatus, comprising: a support frame; a subframe supported by the support frame and configured to rotate relative to the support frame at least partially around a subframe rotation axis, the subframe rotation axis configured to be substantially coaxial with a longitudinal axis of a pipe to which the apparatus may be applied in use; and one or more coating material removal members rotatably mounted to the subframe to remove part of an exterior coating of a said pipe; wherein the apparatus is configured such that the one or more coating material removal members enable the removal of pipe coating material at orientations substantially parallel to, and inclined with respect to, the longitudinal axis of the pipe. 
     A second aspect of the invention provides a method of preparing a pipe coating in readiness for receiving a field-applied coating, comprising removing pipe coating material using an apparatus according to the first aspect of the invention. 
     It is to be understood that any feature, including any preferred feature, of any aspect of the invention may be a feature, including a preferred feature, of any other aspect of the invention. 
     The pipe preferably comprises a joint region, e.g. a field joint, between two pipe sections of a pipeline. 
     Preferably, the removal of pipe coating material at an orientation inclined with respect to the longitudinal axis of the pipe provides part of the pipe coating with a chamfered or bevelled external surface, or provides a clean new external surface of a pre-existing chamfered or bevelled section of the pipe coating material. 
     The one or more coating material removal members preferably is adjustable to enable the removal of pipe coating material at an orientation substantially parallel to, and inclined with respect to, the longitudinal axis of the pipe. 
     Advantageously, the one or more coating material removal members may be adjustable to enable the orientation of pipe coating material removal to be varied throughout a range or series of orientation angles and/or to enable a depth of pipe coating material removal to be varied. The range or series of orientation angles preferably has a lower limit of no more than 0 degrees and preferably has an upper limit of at least 20 degrees, more preferably at least 30 degrees, with respect to the longitudinal axis of the pipe. 
     Preferably, the one or more rotatable coating material removal members each comprise a rotatable milling cutter (e.g. a generally or substantially cylindrical milling cutter) or a rotatable grinding member or abrasive member. The, or each, grinding member or abrasive member preferably comprises a grinding wheel, or a flap wheel, e.g. a ceramic flap wheel, or a wire brush wheel. The, or each, coating material removal member preferably is rotatable by means of a motor, e.g. a pneumatic motor. 
     The pipe coating removal apparatus may further comprise a feed plate located on one side, or on each of two opposite sides, of the, or each, rotatable coating material removal member, preferably arranged to control or determine a depth of coating removed from the pipe by the coating material removal member in use. In some preferred embodiments, the, or each, feed plate is movably, preferably rotatably, mounted with respect to the rotatable coating material removal member, to accommodate misalignments between the coating material removal member and the pipe, in use. 
     An angle of orientation of a rotation axis of one or more coating material removal members (especially the, or each, milling cutter, where present) with respect to the subframe rotation axis preferably is adjustable to enable the orientation of pipe coating material removal to be varied. The angle of orientation of the rotation axis of the, or each, coating material removal member preferably is adjustable by means of an air-hydro cylinder, for example. 
     The depth of coating material removal by one or more coating material removal members (especially the, or each, milling cutter, where present) may advantageously be adjustable by means of a servo motor or actuator, for example. Preferably, the depth of coating material removal is settable with the aid of a mechanical contact member, preferably comprising one or more feed plates and/or one or more ball transfer units, arranged to contact an exterior surface of the pipe coating, in use. 
     The pipe coating material removal apparatus of the invention preferably further comprises one or more distance measuring sensors configured to enable the apparatus to control an operating position of one or more said coating material removal members relative to the external surface of the pipe or pipe coating in use. The, or each, distance measuring sensor may advantageously comprise, for example, at least one of: an inductive sensor; an eddy current sensor; an optical sensor; a laser sensor; a mechanical sensor; an ultrasonic sensor; and a capacitive sensor. 
     The pipe coating material removal apparatus of the invention preferably further comprises a longitudinally movable member, preferably a plate member, movably mounted on the subframe, and to which the, or each, coating material removal member is mounted for longitudinal positioning with respect to a said pipe. The movable member (e.g. plate member) preferably is longitudinally movable (e.g. slidable) with respect to the subframe by means of a linear servo motor or actuator mounted on the subframe. 
     The pipe coating material removal apparatus of the invention preferably includes a plurality of coating material removal members mounted on the subframe in one or more coating material removal subassemblies. Preferably, there may be a plurality of coating material removal subassemblies rotatably mounted to the subframe in one or more opposing pairs of coating material removal subassemblies, for example. Advantageously, the, or each, coating material removal subassembly may be movable relative to the subframe by means of a pneumatic cylinder, for example. 
     In some preferred embodiments of the invention, the pipe coating removal apparatus includes one or more laser sensors, preferably one or more 2-dimensional profile laser sensors, configured to locate profile features of the pipe coating. Preferably, the, or each, laser sensor is configured to locate profile features of the pipe coating, which profile feature locations are used to axially position the coating material removal members with respect to the pipe coating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: 
         FIG.  1    schematically shows a pipe coating material removal apparatus of the invention in situ and working on a section of a pipeline; 
         FIG.  2    shows a view of the apparatus of  FIG.  1    with the outer support frame removed, for clarity; 
         FIG.  3    shows another view of the apparatus of  FIG.  1    with the outer support frame removed, for clarity; 
         FIGS.  4  to  7   ( c ) show various details of the apparatus of  FIG.  1   , from different perspectives; 
         FIGS.  8  to  10    show various details of the apparatus as shown in  FIGS.  2  and  3   , from different perspectives; and 
         FIGS.  11 ( a ) to  15 ( b )  show various details of other embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG.  1   , the illustrated exemplary embodiment of the invention comprises a pipe coating material removal apparatus  1  in situ and working on a pipe section  2  of a pipeline. The pipe coating material removal apparatus  1  comprises a support frame  3  and a subframe  5  supported by the support frame and configured to rotate relative to the support frame at least partially around a subframe rotation axis A. The subframe rotation axis A is configured to be substantially coaxial with the longitudinal axis of the pipe section  2  to which the apparatus  1  is applied, in use. 
     The illustrated support frame  3  comprises an outer steel frame of the apparatus  1 , which supports the rest of the apparatus, including the subframe  5 . The support frame  3  and the subframe  5  include wide slot-like openings  7  and  9  respectively, in respective opposite end wall parts  11  and  13  of each of the support frame  3  and the subframe  5 , to allow the pipe section  2  to extend through the apparatus  1  with the longitudinal axis of the pipe section substantially coaxial with the subframe rotation axis A. The subframe  5  is mounted on the support frame  3  by means of a gear and drive system, comprising a large partial ring gear  15  (see  FIG.  2   ) mounted on each end wall part  13  of the subframe  5 , guided and supported by guide wheels  17  mounted on each end wall part  11  of the support frame. In use, the subframe  5  is rotated with respect to the support frame  3  by means of an electric motor (e.g. a 3-phase motor) mounted on the support frame  3 , which drives at least one of the partial ring gears  15  by means of a chain or belt. 
     As shown in  FIG.  1   , the pipe section  2  comprises a field joint being prepared by the apparatus  1  for a subsequent field joint coating to be applied. The “cutback” regions (end portions containing no coating) of two pipe lengths have previously been welded together to form a weld join  4 . As described above, to prepare the field joint region of the pipe section for the field joint coating, each end region of the factory coating  6  needs to be machined to remove a thin top layer of the polyethylene material (indicated by reference numeral  8 ), so that any dirt and grease is removed, and so that there will be good adhesion between the factory coating material and the field joint coating material. Additionally, chamfers or bevels  10  need to be machined into the polyethylene layer(s) of the factory coating to provide a gradual decrease in thickness of the factory coating material in a direction towards the uncoated lengths of pipe, or pre-existing such chamfers or bevels need to have a top layer of coating material removed from them, to provide a clean new exterior surface. Furthermore, a short length, known as a “toe”, of fusion-bonded epoxy (FBE) material  12  extending from each end portion of the polyethylene material typically needs to be abraded to clean the external surface of the FBE material and to ensure good adhesion to the field joint coating material. The apparatus  1  of the invention is constructed and configured to carry out these preparation steps, preferably automatically, following setup of the apparatus  1  around the field joint. 
       FIGS.  2  and  3    show views of the apparatus of  FIG.  1    with the outer support frame  3  removed, for clarity. In these views, the subframe  5  can clearly be seen; it comprises a pair of opposing end wall parts  13  interconnected by a plurality of elongate struts  19  and an elongate slotted substructure  21 . The substructure  21  is connected to each end wall part  13  at a position diametrically opposite the wide slot-like opening  9  which receives the pipe section  2  in use. The substructure  21  supports a plate member  23  which is movable (e.g. by sliding) longitudinally along the radial interior face of the substructure parallel to the subframe rotation axis A, by means of a linear servo motor  25  located on a radial exterior of the substructure  21 . The plate member  23  includes a spaced-apart pair of short beams  27  which extend across the radial exterior of the substructure  21 . The plate member  23 , including the beams  27 , supports a plurality of coating material removal subassemblies  29 , four of which are included in the illustrated embodiment (but more, or fewer, could instead be included). As also shown in  FIGS.  4  and  5   , each of the coating material removal subassemblies  29  is rotatably mounted to the plate member  23  by hinges  31 , in a generally “gullwing” arrangement of opposing pairs of substantially mirror-image subassemblies  29 , including gullwing arms  33 . Rotation of each coating material removal subassembly  29  about its respective hinge  31  is caused by means of a pneumatic cylinder  35  extending between each subassembly  29  and its respective beam  27 . Pneumatic cylinders  35  are preferred for this purpose because they may advantageously allow a small amount of radial movement of the coating material removal subassemblies  29  as the subframe  5  is rotated at least partially around the pipe section  2 , to allow for any small non-uniformities or misalignments. 
     In use, an opposing pair of coating material removal subassemblies  29  may be moved to the correct longitudinal position along the pipe section  2  to carry out the particular required coating material removal operation at that longitudinal position, by longitudinal movement (e.g. sliding) of the plate member  23  caused by control of the linear servo motor  25 , for example by computer (e.g. utilizing an optical or laser sensor for longitudinal positioning) and/or human operator control via a control panel (not shown). Then, each coating material removal subassembly  29  of the longitudinally positioned opposing pair may be rotated towards the pipe section  2  about its hinge  31  on its gullwing arm  33  by means of its respective pneumatic cylinder  35 , again by computer and/or human operator control, for example. For speed of operation, and also for balance and close positional control, both coating material removal subassemblies  29  of an opposing pair preferably are rotated into position to operate substantially at the same time at different circumferential locations on the pipe section  2 , preferably substantially diametrically opposite locations. 
     In use, the subframe  5  is rotated relative to the support frame  3 , at least partially around the pipe section  2 , by the electric motor and the chain or belt, e.g. by computer and/or human operator control, so that each of the currently operating coating material removal subassemblies  29  removes the required coating from a respective circumferential region of the pipe section  2 . Once the particular coating material removal operation required at that longitudinal position around the entire circumference of the pipe section  2  has been completed, the coating material removal subassemblies  29  may be rotated away from the pipe section  2  about their hinge  31  by their pneumatic cylinders  35 , and any required further coating removal operations at different locations on the pipe section  2  may be carried out in a similar way. 
     As shown in  FIGS.  5  to  10   , each coating material removal subassembly  29  comprises a mounting frame  37  mounted on a respective parallel pair of gullwing arms  33  (i.e. gullwing arms extending from the hinge  31  in the same direction, in parallel), and rotatably mounted on each mounting frame  37  is a subassembly frame  39 . Each subassembly frame  39  supports: (i) a coating removal member in the form of a milling cutter  41  (preferably a generally or substantially cylindrical milling cutter); (ii) a pneumatic motor  43  arranged to rotate the milling cutter  41  by means of a belt drive  45  (see  FIG.  10   ); (iii) a coating removal member in the form of a rotational grinding or abrasive member  47 , e.g. a grinding wheel or an abrasive flap wheel (especially a ceramic flap wheel), driven by a small pneumatic motor  75 ; (iv) a solenoid actuator  77  (the function of which is explained below); and (v) a distance measuring sensor  49 , e.g. an inductive sensor or eddy current sensor. (One or more other types of distance measuring sensor may additionally or alternatively be used, e.g. an optical sensor, a mechanical sensor (i.e. that senses distances by mechanical contact), an ultrasonic sensor, a capacitive sensor.) 
     Each mounting frame  37  also supports a mechanical contact member  51 , e.g. in the form of one or more ball transfer units, arranged to contact the exterior surface of the factory-applied pipe coating  6  to limit the radially-inward travel of the gullwing arms  33  and to assist in setting the depth of coating material removal by each milling cutter  41 . For parallel coating material removal (i.e. parallel to the pipe axis A) the cutting edge(s) of each milling cutter  41  are typically set at a position (equating to a coating material removal depth) approximately 0.5 mm radially inward of the radially inwardmost part(s) of the mechanical contact member  51 . Preferably, as little coating material as possible is removed from the surface of the pipe section  2 . The depth of coating material removal, by each milling cutter  41 , is adjustable by means of a linear servo motor or actuator  53 , preferably a high precision linear servo motor, supported by each mounting frame  37 , which is configured to adjust the position of the mechanical contact member  51  relative to the milling cutter  41 . This is achieved by each mechanical contact member  51  being mounted on a respective pivot arm  55  (in the form of a “quadrant arm” in the illustrated exemplary embodiment) which is pivotably mounted via a pivot  57  to its respective mounting frame  37 , and a movable actuator arm  59  of the linear servo motor  53  being connected to the pivot arm  55  at a position spaced from the pivot  57 . Each linear servo motor  53  is controllable by computer and/or human operator control, for example. 
     In the illustrated embodiment, each mechanical contact member  51  comprises a plurality of ball transfer units which can be kept automatically in contact with the exterior surface of the factory-applied pipe coating  6  by being mounted on a pivoting support  61  which itself is pivotally mounted to the respective pivot arm  55 . The pivoting movement of each pivot arm  55  is guided by means of guide wheels  63  rotationally mounted on the respective mounting frame  37 , and the pivoting movement of each pivoting support  61  is guided by means of projections  65  on the pivot arm  55  movably located in part-circular slots  67  on the pivoting support  61 . (Other mechanical arrangements may additionally or alternatively be used, as will be understood by the skilled person.) 
     Each subassembly frame  39  is rotatably mounted on its mounting frame  37  by means of hinges  69 , to enable the angle of orientation of the subassembly frame, with respect to the longitudinal axis of the pipe section  2 , to be varied, thereby enabling the angle of orientation of each coating removal member  41 ,  47 , with respect to the longitudinal axis of the pipe section  2 , to be varied. The angle of orientation of each subassembly frame  39  is adjustable by means of an air-hydro cylinder  71  which extends between an attachment pivot  72  on each mounting frame  37  and an attachment pivot  74  on each respective subassembly frame  39 . Air-hydro cylinders  71  are preferred for this purpose because of their rigidity once the orientation has been set. The operation of each air-hydro cylinder  71  is controllable by computer and/or human operator control, for example. The angle of orientation of each milling cutter  41  determines the chamfer or bevel angle of each chamfer or bevel  10  which is formed in the factory-applied coating  6  by the apparatus  1 , this preferably being in the range of 20 to 35 degrees, e.g. substantially 30 degrees, for example. However, the operating orientation of each milling cutter  41  preferably is continuously variable through an entire range of angles elative to the subframe rotation axis A (which is substantially coaxial with the longitudinal axis of the pipe section, during operation of the apparatus  1 ). The range of angles preferably has a lower limit of no more than 0 degrees (i.e. at least parallel to axis A, and possibly including “negative” inclined angles). An upper limit of the range of angles may be at least 20 degrees, and preferably at least 30 degrees, for example. The ability to operate the milling cutters  41  at substantially any angle within a range of angles enables different chamfer/bevel angles to be used (e.g. depending on specific requirements), and may also enable the provision of a smooth (rather than stepped) transition between milled and nonmilled coating regions, for example. 
     As mentioned above, and as best shown in  FIGS.  7 ( a )  to  10 , each subassembly frame  39  also supports another coating removal member in the form of a rotational grinding or abrasive member  47 , e.g. a grinding wheel or an abrasive flap wheel. Additionally, each subassembly frame  39  supports a distance measuring sensor  49 , e.g. an inductive sensor or eddy current sensor (and/or one or more other types of sensor, as indicated above). In the illustrated exemplary embodiment, both the rotational abrasive member  47  and the distance measuring sensor  49  are mounted on a mounting plate  73  such that their rotational and longitudinal axes (respectively) are inclined with respect to the rotational axis of the milling cutter  41 . In the illustrated embodiment, the rotational axis of the rotational abrasive member  47  is inclined at approximately 30 degrees to the rotational axis of the milling cutter  41 , and the longitudinal axis of the (elongate) eddy current sensor (or other distance measuring sensor  49 ) is substantially perpendicular the rotational axis of the rotational abrasive member  47  and therefore is inclined at approximately 60 degrees to the rotational axis of the milling cutter  41 . This inclined mounting of the rotational abrasive member  47  and the distance measuring sensor  49  helps to ensure that the milling cutter  41  is spaced from the pipe section  2  during their operation. 
     The rotational abrasive member  47  is powered by a pneumatic motor  75 , and preferably is brought into operation to abrade and clean the external surface of the “toe” of fusion-bonded epoxy (FBE) material  12  and to ensure good adhesion to the subsequently-applied field joint coating material, preferably after the chamfers or bevels  10  have been formed in the polyethylene layer  8  of the factory coating  6 . The correct radial positioning of the rotational abrasive member  47  relative to the exterior surface of the steel pipe section  2  is controlled by means of the distance measuring sensor  49  and a computer controller (not shown). Advantageously, the sensor-computer system may determine the distance between the sensor  49  and the pipe surface, and make any necessary distance corrections, several times per second during operation. In some preferred implementations of the invention, the depth of coating material removal by the milling cutter  41  is also controlled with reference to distance measurements made by the distance measuring sensor  49 , e.g. using proportionality parameters to set the correct material removal depth(s). 
     When not in operation, the rotational abrasive member  47  is held in an extended “parked” position as shown in  FIG.  7 ( a )  in which the abrasive member  47  is tangentially spaced apart from the exterior surface of the pipe section. In order to operate the abrasive member  47 , it is moved into a retracted operational position along the pipe surface tangent so as to be radially closer thereto, as shown in  FIG.  7 ( b ) . In the retracted operational position the abrasive member  47  is able to contact the exterior surface of the pipe section  2  under the distance control of the distance measuring sensor  49  and computer control system. The rotational abrasive member  47  is moved between its extended and retracted positions by means of a solenoid  77 , for example. The operation of the rotational abrasive member  47  preferably is under substantially automatic computer control. 
     As described above, when the pipe coating removal operation(s) required at a particular longitudinal position of the pipe section  2  has/have been completed, the plate member  23  is moved longitudinally by the linear servo motor  25  with respect to the substructure  21  so that the appropriate coating removal subassemblies  29  may carry out any required further pipe coating removal operation(s) at one or more different longitudinal positions on the pipe section  2 . Once all of the necessary pipe coating removal operations have been completed, the apparatus  1  may be removed from the pipe section  2  and the field joint coating operations may commence. 
       FIGS.  11 ( a ) to  15 ( b )  show various details of other slightly modified embodiments of the invention. 
       FIGS.  11 ( a ) and  11 ( b )  and  FIG.  12   , show details of another embodiment of the apparatus of the invention, in which there is a feed plate  79  located on each of two opposites sides of the, or each, milling cutter  41  or other coating material removal member, the feed plates arranged substantially parallel to the axis of rotation of the milling cutter. As the milling cutter  41  (or other coating material removal member) is rotated around part of the circumference of the pipe section  2  to remove pipe coating material  6 , one of the feed plates  79   a  (see  FIG.  12   ) is situated in front of the cutter  41  in the direction of rotation of the milling cutter around the pipe section  2  and is referred to as the “infeed plate”, and the other feed plate  79   b  is situated behind the cutter  41  and is referred to as the “outfeed plate”. 
     The feed plates  79  are rigidly mounted relative to the axis of rotation of the milling cutter  41  such that the distance from the surface of the cutter to the running surface on the feed plates is constant. The feed plates act to passively control the depth of cut without the need for sophisticated controls. The feed plates  79  are configured to ensure a minimum amount of material is always removed from the factory coating  6  to present a clean “virgin” surface for the application of the field joint coating (injection moulded polypropylene), while accommodating variations in diameter, ovality and alignment, for example. Additionally, the feed plates  79  are configured to prevent the milling cutter  41  from removing an excessive amount of factory coating material  6 , which can cause the cutter to stall and disrupt the coating preparation procedure. A further benefit of the feed plates  79  is to help with chip extraction. The feed plates  79  create a narrow channel to direct airflow and contain chips being thrown from the cutter  41 . An extraction port  81  on the back of the cutter box can allow for the connection of dust extraction equipment and removal of the chips from the cutter assembly, for example. 
     A detail of factory coating material removal utilising feed plates  79   a  and  79   b  is shown in  FIG.  12   . At cut initiation, the cutter assembly is moved towards the pipe with the cutter  41  spinning, and the cutter touches the factory coating  6  to begin the cut. The cutter is driven into the factory coating  6  until the feed plates  79  contact the coating, at which point no further depth of material is removed. As cutting progresses, the main rotation assembly of the apparatus starts to revolve the cutter assembly around the pipe section. At this point, (assuming perfect alignment) the outfeed plate  79   b  no longer touches the factory coating  6  and it is only the infeed plate  79   a  that controls/limits the depth of cut. (In cases of extreme misalignment, the outfeed plate  79   b  may be touching the coating  6  and limiting the depth of cut, but this is not normal.) As the cut approaches completion (approaching the start of the cut from the opposing cutter) there comes a point when the infeed plate  79   a  no longer rides on the original outer diameter of the coating  6 , but instead rides on the freshly cut surface. This causes the cutter  41  to briefly cut twice as deep right at the end of the cut. Once the full amount of rotation (e.g. 180 degrees) of the milling cutter  41  around part of the circumference of the pipe section  2  has been completed, the cutter  41  is withdrawn from the pipe. 
       FIGS.  13 ( a ) and  13 ( b )  show a variation on the embodiment shown in  FIGS.  11 ( a )  to  FIG.  12   , in which the feed plates are rotatably mounted with respect to the (or each respective) rotatable coating material removal member, to accommodate misalignments between the coating material removal member and the pipe, in use. In particular, in the embodiment shown in  FIGS.  13 ( a ) and  13 ( b ) , the feed plates  83  are configured to roll around the cutter  41  to maintain a highly consistent depth of cut, regardless of the alignment of the pipe section with respect to the alignment of the main rotation axis of the apparatus. With the previously described embodiment of  FIGS.  11 ( a )  to  FIG.  12   , utilizing fixed feed plates  79 , misalignment of the pipe section  2  skews the pipe section to one side of the cutter  41  which then reduces the depth of cut as the pipe section leans heavily on one feed plate instead of evenly on both. If the misalignment is extreme, the cutter  41  may not cut the surface of the coating  6  at all. With the embodiment shown in  FIGS.  13 ( a ) and  13 ( b ) , the feed plates  83  are mounted to a supporting partial ring or partial cylinder  85  which is arranged around the cutter and rotationally supported by a plurality of guide bearings  87 . With a misaligned pipe section  2 , this mechanism allows the feed plates  83  to roll around the cutter  41  to directly face the pipe section, so that when initial contact is made between the cutter  41  and the pipe coating  6 , both feed plates  83  then engage with the coating surface. This substantially ensures the depth of cut is consistent, regardless of pipe section alignment (preferably, within pre-defined acceptable limits). 
     The factory coating surface  6  to be prepared by the apparatus  1  according to the invention is generally not perfectly round, and its thickness generally varies along with the dimensions of the pipe  2  beneath. In addition to this, the pipe  2  may not be positioned in the exact centre of the apparatus corresponding to the axis of rotation of the subframe  5 . These factors combined mean that as the, or each, coating material removal subassembly  29  rotates around the pipe  2 , the location of the surface of the coating changes. As described above, each coating material subassembly  29  is mounted to a pneumatically actuated gullwing arm  33 , which moves the subassembly  29  towards or away from the pipe and factory coating. With the embodiments of  FIGS.  11 ( a ) to  13 ( b ) , the pneumatic actuator  35  pushes the milling cutter  41  into the factory coating  6  until the feed plates  79 / 83  make contact with the surface of the factory coating and the cutter  41  can advance no further. As the cutter  41  rotates around the pipe  2  and the position of the surface of the coating varies, the pneumatic actuator  35  acts as a spring to accommodate these variations. The feed plates  79 / 83  and pneumatic gullwing arm  33  combined follow the varying surface of the factory coating  6 , while maintaining a consistent depth of cut, preferably without the need for sophisticated controls. 
       FIGS.  14 ( a ) and  14 ( b )  show a subassembly frame  39  of an embodiment of the apparatus of the invention, in two different orientations. Depending on the project specifications, the milling cutters  41  may be used to clean the overlap and/or the chamfer. The overlap region is an area on the outer diameter of the factory coating  6  and requires the cutter to be approximately parallel to the axis of the pipe. The chamfer is a region where the polypropylene factory coating is reducing in diameter forming a conical or tapering surface with a typical angle of 30° to the pipe axis. For the chamfer region the cutter  41  must be positioned at this same 30° angle relative to the pipe axis. The angle of the chamfer can vary significantly (e.g. ˜5° such that this variance must be accounted for. As shown in  FIGS.  14 ( a ) and  14 ( b ) , the cutter assembly is mounted to allow the milling cutter  41  to move between the required angles, with the angle of the milling cutter  41  being controlled primarily by the air-hydro cylinder  71 . 
     In the embodiment shown in  FIGS.  14 ( a ) and  14 ( b ) , the pivoting mechanism includes a pair of stops,  91  and  93 , which are used to control the position of the milling cutter  41  for each pass (overlap and chamfer). In the case of the overlap pass, as shown in  FIG.  14 ( a ) , the cutter angle is fixed, e.g. with the aid of the second stop  93 . It can be fixed to be parallel to the pipe or have a slight angle ˜1° to “feather” the cut and blend it out at the edge. In the case of the chamfer pass, as shown in  FIG.  14 ( b ) , the subassembly frame  39  of the milling cutter  41  is moved to the first stop  91 , which sets the cutter  41  at an angle just greater than the maximum specified angle of the chamfer. The cutter subassembly frame  39  is then moved towards the pipe such that the cutter  41  first engages with the pipe close to the vertex between the chamfer and the polypropylene toe (on the chamfer face). The milling cutter  41  starts to cut and the feed plates  79 / 83  engage with the surface of the chamfer. The respective pneumatic cylinder  35  continues to push the milling cutter  41  towards the chamfer which in turn partially retracts the piston  76  of the air-hydro cylinder  71  and reduces the angle of the cutter until the angle of the cutter matches the angle of the chamfer. At this point, the system is stable and the milling cutter  41  can follow and adapt to the varying chamfer angle as the cutter assembly rotates around the pipe. 
     The chamfering pivot point preferably is substantially in-line with the chamfer coating surface from the reference point of the line of action of the swing arm. The force imparted by the air-hydro cylinder  71  to extend the milling cutter subassembly  39  to the initial chamfer angle against the first stop  91  must be light enough to be overcome by the force from the pneumatic cylinder  35 , so that the piston  76  of the air-hydro cylinder  71  can be compressed to allow the milling cutter  41  to sit “flat” against the chamfer surface, thereby providing a passive system that matches the angle of the cutter to the angle of the chamfer without the need for sophisticated controls. 
     In some preferred embodiments of the invention, a first step in the method of preparing a pipe coating using the apparatus of the invention is to scan, e.g. laser scan, the factory coating and determine the location of the critical inflection points on the factory coating. The position of the pipe within the apparatus can vary along with the dimensions of the factory coating and the apparatus preferably is able to accommodate these variations automatically, without any operator input. Accordingly, in some preferred embodiments of the invention, and as shown in  FIG.  15 ( a ) , the pipe coating removal apparatus includes one or more (preferably two) laser sensors  97 , preferably one or more 2-dimensional profile laser sensors, configured to locate profile features of the pipe coating. Preferably, the laser sensor(s) are configured to locate profile features of the pipe coating, which profile location information is used to position the coating material removal subassemblies  29  linearly relative to the substructure  21 , parallel to the axis of the pipe section, by the movement and positioning of the plate member  23  by means of a linear servo motor  25 , as described above. 
     The, or each, laser sensor  97  may be selected such that its field of view is sufficient to detect all of the critical points across the acceptable range of pipe locations without the need to move or reposition the laser. Once the apparatus has been moved to the pipe, and the pipe is at a suitable position within the apparatus, the, or each, laser sensor  97  preferably scans the factory coating and using software and computer control determines X (pipe axial) and Y (pipe radial) coordinates for each of the inflection points  99 ,  101 ,  103 , and  105 , of the coating (see  FIG.  15 ( b ) ). A computer control system of the apparatus preferably then uses the measured X coordinates to move the coating material removal subassemblies  29 , including cutters  41 , to the correct positions for each rotational pass of the subframe  5 .  FIG.  15 ( b )  shows end regions of two different types of pipe sections, and indicates the critical points on the pipe coating that may be detected by the laser sensor(s), i.e. the “overlap start”  99 , the “chamfer start”  101 , the “polypropylene toe start”  103 , and the “FBE toe start”  105 . 
     It will be understood that the above description and the drawings are of particular example embodiments of the invention, but that other implementations and embodiments of the invention are included in the scope of the claims.