AU2012204047B2

AU2012204047B2 - Underwater pipeline coating removal tool

Info

Publication number: AU2012204047B2
Application number: AU2012204047A
Authority: AU
Inventors: Shane Spencer Parker; Brett Silich; Ian Alistair Wilson
Original assignee: Woodside Energy Ltd
Current assignee: Woodside Energy Ltd
Priority date: 2011-07-08
Filing date: 2012-07-09
Publication date: 2014-09-18
Anticipated expiration: 2032-07-09
Also published as: AU2012204047A1

Abstract

A method and apparatus of removing a coating from an underwater pipe is described. The method includes the step of applying a rotating milling tool to 5 a polymer coating, the milling tool arranged to move radially towards the pipe in response to a signal generated from a depth guide sensor that is indicative of the distance between the milling tool and the pipe. Milling is ceased when the milling tool has reached a predetermined offset distance from the pipe, thereby leaving a residual polymer coating layer on the pipe when the milling 10 tool is removed. Thereafter, a rotating brushing tool is applied to remove the residual polymer coating layer and an epoxy coating from the pipe, the brushing tool being arranged to move radially towards the pipe. Figure 2. '3N 2IQ) ~~71 0o

Description

1 UNDERWATER PIPELINE COATING REMOVAL TOOL Field of the Invention The present invention relates to the maintenance and repair of underwater 5 pipelines, such as those used in the oil-and-gas industry. Background to the Invention The production of oil and gas offshore frequently requires the piping of product through pipelines laid on the seabed. These pipelines can be at 10 significant depths, potentially in excess of 2000m. Even at shallower depths, in the order of 500m to 1500m, access to the pipes by divers can be problematic. When maintenance or inspection of pipes is required, this is generally done by use of a remotely operated vehicle (ROV) controlled from about the sea surface. 15 Pipes are typically formed of steel (for instance having an internal diameter of 500mm to 760mm) which is then coated in layers of protective material. Typical coatings include an outer layer of steel reinforced concrete (commonly referred to in the art as 'weight coat'), a middle layer of polypropylene and an 20 inner layer of fusion bonded epoxy which bonds the polypropylene to the steel of the pipe. The outer concrete layer may be in the order of 100mm thick. The polypropylene layer may be in the order of 25mm to 100mm thick. If a leak or other weakness is found in an underwater pipe, it may be 25 necessary to replace a length of the pipe. Such an operation requires the removal of the protective coatings, so that a pipe repair device can engaged the pipe. It is considered desirable to remove the protective coatings so thoroughly that the resulting pipe surface is equivalent to a sand-blasted surface, while not removing virtually any of the steel from the pipe. Such an 30 operation, conducted deep underwater, is highly problematic. Various attempts have been made to construct tools which can assist in this process. One such tool, manufactured by Sonsub Inc. of Texas, achieves its end by clamping onto the pipe and removing the concrete with cutting blades.

2 The polypropylene and epoxy are removed using a cylindrical brush arrangement where each of the bristles of the brush has a tungsten carbide or diamond tip. This system necessarily involves the removal of a small amount of the steel of the pipe as a result of contact between the carbide-tipped 5 bristles of the brushes and the steel of the pipe. This damage to the steel of the pipe can interfere with the ability to apply a new coating at a later time. In addition, this system requires a high degree of accuracy, both in axial placement about the section to be cleaned and in the degree of radial movement of the cutting blades and the bristles of the brushes. 10 Other attempts have been made to remove protective coatings using high pressure water jets. These have proved to be of limited utility, generally operating only in shallow water and being unable to remove reinforcing from the concrete. 15 The present invention seeks to provide a tool for the removal of protective coatings which addresses, at least in part, a limitation of the prior art. Summary of the Invention 20 According to a first aspect of the present invention there is provided a method of removing a coating from an underwater pipe, the method including the steps of: applying a rotating milling tool to a polymer coating, the milling tool arranged to move radially towards the pipe in response to a signal generated 25 from a depth guide sensor that is indicative of the distance between the milling tool and the pipe; ceasing milling when the milling tool has reached a predetermined offset distance from the pipe, thereby leaving a residual polymer coating layer on the pipe when the milling tool is removed; and, 30 applying a rotating brushing tool to remove the residual polymer coating layer and an epoxy coating from the pipe, the brushing tool being arranged to move radially towards the pipe.

3 In one form, the method includes the step of operating hydraulic rams to bring the milling tool into contact with the polymer layer, the hydraulic rams biasing the milling tool towards the pipe. In one form, the method includes the step of operating hydraulic rams to bring the brushing tool into contact with the 5 polymer layer, the hydraulic rams biasing the brushing tool towards the pipe. In one form, the method includes the step of operating the milling tool under constant power until the preset offset distance has been reached. In one form, the method includes the step of moving the rotating brushing tool at least twice around the circumference of the underwater pipe, with the direction 10 of brush rotation reversing after a first revolution around the circumference of the underwater pipe. When the pipeline includes a layer of concrete as a coating, the method may include, prior to applying the milling tool, the step of: 15 applying a rotating cutting tool to the underwater pipe to remove a layer of concrete, the cutting tool arranged to move radially towards the pipe, the cutting tool being powered by a cutting tool motor operating at constant power; monitoring the load on the cutting tool motor; and 20 ceasing to apply the cutting tool when the load on the motor reaches a predetermined level indicating completion of the removal of the layer of concrete. In one form, the method comprises the step of operating hydraulic rams to 25 rotate a pair of pivot arms about their pivot, thereby bringing the milling tool into engagement with the polymer layer. According to a second aspect of the present invention there is provided an apparatus for removing a coating from an underwater pipe, the apparatus 30 comprising: a rotating milling tool for removal of a polymer coating from the underwater pipe, the milling tool arranged to move radially towards the pipe in response to a signal generated from a depth guide sensor that is indicative of the distance between the milling tool and the pipe; 4 a control system for ceasing milling when the milling tool has reached a predetermined offset distance from the pipe, thereby leaving a residual polymer coating layer on the pipe when the milling tool is removed; and, a rotating brushing tool for removing the residual polymer coating layer 5 from the pipe, the brushing tool being arranged to move radially towards the pipe. In one form, the apparatus includes a working carriage arranged to move circumferentially around the pipe and the milling tool and brushing tool are 10 mounted to the working carriage. In one form, the apparatus includes a supporting carriage with pipe clamps located at either end thereof, arranged to mount to the pipe. In one form, the working carriage is mounted within the supporting carriage and the working carriage is arranged for axial movement along the supporting carriage with axial-movement rails supporting the 15 working carriage relative to the pipe. In one form, the working carriage is arranged for circumferential movement around the pipe, with circumferential movement rollers mounted to a supporting frame. In one form, the supporting frame is connected to an interface skid for 20 engagement with a remotely operated vehicle to provide one or both of electrical or hydraulic power to the apparatus. In one form, the remotely operated vehicle and interface skid are arranged to move axially with the working carriage without circumferential rotation around the pipe. 25 In one form, the working carriage is mounted between two semi-circular rails and the rails are connected by one or more cross-members. In one form, a pair of hydraulic rams extend between the rails and an outer end of a pair of pivot arms such that operation of the hydraulic rams causes rotation of the pair of pivot arms about a pivot and wherein the pair of pivot arms is attached 30 to a cross member at the pivot. In one form, the brushing tool and the milling tool are mounted on either end of the pair of pivot arm to facilitate radial movement of each tool towards the pipe and whereby the milling tool acts as counterweight for the brushing tool.

5 In one form, the milling tool comprises a roller onto which plurality of carbide cutting tips are arranged in a generally helical configuration. In one form, the plurality of carbide cutting tips are provided in a spaced apart arrangement 5 along the length of each of a plurality of inserts, the inserts being removably fixed around the circumference of the roller In one form, the milling tool is rotated by means of a hydraulic milling tool motor and the hydraulic power characteristics of the milling tool motor are 10 matched to the specific power requirements of the milling tool. In one form, the brushing tool is a cylindrical wheel with stiff wire bristles. In one form, the brushing tool is rotatable about its central axis by means of a hydraulic brushing tool motor and the hydraulic power characteristics of the 15 brushing tool motor are matched to the specific power requirements of the brushing tool. When used to remove a layer of concreted from an underwater pipe, the apparatus may further include: 20 a rotating cutting tool for removal of a layer of concrete prior to milling, the cutting tool arranged to move radially towards the pipe, the cutting tool being powered by a cutting tool motor operating at constant power; a sensor for monitoring the load on the cutting tool motor; and a control system for ceasing to apply the cutting tool when the load on 25 the motor reaches a predetermined level indicating completion of the removal of the layer of concrete. In one form, the cutting tool and the milling tool are mounted on either end of the pair of pivot arms to facilitate radial movement of each tool towards the 30 pipe and whereby the milling tool acts as counterweight for the cutting tool. In one form, the cutting tool comprises a plurality of spaced cutting discs, located along a central axle so that the cutting discs are rotated about a common axis by the cutting tool motor. In one form, each of the cutting tool motor, the 6 milling tool motor and the brushing tool motor operates independently using power supplied via the interface skid connection with the ROV. According to a third aspect of the present invention there is provided a method 5 of removal of a coating for an underwater pipe substantially as herein described with reference to and as illustrated in the accompanying figures. According to a fourth aspect of the present invention there is provided an apparatus for removal of a coating for an underwater pipe substantially as 10 herein described with reference to and as illustrated in the accompanying figures. Brief Description of the Drawings It will be convenient to further describe the invention with reference to 15 preferred embodiments of the underwater pipeline tool of the present invention. Other embodiments are possible, and consequently the particularity of the following discussion is not to be understood as superseding the generality of the preceding description of the invention. In the drawings: Figure la is a schematic side view of an underwater pipeline tool in 20 accordance with the present invention; Figure 1b is a schematic cross section of the underwater pipeline tool of Figure 1a; Figure 1c is a schematic plan view of the underwater pipeline tool of Figure 1a; 25 Figure 2 is a first perspective of a working carriage from the underwater pipeline tool of Figure 1 a; Figure 3 is a second perspective of the working carriage of Figure 2; Figure 4 is a perspective of a cutting tool from within the working carriage of Figure 2; 30 Figure 5 is a cross section of the cutting tool of Figure 4; and, Figure 6 is a perspective of a milling tool from within the working carriage of Figure 2.

7 Detailed Description of Preferred Embodiments Referring to the Figures, Figure 1 shows a general schematic arrangement of an underwater pipeline coating removal tool 10, shown attached to a coated pipe 12. The coated pipe 12 consists of a steel pipe 14 coated with fusion 5 bonded epoxy, surrounded by a polymer coating in the form of a polypropylene layer 16 and an optional additional coating in the form of a reinforced concrete layer 18. In the example of the drawings, the fusion bonded epoxy coating (not shown) is less than 500 microns thick, the polypropylene layer 16 is in the range of 25mm to 6mm thick, and the 10 concrete layer 18 is in the range of 40 to 11 5mm thick. The pipeline tool 10 has a supporting carriage 20 which extends along a length of the pipe 12 from which coating is to be removed. The supporting carriage 20 has pipe clamps 22 located at either end thereof, arranged to 15 mount to the coated pipe 12. In the embodiment shown, where coating is to be removed along a length of pipe of about 4m, the supporting carriage 20 is about 10m in length with the clamps 22 spaced about 8.4m from each other. A working carriage 30 is mounted within the supporting carriage 20. The 20 working carriage 30 is arranged for axial movement along the supporting carriage 20, with axial-movement rails 32 supporting the working carriage relative to the pipe 12. The working carriage 30 is also arranged for circumferential movement around the pipe 12, with circumferential movement rollers 34 mounted to a supporting frame 36. 25 The supporting frame 36 is connected to an interface skid 38 to which an ROV 40 (shown schematically) can be engaged. The ROV 40 and interface skid 38 are thus arranged to move axially with the working carriage 30, but not to rotate circumferentially. Upon connection to the interface skid 38, the ROV 30 serves as a hydraulic power supply for each of the motors and other hydraulic equipment of the pipeline tool 10. The ROV also serves as an electrical connection for a control system located at the surface which monitors and controls the operation of the pipeline tool 10.

8 The working carriage 30 can be seen in better details in Figures 2 and 3. The working carriage 30 is mounted between two semi-circular rails 50, which are connected by cross-members 52. Three tools are mounted to the cross members 52: a cutting tool 54, a milling tool 56 and a brushing tool 58. The 5 cutting tool 54 is designed and sized to remove the reinforced concrete layer 18. The milling tool 56 is designed and sized to remove the bulk of the polypropylene layer 16 (as described in greater detail below) and the brushing tool 58 is designed and sized to remove the remainder of the polypropylene layer and the epoxy coating from the steel pipe. 10 In the illustrated embodiments, the cutting tool 54 and the milling tool 56 are mounted at either end of a first pair of pivot arms 60. The pivot arms 60 are attached to a cross member 52 at a pivot 62. A pair of hydraulic rams 64 extend between the rails 50 and an outer end of the pivot arms 60. Operation 15 of the hydraulic rams 64 causes rotation of the pivot arms 60 about the pivot 62. In this way both the cutting tool 54 and the milling tool 56 can be moved radially relative to the pipe 12, albeit being constrained to move in opposite directions. Using this arrangement, the cutting tool 54 effectively acts as a counterweight for the milling tool 56 and vice versa. The brushing tool 58 is 20 similarly located at the end of a second pair of pivot arms 66, opposite the first pair 60. The arrangement of the second pair of pivot arms 66 is similar to that of the first pair 60, with the difference being that the other end of the second pair of pivot arms 66 holds a counter-weight 68. 25 The cutting tool 54 can be seen in Figures 4 and 5. It consists of a plurality of spaced cutting discs 70, which are located along a central axle 72. There is a gap of about 6mm between each cutting disc. In use, the central axle 72 and hence the cutting discs 70 are rotated about their common axis, by means of a hydraulic cutting tool motor 74. The cutting discs 70 have outer blades 30 designed for cutting through reinforced concrete. As the disc 70 penetrate the concrete, concrete between adjacent discs 70 shears away. The milling tool 56 can be seen in Figure 6. It consists of a roller 76 onto which plurality of carbide cutting tips 78 are arranged in a generally helical 9 configuration. In the illustrated embodiment, the plurality of carbide cutting tips 78 are provided in a spaced apart arrangement along the length of each of a plurality of inserts 79 which are removably fixed around the circumference of the roller 76 using mechanical fastening means such as a plurality of cap 5 screws 81. In use, the roller 76 is rotated about a central axis of the milling tool 56 by means of a hydraulic milling tool motor 75. The plurality of inserts 79 are offset from each other in such a way that when the roller 76 is brought to bear against the polypropylene coating, the carbide tips 78 act to slice away layers of the polypropylene coating in a helical pattern. 10 Alternatively, the milling tool may comprise a plurality of discs on a common shaft, each of the plurality of disc separate by spacers. In this embodiment, the carbide cutting tips 78 are arranged at regular intervals around the outside circumference of each of the plurality of discs. The plurality of discs is 15 indexed so that a helical array of carbide tips was presented to the surface of the polypropylene coating. With reference to Figure 3, the brushing tool 58 is a cylindrical wheel with stiff wire bristles. In use, the brushing tool 58 is rotated about its central axis by 20 means of a hydraulic brushing tool motor 77. When rotated against a steel surface, the stiff wire bristles of the brushing tool 58 loosen and remove small remnant particles of polypropylene or fusion bonded epoxy from the steel surface without removing steel from the pipe surface. 25 Each of the cutting tool motor 74, the milling tool motor 75 and the brushing tool motor 77 operates independently using power supplied via the interface skid connection with the ROV 40. The hydraulic power characteristics of each of the cutting tool motor 74, the milling tool motor 75 and the brushing tool motor 77 are matched to the specific power requirements of the cutting tool 30 54, milling tool 56, and brushing tool 58, respectively. Use of the pipeline tool 10 will now be described in the context of a pipeline that includes a reinforced concrete layer 18.

10 When a section of coated pipe 12 to be maintained is identified, the pipeline tool 10 may be brought to the pipe 12 by means of the ROV 40, and positioned such that the ROV establishes electrical and hydraulic power connections with the interface skid 38, and the pipe clamps 22 are located on 5 either side of the pipe section to be maintained. The pipe clamps 22 can then be engaged, fixing the supporting carriage 20 in position. The working carriage 30 can be moved axially along the supporting carriage to an outer end of the pipe section 12 being maintained. 10 The first action in removing the pipe coating is to remove the reinforced concrete layer 18. This is achieved by operating the hydraulic rams 64 connected to the first pair of pivot arms 60 in order to bring the cutting tool 54 into engagement with the concrete layer 18. It will be appreciated that this action also causes the milling tool to be spaced from the pipe 12. The 15 hydraulic rams 64 connected to the second pair of pivot arms 66 act to keep the brushing tool 58 in a neutral position. The cutting tool motor 74 operates the cutting tool 54 at a constant power, while monitoring the load on the cutting tool 54. The hydraulic rams 64 bias 20 the cutting tool 54 towards the steel pipe 14. In this way the cutting tool 54 can be applied against the concrete layer 18 until such time as a control system receives a signal responsive to an increase in resistance that indicates that the polypropylene layer 16 has been reached. 25 During cutting of the concrete layer 18 the working carriage 30 is moved about the circumference of the pipe 12. The arrangement is such that the working carriage is enabled to move about 1850 either side of a neutral position, thus enabling the entire circumference of the concrete layer 18 to be removed. 30 When the load on the cutting tool motor 74 indicates that the entire concrete layer has been removed, the hydraulic rams 64 are operated using power from the ROV so as to rotate the first pair of pivot arms 60 about their pivot 62, and to bring the milling tool 56 into engagement with the polypropylene layer 16. It will be appreciated that this action also causes the cutting tool 54 11 to be spaced from the pipe 12. The hydraulic rams 64 connected to the second pair of pivot arms 66 continue to act to keep the brushing tool 58 in a neutral position. 5 The thickness of the polypropylene layer 16 to be removed is known. The rotating milling tool 56 is applied to the polymer coating 16, the milling tool being arranged to move radially with respect to the pipe 14. The milling tool 56 is operatively connected to a depth guide sensor 57 that generates a signal that is indicative of the distance between the milling tool 56 and the pipe 14. 10 This signal is received by a control system (not shown) located on the tool 10, the ROV 40 or at the surface and is used to control further radial movement of the milling tool 56 towards the pipe 14 during milling. Using the method and apparatus of the present invention, milling is ceased when the depth guide sensor 57 generates a signal indicating that the milling tool 56 has reached a 15 predetermined offset distance from the pipe 14 beneath the polymer coating so that a residual polymer coating layer is left on the pipe 14 to protect it from damage. When the predetermined offset distance has been reached, the rotating brushing tool is applied to remove the residual polymer coating layer along with the fusion bonded coating layer from the pipe and reveal the pipe 20 surface. The depth guide sensor may form an integral part of the milling tool 56 or be provided elsewhere on the tool 10. By way of example, the depth guide sensor 57 may be located axially alongside the milling tool 56 and positioned 25 such that contact of the depth guide sensor 57 with the polypropylene layer 16 (alongside the removed section) generates a signal that prevents further radial movement of the milling tool 56 towards the steel pipe 14. Alternatively, the depth guide sensor 57 may generate a continuous signal that is indicative of the offset distance between the milling tool 56 and the steel pipe 14 from 30 which the coatings are being removed. In either case, the depth guide sensor 57 is set such that the milling tool 56 is allowed to cut through polypropylene layer 16 until through to a predetermined offset distance from the underlying steel pipe. When the milling tool 56 has reached the preset offset distance, the milling operation ends so that contact between the steel pipe 14 and the 12 carbide cutting tips 78 cannot occur. In contrast to the prior art, the milling tool 56 is disengaged prior to complete removal of the polypropylene layer and thus prior to coming into contact with the steel pipe 14 so as to ensure that the carbide tips 78 of the milling tool 56 do not come into contact with the 5 steel pipe 14. The predetermined offset distance is set a distance that provides a sufficient margin of error for inconsistencies in the thickness of the polypropylene layer. By way of example, when the polymer coating layer is 63 mm thick, the preset 10 offset distance may be set at 3mm so as to provide a sufficient margin of error for inconsistencies in the thickness of the polypropylene layer. During milling of the polypropylene layer 16, the working carriage 30 is moved about the circumference of the pipe 12 in an analogous manner to the way in 15 way in which the working carriage moves during the cutting of the reinforced concrete layer. The hydraulic rams 64 bias the milling tool 54 towards the steel pipe 14 and the milling tool motor 75 operates to rotate the milling tool 54 under constant 20 power until the sensor(s) indicate that the preset offset distance has been reached. When the polypropylene layer has been removed using the milling tool 56 to its preset offset thickness from the circumference of the steel pipe 14, the hydraulic rams 64 may be activated to bring both the cutting tool 54 and the milling tool 56 into neutral positions. Once this is achieved, the 25 brushing tool 58 may be moved towards the surface of the steel pipe 14 by rotation of the second pair of pivot arms 66. The brushing tool 58 is caused to rotate using the brushing tool motor 77 and is rotated along the surface of the steel pipe 14 to remove the remaining polypropylene layer and the fusion bonded epoxy coating. 30 The working carriage 30 completes two revolutions about the circumference of the pipe 12 during the brushing operation. During the first revolution, about 90% of the remaining polypropylene layer is removed. The direction of brush rotation is then reversed for the second revolution. During this second 13 revolution the remaining polypropylene and the fusion bonded epoxy coating is removed, and the action of the steel pipe 14 against the reverse side of the brush bristles acts to sharpen the bristles of the brushing tool 58. 5 Once a circumference of the pipe 14 has thus been uncovered, the working carriage 30 can be moved axially in order to uncover a further portion of the pipe 14. This process can be repeated until the coatings of the entire length of pipe 12 to be maintained have been removed. 10 Once the entire length has been treated, the pipe clamps 22 can be disengaged to permit removal of the pipeline tool 10. It is envisaged that the tool 10 will be constructed with a number of backup features in the event of a hydraulic failure. Each of the pairs of pivot arms 60, 15 66 are spring-biased to a neutral position. A hydraulic override switch is also provided as a backup mechanism. Although speed of coating removal will be dependent on many factors, modelling suggests that the embodiment described will be able to remove 20 coating from underwater pipe (at depths up to about 1500m) at a rate of about 1 m every three hours. This represents a speed in the order of double that of other pipe coating removal systems currently available. The underwater pipeline coating removal tool described above is provided 25 with three tools - the cutting tool 54, the milling tool 56 and the brushing tool 58. It is to be understood that not all of the tools need be used for coating removal job. By way of example, when the coated pipe is coated only with fusion bonded epoxy and a polymer or polypropylene layer, and the reinforced concrete layer is not present, the pipeline tool of the present invention may be 30 provided without using the cutting tool. As an alternative, the tool 10 may be provided with only two of the tools, instead of three. In this example, only two tools are mounted to the cross members 52: the milling tool 56 and the brushing tool 58. Using this arrangement, the milling tool 56 and the brushing tool 58 may be mounted at either end of the first pair of pivot arms 60. In this 14 embodiment, the brushing tool 58 effectively acts as a counterweight for the milling tool 56 and vice versa. There is no requirement to provide the second pair of pivot arms. 5 Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.

Claims

1. A method of removing a coating from an underwater pipe, the method including the steps of: 5 applying a rotating milling tool to a polymer coating, the milling tool arranged to move radially towards the pipe in response to a signal generated from a depth guide sensor that is indicative of the distance between the milling tool and the pipe; ceasing milling when the milling tool has reached a predetermined 10 offset distance from the pipe, thereby leaving a residual polymer coating layer on the pipe when the milling tool is removed; and, applying a rotating brushing tool to remove the residual polymer coating layer and an epoxy coating from the pipe, the brushing tool being arranged to move radially towards the pipe. 15

2. The method of claim 1 including the step of operating hydraulic rams to bring the milling tool into contact with the polymer layer, the hydraulic rams biasing the milling tool towards the pipe. 20

3. The method of claim 1 or 2 including the step of operating hydraulic rams to bring the brushing tool into contact with the polymer layer, the hydraulic rams biasing the brushing tool towards the pipe.

4. The method of any one of the preceding claims including the step of 25 operating the milling tool under constant power until the preset offset distance has been reached.

5. The method of any one of the preceding claims including the step of moving the rotating brushing tool at least twice around the circumference of 30 the underwater pipe, with the direction of brush rotation reversing after a first revolution around the circumference of the underwater pipe.

6. The method of any one of the preceding claims wherein, prior to applying the milling tool, the method includes the steps of: 16 applying a rotating cutting tool to the underwater pipe to remove a layer of concrete, the cutting tool arranged to move radially towards the pipe, the cutting tool being powered by a cutting tool motor operating at constant power; 5 monitoring the load on the cutting tool motor; and ceasing to apply the cutting tool when the load on the motor reaches a predetermined level indicating completion of the removal of the layer of concrete. 10

7. The method of any one of the preceding claims comprising the step of operating hydraulic rams to rotate a pair of pivot arms about their pivot, thereby bringing the milling tool into engagement with the polymer layer.

8. An apparatus for removing a coating from an underwater pipe, the 15 apparatus comprising: a rotating milling tool for removal of a polymer coating from the underwater pipe, the milling tool arranged to move radially towards the pipe in response to a signal generated from a depth guide sensor that is indicative of the distance between the milling tool and the pipe; 20 a control system for ceasing milling when the milling tool has reached a predetermined offset distance from the pipe, thereby leaving a residual polymer coating layer on the pipe when the milling tool is removed; and, a rotating brushing tool for removing the residual polymer coating layer from the pipe, the brushing tool being arranged to move radially towards the 25 pipe.

9. The apparatus of claim 8 wherein the apparatus includes a working carriage arranged to move circumferentially around the pipe and the milling tool and brushing tool are mounted to the working carriage 30

10. The apparatus of claim 8 or 9 wherein the apparatus includes a supporting carriage with pipe clamps located at either end thereof, arranged to mount to the pipe. 17

11. The apparatus of claim 10 wherein the working carriage is mounted within the supporting carriage and the working carriage is arranged for axial movement along the supporting carriage with axial-movement rails supporting the working carriage relative to the pipe. 5

12. The apparatus of any one of claims 9 to 11 wherein the working carriage is arranged for circumferential movement around the pipe, with circumferential movement rollers mounted to a supporting frame. 10

13. The apparatus of claim 12 wherein the supporting frame is connected to an interface skid for engagement with a remotely operated vehicle to provide one or both of electrical or hydraulic power to the apparatus.

14. The apparatus of claim 13 wherein the remotely operated vehicle and 15 interface skid are arranged to move axially with the working carriage without circumferential rotation around the pipe.

15. The apparatus of any one of claims 9 to 14 wherein the working carriage is mounted between two semi-circular rails and the rails are 20 connected by one or more cross-members.

16. The apparatus of claim 15 wherein a pair of hydraulic rams extend between the rails and an outer end of a pair of pivot arms such that operation of the hydraulic rams causes rotation of the pair of pivot arms about a pivot 25 and wherein the pair of pivot arms is attached to a cross member at the pivot.

17. The apparatus of claim 16 wherein the brushing tool and the milling tool are mounted on either end of the pair of pivot arm to facilitate radial movement of each tool towards the pipe and whereby the milling tool acts as 30 counterweight for the brushing tool.

18. The apparatus of any one of claims 8 to 17 wherein the milling tool comprises a roller onto which plurality of carbide cutting tips are arranged in a generally helical configuration. 18

19. The apparatus of claim 18 wherein the plurality of carbide cutting tips are provided in a spaced apart arrangement along the length of each of a plurality of inserts, the inserts being removably fixed around the circumference 5 of the roller

20. The apparatus of any one of claims 8 to 19 wherein the milling tool is rotated by means of a hydraulic milling tool motor and the hydraulic power characteristics of the milling tool motor are matched to the specific power 10 requirements of the milling tool.

21. The apparatus of any one of claims 8 to 20 wherein the brushing tool is a cylindrical wheel with stiff wire bristles. 15

22. The apparatus of any one of claims 8 to 21 wherein the brushing tool is rotatable about its central axis by means of a hydraulic brushing tool motor and the hydraulic power characteristics of the brushing tool motor are matched to the specific power requirements of the brushing tool. 20

23. The apparatus of any one of claims 8 to 22 wherein the apparatus includes: a rotating cutting tool for removal of a layer of concrete prior to milling, the cutting tool arranged to move radially towards the pipe, the cutting tool being powered by a cutting tool motor operating at constant power; 25 a sensor for monitoring the load on the cutting tool motor; and a control system for ceasing to apply the cutting tool when the load on the motor reaches a predetermined level indicating completion of the removal of the layer of concrete. 30

24. The apparatus of claim 23 wherein the cutting tool and the milling tool are mounted on either end of the pair of pivot arms to facilitate radial movement of each tool towards the pipe and whereby the milling tool acts as counterweight for the cutting tool. 19

25. The apparatus of claim 23 or 24 wherein the cutting tool comprises a plurality of spaced cutting discs, located along a central axle so that the cutting discs are rotated about a common axis by the cutting tool motor. 5

26. The apparatus of any one of claims 23 to 25 wherein each of the cutting tool motor, the milling tool motor and the brushing tool motor operates independently using power supplied via the interface skid connection with the ROV. 10

27. A method of removal of a coating for an underwater pipe substantially as herein described with reference to and as illustrated in the accompanying figures.

28. An apparatus for removal of a coating for an underwater pipe 15 substantially as herein described with reference to and as illustrated in the accompanying figures.