Method for manufacturing a fluid conduit

Abstract

A method for manufacturing a fluid conduit having at least one tubular metal fitting in fluid communication with a metal tubular body, the fluid conduit often being incorporated into a subsea umbilical. The fitting is formed by cold forming a workpiece in a spinning operation. The workpiece material has an initial yield strength lower than the yield strength of the tubular body material. The present invention corrects or reduces the mismatch between the yield strength of the tubular body material and the initial yield strength of the workpiece material by assessing the yield strength of the tubular body material, assessing the increased yield strength of the workpiece material due to the spinning operation, and comparing the increased yield strength of the workpiece material to the yield strength of the tubular body material prior to welding the fitting to the tubular body.

Description

Technical Field

[0001] The present invention relates to incorporating metal fittings into a plurality of metal tubular bodies, and more specifically, to the manufacture of fluid conduits provided with fittings and tubular bodies, which can be used, for example, in the umbilicals of the subsea oil and gas industry. In such applications, the inventors have found that it is effective to modify the mechanical properties of the fittings welded to the tubular body, particularly when the application standard requires that both the fittings and the tubular body be of the same material grade.

Background Art

[0002] A subsea umbilical consists of an elongated body extending between the sea surface and subsea facilities on the seabed or between a plurality of facilities installed at spaced-apart subsea locations. In addition to power and control signals, an integrated umbilical supplies fluids such as hydraulic oil for valve control, hydrate inhibitors for flow assurance and other chemicals, and removal fluids such as dead oil or methanol. A typical integrated umbilical also provides, for example, data communication via optical fibers.

[0003] WO 2011 / 045582 exemplifies an umbilical. WO 2018 / 052311 discloses improvements for protecting the steel fluid lines of an umbilical from corrosion. WO 2016 / 061235 proposes reinforcing the metal tubes of an umbilical with carbon fiber composites. In WO 2018 / 148718, the welding area of a tube is reinforced by expanding the tube. US 2015 / 361728 discloses a method of cold-forming the joints of subsea pipes by rotational movement, while WO 2015 / 200325 discloses a method of manufacturing tubes by flow-forming a hollow cylindrical preform.

Summary of the Invention

Problems to be Solved by the Invention

[0004] Figure 1 shows a cross-section of a conventional integrated subsea umbilical 10. The umbilical 10 comprises a bundle of elongated functional elements including a plurality of cables and fluid conduits bound together by a transverse spacer structure 12 within a cylindrical polymer outer sheath 14. Although not shown in this schematic, the outer sheath may include an inner sheath, an outer sheath, and one or more steel wire armor layers disposed therebetween.

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

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

[0007] The fluid conduit also includes a cylindrical steel fitting 26 that is continuously welded to the steel tube 24 to provide fluid communication, as illustrated in Figures 2 and 3. Such fittings 26 may be required at the end connections of the conduit and at one or more intermediate positions along the conduit length. Examples of such intermediate fittings 26 shown in Figures 2 and 3 include transition pieces that enable fluid communication between tubes 24 having different inner diameters and / or outer diameters and / or wall thicknesses. The fitting 26 is rotationally symmetric about a central longitudinal axis 28 and includes a wide end portion 26A that faces a narrow end portion 26B. A circumferential frustoconical step or shoulder 30 defines the boundary between the end portions 26A and 26B and corresponds to the change in diameter therebetween.

[0008] Figure 3 shows the wide end portion 26A of the fitting 26 attached to the large-diameter tube 24A of the fluid conduit by circumferential butt welding 32, and the narrow end portion 26B of the fitting 26 attached to the small-diameter tube 24B of the fluid conduit by similar welding 32. All of these are arranged in a row on the central longitudinal axis 28. For this reason, the large-diameter tube 24A, the fitting 26, and the small-diameter tube 24B are continuously arranged and are in fluid communication with each other.

[0009] Conventionally, fittings used for ambilical fluid conduits are manufactured by forging. Standards such as the ASTM (American Society for Testing and Materials) specification A815 require welding such fittings to tubular bodies of the same material grade. However, due to the forging process, the mechanical properties of the fittings become inferior to those of the tubular bodies to which the fittings are welded.

[0010] In this regard, the inventors focused on the fact that by designing the ambilical in a bundled form, the yield strength value guaranteed by the manufacturer of the processed tubular body is improved. However, similar properties are not guaranteed by the manufacturers of the corresponding fittings welded to such tubular bodies.

[0011] Specifically, the manufacturer of the tubular body guarantees a SMYS (Specified Minimum Yield Strength) that is greater than the industry-standard required minimum yield strength for the same material grade. SMYS serves as a measure of the minimum stress that causes permanent plastic deformation. The SMYS of fittings of the same material grade manufactured by other manufacturers may be 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-specifying the forged fittings and increasing the overall wall thickness of the tubular body to compensate for the low yield strength of the fittings. In this approach, the ambilical fluid conduit becomes unnecessarily bulky and costly.

[0013] Under such circumstances, the present invention provides a method for manufacturing a fluid conduit having at least one cylindrical metal fitting in fluid communication with a metal tubular body. This fluid conduit is often incorporated into the umbilical during the assembly of the subsea umbilical.

Brief Description of the Drawings

[0014] To explain the present invention in context, reference is made to FIGS. 1 to 3 of the accompanying drawings.

Figure 1

Figure 2

Figure 3

[0015] To make the present invention easier to understand, reference is also made to the remaining accompanying drawings as an example.

Figure 4

Figure 5a

Figure 5b

Figure 5c

Figure 5d

Figure 6a

Figure 6b

Figure 6c

Mode for Carrying Out 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 the material of the workpiece by the spinning operation of forming the fitting from the workpiece, and welding the fitting to a tubular body, wherein the initial yield strength of the material of the workpiece is lower than the yield strength of the material of the tubular body. Therefore, the materials of the fitting and the tubular body are weld-compatible with each other, for example, of the same grade.

[0017] In order to enhance ductility, when performing cold-forming, the workpiece may be heated to a temperature higher than the ambient temperature but lower than the recrystallization temperature of the material of the workpiece. The fitting may also be heat-treated, for example, by annealing or stress relief, before being welded to the tubular body.

[0018] Before being cold-formed in a spinning operation, the workpiece may be formed into, for example, a plate shape or a rod shape by forging or other processing.

[0019] Preferably, the workpiece is suitably cold-formed around a spinning mandrel using a spinning forming tool.

[0020] In a sense, the principle of the present invention can be expressed as correcting or reducing the discrepancy between the yield strength of the material of the tubular body and the initial yield strength of the material of the workpiece. In this case, the method may include evaluating the yield strength of the material of the tubular body and evaluating the yield strength of the material of the workpiece increased by the spinning operation. Then, before welding the fitting to the tubular body, the increased yield strength of the material of the workpiece and the yield strength of the material of the tubular body may be compared.

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

[0022] The concept of the present invention encompasses a subsea fluid conduit comprising at least one tubular cold-spun metal fitting welded to a metal tubular body and in fluid communication with each other, wherein the fitting and the tubular body are composed of materials of grades that are compatible with each other. The concept of the present invention also encompasses a subsea umbilical comprising at least one fluid conduit of the present invention.

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

[0024] Therefore, the present invention takes a beneficial approach of reducing unnecessary materials by adding a reinforcement process including spinning of the fitting. Spinning has the advantage of improving certain mechanical properties of the fitting, particularly its yield strength.

[0025] A preferred embodiment of the present invention uses metal spinning technology to improve the mechanical properties of duplex or super-duplex stainless steel material grades. Due to the metallurgy of duplex or super-duplex grades, cold working from the spinning process becomes possible and beneficial. Therefore, the present invention proposes a fitting manufacturing technology including cold working instead of a hot forging process.

[0026] Duplex steel has a duplex microstructure consisting of a mixture of austenitic steel and ferritic steel. Improvement of the yield strength of duplex steel enables the formation of thin sections and significant weight reduction. These benefits also apply to super-duplex steel, but due to its high chromium and molybdenum content, its corrosion resistance increases.

[0027] It is advantageous to achieve the work hardening effect by cold forming the metal during the spinning operation. Since work hardening decreases with an increase in the processing temperature of the material, hot spinning is not preferred. However, it is considered desirable to slightly increase the temperature of the workpiece without tearing the material to achieve the desired plasticity and ductility. Solution annealing heat treatment may also be required subsequently to evaluate the state after cold forming. However, the shaping process of the workpiece is still regarded and managed as a cold forming operation.

[0028] A variety of spinning tools can be used. For example, a hemispherical hardened steel roller mounted on a bearing to reduce friction with the workpiece is exemplified.

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

[0030] In the present invention, the yield and tensile properties of the material can be improved by spinning a plate-shaped material or a forging bar-shaped material with holes into a tubular shape to work harden. Such spun items define control tubular fittings such as transition pieces between tubes with different wall thicknesses and / or diameters.

[0031] Embodiments of the present invention implement a method for correcting or reducing the strength mismatch between a metal tubular body and a metal fitting. The method includes a step of evaluating the SMYS and material grade of the tubular body, a step of providing a base material of a fitting with a material grade compatible with the tubular body, a step of preforming the base material to obtain an initial piece, a step of cold spinning the initial piece to reshape the fitting while improving or enhancing the properties of the fitting material, a step of re-certifying the fitting material according to domestic or international standards, and a step of welding the fitting to the tubular body.

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

[0033] The initial piece may be, for example, a plate-like body or a hollow rod-like body. The initial piece is preferably often processed by forging.

[0034] In short, the present invention provides a method for manufacturing a fluid conduit provided with at least one cylindrical metal fitting in fluid communication with a metal tubular body. This fluid conduit is often incorporated particularly into a subsea umbilical, which is the cause of technical problems, and thus falls within the concept of the present invention.

[0035] The fitting is formed by cold-forming a workpiece in a spinning operation. The initial yield strength of the workpiece material is lower than the yield strength of the tubular body material. The present invention evaluates the yield strength of the tubular body material, evaluates the yield strength of the workpiece material increased by the spinning operation, and compares the increased yield strength of the workpiece material with the yield strength of the tubular body material before welding the fitting to the tubular body, thereby correcting or reducing the discrepancy between the yield strength of the tubular body material and the initial yield strength of the workpiece material.

[0036] Next, refer to FIG. 4. This flow diagram shows that the method of the present invention includes a preliminary step consisting of step 34 of evaluating the SMYS and material grade of the tubular body and step 36 of providing a workpiece of a material of a grade compatible with the grade of the tubular body.

[0037] In this regard, the compatibility of the tubular body and the workpiece material grades requires that these grades be approved for welding the components of one material to the components of the other material when assembling the assemblies used in the required technical applications. For example, the compatibility can be determined by industry standards such as ASTM A815 mentioned above, which apply to fabricated pipe fittings made of ferritic steel, ferritic steel / austenitic steel, and martensitic stainless steel as 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 in the manufacture of fluid conduits used in the subsea oil and gas industry, similar to the fluid conduits used in umbilicals. However, in a broad sense, compatibility does not necessarily exclude different material grades as long as the applicable standards consider the components of the technical application to be weld-compatible with each other.

[0039] When a suitable material grade for the workpiece is selected in step 36, in step 38, the workpiece is shaped by cold spinning and formed into the desired tubular fitting.

[0040] Between step 36 and step 38, the workpiece may be subjected to any intermediate steps of the pre-treatment in step 40 to prepare the workpiece for cold spinning. For example, it is possible to form a longitudinally perforated tubular workpiece from a cylindrical round bar, or to cut a flat plate to form a workpiece with an initial disk shape.

[0041] In the intermediate pretreatment step 40, the mechanical properties of the workpiece may be checked. If necessary, it may be modified to facilitate cold spinning. This can be done, for example, by heating the workpiece so that its flexibility and ductility are improved, but at this time, it is not heated to a temperature that deviates from the cold forming domain. In this regard, in contrast to hot working or hot forming that causes plastic deformation of the metal at a temperature higher than the recrystallization temperature of the metal, cold working or cold forming causes plastic deformation of the metal at a temperature lower than the recrystallization temperature of the metal. The recrystallization temperature of steel is typically between 400°C and 700°C, but stainless steel tends to be higher.

[0042] In step 38, since the workpiece is cold spun to form a fitting, the material of the workpiece work hardens. Therefore, the mechanical properties of the workpiece change from its initial state. The obtained mechanical properties are certified in step 42. It is confirmed that the fitting is suitable for the intended purpose, for example, due to work hardening, the yield strength of the material has increased to the extent that it compensates for a beneficial reduction in the wall thickness of the fitting. Before the certification of the fitting in step 42, additional processing steps such as annealing or other heat treatments, for example, stress relaxation, may be required as the case may be to adjust the properties of the cold formed material.

[0043] Finally, once certified, in step 44, the fitting is welded to the tubular body.

[0044] Figures 5a - 5d and 6a - 6c illustrate how the fitting 26 shown in FIGS. 2 and 3 can be cold formed from the workpiece in the spinning operation. In FIG. 5a, the workpiece is preferably a round bar tube 46 with holes that can be formed into a cylinder, in principle, by other known techniques such as extrusion. On the other hand, in FIG. 6a, the workpiece 48 has an initial shape cut from a flat plate that is disc - shaped.

[0045] Figure 5b shows a tube 46 surrounding an internal mandrel 50 inserted into its central cavity. Since the mandrel 50 is longer than the tube, it projects longitudinally from both open ends of the tube 46.

[0046] The outer surface of the mandrel 50 having a step in the circumferential direction reflects the corresponding step shape of the fitting 26 and determines the internal contour of the fitting 26. For this reason, the mandrel 50 is rotationally symmetric with respect to the central longitudinal axis 28 and includes a wide end portion 50A facing the narrow end portion 50B. The circumferential frustoconical step or shoulder portion 52 defines the boundary between the end portions 50A and 50B of the mandrel 50 and corresponds to the diameter change therebetween. The wide end portion 50A of the mandrel 50 is a close sliding or interference fit that fits inside the surrounding tube 46.

[0047] It is advantageous that the smooth outer surface of the mandrel 50 forms a corresponding smooth inner surface inside the fitting 26. Thereby, the flow of fluid through the fitting 26 is promoted and the deposition of solids inside the fitting 26 during use is reduced.

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

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

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

[0051] The forming tool 54 shown in Figure 5c rotates about a spin axis 56 that intersects orthogonally with the central longitudinal axis 28. The forming tool 54 shown here is rotationally symmetric with respect to the spin axis 56 and includes a frustum-shaped head 58 that tapers (narrows) toward the mandrel 50. The taper angle of the head 58 substantially matches to determine the inclination of the shoulder portion 30 of the fitting 26.

[0052] On the other hand, the forming tool 54 shown in Figure 5d rotates about a spin axis 56 parallel to the central longitudinal axis 28. The forming tool 54 shown here includes an oblate spherical roller 60 that is rotationally symmetric with respect to the spin axis 56.

[0053] As can be seen from Figures 5c and 5d, the wall of the compressed and radially thinned tube 46 extends longitudinally and stretches along the mandrel 50, and finally, the tube 46 becomes longer than the desired length of the fitting 26. Thereafter, the excess length of the fitting 26 is cut, and surface finishing and chamfering are performed on the ends of the fitting 26 to obtain the fitting 26 to be welded to the tubes 24A and 24B as shown in Figure 3.

[0054] Finally, refer to FIGS. 6a through 6c. These figures illustrate a method of cold spinning the fitting 26 from a workpiece 48 having an initial shape of a disc. FIGS. 6a through 6c also show a forming tool 54 that applies pressure to the workpiece 48 to impart the final shape of the fitting 26 to the workpiece 48 clamped between the forming tool 54 and the mandrel 50.

[0055] FIG. 6a shows a workpiece 48 having an initial shape of a flat disc shape, fixed to the narrow end portion of the mandrel 50 in a plane perpendicular to the central longitudinal axis 28 and rotationally symmetric with respect to the central longitudinal axis 28. Also, the workpiece 48 spins together with the mandrel 50 about the central longitudinal axis 28. During the spinning operation, the forming tool 54 folds and deforms the workpiece 48 against and along the mandrel 50 from the planar initial state of the workpiece 48 shown in FIG. 6a through an intermediate state in which the shape changes to a frustum shape shown in FIG. 6b to a cylindrical state substantially conforming to the shape of the mandrel 50 shown in FIG. 6c.

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

[0057] The forming tool 54 illustrated in FIGS. 6a through 6c is the same as that shown in FIG. 5d. For this reason, the forming tool 54 includes an oblate spherical roller 60 that is rotationally symmetric with respect to the spin axis 56. However, in this case, since the spin axis 56 is kept substantially parallel to a part of the workpiece 48 that the roller 60 contacts, the spin axis 56 turns during the spinning operation as the workpiece 48 folds and deforms with respect to the mandrel 50.

[0058] When the workpiece 48 is fully formed, the closed end of the workpiece 48 surrounding the narrow end portion of the mandrel 50 is cut to form the fitting 26 having an open end as shown in FIG. 2.

[0059] In all embodiments, it is preferred that the spin axis 56 of the forming tool 54 is in the same plane as the central longitudinal axis 28 about which the workpiece rotates during the spinning operation.

[0060] Various modifications are possible within the scope of the concept of the present invention. For example, during spinning, instead of pressing the workpiece radially inward around the internal male mandrel, it can be formed by pressing it radially outward inside the external female mold.

[0061] The forming tool used in the present invention preferably rotates around the spin axis to reduce friction. However, in principle, since the forming tool does not rotate, it can be preferably lubricated and brought into sliding contact with the rotating workpiece.

Claims

A method for manufacturing a fluid conduit comprising at least one forged (different from the first processing) tubular metal fitting in fluid communication with a first processed metal tubular body, the fitting and the tubular body being made of a material of a grade suitable for being welded to each other, comprising: Cold forming the workpiece in a spinning operation to form the 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, A method for manufacturing a fluid conduit, wherein the initial yield strength of the material of the workpiece is lower than the yield strength of the material of the tubular body.

2. The method according to claim 1, comprising cold forming the workpiece at a temperature lower than the recrystallization temperature of the material of the workpiece and higher than the ambient temperature.

3. The method according to claim 1 or 2, comprising annealing or stress relieving the fitting before welding the fitting to the tubular body.

4. The method according to any one of claims 1 to 3, comprising providing the workpiece as a plate-like body or a hollow rod-like body before cold forming the workpiece in the spinning operation.

5. The method according to any one of claims 1 to 4, comprising cold forming the workpiece around a spinning mandrel.

6. The method according to any one of claims 1 to 5, comprising cold forming the workpiece using a spinning forming tool.

7. The method according to any one of claims 1 to 6, comprising correcting or reducing the difference between the yield strength of the material of the tubular body and the initial yield strength of the material of the workpiece.

8. Evaluating the yield strength of the material of the tubular body, and Evaluating the yield strength of the material of the workpiece increased by the spinning operation, the method according to any one of claims 1 to 7.

9. The method according to claim 8, comprising comparing the increased yield strength of the material of the workpiece with the yield strength of the material of the tubular body before welding the fitting to the tubular body.

10. The method according to any one of claims 1 to 9, including determining the mechanical properties of the material of the fitting in order to confirm that the fitting is suitable for the intended purpose after cold forming of the workpiece by the spinning operation and before welding the fitting to the tubular body.

11. The method according to claim 10, including determining the mechanical properties of the material of the fitting in accordance with ASTM A815.

12. The method according to any one of claims 1 to 11, including incorporating the fluid conduit into the umbilical during the assembly of the subsea umbilical.

13. The metal of the tubular body and the fitting 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-hardening steel, and work-hardening alloy steel, according to any one of claims 1 to 12.

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