A Method of Making a Fluid Conduit

Abstract

A method of making a fluid conduit includes at least one tubular metal fitting in fluid communication with metal tubing. The conduit is apt to be incorporated into a subsea umbilical. The fitting is made by cold-forming a workpiece in a spinning operation. The workpiece material initially has a yield strength below that of the tubing material. The invention corrects or reduces this mismatch between the yield strength of the tubing material and the initial yield strength of the workpiece material by: assessing the yield strength of the tubing material; assessing the increased yield strength of the workpiece material by virtue of the spinning operation; and before welding the fitting to the tubing, comparing the increased yield strength of the workpiece material with the yield strength of the tubing material.

Claims

1. A method of making a fluid conduit that comprises at least one tubular metal fitting in fluid communication with metal tubing, the fitting and the tubing being of materials of mutually-compatible grades for welding to each other, the method comprising: cold-forming a workpiece in a spinning operation to make the fitting, the material of the workpiece initially having a yield strength below that of the material of the tubing; increasing the yield strength of the material of the workpiece by virtue of the spinning operation that forms the fitting from the workpiece; and welding the fitting to the tubing.

2. The method of claim 1, wherein the materials of the fitting and the tubing are of the same grade.

3. The method of claim 1, comprising cold-forming the workpiece at an above-ambient temperature below the recrystallisation temperature of the material of the workpiece.

4. The method of claim 1, comprising annealing or stress-relieving the fitting before welding the fitting to the tubing.

5. The method of claim 1, comprising providing the workpiece as a forging before cold-forming the workpiece in the spinning operation.

6. The method of claim 1, comprising providing the workpiece as a plate or a hollow bar before cold-forming the workpiece in the spinning operation.

7. The method of claim 1, comprising cold-forming the workpiece around a spinning mandrel.

8. The method of claim 1, comprising cold-forming the workpiece with a spinning forming tool.

9. The method of claim 1, comprising correcting or reducing a mismatch between the yield strength of the material of the tubing and the initial yield strength of the material of the workpiece.

10. The method of claim 1, comprising: assessing the yield strength of the material of the tubing; and assessing the increased yield strength of the material of the workpiece by virtue of the spinning operation.

11. The method of claim 10, comprising comparing the increased yield strength of the material of the workpiece with the yield strength of the material of the tubing, before welding the fitting to the tubing.

12. The method of claim 1, comprising re-certifying the material of the fitting in accordance with applicable standards before welding the fitting to the tubing.

13. The method of claim 1, comprising re-certifying the material of the fitting in accordance with ASTM A815.

14. The method of claim 1, comprising incorporating the fluid conduit into a subsea umbilical during assembly of the umbilical.

15. The method of claim 1, wherein the metal of the tubing and of the fitting are selected from the following group: steels; steel alloys, such as duplex, super duplex or Inconel; duplex and super-duplex stainless steels; and work-hardenable steels and steel alloys.

16. A subsea fluid conduit comprising at least one tubular cold-spun metal fitting welded to metal tubing in mutual fluid communication, the fitting and the tubing being of materials of mutually compatible grades.

17. The fluid conduit of claim 16, wherein the materials of the fitting and the tubing are of the same grade.

18. The fluid conduit of claim 16, wherein the materials of the fitting and the tubing have substantially identical yield strengths.

19. The fluid conduit of claim 16, wherein the tubing has an inner diameter of up to two inches (50.8 mm) and a length of at least 100 metres.

20. The fluid conduit of claim 16, wherein the metal of the tubing and of the fitting are selected from the following group: steels; steel alloys, such as duplex, super duplex or Inconel: duplex and super-duplex stainless steels; and work-hardenable steels and steel alloys.

21. A subsea umbilical comprising at least one fluid conduit of claim 16.

Description

[0034] To put the invention into context, reference has already been made to FIGS. 1 to 3 of the accompanying drawings, in which:

[0035] FIG. 1 is a schematic cross-sectional view through a subsea umbilical;

[0036] FIG. 2 is a schematic side view of a fitting for a fluid conduit of the umbilical shown in FIG. 1; and

[0037] FIG. 3 is a schematic side view of the fitting welded between pipes of the conduit.

[0038] In order that the invention may be more readily understood, reference will now be made, by way of example, to the remainder of the accompanying drawings in which:

[0039] FIG. 4 is a flow diagram illustrating a method of the invention;

[0040] FIG. 5a is a schematic side view of a tubular workpiece for cold-forming into the fitting;

[0041] FIG. 5b is a schematic sectional side view that shows the workpiece of FIG. 5a surrounding a mandrel;

[0042] FIGS. 5c and 5d correspond to FIG. 5b but show the workpiece being cold-formed around the mandrel during a spinning operation; and

[0043] FIGS. 6a to 6c are a sequence of schematic sectional side views that show a workpiece initially in the form of a disc or plate being cold-formed around a mandrel in a spinning operation.

[0044] Turning next, then, to FIG. 4, this flow diagram shows that the method of the invention comprises the preliminary steps of assessing the SMYS and material grade of tubing at 34 and then, at 36, providing a workpiece in a material of a grade that is compatible with the grade of the tubing.

[0045] In this context, compatibility between the material grades of the tubing and the workpiece requires those grades to be approved for welding a component of one material to a component of the other material when fabricating an assembly for use in the technical application required. For example, compatibility may be determined by industry standards such as the aforementioned ASTM A815, which applies to wrought piping fittings of ferritic, ferritic/austenitic and martensitic stainless steel as used in subsea umbilicals.

[0046] In some such cases, compatibility may require the material grades to be the same, or at least substantially the same. This is accepted to be the case when manufacturing fluid conduits for use in the subsea oil and gas industry, like those used in umbilicals. However, in a broad sense, compatibility does not necessarily preclude the material grades being different, provided that the applicable standards deem them to be compatible for welding components to each other for the technical application in question.

[0047] Once a suitable material grade has been selected for the workpiece at step 36, the workpiece is shaped by cold-spinning at step 38 to form a desired tubular fitting.

[0048] Between steps 36 and 38, the workpiece may also be subjected to an optional intermediate step of preliminary processing at 40 to prepare the workpiece for cold-spinning. For example, a cylindrical round bar could be bored longitudinally to make a workpiece in the form of a tube, or a plate could be cut to make a workpiece that is initially in the form of a circular disc.

[0049] In an intermediate preliminary processing step 40, the mechanical properties of the workpiece may also be checked and if necessary modified to facilitate cold-spinning, for example by heating the workpiece to improve its flexibility and ductility but not to a temperature that departs from the cold-forming domain. In this respect, cold working or cold forming effects plastic deformation of a metal below its recrystallisation temperature, in contrast to hot working or hot forming which effects plastic deformation of a metal above its recrystallisation temperature. The recrystallisation temperature for steels is typically between 400° C. and 700° C. but tends to be higher for stainless steels.

[0050] As the workpiece is cold-spun to form the fitting in step 38, the material of the workpiece undergoes work-hardening and therefore its mechanical properties will change from its initial state. The resulting mechanical properties are certified at step 42 to ensure that the fitting is suitable for its intended purpose, for example to ensure that work-hardening has increased the yield strength of the material to an extent that compensates for a beneficial reduction in the wall thickness of the fitting. Potentially, an additional treatment step such as annealing or other heat treatment such as stress-relieving may be required to adjust the properties of the cold-formed material before certification of the fitting at step 42.

[0051] Finally, once certified, the fitting is welded to the tubing at step 44.

[0052] FIGS. 5a to 5d and FIGS. 6a to 6c exemplify how the fitting 26 shown in FIGS. 2 and 3 may be cold-formed from a workpiece in a spinning operation. In FIG. 5a, the workpiece is a tube 46 that is preferably a bored round bar but could, in principle, be formed into a tubular shape by other known techniques such as extrusion. Conversely, in FIG. 6a, the workpiece 48 is initially a circular disc that is cut from a flat plate.

[0053] FIG. 5b shows the tube 46 surrounding an internal mandrel 50 that has been inserted into the central lumen of the tube 46. The mandrel 50 is longer than the tube and so protrudes longitudinally beyond the opposed open ends of the tube 46.

[0054] The circumferentially-stepped outer surface of the mandrel 50 reflects the correspondingly-stepped shape of the fitting 26 and determines the internal contour of the fitting 26. Thus, the mandrel 50 is rotationally symmetrical about a central longitudinal axis 28 and comprises a wide end portion 50A opposed to a narrow end portion 50B. A circumferential frusto-conical step or shoulder 52 defines the boundary between the end portions 50A, 50B of the mandrel 50 and effects the change of diameter between them. The wide end portion 50A of the mandrel 50 is a close sliding or interference fit within the surrounding tube 46.

[0055] Advantageously, the smooth outer surface of the mandrel 50 produces a correspondingly smooth internal surface finish within the fitting 26, hence promoting fluid flow through the fitting 26 and reducing deposition of solids on the interior of the fitting 26 in use.

[0056] FIGS. 5c and 5d show the tube 46 being cold-formed around the mandrel 50 during the spinning operation. In each case, the mandrel 50 and the tube 46 are spun together about the central longitudinal axis 28 while a forming tool 54 is pressed radially inwardly against the exterior of the tube 46 and advanced longitudinally, parallel to the central longitudinal axis 28. In this way, the wall of the tube 46 is progressively squeezed and cold-formed between the forming tool 54 and the mandrel 50, becoming radially thinner in the process.

[0057] FIG. 5c shows a first stage of the spinning operation, in which the narrow end portion 26B of the fitting 26 is being formed as a forming tool 54 presses the tube 46 radially inwardly against the narrow end portion 50B of the mandrel 50. Conversely, FIG. 5d shows a second stage of the spinning operation, in which the wide end portion 26A of the fitting 26 is being formed as a forming tool 54 presses the tube 46 radially inwardly against the wide end portion 50A of the mandrel 50.

[0058] In principle, the same forming tool 54 could be used for both stages of the spinning operation. However, to show another possibility, FIGS. 5c and 5d show different forming tools 54 being used at each stage.

[0059] The forming tool 54 shown in FIG. 5c turns about a spin axis 56 that intersects the central longitudinal axis 28 orthogonally. The forming tool 54 shown here comprises a frusta-conical head 58 that is rotationally symmetrical about the spin axis 56 and that tapers toward the mandrel 50. The taper angle of the head 58 determines, and hence substantially matches, the inclination of the shoulder 30 of the fitting 26.

[0060] In contrast, the forming tool 54 shown in FIG. 5d turns about a spin axis 56 that is parallel to the central longitudinal axis 28. The forming tool 54 shown here comprises a roller 60 in the shape of an oblate spheroid that is rotationally symmetrical about the spin axis 56.

[0061] It will be evident from FIGS. 5c and 5d that the squeezed, radially-thinned wall of the tube 46 elongates longitudinally and hence stretches along the mandrel 50, eventually lengthening the tube 46 to exceed the desired length of the fitting 26. The excess length of the fitting 26 is subsequently cut away and the ends of the fitting 26 are faced and chamfered to prepare the fitting 26 to be welded to respective tubes 24A, 24B as shown in FIG. 3.

[0062] Turning finally to FIGS. 6a to 6c, these drawings show how a fitting 26 may instead be cold-spun from a workpiece 48 that is initially in the form of a circular disc. FIGS. 6a to 6c also show a forming tool 54 that bears against the workpiece 48 to impart the eventual shape of the fitting 26 as the workpiece 48 is sandwiched between the forming tool 54 and the mandrel 50.

[0063] FIG. 6a shows the workpiece 48 in its initial flat disc shape, in a plane orthogonal to the central longitudinal axis 28, fixed to the narrow end of the mandrel 50 and also rotationally symmetrical about the central longitudinal axis 28. Again, the workpiece 48 spins with the mandrel 50 about the central longitudinal axis 28. During the spinning operation, the forming tool 54 deforms and collapses the workpiece 48 onto and along the mandrel 50 from a planar initial state of the workpiece 48 shown in FIG. 6a through a frusto-conical dished intermediate state shown in FIG. 6b to a tubular state substantially conforming to the shape of the mandrel 50 as shown in FIG. 6c.

[0064] FIG. 6b shows a first stage of the spinning operation, in which the narrow end portion 26B of the fitting 26 is being formed as the forming tool 54 presses the workpiece 48 radially inwardly against the narrow end portion 50B of the mandrel 50. Conversely, FIG. 6c shows a second stage of the spinning operation, in which the wide end portion 26A of the fitting 26 is being formed as the forming tool 54 presses the workpiece 48 radially inwardly against the wide end portion 50A of the mandrel 50.

[0065] The forming tool 54 exemplified in FIGS. 6a to 6c is like that shown in FIG. 5d. Thus, the forming tool 54 comprises an oblate spheroidal roller 60 that is rotationally symmetrical about a spin axis 56. In this case, however, the spin axis 56 is kept generally parallel to the part of the workpiece 48 contacted by the roller 60 and hence pivots during the spinning operation as the workpiece 48 deforms and collapses onto the mandrel 50.

[0066] Once the workpiece 48 is fully formed, the closed end of the workpiece 48 that surrounds the narrow end of the mandrel 50 is cut away to form an open-ended fitting 26 like that shown in FIG. 2.

[0067] In all embodiments, the spin axis 56 of the forming tool 54 is suitably coplanar with the central longitudinal axis 28 about which the workpiece rotates during the spinning operation.

[0068] Many variations are possible within the inventive concept. For example, the workpiece could be formed during spinning by pressing radially outwardly within an external female die rather than by pressing radially inwardly around an internal male mandrel.

[0069] A forming tool used in the invention preferably turns about a spin axis to reduce friction but, in principle, forming tools could be non-rotating, hence being in sliding contact with a suitably-lubricated rotating workpiece.