Method of welding duplex stainless steel strip for the production of an armouring layer of a flexible pipe

• US 8,350,178 B2
• Filed: 03/14/2008
• Issued: 01/08/2013
• Est. Priority Date: 03/23/2007
• Status: Expired due to Fees

• Current Assignee: National Oilwell Varco Denmark I/S (NOV, Inc.)
• Original Assignee: National Oilwell Varco Denmark I/S (NOV, Inc.)
• Inventors: Inselmann, Steen
• Primary Examiner(s): Lebentritt, Michael
• Application Number:
  US12/532,732
• Publication Number:
  US 20100059488A1
• Time in Patent Office: 1,761 Days
• Field of Search: 219/60.2, 219/61, 219/79, 219/101, 219/103, 219/104, 219/121.64, 219/137.R, 219/137.WM, 219/146.23
• US Class Current: 219/100
• CPC Class Codes: B21C 37/124 the tubes having a special shape, e.g. with corrugated wall, flexible tubes
• CPC Class Codes: B23K 11/04 Flash butt welding
• CPC Class Codes: B23K 11/16 taking account of the properties of the material to be welded
• CPC Class Codes: B23K 2103/05 Stainless steel

Abstract

The invention relates to a method of providing a duplex stainless steel strip for an armouring layer of a flexible pipe, said method comprising

• • providing at least two duplex stainless steel pieces each having an end-face,
  • welding said end-faces of said two pieces of duplex stainless steel together to form a welded interface section using resistance welding, such as dual upset welding or flash welding, and
  • providing a controlled cooling of said welded interface section.

The duplex stainless steel strip should preferably be heated to a temperature of at least about 1100° C., preferably at least about 1200° C., such as at least about 1500° C. in said interface section, and the controlled cooling of said welded interface section preferably comprises prolonging the cooling of the interface section preferably at least for the cooling from about 800° C. to about 600° C., more preferably at least for the cooling from about 1100° C. to about 600° C.

Claims

1. A method of providing a flat duplex stainless steel strip for an armouring layer of a flexible pipe, said method comprising

providing at least two duplex stainless steel pieces each having an end-face and a thickness which is at least 10 times less than its width,

welding said end-faces of said two pieces of duplex stainless steel together to form a welded interface section using resistance welding,

allowing the welded interface section to cool down from an initial temperature during the welding where it is at least partly in liquid phase to a temperature y₁ of at least about 1100°

C. without application of additional heat,

whereafter a controlled cooling of said welded interface section is provided by applying heat in the form of current to reduce the cooling rate of the interface section at least for the cooling from about the temperature y₁ until the temperature of the interface section is a temperature y₂ of about 600°

C. or less.

2. A method as claimed in claim 1 wherein the duplex stainless steel pieces is in the form of oblong elements with a substantially identical cross-sectional profile, the method comprising welding the duplex stainless steel pieces to each other in an end-to-end configuration.

3. A method as claimed in claim 1, wherein the resistance welding is a dual upset welding.

4. A method as claimed in claim 3

wherein the dual upset welding comprises

bringing said two end faces in contact with each other

heating at least a section of said two duplex stainless steel pieces by applying a current, the heated sections comprising said end-faces, and

pressing said end-faces against each other.

5. A method as claimed in claim 4, wherein the application of current is sufficient to raise the temperature of the duplex stainless steel pieces at their respective end-faces to at least soften said duplex stainless steel, preferably the application of current being sufficiently low to avoid burning off the duplex stainless steel.

6. A method as claimed in claim 1, wherein the resistance welding is a flash welding, the method comprising

bringing said two end faces in contact with each other

heating at least a section of said two duplex stainless steel pieces by applying a current, the heated sections comprising said end-faces, and

applying a pressure.

7. A method as claimed in claim 6, wherein the application of current is in the form of a current density above about 25 A/mm² surface to be welded, for a period of about 1 second or less.

8. A method as claimed in claim 6 wherein the application of current is sufficiently high to at least burn off a surface layer of at least one of the abutting end-faces.

9. A method as claimed in claim 1, wherein the welding comprises heating the duplex stainless steel strip at least in said interface section to a temperature of at least about 1100°

C.

10. A method as claimed in claim 1, wherein the controlled cooling of said welded interface section comprises prolonging the cooling of the interface section at least for the cooling from about 800°

C. to about 600°

C.

11. A method as claimed in claim 1, wherein the controlled cooling is provided by applying heat to reduce the cooling rate in at least a part of the cooling of the interface section, the controlled cooling is provided by applying heat in the form of current to reduce the cooling rate in at least a part of the cooling of the interface section.

12. A method as claimed in claim 1, wherein the interface section is allowed to cool down to at least about 1200°

C. without application of additional heat where after the cooling rate is controlled by applying heat, until the temperature of the interface section is about 600°

C. or less.

13. A method as claimed in claim 1, wherein the interface of the temperature of the interface section is measured using Infra Red Pyrometer.

14. A method as claimed in claim 1, wherein the duplex stainless steel strip is of a duplex comprising an austenite and ferrite mixture in the interval from about 40:

60 volume %, to about 60;

40 volume %.

15. A method as claimed in claim 1, wherein the flexible pipe comprises an inner armouring layer (carcass) inside the inner liner, said inner armouring layer being provided by the duplex stainless steel strip(s).

16. An apparatus for welding flat duplex stainless steel pieces together wherein the duplex stainless steel pieces each comprise an end part with an end-face and a thickness which is at least 10 times less than its width, said apparatus comprises a pair of conductive platens capable of holding the duplex stainless steel pieces in their respective end parts, said pair of conductive platens being movable relative to each other and arranged to bring said end faces in contact with each other, and said pair of conductive platens being capable of heating said respective end parts of said duplex stainless steel pieces, wherein said apparatus further comprises a computer programmed to apply current through said conductive platens to heat said end parts to a sufficient level to weld said end-faces together to form an interface section, and to allow said interface section to cool under controlled conditions by applying current through said conductive platens during at least a part of the cooling of the interface section, such that said welded interface section is allowed to cool down from an initial temperature during the welding where it is at least partly in liquid phase to a temperature y₂ of at least about 1100°

C. without application of additional heat whereafter the cooling rate is controlled by applying heat in the form of current to reduce the cooling rate at least for the cooling from the temperature y₂ until the temperature of the interface section is a temperature y₂ of about 600°

C. or less.

17. An apparatus as claimed in claim 16 wherein said pair of conductive platens are to press said end faces together, said computer being programmed to press said end faces together during or after applying current through said conductive platens to heat said end parts to a sufficient level to weld said end-faces together to form the interface section.

18. An apparatus as claimed in claim 16, wherein said computer is programmed to allow the interface section to cool down to at least about 1200°

C. without application of additional heat where after the cooling rate is controlled by applying heat, until the temperature of the interface section is about 600°

C. or less.

19. An apparatus as claimed in claim 16, wherein said apparatus comprises an Infra Red Pyrometer temperature sensor arranged to determine the temperature of an interface section.