Method of manufacturing a fluid conduit

Abstract

A method of manufacturing a fluid conduit comprising at least one tubular metal fitting in fluid communication with a metal pipe. The conduit is readily 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 lower than the pipe material. The present invention corrects or reduces this mismatch between the yield strength of the pipe material and the initial yield strength of the workpiece material by: assessing the yield strength of the pipe material; assessing the increased yield strength of the workpiece material through the spinning operation; and comparing the increased yield strength of the workpiece material to the yield strength of the pipe material before welding the fitting to the pipe.

Description

Technical Field

[0001] The present invention relates to the incorporation of metal fittings into metal pipe sections. The invention relates in particular to the manufacture of fluid conduits comprising such fittings and pipes, such as umbilicals useful in the subsea oil and gas industry. In such applications, the inventors have identified the benefits of modifying the mechanical properties of a fitting to be welded to the pipe, particularly where applicable standards require the fitting and pipe to be of the same material grade. Background Art

[0002] A subsea umbilical cable comprises an elongated body extending between subsea installations on the sea surface and the seabed or between items of equipment at mutually spaced subsea locations. In addition to power and control signals, an integrated umbilical cable also provides service fluids, such as: oil for valve hydraulic control; hydrate inhibitors and other chemicals for flow assurance; and repair fluids, such as dead oil or methanol. A typical integrated umbilical cable also provides data communications, such as via optical fiber.

[0003] WO 2011 / 045582 illustrates an umbilical cable. WO 2018 / 052311 describes an improvement for protecting the steel fluid line of an umbilical cable from corrosion. WO 2016 / 061235 proposes reinforcing the metal tube of an umbilical cable with a carbon fiber composite material. In WO 2018 / 148718, the weld zone of the tube is strengthened by expanding the tube. US2015 / 361728 describes a method for cold forming a connection section for a submarine pipeline by a rotary action, while WO 2015 / 200325 discloses a method for manufacturing a tube by flow forming a hollow cylindrical preform.

[0004] Figure 1 Shown is a cross section of a conventional integrated submarine umbilical 10. The umbilical 10 comprises a bundle of a plurality of elongated functional elements, including electrical cables and fluid conduits, held together by a transverse spacing structure 12 within a tubular polymeric sheath 14. Although not shown in this schematic diagram, the sheath may include an inner sheath and an outer sheath with one or more steel wire armor layers disposed between the sheaths.

[0005] Power cables 16, also called power cores, typically supply three-phase AC power at high voltage along the umbilical 10. Functional elements of the umbilical 10 also include electrical signal cables 18, fiber optic cables 20, and strength members 22, such as steel or carbon wires, ropes or rods.

[0006] The fluid conduit of the umbilical cable 10, also known as the fluid core or fluid line, includes an elongated pipe or tube 24 that conveys the service fluid under pressure. The tube 24 can be made of carbon steel, but is more typically made of a corrosion-resistant alloy, such as duplex stainless steel or super duplex stainless steel. The tube 24 has a small inner diameter, typically less than two inches (50.8 mm), but can be tens or hundreds of meters long.

[0007] The fluid conduit also includes a tubular steel fitting 26, such as Figure 2 and Figure 3 As shown, they are welded in series to, and in fluid communication with, the tube 24. Such fittings 26 are required to be connected at the ends of the conduit and may also be required at one or more intermediate locations along the length of the conduit. Figure 2 and Figure 3 The example of such an intermediate fitting 26 shown in is a transition piece that enables fluid communication between tubes 24 of different inner and / or outer diameters and / or different wall thicknesses. The fitting 26 is rotationally symmetrical about a central longitudinal axis 28 and includes a wide end 26A opposite a narrow end 26B. A circumferential frustoconical step or shoulder 30 defines the boundary between the wide end 26A, the narrow end 26B and effects a diameter change therebetween.

[0008] Figure 3 The wide end 26A of the fitting 26 is shown attached to the larger tube 24A of the fluid conduit by a circumferential butt weld 32, while the narrow end 26B of the fitting 26 is attached to the smaller tube 24B of the fluid conduit by another similar weld 32, all aligned on the central longitudinal axis 28. Thus, the larger tube 24A, the fitting 26, and the smaller tube 24B are arranged in series and in fluid communication with each other.

[0009] Traditionally, fittings for fluid conduits used in umbilicals are manufactured by forging. Compliance with standards such as ASTM (American Society for Testing and Materials) Standard Specification A815 requires that such fittings be welded to pipe fittings of the same material grade. However, the forging process leaves fittings with lower mechanical properties than the pipe fittings to which they are welded.

[0010] In this regard, the inventors have observed that umbilical cable bundle designs utilize increased yield strength values ​​guaranteed by manufacturers of deformed tubulars. However, manufacturers of corresponding fittings to be welded to such tubulars are unable to guarantee similar performance.

[0011] In particular, fitting manufacturers guarantee a specified minimum yield strength (SMYS) greater than the industry standard SMYS for the same material grade. SMYS represents the minimum stress that will cause permanent plastic deformation. The SMYS of fittings of the same material grade produced by other manufacturers may be approximately 100 MPa lower than the SMYS of the corresponding fitting.

[0012] The conventional solution to this problem is to over-design the swaged fittings and increase all pipe wall thicknesses to compensate for the lower yield strength of the fittings. This approach makes the umbilical's fluid conduit unnecessarily bulky and expensive. Summary of the invention

[0013] Against this background, the present invention provides a method of manufacturing a fluid conduit comprising at least one tubular metal fitting in fluid communication with a metal pipe. The fluid conduit is readily incorporated into a subsea umbilical during assembly of the umbilical.

[0014] The method of the invention comprises: cold forming a workpiece in a spinning operation to produce a fitting, the material of the workpiece initially having a yield strength lower than that of the pipe material; increasing the yield strength of the workpiece material by the spinning operation to form the fitting from the workpiece; and welding the fitting to the pipe. The materials of the fitting and the pipe are thus compatible for welding to each other, for example being of the same grade.

[0015] To increase ductility, the workpiece may be heated to cold form at an elevated temperature above ambient but below the recrystallization temperature of the workpiece material. The fitting may also be heat treated, such as by annealing or stress relieving, prior to welding to the tubular.

[0016] The workpiece may be forged or otherwise formed, for example in the form of a plate or hollow bar, prior to cold forming in a spinning operation.

[0017] The workpiece may suitably be cold formed about a rotating mandrel, preferably using a rotational forming tool.

[0018] In a sense, the principles of the present invention can be stated as correcting or reducing the mismatch between the yield strength of the tube material and the initial yield strength of the workpiece material. In that case, the method can include evaluating the yield strength of the tube 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 tube material before welding the fitting to the tube.

[0019] Prior to welding the fitting to the tubular, the fitting's material may be requalified to the applicable standard (e.g., ASTM A815, ASTM A815 / A815M-18, or other current revision).

[0020] The inventive concept includes a subsea fluid conduit comprising at least one tubular cold spun metal fitting welded in fluid communication with a metal pipe, the fitting and pipe being of mutually compatible grades of material. The inventive concept also extends to a subsea umbilical comprising at least one fluid conduit of the invention.

[0021] The material of the fitting and the pipe may have substantially the same yield strength.The pipe may be highly elongated, for example having an internal diameter of up to two inches (50.8 mm) and a length of at least 100 metres.

[0022] The present invention therefore takes the beneficial approach of reducing unnecessary material by adding a strengthening step that includes spinning the fitting. Spinning has the advantage of improving certain mechanical properties of the fitting, particularly its yield strength.

[0023] The preferred embodiment of the present invention uses metal spinning technology to improve the mechanical properties of duplex or super duplex stainless steel material grades. The metallurgy of duplex or super duplex grades makes cold working during the spinning process both feasible and beneficial. Therefore, the present invention proposes a manufacturing technology for fittings that involves cold working rather than hot forging processes.

[0024] Duplex steels have a two-phase microstructure, comprising a mixture of austenitic and ferritic steels. Their enhanced yield strength allows for thinner sections and significant weight savings. Superduplex steels have these advantages, but with higher chromium and molybdenum contents to provide even greater corrosion resistance.

[0025] Advantageously, cold forming the metal during the spinning operation achieves a work hardening effect. As the material's operating temperature increases, the degree of work hardening decreases, so hot spinning is not a preferred option. However, it may be necessary to slightly increase the temperature of the workpiece to achieve the required plasticity and ductility levels without tearing the material. A subsequent solution annealing heat treatment may also be required when evaluating cold forming conditions. However, the forming process of the workpiece is still considered a cold forming operation and is managed as a cold forming operation.

[0026] Various spinning tools can be used, such as hemispherical hardened steel rollers mounted on bearings to reduce friction with the workpiece.

[0027] The present invention lends itself to a repeatable process that can be automated to ensure control of process variables and achieve guaranteed quality, thereby defining batches and lot testing requirements.

[0028] By means of the present invention, material in the form of a plate or a drilled forged bar can be spun into a tubular form to work harden the material, thereby improving the yield and tensile properties of the material. For example, such a spun product can define a controlled pipe fitting, such as a transition piece between pipes of different wall thicknesses and / or diameters.

[0029] Embodiments of the present invention implement a method to correct or reduce the strength mismatch between a metal pipe and a metal fitting. The method includes the steps of evaluating the SMYS and material grade of the pipe; providing a base material for installing a compatible material grade to the pipe; preforming the base material into an initial piece; cold spinning the initial piece to reshape the fitting while improving or enhancing the properties of the fitting material; recertifying the fitting material according to national or international standards; and welding the fitting to the pipe.

[0030] The metal of the pipes and fittings may be selected from the group consisting of: steel; steel alloys such as duplex, super duplex or Inconel; duplex and super duplex stainless steels; and work hardenable steels and steel alloys.

[0031] For example, the initial piece may be a plate or a hollow bar. The initial piece is easily forged, preferably by forging.

[0032] 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 pipe. The conduit is particularly easy to incorporate into a submarine umbilical, which is the source of the technical problems and therefore also falls within the concept of the present invention.

[0033] The fitting is made by cold forming a workpiece in a spinning operation. The workpiece material initially has a yield strength lower than the tube material. The present invention corrects or reduces this mismatch between the yield strength of the tube material and the initial yield strength of the workpiece material by: assessing the yield strength of the tube material; assessing the increased yield strength of the workpiece material through the spinning operation; and comparing the increased yield strength of the workpiece material to the yield strength of the tube material before welding the fitting to the tube. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to place the present invention into context, reference has been made to the drawings Figures 1 to 3 ,in:

[0035] Figure 1 is a schematic cross-sectional view through a submarine umbilical cable;

[0036] Figure 2 yes Figure 1 A schematic side view of a fitting for a fluid conduit of an umbilical cable is shown; and

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

[0038] In order that the present 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] Figure 4 is a flow chart illustrating the method of the present invention;

[0040] Figure 5a is a schematic side view of a tubular workpiece for cold forming into a fitting;

[0041] Figure 5b It is shown Figure 5a A schematic cross-sectional side view of a workpiece around a mandrel;

[0042] Figure 5c and Figure 5d Corresponds to Figure 5b , but showing the workpiece cold formed about a mandrel during a spinning operation; and

[0043] Figures 6a to 6c is a series of schematic cross-sectional side views showing a workpiece, initially in the form of a disk or plate, being cold formed about a mandrel in a spinning operation. DETAILED DESCRIPTION

[0044] Next turn Figure 4 , the flow chart shows that the method of the present invention includes a preliminary step of evaluating the SMYS and material grade of the pipe at 34 and then providing a workpiece having a grade of material compatible with the grade of the pipe at 36.

[0045] In this case, compatibility between the material grades of the pipe fitting and the workpiece requires that these grades be approved for welding components of one material to components of another material when manufacturing components for the required technical application. For example, compatibility may be determined by an industrial standard such as the above-mentioned ASTM A815, which applies to wrought pipeline fittings of ferritic, ferritic / austenitic and martensitic stainless steels used in submarine umbilicals.

[0046] In some such cases, compatibility may require that the material grades be identical, or at least substantially identical. This is a recognized situation in the manufacture of fluid conduits for the subsea oil and gas industry, such as those used in umbilical cables. However, in a broad sense, compatibility does not necessarily exclude differences in material grades, as long as the applicable standards consider them compatible for mutually welded assemblies for the technical application in question.

[0047] Once the appropriate material grade is selected for the workpiece at step 36 , the workpiece is formed by cold spinning at step 38 to form the desired tubular fitting.

[0048] Between steps 36 and 38, the workpiece may also undergo an optional pre-treatment intermediate step at 40 to prepare the workpiece for cold spinning. For example, a cylindrical round bar may be drilled longitudinally to produce a workpiece in the form of a tube, or a plate may be cut to produce a workpiece initially in the form of a disc.

[0049] In the intermediate preliminary processing step 40, the mechanical properties of the workpiece may also be checked and modified if necessary to facilitate cold spinning, such as by heating the workpiece to increase its flexibility and ductility, but not to a temperature that deviates from the cold forming domain. In this regard, cold working or cold forming affects the plastic deformation of a metal below its recrystallization temperature, while hot working or hot forming affects the plastic deformation of a metal above its recrystallization temperature. The recrystallization temperature of steel is generally between 400°C and 700°C, but stainless steels tend to have higher recrystallization temperatures.

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

[0051] Finally, once certified, the fitting is welded to the pipe in step 44 .

[0052] Figure 5a to Figure 5d and Figures 6a to 6c An example is given Figure 2 and Figure 3 How the fitting 26 shown in FIG. can be cold formed from a workpiece in a spinning operation. Figure 5a In the embodiment, the workpiece 48 is a tube, which is preferably a bored round bar, but in principle can be formed into a tube by other known techniques such as extrusion. Figure 6a In FIG. 4 , the workpiece 48 is initially a disk cut from a flat plate.

[0053] Figure 5b The tube is shown surrounding an internal mandrel 50 which has been inserted into the central lumen of the tube. The mandrel 50 is longer than the tube and therefore protrudes longitudinally beyond the opposite open end of the tube.

[0054] The circumferential stepped outer surface of the mandrel 50 reflects the corresponding stepped shape of the fitting 26 and determines the internal profile of the fitting 26. Thus, the mandrel 50 is rotationally symmetrical about the central longitudinal axis 28 and includes a wide end 50A opposite a narrow end 50B. A circumferential frustoconical step or shoulder defines the boundary between the wide end 50A, the narrow end 50B of the mandrel 50 and affects the diameter change therebetween. The wide end 50A of the mandrel 50 is a close sliding fit or interference fit within the surrounding tube.

[0055] Advantageously, the smooth outer surface of the mandrel 50 creates a correspondingly smooth inner surface finish within the fitting 26, thereby promoting fluid flow through the fitting 26 and reducing the deposition of solids within the interior of the fitting 26 during use.

[0056] Figure 5c and Figure 5d A tube is shown being cold formed about a mandrel 50 during a spinning operation. In each case, the mandrel 50 and tube are rotated together about the central longitudinal axis 28 while a forming tool 54 is pressed radially inwardly against the exterior of the tube and is advanced longitudinally parallel to the central longitudinal axis 28. In this manner, the wall of the tube is gradually squeezed and cold formed between the forming tool 54 and the mandrel 50, becoming radially thinned in the process.

[0057] Figure 5c A first stage of the spinning operation is shown in which the narrow end 26B of the fitting 26 is being formed as the forming tool 54 presses the tube radially inwardly against the narrow end 50B of the mandrel 50 . Figure 5d A second stage of the spinning operation is shown wherein the wide end 26A of the fitting 26 is being formed as the forming tool 54 presses the tube radially inwardly against the wide end 50A of the mandrel 50 .

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

[0059] Figure 5c The forming tool 54 shown rotates about an axis of rotation 56 that intersects the central longitudinal axis 28 orthogonally. The forming tool 54 shown here comprises a frustoconical head 58 which is rotationally symmetrical about the axis of rotation 56 and tapers toward the mandrel 50. The cone angle of the head 58 determines and therefore substantially matches the inclination of the shoulder 30 of the fitting 26.

[0060] on the contrary, Figure 5d The forming tool 54 shown in FIG. 1 rotates about an axis of rotation 56 which 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 which is rotationally symmetrical about the axis of rotation 56 .

[0061] from Figure 5c and Figure 5d It can be readily seen that the extruded radially thinned wall of the tube stretches longitudinally and thus along the mandrel 50, ultimately lengthening the tube to exceed the desired length of the fitting 26. The excess length of the fitting 26 is then cut off and the ends of the fitting 26 are faceted and chamfered in preparation for welding the fitting 26 to the corresponding larger tube 24A, smaller tube 24B, as shown. Figure 3 shown.

[0062] Final Turn Figures 6a to 6c , these figures show how the fitting 26 can alternatively be cold spun from a workpiece 48 which is initially in the form of a disc. Figures 6a to 6c Also shown is a forming tool 54 which bears against the workpiece 48 to impart the final shape to the fitting 26 when the workpiece 48 is clamped between the forming tool 54 and the mandrel 50 .

[0063] Figure 6a The workpiece 48 is shown in its initial flat disk shape, fixed to the narrow end of the mandrel 50 in a plane orthogonal to the central longitudinal axis 28 and also rotationally symmetrical about the central longitudinal axis 28. Likewise, the workpiece 48 rotates with the mandrel 50 about the central longitudinal axis 28. During the spinning operation, the forming tool 54 causes the workpiece 48 to be spun from the central longitudinal axis 28 to the narrow end of the mandrel 50. Figure 6a The planar initial state of the workpiece 48 shown is as shown in FIG. Figure 6b The frustoconical dish shown in the intermediate state to Figure 6c The shape of the mandrel 50 is shown as being substantially uniformly tubular in shape and deforming and collapsing to and along the mandrel 50 .

[0064] Figure 6b The first stage of the spinning operation is shown, wherein the narrow end 26B of the fitting 26 is being formed as the forming tool 54 presses the workpiece 48 radially inwardly against the narrow end SOB of the mandrel 50. Figure 6c A second stage of the spinning operation is shown in which the wide end 26A of the fitting 26 is being formed as the forming tool 54 presses the workpiece 48 radially inwardly against the wide end 50A of the mandrel 50 .

[0065] Figures 6a to 6c The molding tool 54 illustrated in Figure 5d Thus, forming tool 54 includes an oblate spherical roller 60 that is rotationally symmetrical about an axis of rotation 56. However, in this case, axis of rotation 56 remains generally parallel to the portion of workpiece 48 that contacts roller 60 and thus pivots as workpiece 48 deforms and collapses onto mandrel 50 during the rotating operation.

[0066] Once the workpiece 48 is fully formed, the closed end of the workpiece 48 around the narrow end of the mandrel 50 is cut away to form the open-ended fitting 26, as shown in FIG. Figure 2 As shown.

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

[0068] Many variations are possible within the concept of the invention. For example, the workpiece can be formed during rotation by extrusion radially outwards in an outer female die rather than by extrusion radially inwards around an inner male mandrel.

[0069] The forming tool used in the present invention preferably rotates about an axis of rotation to reduce friction, but in principle the forming tool may be non-rotating and thus in sliding contact with a suitably lubricated rotating workpiece.

Claims

1. A method of manufacturing a fluid conduit comprising at least one tubular metal fitting in fluid communication with a metal pipe, the method comprising: cold forming a workpiece in a spinning operation to produce the fitting, the workpiece material having an initial yield strength lower than the yield strength of the tube material; increasing the yield strength of the workpiece material by a spinning operation to form the fitting from the workpiece; as well as The fitting is welded to the pipe.

2. The method of claim 1 comprising cold forming the workpiece at a temperature above ambient that is below the recrystallization temperature of the workpiece material.

3. A method according to claim 1 or 2, comprising annealing or stress relieving the fitting before welding it to the pipe.

4. A method according to claim 1 or 2, comprising providing the workpiece as a forging before cold forming the workpiece in the spinning operation.

5. A method according to claim 1 or 2, comprising providing the workpiece as a plate or hollow bar before cold forming the workpiece in the spinning operation.

6. A method according to claim 1 or 2, comprising cold forming the workpiece about a rotating mandrel.

7. The method of claim 1 or 2, comprising cold forming the workpiece with a spin forming tool.

8. The method of claim 1 or 2, comprising correcting or reducing a mismatch between the yield strength of the material of the tube and the initial yield strength of the material of the workpiece.

9. The method according to claim 1 or 2, comprising: evaluating the yield strength of the material of the pipe; as well as The material of the workpiece is evaluated for an increase in yield strength resulting from the spinning operation.

10. The method of claim 9, comprising comparing the increased yield strength of the material of the workpiece with the yield strength of the material of the pipe before welding the fitting to the pipe.

11. The method of claim 1 including verifying the mechanical properties of the material of the fitting after cold forming the workpiece in a spinning operation and before welding the fitting to the pipe to ensure that the fitting is suitable for its intended purpose.

12. The method of claim 1, comprising recertifying the mechanical properties of the material of the fitting in accordance with ASTM A815.

13. The method of claim 1 or 2, comprising incorporating the fluid conduit into a subsea umbilical during assembly of the subsea umbilical.

14. The method according to claim 1 or 2, wherein: The metals of the pipes and fittings are selected from the following group: Steels; steel alloys; duplex and super duplex stainless steels; and machine hardenable steels and steel alloys.

Images