Repair of Pipeline Welds Using Friction Stir Processing

Abstract

A method of using friction stir processing to repair a defect in a pre-existing weld of a pipeline of steel or other ferrous alloy. The method comprises attaching run-on and run-off tabs to the pipeline on respective sides of the defect and then advancing a friction stir processing tool into the nm-on tab. The tool is moved from the nm-on tab to the run-off tab along a weld-processing path that incorporates the defect. Once the defect is repaired, the tool is removed from the run-off tab. The run-on and run-off tabs may then be removed from the pipeline. The nm-on and nm-off tabs are generally wedge-shaped, each comprising an inner seating face and an outer running face that converges with the inner seating face. When attached to the pipeline, the tabs toward each other about the circumference of the pipeline.

Claims

1. A method of repairing a defect in a pre-existing weld of a pipeline of steel or other ferrous or non-ferrous alloy, comprising: attaching run-on and run-off tabs to the pipeline on respective sides of the defect; advancing a friction stir processing tool into the run-on tab; effecting relative movement of the tool from the run-on tab to the run-off tab along a weld-processing path that incorporates the defect, to repair the defect by friction stir processing of a portion of the weld along the path; removing the tool from the run-off tab; and removing the run-on and run-off tabs from the pipeline.

2. The method of claim 1, wherein the weld is a circular butt weld and the weld-processing path extends along the weld in a generally circumferential direction with respect to the pipeline.

3. The method of claim 1 or claim 2, comprising pressing a shoulder of the tool against a running surface while moving the tool along the weld-processing path and spinning the tool around an axis of rotation, the running surface comprising respective running faces of the run-on and run-off tabs.

4. The method of claim 3, comprising keeping the axis of rotation substantially orthogonal to the running surface while moving the tool along the weld-processing path.

5. The method of claim 3 or claim 4, wherein the running surface lies wholly outside the pipeline.

6. The method of claim 3 or claim 4, comprising pressing the shoulder of the tool against an outer surface of the pipeline disposed between the running faces of the run-on and run-off tabs during movement of the tool along the weld-processing path.

7. The method of claim 6, comprising reorienting the tool as the shoulder runs over the outer surface of the pipeline to correspond with circumferential progress of the tool around that outer surface.

8. The method of any of claims 3 to 5, comprising moving the shoulder of the tool along the weld-processing path directly between the running faces of the run-on and run-off tabs.

9. The method of any of claims 3 to 8, comprising moving the tool in a substantially straight line along the weld-processing path.

10. The method of claim 9, comprising maintaining the orientation of the axis of rotation while moving the tool along the weld-processing path.

11. The method of any of claims 3 to 8, comprising moving the shoulder of the tool from a running face portion of the run-on tab to a running face portion of the run-off tab, which portions lie in respective mutually-intersecting planes, and reorienting the axis of rotation while the shoulder moves between those portions.

12. The method of claim 11, wherein the mutually-intersecting planes also intersect the pipeline.

13. The method of claim 11 or claim 12, wherein the axes of rotation when the shoulder is on the respective running face portions converge in a radially inward direction with respect to the pipeline.

14. The method of claim 11 or claim 12, wherein the axes of rotation when the shoulder is on the respective running face portions diverge in a radially inward direction with respect to the pipeline.

15. The method of any preceding claim, comprising, with movement of the tool along the weld-processing path, ramping a probe of the tool from the run-on tab radially inwardly into a wall of the pipeline toward the defect, then radially outwardly out of the wall of the pipeline from the defect into the run-off tab.

16. The method of any preceding claim, comprising attaching the run-on and run-off tabs to the pipeline together as parts of a single repair fitting.

17. The method of claim 16, wherein the run-on and run-off tabs are integral parts of a single body that is attached to the pipeline.

18. The method of any preceding claim, comprising heating the run-on and/or run-off tabs before advancing the tool into the run-on tab and/or while effecting relative movement of the tool from the run-on tab toward the run-off tab along the weld-processing path.

19. The method of any preceding claim, wherein an internal back-up member is positioned against an internal surface of the pipeline in opposition to inward force applied by the tool.

20. A friction stir processing system for repairing a defect in a pre-existing weld of a pipeline of steel or other ferrous alloy, the system comprising generally wedge-shaped run-on and run-off tabs that each comprise an inner seating face and an outer running face that converges with the inner seating face, the tabs being attachable to the pipeline to taper toward each other about the circumference of the pipeline.

21. The system of claim 20, wherein the inner seating face of each tab is concave-curved with a part-circular cross-section.

22. The system of claim 20 or claim 21, wherein the tabs are in mirror-image mutual opposition.

23. The system of any of claims 20 to 22, wherein the outer running face and the inner seating face meet at an edge.

24. The system of claim 23, wherein the curvature of the inner seating face follows a circle that the outer running face substantially intersects tangentially.

25. The system of any of claims 20 to 24, wherein the tabs are spaced apart by a gap where an outer surface of the pipeline is exposed.

26. The system of any of claims 20 to 24, wherein the tabs are conjoined by a web or abut each other.

27. The system of any of claims 20 to 26, wherein the outer running faces of the tabs are substantially coplanar.

28. The system of claim 27, wherein the outer running faces of the tabs are parts of a running surface that is substantially planar along its length.

29. The system of any of claims 20 to 26, wherein at least portions of the running faces of the respective tabs lie in respective mutually-intersecting planes.

30. The system of claim 29, wherein the mutually-intersecting planes also intersect the pipeline.

31. The system of claim 29 or claim 30, wherein the mutually-intersecting planes intersect at an angle of less than 180 on their radially outer side with respect to the pipeline.

32. The system of claim 29 or claim 30, wherein the mutually-intersecting planes intersect at an angle of greater than 180 on their radially outer side with respect to the pipeline.

33. The system of any of claims 20 to 32, wherein the tabs comprise heat exchange elements being electrical heating wires or channels for circulation of heating and/or cooling fluid.

Description

[0065] In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which:

[0066] FIG. 1 is a schematic side view of abutting pipe sections joined by a circumferential girth weld;

[0067] FIG. 2 is a schematic cross-sectional view taken through the weld on line II=II of FIG. 1, showing a defect in the weld;

[0068] FIG. 3 is a schematic plan view of the abutting pipe sections shown in FIG. 1 ready for repairing the weld in accordance with the invention, with run-on and run-off tabs secured to the pipe sections at circumferentially-offset positions around the defect in the weld;

[0069] FIG. 4 is a schematic cross-sectional view taken through the weld and the run-on and run-off tabs on line IV-IV of FIG. 3, showing the weld now undergoing FSP in accordance with the invention;

[0070] FIG. 5 is a schematic cross-sectional view taken through the weld as in FIG. 2, showing a variant of the invention in which the run-on and run-off tabs are integrated into a single body and an FSP tool follows a straight path;

[0071] FIG. 6 is a schematic cross-sectional view taken through the weld as in FIG. 2, showing another variant of the invention in which an FSP tool follows a convex path with respect to the pipe sections; and

[0072] FIG. 7 is a schematic cross-sectional view taken through the weld as in FIG. 2, showing another variant in which an FSP tool follows a concave path with respect to the pipe sections.

[0073] FIG. 1 shows a pipeline 10 that comprises abutting pipe sections 12 of carbon steel joined end-to-end by a planar circumferential girth weld 14. The weld 14 will typically have been produced by multi-pass fusion welding although, in principle, other welding techniques such as FSW could have been used instead.

[0074] A defect 16 in the weld 14 is apparent in the cross-sectional view of FIG. 2, which defect 16 may be detected by NDT.

[0075] FIGS. 3 and 4 show a repair fitting 18 in accordance with the invention. The repair fitting 18 is attached to the pipeline 10 in alignment with the weld 14 for repairing the defect 16.

[0076] The repair fitting 18 comprises a run-on tab 20 and a run-off tab 22 that are circumferentially aligned with each other and placed onto the pipeline 10 on respective opposed circumferential sides of the defect 16. The tabs 20, 22 are substantially solid, generally wedge-shaped bodies of cast and/or machined steel, preferably of the same carbon steel as the pipe sections 12 of the pipeline 10. The wedge shapes of the tabs 20, 22 taper toward each other about the circumference of the pipeline 10.

[0077] The cross-sectional view of FIG. 4 shows that each tab 20, 22 comprises a concave-curved inner face 24 of part-circular cross-section, whose radius of curvature substantially matches that of the outer surface of the pipeline 10. This curvature of the inner face 24 allows the tab 20, 22 to seat closely against the pipeline 10. Each tab 20, 22 further comprises a substantially flat outer face 26. The outer face 26 converges with the inner face 24 to meet at a thin edge 28, as can be seen in the plan view of FIG. 3.

[0078] The curvature of the inner face 24 follows a circle that lies in the plane of the weld 14 when the tab 20, 22 is seated against the pipeline 10. The diameter of that circle substantially matches the outer diameter of the pipeline 10. Allowing for the minimal thickness of the edge 28, the outer face 26 nearly intersects that circle tangentially, at least where the outer face 26 approaches the edge 28. Thus, when the inner face 24 of a tab 20, 22 is seated onto the pipeline 10, the outer face 26 extends substantially tangentially with respect to the outer surface of the pipeline 10.

[0079] The tabs 20, 22 are placed over the weld 14 on the pipeline 10 in mirror-image mutual opposition centred on the radial position of the defect 16. The tabs 20, 22 taper toward each other and so present their edges 28 to each other in mutual opposition, in this example across a narrow gap 30 between the tabs 20, 22 where the outer surface of the pipeline 10 is exposed, as best appreciated in the plan view of FIG. 3. The gap 30 is aligned radially with the defect 16, although the defect 16 may extend circumferentially beyond the gap 30.

[0080] In the example shown in FIGS. 3 and 4, the flat outer faces 26 of the tabs 20, 22 substantially align in a common plane that is substantially tangential to the outer surface of the pipeline 10. That plane intersects the outer surface of the pipeline 10 in the gap 30 between the tabs 20, 22.

[0081] In this example, the tabs 20, 22 are attached to the pipeline 10 by clamping forces exerted by tightening longitudinally-spaced straps 32. The straps 32 encircle the pipeline 10 under tension and lie in spaced parallel planes to straddle the weld 14 between them. Intermediate strap portions 34 also connect and space apart the tabs 20, 22 to define the gap 30 between the edges 28 of the tabs 20, 22.

[0082] FIG. 4 shows a weld-backing device 36 inserted into the pipeline 10 in axial alignment with the weld 14. The weld-backing device 36 comprises back-up ring segments 38 that are driven radially by actuators 40. Initially the ring segments 38 are retracted radially inwardly for insertion of the weld-backing device 36 into the pipeline 10. When the weld-backing device 36 is in alignment with the weld 14, the ring segments 38 are extended radially outwardly as shown in FIG. 4 to apply back-up force against the inner wall of the pipeline inside the weld 14.

[0083] The radially-outward back-up force applied by the ring segments 38 resists high inward z-axis loads that are applied in use through a rotary FSP tool 42 facing toward the pipeline 10. In this way, the back-up force resists inward radial deformation or deflection of the pipeline 10 or the weld 14 during an FSP operation.

[0084] The FSP tool 42 is rotationally symmetrical about its central longitudinal axis 44, comprising: [0085] a shank 46 coupled at its proximal end to a motor 48 that supports and spins the FSP tool 42 about the axis 44; [0086] a probe holder 50 at the distal end of the shank 46; and [0087] a probe 52 protruding distally from the probe holder 50 toward the pipeline 10.

[0088] The probe holder 50 defines a shoulder 54 around the probe 52 to exert inward forging pressure on the softened metal during an FSP operation. Thus, the shoulder 54 extends substantially orthogonally from the central longitudinal axis 44.

[0089] The probe 52 has a frusto-conical shape that tapers distally from the shoulder 54. The length of the probe 52 measured from the shoulder 54 to the tip of the probe 52 is slightly less than the wall thickness of the pipeline 10.

[0090] The FSP tool 42 is driven by the motor 48 to spin at a controlled speed about the central longitudinal axis 44, which therefore serves as an axis of rotation for the FSP tool 42. As will be exemplified in later embodiments, the FSP tool 42 is also mounted and driven for translational movements with respect to the pipeline 10. Those translational movements comprise advancing or retracting the spinning FSP tool 42 on the z-axis along the central longitudinal axis 44 and traversing the spinning FSP tool 42 transversely to the central longitudinal axis 44, on the x-axis along the weld 14. The direction of traverse may, for example, be substantially orthogonal to the central longitudinal axis 44. As will also be explained, there is a further option to pivot the spinning FSP tool 42 with respect to the pipeline 10.

[0091] To illustrate the z-axis and x-axis movements of the FSP tool 42, FIG. 4 shows the FSP tool 42 in three states during an FSP pass.

[0092] To the left in FIG. 4, in dashed outline, the FSP tool 42 is shown near the start of the FSP pass where the probe 52 begins to enter the run-on tab 20. Here, the spinning FSP tool 42 is being advanced distally along the z-axis into the flat outer face 26 of the run-on tab 20. It will be noted that the probe 52 enters the outer face 26 substantially orthogonally and that the shoulder 54 lies in a plane parallel to the outer face 26. Insertion of the probe 52 into the run-on tab 20 starts to create a thermo-mechanically affected zone (TMAZ) 56, which is a region that is affected metallurgically by both temperature cycling and plastic deformation. It is in the TMAZ 56 that the metal of the run-on tab 20 is softened and stirred.

[0093] The z-axis movement of the FSP tool 42 continues until the probe 52 is fully inserted into the run-on tab 20, with the shoulder 54 of the probe holder 50 then bearing flat against the outer face 26. The shoulder 54 bears against the outer face 26 to limit insertion of the probe 52 and to apply forging pressure to the softened metal in the TMAZ 56.

[0094] The TMAZ 56 extends around the frusto-conical side wall of the probe 52 and also distally beyond the tip of the probe 52. Thus, the TMAZ 56 is slightly wider than the width of the probe 52 and extends slightly deeper into the run-on tab 20 than the depth or length of the probe 52.

[0095] Next, while maintaining this depth of insertion of the probe 52 as the shoulder 54 continues to bear against the outer face 26, the FSP tool 42 is traversed along the x-axis across the run-on-tab 20 toward the run-off tab 22. During this x-axis traversal, the probe 52 crosses the thin convex-curved interface 58 between the run-on tab 20 and the outer surface of the pipeline 10, more specifically the outer surface of the weld 14. The TMAZ 56 grows laterally to follow the progress of the probe 52 during x-axis traversal of the FSP tool 42, now therefore extending from the run-on tab 20 into the wall of the pipeline 10.

[0096] It will be seen in FIG. 4 that the probe 52 intersects the interface 58 at a shallow angle of incidence between the x-axis and the outer surface of the pipeline 10. In this example, the angle of incidence is never greater than about 30, which is when the tip of the probe 52 first encounters the outer surface of the pipeline 10. The angle of incidence then reduces with further progress of the probe 52 into the wall of the pipeline 10. This minimises loads on the probe 52 arising from the transition from the run-on tab 20 to the pipeline 10.

[0097] The probe 52 encounters the interface 58 initially at the distal tip of the probe 52. Then, as the FSP tool 42 traverses the weld 14 in the pipeline 10, the intersection between the probe 52 and the interface 58 moves progressively along the probe 52 in the proximal direction away from the tip. With continued proximal movement along the probe 52, the intersection between the probe 52 and the interface 58 eventually reaches the root of the probe 52 near the level of the shoulder 54. This occurs as the probe 52 exits the tapering part of the run-on tab 20 via the thin edge 28.

[0098] At this point, the probe 52 of the FSP tool 42 is fully contained in the wall thickness of the pipeline 12 as shown centrally in FIG. 4, in solid outline. This shows the still-spinning FSP tool 42 between the tabs 20, 22 with the probe 52 fully inserted into the wall of the pipeline 10 up to the shoulder 54 of the probe holder 50. The TMAZ 56 extends slightly ahead of the probe 52 and has now incorporated and hence repaired part of the defect 16.

[0099] Continued x-axis traversal of the FSP tool 42 further extends the TMAZ 56 circumferentially and so repairs the remainder of the defect 16. Gradually, the probe 52 exits the wall of the pipeline 10 on the opposite side to its entry point, passing through another thin convex-curved interface 60 between the outer surface of the pipeline 10 and the run-off tab 22.

[0100] Again, the probe 52 intersects the interface 60 at a shallow angle of incidence to minimise loads on the probe 52 arising from the transition from the pipeline 10 to the run-off tab 22. This angle of incidence is initially near zero where the probe 52 enters the tapering part of the run-off tab 22 via the thin edge 28. The angle of incidence then increases with further progress of the probe 52 into the run-off tab 22 but is never greater than about 30, which is when the tip of the probe 52 leaves the wall of the pipeline 10. The TMAZ 56 now extends as a segment across the wall of the pipeline 10. That segment encompasses the former defect 16, which has therefore been stirred into the TMAZ 56 and so has been fully repaired.

[0101] When the full length of the probe 52 is embedded in the run-off tab 22 and the probe 52 has sufficiently cleared the pipeline 10, the FSP tool 42 can then be lifted out of the run-off tab 22 by retracting movement along the z-axis. The FSP tool 42 is shown at the end of the FSP pass to the right in FIG. 4 in dashed lines, having been retracted from the run-off tab 22 and now no longer spinning. The full extent of the TMAZ 56 left behind by the probe 52 is also shown in dashed lines.

[0102] When the FSP pass is complete, the tabs 20, 22 are cut from the pipeline and the outer surface of the repaired weld 14 is ground flush with the adjoining pipe sections 12.

[0103] The transitions across the interfaces 58, 60 from the run-on tab 20 to the pipeline 10 and from the pipeline 10 to the run-off tab 22 are further eased because those parts are of similar materials and are at similar temperatures. Balancing the temperatures of the run-on tab 20, the pipeline 10 and the run-off tab 22 is improved by thermal coupling and conduction between the close-fitting inner faces 22 of the tabs 20, 22 and the outer face of the pipeline 10. Controlled selective heating of the tabs 20, 22 and the pipeline 10 is also possible, as will now be explained.

[0104] When the repair fitting 18 comprising the tabs 20, 22 has been attached to the pipeline 10, the pipeline 10 and the tabs 20, 22 are optionally heated to bring them closer to their softening temperature. This reduces stress and wear on the FSP tool 42. Heating suitably takes place before the probe 52 of the FSP tool 42 enters the run-on tab 20 with z-axis movement and may continue while the probe 52 traverses the pipeline 10 and the tabs 20, 22 with x-axis movement.

[0105] Cooling may also be applied selectively, locally or generally to the pipeline 10 or to the tabs 20, 22. Cooling may accelerate and control solidification of the part of the TMAZ 56 that trails the traversing probe 52. Cooling may also be used to bring the pipeline 10 and the tabs 20, 22 quickly down to a safe temperature for further operations after the FSP pass is complete.

[0106] Heating and cooling may be effected in various ways. For example, gas burners may be applied to heat the pipeline 10 and the tabs 20, 22 before the FSP pass begins. Alternatively, FIG. 4 shows heat-exchange elements 62 provided in the ring segments 38 of the weld-backing device 36 to heat or cool the pipeline 10 directly. Similarly, FIG. 3 shows that other heat-exchange elements 64 may be attached externally to or incorporated into the tabs 20, 22 to heat or cool the tabs 20, 22 directly. FIG. 3 shows that the heat-exchange elements 64 are spaced apart across the plane of the weld 14 to allow the probe 52 of the FSP tool 42 to pass between them during traversal along the x-axis.

[0107] The heat-exchange elements 62, 64 may comprise heating elements such as electrical resistance wires or induction coils and/or cooling elements such as pipes for a cooling flow of water or gas. Indeed, a heat-exchange element 62, 64 may serve as both a heating element and a cooling element, for example by carrying a switchable flow of steam, water or gas to heat and to cool as required. Some heat-exchange elements 62, 64 behind the traversing probe 52 could be activated to cool the pipeline 10 or the tabs 20, 22 at the same time as other heat-exchange elements 62, 64 ahead of the traversing probe 52 are activated to heat the pipeline 10 or the tabs 20, 22.

[0108] Another way to heat a ferrous alloy such as steel is to use induction heating. In this respect, FIG. 5 shows an embodiment of the invention in which a pre-heat system 66 is mounted on a carriage 68 to face the pipeline 10 and the tabs 20, 22. The pre-heat system 66 may comprise one or more inductive heating coils and/or high-frequency resistance preheaters. The pre-heat system 66 creates a pre-heated zone 70 of the pipeline 10 and the tabs 20, 22.

[0109] The FSP tool 42 is also mounted on the carriage 68, in a trailing position with respect to the pre-heat system 66 in the direction of traversal along the x-axis. Thus, with continued x-axis movement, the probe 52 of the FSP tool 42 following the pre-heat system 66 enters the pre-heated zone 70. This ensures that the FSP tool 42 encounters metal that is already hot, although not yet softened. As a result, the FSP tool 42 needs to input less heat energy generated by friction and stirring to achieve and to maintain the plasticised conditions that are necessary for the FSP operation.

[0110] FIG. 5 also shows a variant of the repair fitting 18 in which the tabs 20, 22 are not spaced as in the previous embodiment but instead are conjoined or integrated into a single body. Thus, the edges 28 opposed about a gap 30 of the previous embodiment are replaced by a thin web 72 that joins the tabs 20, 22 integrally. Now, the tabs 20, 22 share a single flat outer face 26. The plane of the outer face 26 no longer intersects the outer surface of the pipeline 10 but instead lies slightly outside it.

[0111] As FIG. 5 shows, the web 72 is thin enough that the probe of the FSP tool 42 extends through the web 72 and projects into the wall of the pipeline 10. The FSP tool 42 projects into the wall of the pipeline 10 to a depth that is sufficient for the TMAZ 56 to encompass and repair the defect 16 as shown.

[0112] FIG. 5 also shows, schematically, arrangements for effecting translational movements of the FSP tool 42 on the z- and x-axes with respect to the pipeline 10. In this respect, the carriage 68 is mounted for movement along a straight rail 74 that lies parallel to the flat outer face 26 of the tabs 20, 22. Thus, movement of the carriage 68 along the rail 74 effects x-axis translation of the FSP tool 42 across the flat outer face 26. Also, an actuator 76 is interposed between the carriage 68 and the motor 48 that supports and drives the FSP tool 42. Extension or retraction of the actuator 76 effects z-axis translation of the FSP tool 42 to drive the probe 52 into and out of the flat outer face 26.

[0113] A controller 78 provides central coordinated control of the system. Thus, the controller 78 controls the motor 48 to determine the spin speed of the FSP tool 42. The controller 78 also controls movement of the FSP tool 42 on the z- and x-axes by controlling movement of the carriage 68 and the actuator 76. Where provided, heating and/or cooling facilities such as the pre-heat system 66 and the heat-exchange elements 62,64 provided in the ring segments 38 and on the tabs 20, 22 are also controlled by the controller 78. Ideally, the controller 76 can initiate and control all operations that are necessary to execute a successful FSP pass.

[0114] As is well known in the FSW art, sensors (not shown) such as strain gauges or thermocouples may provide stress and temperature inputs to the controller 76 that are indicative of the condition of the FSP tool 42. The controller 76 may generate various control outputs in response to those inputs and also in response to operator commands.

[0115] The variants of the repair fitting 18 in FIGS. 6 and 7 show that outer faces 26 of the tabs 20, 22 need not be coplanar and indeed need not be flat. Consequently, unlike the embodiment shown in FIGS. 3 and 4, the FSP tool 42 is supported to swing or pivot about an axis orthogonal to its central longitudinal axis 44 so that the orientation of the central longitudinal axis 44 changes during the FSP pass. However, as before, the FSP tool 42 may be supported by a carriage 68 via an actuator 76 and a motor 48.

[0116] To illustrate this principle clearly, FIGS. 6 and 7 omit the pre-heat system 66 and the controller 78 of FIG. 5. It should, however, be understood that a pre-heat system 66 and a controller 78 may be present in these or any other embodiments of the invention.

[0117] In FIG. 6, the repair fitting 18 is essentially the same as that shown in FIGS. 3 and 4, with the exception that the gap 30 shown between the opposed edges 28 in FIG. 3 is wider. This may be achieved simply by lengthening the intermediate strap portions 34, also shown in FIG. 3, that connect and space apart the tabs 20, 22.

[0118] The wider gap 30 between the tabs 20, 22 in FIG. 6 shifts the tabs 20, 22 apart in opposite circumferential directions around the pipeline 10. Thus, the flat outer faces of the tabs 20, 22 remain substantially tangential with respect to the outer surface of the pipeline 10, but now form tangents with points on that surface that are circumferentially spaced rather than coincident.

[0119] It follows that the flat outer faces 26 of the tabs 20, 22 now lie in respective non-parallel planes that intersect along a line in the gap 30 between the tabs 20, 22. Those planes intersect with an external angle of greater than 180 between them on the radially outer side with respect to the pipeline 10.

[0120] The effect of this geometry is that the exposed portion of the pipeline 10 in the gap 30 between the tabs 20, 22 in FIG. 6 is longer in the circumferential direction than it is in the embodiments shown in FIGS. 3 and 4. The probe 52 of the FSP tool 42 will therefore cover a greater angular span of the wall of the pipeline 10, across a wider arc with respect to the central longitudinal axis of the pipeline 10. This allows a larger defect 16 in the weld 14 to be repaired in one FSP pass.

[0121] It will be apparent from FIG. 6 that the tabs 20, 22 and the exposed portion of the pipeline 10 in the gap 30 between the tabs 20, 22 together define a running surface 80 for the shoulder 56 of the FSP tool 42. That running surface 80 extends from the flat outer face 26 of the run-on tab 20 onto the flat outer face 26 of the run-off tab 22 via the outer surface of the exposed portion of the pipeline 10, which is part-circular in cross-section. Thus, the running surface 80 is generally convex when seen from the radially outer side, that is, from above as viewed in FIG. 6.

[0122] FIG. 6 shows that the central longitudinal axis 44 of the FSP tool 42 is kept substantially orthogonal to the running surface 80 throughout an FSP pass. In this example, this is achieved by moving the carriage 68 along a curved rail 82 that is shaped and oriented to run parallel to, and remain equidistant from, the running surface 80 along its length. Consequently, the rail 82 comprises straight end sections that correspond to the flat outer faces 26 of the tabs 20, 22, joined by a central section of part-circular curvature that corresponds to the exposed portion of the pipeline 10.

[0123] To the left in FIG. 6, the FSP tool 42 is shown in dashed outline just after the start of the FSP pass. The FSP tool 42 is spinning and its probe 52 has just reached its full depth in the run-on tab 20 after being advanced on the z-axis. The probe 52 has entered the outer face 26 of the run-on tab 20 substantially orthogonally so that the shoulder 54 of the FSP tool 42 lies in a plane parallel to the outer face 26. A TMAZ 56 has begun to form around the probe 52.

[0124] Traversing movement of the FSP tool 42 on the x-axis along the running surface 78 toward the run-off tab 22 now begins. The depth of insertion of the probe 52 is kept constant as the shoulder 54 continues to bear against the outer face 26 of the run-on tab 20 and then against the outer surface of the exposed portion of the pipeline 10 between the tabs 20, 22.

[0125] As the slope of the flat outer surface 26 of the run-on tab 20 remains constant, the orientation of the central longitudinal axis 44 of the FSP tool 42 is kept substantially constant until the FSP tool 42 encounters the exposed portion of the pipeline 10 between the tabs 20, 22. Here, the orientation of the central longitudinal axis 44 is changed continuously during continued circumferential movement of the FSP tool 42 around the wall of the pipeline 10, so as to remain radially aligned with respect to the curvature of the pipeline 10. For example, the FSP tool 42 is shown in a central position in solid lines in FIG. 6, with the probe 52 now engaged in the exposed portion of the pipeline 10 where it has repaired a portion of the defect 16 in the weld 14.

[0126] By the time that the FSP tool 42 encounters the flat outer face 26 of the run-off tab 22 with further x-axis movement, the central longitudinal axis 44 is orthogonal to that face 26. The FSP tool 42 is now ready to proceed with the same orientation into the run-off tab 22, eventually reaching a position like that shown in dashed lines to the right in FIG. 6 where the probe 52 has cleared the wall of the pipeline 10. The FSP tool 42 is now ready to be retracted from the run-off tab 22 with reverse movement on the z-axis.

[0127] As before, during x-axis movement of the FSP tool 42, the probe 52 crosses the interfaces 58, 60 between the run-on tab 20, the wall of the pipeline 10 and the run-off tab 22. The TMAZ 56 grows laterally to follow the progress of the probe 52. The full extent of the TMAZ 56 left behind by the probe 52 is shown in dashed lines. It will be apparent that the TMAZ 56 follows the shape of the running surface 78 and so is similarly convex-curved in this example.

[0128] It is convenient that the outer faces 26 of the tabs 20, 22 are flat as shown in FIG. 6 but they need not be flat, at least not along their full circumferential length. Also, the tabs 20, 22 need not be separate bodies but could instead be joined by a thin integral web, in much the same way as the embodiment of FIG. 5 relates to the embodiment of FIGS. 3 and 4.

[0129] In the embodiment shown in FIG. 7, the outer face 26 of each tab 20, 22 has a flat outward portion 84 and a concave-curved inward portion 86 that converges with the concave-curved inner face 24. The curved inward portion 86 of the outer face 26 meets the inner face 24 at a thin edge 28 like that shown in FIG. 3.

[0130] The flat outward portions 84 of the outer faces 26 of the tabs 20, 22 lie in respective non-parallel planes that intersect along a line aligned with the gap between the tabs 20, 22. In this embodiment, those planes intersect with an external angle of less than 180 between them on the radially outer side with respect to the pipeline 10. The line of intersection between those planes lies under the outer surface of the pipeline 10, within the wall of the pipeline 10.

[0131] It will be apparent from FIG. 7 that the tabs 20, 22 and the exposed portion of the pipeline 10 in the gap 30 between the tabs 20, 22 together define a running surface 88 for the shoulder 56 of the FSP tool 42. That running surface 88 extends from the flat outward portion 84 of the outer face 26 of the run-on tab 20 onto the flat outward portion 84 of the outer face 26 of the run-off tab 22. The running surface 88 extends via the outer surface of the exposed portion of the pipeline 10, which is part-circular in cross-section. By virtue of the radially-inward convergence of the planes of the flat outward portions 84 of the outer faces 26, the running surface 88 is generally concave when seen from the radially outer side, that is, from above as viewed in FIG. 7.

[0132] To avoid a sharp step in the running surface 88 at the interfaces between the pipe 10 and the tabs 20, 22, the inward portion 86 of the outer face 26 of each tab 20, 22 curves as it nears the edge 28 so as to approach the outer surface of the pipeline 10 substantially tangentially.

[0133] The effect of this geometry is that in comparison to the preceding embodiments, the probe 52 of the FSP tool 42 will cover a lesser angular span of the wall of the pipeline 10, across a smaller arc with respect to the central longitudinal axis of the pipeline 10. This reduces the length of the FSP pass that is necessary to repair a small defect 16 in the weld 14, while allowing the probe 52 and hence the TMAZ 56 to reach the same depth into the wall of the pipeline 10 as in the preceding embodiments.

[0134] As in the embodiment shown in FIG. 6, the central longitudinal axis 44 of the FSP tool 42 is kept substantially orthogonal to the running surface 88 throughout an FSP pass. Again, this is achieved by moving the carriage 68 along a curved rail 90 that is shaped and oriented to run parallel to, and remain equidistant from, the running surface 88 along its length.

[0135] To the left in FIG. 7, the FSP tool 42 is shown in dashed outline just after the start of the FSP pass. The probe 52 of the FSP tool 42 has entered the flat outward portion 84 of the outer face 26 of the run-on tab 20 substantially orthogonally. A TMAZ 56 has begun to form around the probe 52.

[0136] Traversing movement of the FSP tool 42 on the x-axis along the running surface 78 toward the run-off tab 22 now begins. The shoulder 54 of the FSP tool 42 continues to bear against the outer face 26 of the run-on tab 20 as it follows the curve of the inward portion 86 of the outer face 26. Next, the shoulder 54 bears against the outer surface of the exposed portion of the pipeline 10 between the tabs 20, 22. In this respect, the FSP tool 42 is shown in a central position in solid lines in FIG. 7, with the probe 52 now engaged in the exposed portion of the pipeline 10 where it has repaired a portion of the defect 16 in the weld 14.

[0137] Eventually the FSP tool 42 reaches the position shown in dashed lines to the right in FIG. 7, where the probe 52 has cleared the wall of the pipeline 10. The full extent of the TMAZ 56 left behind by the probe 52 is shown in dashed lines. The FSP tool 42 is now ready to be retracted from the run-off tab 22 with reverse movement on the z-axis.

[0138] Again, during the x-axis movement of the FSP tool 42, the probe 52 crosses the interfaces 58, 60 between the run-on tab 20, the wall of the pipeline 10 and the run-off tab 22. The TMAZ 56 grows laterally to follow the progress of the probe 52. Hence, the TMAZ 56 follows the shape of the running surface 88 and so is similarly concave-curved.

[0139] Again, the tabs 20, 22 shown in FIG. 7 need not be separate bodies but could instead be joined by a thin integral web, in much the same way as the embodiment of FIG. 5 relates to the embodiment of FIGS. 3 and 4.

[0140] Many other variations are possible without departing from the inventive concept. For example, the thin edges of the run-on and run-off tabs may abut, leaving no gap between the tabs even if the tabs are separate bodies. Also, the tabs may be attached to the pipeline by attachment means other than straps, such as by radially-inward pressure from external clamps, by adhesive bonding to the pipeline across the concave inner face or by tack welding to the pipeline around that inner face. Alternatively, the tabs can be bolted or otherwise attached to the clamping part of an orbital FSW system.

[0141] Whatever structure supports the FSP tool for movement relative to the pipeline may be anchored to the pipeline for stability, for example by a clamp ring that encircles the pipeline or clamp jaws that embrace the pipeline.

[0142] It is not essential that the central longitudinal axis of the FSP tool is exactly orthogonal to the surface of the workpiece into which the probe of the tool is pressed. A small departure from true orthogonality, by say 5, is possible. In that case, the shoulder of the tool suitably has a shallow frusto-conical shape to bear flat on one side against the surface of the workpiece.

[0143] It is not essential that the probe of the FSP tool is fully inserted into the run-on tab before traversal on the x-axis begins, or that retraction of the probe on the z-axis from the run-off tab can only begin after traversal on the x-axis has ended. For example, the excursion profile of the probe may be ramped such that the probe is advanced and/or retracted on the z-axis while x-axis movement takes place.

[0144] Guide means other than a shaped rail may be used to control the orientation of the FSP tool. For example, the tool could be supported and moved about various axes by a linkage or a system of actuators whose movements are controlled by the controller.

[0145] It is even possible to use a straight rail to traverse the FSP tool across a curved running surface. For example, the FSP tool could be pivotably mounted with respect to the rail and actuators could advance the FSP tool away from the rail or retract it toward the rail.