Method of making fluid conduit

Abstract

To provide a method of making a fluid conduit that includes at least one tubular metal fitting in fluid communication with metal tubing, the conduit being apt to be incorporated into a subsea umbilical.SOLUTION: 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 the 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.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to the incorporation of metal fittings into metal tubular bodies, and in particular to the manufacture of fluid conduits comprising fittings and tubular bodies, such as may be used in umbilicals in the offshore oil and gas industry. In such applications, the inventors have found it advantageous to modify the mechanical properties of the fittings welded to the tubular bodies, especially when applicable standards require that both the fittings and the tubular bodies be of the same material grade. [Background technology]

[0002] A subsea umbilical consists of an elongated body that extends between the sea surface and subsea equipment on the sea floor, or between multiple equipment installed at spaced-apart subsea locations. In addition to power and control signals, an integrated umbilical supplies fluids such as oil for hydraulic control of valves, hydrate inhibitors and other chemicals for flow assurance, and removal fluids such as dead oil or methanol. A typical integrated umbilical also provides data communications, for example, via fiber optics.

[0003] WO 2011 / 045582 exemplifies an umbilical. WO 2018 / 052311 discloses improvements for protecting steel fluid lines of an umbilical from corrosion. WO 2016 / 061235 proposes reinforcing the metal tube of an umbilical with a carbon fiber composite. WO 2018 / 148718 reinforces the welded area of the tube by expanding the tube. U.S. Patent Publication No. 2015 / 361728 discloses a method for cold-forming subsea pipe joints by a rotary action, while WO 2015 / 200325 discloses a method for producing tubes by flow-forming a hollow cylindrical preform. Summary of the Invention [Problem to be solved by the invention]

[0004] 1 shows a cross section of a conventional integrated subsea umbilical 10. The umbilical 10 comprises a bundle of elongated functional elements including multiple cables and fluid conduits bundled together by transverse spacer structures 12 within a tubular polymeric outer sheath 14. Although not shown in this schematic view, the outer sheath may include an inner sheath, an outer sheath, and one or more layers of steel wire armor disposed between the sheaths.

[0005] Power cable 16, also known as a power core, typically carries three-phase AC power at high voltage along umbilical 10. The functional elements of umbilical 10 further include a plurality of electrical signal cables 18, a fiber optic cable 20, and a plurality of reinforcing elements 22, such as steel or carbon wires, ropes, or rods.

[0006] The fluid conduit of the umbilical 10, also known as the fluid core or fluid line, consists of an elongated pipe or tube 24 that transports the supply fluid under pressure. The tube 24 may be made of carbon steel, but is more commonly made of a corrosion-resistant alloy such as duplex or superduplex stainless steel. The inside diameter of the tube 24 is small, typically less than 2 inches (50.8 mm), but the length can be tens to hundreds of meters.

[0007] The fluid conduit also includes tubular steel fittings 26, as illustrated in Figures 2 and 3, which are continuously welded to and in fluid communication with the steel tube 24. Such fittings 26 may be required at the end connections of the conduit and at one or more intermediate locations along the length of the conduit. Examples of such intermediate fittings 26 shown in Figures 2 and 3 include fittings 26 on the inner and / or outer diameter and / or or a transition piece that allows fluid communication between tubes 24 of different wall thicknesses. Fitting 26 is rotationally symmetrical about a central longitudinal axis 28 and includes a wide end portion 26A opposed by a narrow end portion 26B. A circumferential frustoconical step or shoulder 30 defines the boundary between end portions 26A and 26B and accommodates the diameter change therebetween.

[0008] 3 shows a wide end portion 26A of fitting 26 attached to large diameter fluid conduit tube 24A by a circumferential butt weld 32, and a narrow end portion 26B of fitting 26 attached to small diameter fluid conduit tube 24B by a similar weld 32, all aligned along central longitudinal axis 28. Thus, large diameter tube 24A, fitting 26, and small diameter tube 24B are disposed contiguously and in fluid communication with one another.

[0009] Traditionally, fittings used in umbilical fluid conduits are manufactured by forging. Standards such as ASTM A815 require such fittings to be welded to tubular members of the same material grade. However, the forging process results in fittings with inferior mechanical properties compared to the tubular members to which they are welded.

[0010] In this regard, the inventors have noted that the bundled design of the umbilical improves the yield strength values guaranteed by manufacturers of fabricated tubular bodies, but the same properties are not guaranteed by manufacturers of the corresponding fittings welded to such tubular bodies.

[0011] Specifically, tubular manufacturers guarantee a minimum yield strength (SMYS) that is greater than the industry-standard minimum yield strength (SMYS) for the same material grade. SMYS is a measure of the minimum stress that causes permanent plastic deformation. Other manufacturers' fittings of the same material grade may have an SMYS that is approximately 100 MPa lower than the SMYS of the corresponding tubular body. [Means for solving the problem]

[0012] Conventional solutions to this problem include over-performing the forged hardware and increasing the overall wall thickness of the tubular body to compensate for the hardware's low yield strength, an approach that adds unnecessary bulk and cost to the umbilical's fluid conduits.

[0013] Against this background, the present invention provides a method for manufacturing a fluid conduit comprising at least one tubular metal fitting in fluid communication with a metal tubular body, the fluid conduit often being incorporated into a subsea umbilical during assembly of the umbilical. [Brief explanation of the drawings]

[0014] To put the present invention into context, reference is made to Figures 1 to 3 of the accompanying drawings. [Figure 1] FIG. 1 is a schematic cross-sectional view of a subsea umbilical. [Figure 2] 2 is a schematic side view of a fitting for use with the fluid conduit of the umbilical shown in FIG. 1. FIG. [Figure 3] FIG. 1 is a schematic side view of a fitting welded between pipes of a conduit.

[0015] For a better understanding of the present invention, reference will now be made, by way of example only, to the remaining accompanying drawings in which: [Figure 4] FIG. 1 is a flow diagram illustrating the method of the present invention. [Figure 5a] FIG. 1 is a schematic side view of a tubular workpiece being cold formed into a fitting. [Figure 5b] 5b is a schematic cross-sectional side view showing the workpiece of FIG. 5a surrounding a mandrel. [Figure 5c] 5b corresponds to FIG. 5b but shows the workpiece being cold formed around the mandrel during the spinning operation. [Figure 5d] 5b corresponds to FIG. 5b but shows the workpiece being cold formed around the mandrel during the spinning operation. [Figure 6a] 1A-1C are a series of schematic cross-sectional side views illustrating a workpiece having an initial disk or plate shape being cold formed around a mandrel in a spinning operation. [Figure 6b] 1A-1C are a series of schematic cross-sectional side views illustrating a workpiece having an initial disk or plate shape being cold formed around a mandrel in a spinning operation. [Figure 6c]1A-1C are a series of schematic cross-sectional side views illustrating a workpiece having an initial disk or plate shape being cold formed around a mandrel in a spinning operation. DETAILED DESCRIPTION OF THE INVENTION

[0016] The method of the present invention includes cold forming a workpiece in a spinning operation to form a fitting, increasing the yield strength of a material of the workpiece by spinning the workpiece to form the fitting, and welding the fitting to a tubular body, the workpiece material having an initial yield strength lower than the yield strength of a material of the tubular body, such that the fitting and tubular body materials are weld-compatible, e.g., of the same grade.

[0017] To enhance ductility, the workpiece may be heated above ambient temperature but below the recrystallization temperature of the workpiece material during cold forming. The metal fittings may also be heat treated, for example by annealing or stress relieving, before being welded to the tubular body.

[0018] The workpiece may be forged or otherwise formed into, for example, a plate or bar before being cold formed in a spinning operation.

[0019] The workpiece is preferably cold formed around a spinning mandrel using a spinning forming tool.

[0020] In one sense, the principles of the present invention can be expressed as correcting or reducing the mismatch between the yield strength of the tubular body material and the initial yield strength of the workpiece material. In this case, the method can include evaluating the yield strength of the tubular body material and evaluating the increased yield strength of the workpiece material due to the spinning operation. The increased yield strength of the workpiece material can then be compared to the yield strength of the tubular body material prior to welding the fitting to the tubular body.

[0021] The material of the fitting may be re-certified according to applicable standards, such as ASTM A815, ASTM A815 / A815M-18, or other current revisions, prior to welding the fitting to the tubular body.

[0022] The present invention also encompasses a subsea fluid conduit comprising at least one tubular cold-spun metal fitting welded to a metal tubular body in fluid communication therewith, said fitting and said tubular body being constructed of compatible grades of material. The present invention also encompasses a subsea umbilical comprising at least one fluid conduit of the present invention.

[0023] The metal fitting material and the tubular body material may have substantially the same yield strength. The tubular body may be very elongated, for example, with an inner diameter of up to 2 inches (50.8 mm) and a length of at least 100 meters.

[0024] Therefore, the present invention provides a beneficial approach to reducing unnecessary material by adding a reinforcing process that involves spinning the hardware, which has the advantage of improving certain mechanical properties of the hardware, particularly its yield strength.

[0025] A preferred embodiment of the present invention utilizes metal spinning techniques to improve the mechanical properties of duplex or super duplex stainless steel material grades. The metallurgy of duplex or super duplex grades Therefore, the present invention proposes a metal fitting manufacturing technique that includes cold working instead of a hot forging process.

[0026] Duplex stainless steels have a two-phase microstructure consisting of a mixture of austenitic and ferritic steels. The increased yield strength of duplex stainless steels allows for the creation of thinner sections and significantly reduced weight. Super duplex stainless steels also offer these benefits, but their high chromium and molybdenum content increases their corrosion resistance.

[0027] It is advantageous to achieve work-hardening effects by cold-forming the metal during the spinning operation. Hot spinning is not preferred because work-hardening decreases as the working temperature of the material increases. However, it may be desirable to slightly increase the temperature of the workpiece to achieve the desired plasticity and ductility without tearing the material. A subsequent solution annealing heat treatment may also be required to evaluate the condition after cold forming. However, the workpiece shaping process is still considered and managed as a cold-forming operation.

[0028] A variety of spinning tools can be used, including hemispherical hardened steel rollers mounted on bearings to reduce friction with the workpiece.

[0029] The present invention is suitable for a repeatable process that can be automated, ensuring control of process variables and achieving guaranteed quality, which defines the requirements for batch and batch testing.

[0030] In the present invention, sheet material or forged bar material with holes can be spun into a cylindrical shape and work-hardened to improve material yield and tensile properties. Such spun items define control tubular fittings, such as transition pieces between tubes of different wall thicknesses and / or diameters.

[0031] An embodiment of the present invention provides a method for correcting or reducing strength mismatch between a metal tubular body and a metal fitting, the method comprising the steps of assessing the SMYS and material grade of the tubular body, providing a fitting base material of a material grade compatible with the tubular body, preforming the base material to obtain an initial piece, cold spinning the initial piece to reshape the fitting while improving or enhancing properties of the fitting material, recertifying the fitting material according to national or international standards, and welding the fitting to the tubular body.

[0032] The metal of the tubular body and the fittings may be selected from the group consisting of steel, alloy steel such as duplex, super duplex or Inconel, duplex stainless steel, super duplex stainless steel, work hardening steel, and work hardening alloy steel.

[0033] The initial piece may be, for example, a plate-shaped body or a hollow rod-shaped body, and is preferably fabricated by forging.

[0034] In summary, the present invention provides a method for manufacturing a fluid conduit comprising at least one tubular metal fitting in fluid communication with a metal tubular body, which fluid conduits are particularly often incorporated into subsea umbilicals, which present technical challenges and therefore fall within the scope of the present invention.

[0035] The fitting is formed by cold forming a workpiece in a spinning operation, the workpiece material having an initial yield strength lower than the yield strength of the tubular body material. The present invention provides a method for evaluating the yield strength of the tubular body material, evaluating the increased yield strength of the workpiece material due to the spinning operation, and applying the increased yield strength of the workpiece material to the tubular body prior to welding the fitting to the tubular body. The yield strength of the tubular body material is compared to the initial yield strength of the workpiece material to compensate for or reduce any discrepancy between the yield strength of the tubular body material and the initial yield strength of the workpiece material.

[0036] Referring now to Figure 4, this flow diagram illustrates that the method of the present invention includes the preliminary steps of assessing the SMYS and material grade of the tubular body at step 34 and providing a workpiece of a grade of material compatible with the grade of the tubular body at step 36.

[0037] In this regard, compatibility of the material grades of the tubular body and the workpieces requires that these grades are approved for welding components of one material to components of the other material in assembling assemblies used in the required technical application. For example, compatibility may be determined by industry standards such as the above-mentioned ASTM A815, which applies to fabricated pipe fittings made of ferritic, ferritic / austenitic, and martensitic stainless steels, such as those used in subsea umbilicals.

[0038] In such cases, compatibility may require that the material grades be the same, or at least substantially the same. This is recognized when manufacturing fluid conduits for use in the offshore oil and gas industry, as well as those used in umbilicals. However, in its broadest sense, compatibility does not necessarily exclude different material grades, as long as the applicable standard deems the components of the technical application compatible for welding together.

[0039] Once the appropriate material grade for the workpiece has been selected in step 36, the workpiece is shaped by cold spinning to form the desired tubular member in step 38.

[0040] Between steps 36 and 38, the workpiece may be subjected to an optional intermediate step of pre-processing at step 40 to prepare the workpiece for cold spinning. For example, a cylindrical rod may be cut to form a tubular workpiece with a longitudinal hole, or a flat plate may be cut to form a workpiece having an initial disk shape.

[0041] In an intermediate pre-treatment step 40, the mechanical properties of the workpiece may be confirmed and, if necessary, modified to facilitate cold spinning. This may be done, for example, by heating the workpiece to improve its flexibility and ductility, but without heating it to a temperature that would cause it to deviate from the cold-forming domain. In this regard, cold working or forming causes plastic deformation of the metal below its recrystallization temperature, as opposed to hot working or forming, which causes plastic deformation of the metal above its recrystallization temperature. The recrystallization temperature of steels is typically between 400°C and 700°C, although stainless steels tend to be higher.

[0042] As the workpiece is cold spun into the metal fitting in step 38, the workpiece material undergoes work hardening. Thus, the mechanical properties of the workpiece change from their initial state. The resulting mechanical properties are qualified in step 42 to ensure that the metal fitting is suitable for its intended purpose, e.g., work hardening has increased the yield strength of the material to an extent that complements the beneficial reduction in wall thickness of the metal fitting. Prior to qualifying the metal fitting in step 42, additional processing steps, such as annealing or other heat treatments, e.g., stress relief, may be required to adjust the properties of the cold-formed material.

[0043] Finally, once qualified, the fittings are welded to the tubular body in step 44.

[0044] 5a-5d and 6a-6c illustrate how the fitting 26 shown in FIGS. 2 and 3 can be cold formed from a workpiece in a spinning operation. In Figure 5a, the workpiece is preferably a tube 46, which is essentially a round bar with a hole, which may be formed into a cylindrical shape by other known techniques such as extrusion, while in Figure 6a, the workpiece 48 has an initial disk-like shape cut from a flat plate.

[0045] 5b shows the tube 46 surrounding an inner mandrel 50 inserted into its central cavity. The mandrel 50 is longer than the tube and therefore protrudes longitudinally from both open ends of the tube 46.

[0046] The circumferentially stepped outer surface of the mandrel 50 mirrors the corresponding stepped shape of the fitting 26 and determines the interior contour of the fitting 26. As such, the mandrel 50 is rotationally symmetrical about the central longitudinal axis 28 and includes a wider end portion 50A opposed by a narrower end portion 50B. A circumferential frustoconical step or shoulder 52 defines the boundary between the end portions 50A and 50B of the mandrel 50 and accommodates the diameter change therebetween. The wider end portion 50A of the mandrel 50 is a close sliding or interference fit within the surrounding tube 46.

[0047] Advantageously, the smooth outer surface of the mandrel 50 provides a correspondingly smooth inner surface on the inside of the fitting 26. This promotes fluid flow through the fitting 26 and reduces the buildup of solids inside the fitting 26 during use.

[0048] 5c and 5d show the tube 46 being cold-formed around the mandrel 50 during the spinning operation. In either case, the mandrel 50 and tube 46 are spun together about the central longitudinal axis 28 while the forming tool 54 is advanced longitudinally parallel to the central longitudinal axis 28 while pressing radially inward against the outside of the tube 46. In this manner, the wall of the tube 46 is gradually compressed and cold-formed between the forming tool 54 and the mandrel 50, thinning radially in the process.

[0049] Figure 5c illustrates a first stage of the spinning operation in which narrow end portion 26B of fitting 26 is formed as forming tool 54 presses tube 46 radially inward against narrow end portion 50B of mandrel 50. Figure 5d illustrates a second stage of the spinning operation in which wide end portion 26A of fitting 26 is formed as forming tool 54 presses tube 46 radially inward against wide end portion 50A of mandrel 50.

[0050] In principle, the same forming tool 54 can be used for both stages of the spinning operation, but to illustrate another possibility, Figures 5c and 5d show the case where different forming tools 54 are used for each stage.

[0051] 5c, the forming tool 54 rotates about a spin axis 56 that intersects the central longitudinal axis 28 at a right angle. The forming tool 54 shown here is rotationally symmetric about the spin axis 56 and includes a frusto-conical head 58 that tapers toward the mandrel 50. The taper angle of the head 58 is substantially consistent since it determines the slope of the shoulder 30 of the fitting 26.

[0052] 5d, on the other hand, rotates about a spin axis 56 that is parallel to the central longitudinal axis 28. The illustrated forming tool 54 includes an oblate spherical roller 60 that is rotationally symmetric about the spin axis 56.

[0053] 5c and 5d, the compressed, radially thinned walls of the tube 46 expand longitudinally and stretch along the mandrel 50, eventually resulting in a tube 46 that exceeds the desired length of the fitting 26. The excess length of the fitting 26 is then cut off, and the fitting 26 is then reassembled. The ends of 26 are surface finished and chamfered to provide fitting 26 which is welded to tubes 24A and 24B as shown in FIG.

[0054] Finally, reference is made to Figures 6a-6c, which illustrate a method of cold spinning fitting 26 from a workpiece 48 having an initial disk-like shape. Figures 6a-6c also show a forming tool 54 that applies pressure to workpiece 48 to impart the final shape of fitting 26 to workpiece 48, which is sandwiched between forming tool 54 and mandrel 50.

[0055] Figure 6a shows a workpiece 48 having an initial flat, disk-shaped configuration secured to the narrow end of a mandrel 50 in a plane perpendicular to the central longitudinal axis 28 and rotationally symmetric about the central longitudinal axis 28. The workpiece 48 also spins together with the mandrel 50 about the central longitudinal axis 28. During the spinning operation, a forming tool 54 folds and deforms the workpiece 48 against and along the mandrel 50 from the initial planar state of the workpiece 48 shown in Figure 6a, through an intermediate, frusto-conical state shown in Figure 6b, to a cylindrical state that substantially conforms to the shape of the mandrel 50 shown in Figure 6c.

[0056] Figure 6b illustrates a first stage of the spinning operation in which narrow end portion 26B of fitting 26 is formed as forming tool 54 presses workpiece 48 radially inward against narrow end portion 50B of mandrel 50. Figure 6c illustrates a second stage of the spinning operation in which wide end portion 26A of fitting 26 is formed as forming tool 54 presses workpiece 48 radially inward against wide end portion 50A of mandrel 50.

[0057] The forming tool 54 illustrated in Figures 6a-6c is the same as that shown in Figure 5d. To this end, the forming tool 54 includes an oblate spherical roller 60 that is rotationally symmetric about a spin axis 56. However, in this case, the spin axis 56 remains substantially parallel to the portion of the workpiece 48 that the roller 60 contacts, and thus pivots during the spinning operation as the workpiece 48 folds and deforms relative to the mandrel 50.

[0058] Once the workpiece 48 is fully formed, the closed end of the workpiece 48 surrounding the narrow end of the mandrel 50 is cut to form the open-ended fitting 26 as shown in FIG.

[0059] In all embodiments, the spin axis 56 of the forming tool 54 is preferably coplanar with the central longitudinal axis 28 about which the workpiece rotates during the spinning operation.

[0060] Many variations are possible within the concept of the invention, for example, the workpiece could be formed during spinning by forcing it radially outward inside an outer female die instead of radially inward around an inner male mandrel.

[0061] The forming tools used in the present invention preferably rotate about a spin axis to reduce friction, but in principle the forming tools do not rotate and can be suitably lubricated and come into sliding contact with the rotating workpiece.

Claims

1. 1. A method of manufacturing a fluid conduit comprising at least one tubular metal fitting in fluid communication with a metal tubular body, said fitting and said tubular body being made of mutually compatible grades of material, comprising: cold forming a workpiece in a spinning operation to form said fitting; increasing the yield strength of the material of the workpiece by the spinning operation of forming the fitting from the workpiece; and welding the fitting to the tubular body; The method for manufacturing a fluid conduit, wherein the workpiece material has an initial yield strength that is lower than the yield strength of the tubular body material.

2. The method of claim 1 , wherein the metal fitting and the tubular body are made of the same grade of material.

3. 3. The method of claim 1 or 2, comprising cold forming the workpiece at a temperature below the recrystallization temperature of the material of the workpiece and above ambient temperature.

4. 10. A method according to any preceding claim, including annealing or stress relieving the fitting prior to welding it to the tubular body.

5. 10. A method according to any preceding claim, comprising providing the workpiece as a forging prior to cold forming the workpiece in the spinning operation.

6. 10. A method according to any preceding claim, comprising providing the workpiece as a plate or hollow rod prior to cold forming the workpiece in the spinning operation.

7. 10. A method according to any preceding claim, comprising cold forming the workpiece around a spinning mandrel.

8. 10. A method according to any preceding claim, comprising cold forming the workpiece using a spinning forming tool.

9. 10. A method according to any preceding claim, comprising correcting or reducing a mismatch between the yield strength of the material of the tubular body and the initial yield strength of the material of the workpiece.

10. assessing the yield strength of the material of the tubular body; and 10. A method according to any preceding claim, comprising assessing the yield strength of the material of the workpiece increased by the spinning operation.

11. 11. The method of claim 10, including comparing the increased yield strength of the workpiece material to the yield strength of the tubular material before welding the fitting to the tubular body.

12. 10. A method according to any preceding claim, including recertifying the material of the fitting in accordance with applicable standards before welding the fitting to the tubular body.

13. 10. The method of any preceding claim, including recertifying the material of the fittings in accordance with ASTM A815.

14. 10. A method according to any preceding claim, comprising incorporating the fluid conduit into a subsea umbilical during assembly of the umbilical.

15. 10. The method of claim 1, wherein the metal of the tubular body and the fittings is selected from the group consisting of steel, alloy steel such as duplex, super duplex or Inconel, duplex stainless steel, super duplex stainless steel, work hardenable steel, and work hardenable alloy steel.

16. 1. A submarine fluid conduit comprising at least one tubular, cold-spun metal fitting welded to a metal tubular body in fluid communication therewith, said fitting and said tubular body being made of mutually compatible grades of material.

17. 17. The fluid conduit of claim 16, wherein the material of the fitting and the tubular body is the same grade.

18. 18. A fluid conduit according to claim 16 or 17, wherein the materials of the metal fitting and the tubular body have substantially the same yield strength.

19. A fluid conduit according to any one of claims 16 to 18, wherein the inner diameter of the tubular body is at most 2 inches (50.8 mm) and the length of the tubular body is at least 100 metres.

20. 20. The fluid conduit of claim 16, wherein the metal of the tubular body and the metal fittings is selected from the group consisting of steel, alloy steel such as duplex, super duplex or Inconel, duplex stainless steel, super duplex stainless steel, work hardenable steel, and work hardenable alloy steel.

21. A submarine umbilical comprising at least one fluid conduit according to any one of claims 16 to 20.

Images