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EP-4739986-A1 - A METHOD AND APPARATUS FOR MONITORING STRESS IN STEEL LIGAMENTS WITHIN A FLEXIBLE PIPE

EP4739986A1EP 4739986 A1EP4739986 A1EP 4739986A1EP-4739986-A1

Abstract

The present invention relates to a method (1200) and apparatus for monitoring stress in steel ligaments within a flexible pipe. The method comprises: subjecting (1202) a ligament to a first magnetic field and obtaining a first measurement indicative of a magnetic flux density near an outer surface of the pipe adjacent to the ligament; and subjecting (1204) the ligament to a second magnetic field and obtaining a second measurement indicative of the magnetic flux density near the outer surface of the pipe adjacent to the ligament. The first magnetic field comprises an alternating magnetic field and wherein the second magnetic field comprises a combination of the alternating magnetic field and a static magnetic field. The method further comprises comparing (1205) the first and second measurements and evaluating (1209) stress in the ligament from the comparison of the first and second measurements.

Inventors

  • BUTTLE, DAVID JOHN

Assignees

  • Baker Hughes Energy Technology UK Limited

Dates

Publication Date
20260513
Application Date
20240705

Claims (20)

  1. 1. A method for monitoring stress in steel ligaments within a flexible pipe, the method comprising: subjecting a ligament to a first magnetic field and obtaining a first measurement indicative of a magnetic flux density near an outer surface of the pipe adjacent to the ligament; subjecting the ligament to a second magnetic field and obtaining a second measurement indicative of the magnetic flux density near the outer surface of the pipe adjacent to the ligament; comparing the first and second measurements; and evaluating stress in the ligament from the comparison of the first and second measurements; wherein the first magnetic field comprises an alternating magnetic field and wherein the second magnetic field comprises a combination of the alternating magnetic field and a static magnetic field.
  2. 2. The method according to claim 1, wherein the static magnetic field has a magnitude greater than a magnitude of the alternating magnetic field.
  3. 3. The method according to any one of the preceding claims, wherein comparing the first and second measurements comprises: determining a parameter sensitive to stress from each of the first and second measurements; and comparing the parameter sensitive to stress from each of the first and second measurements.
  4. 4. The method according to claim 3, wherein comparing the parameter sensitive to stress from each of the first and second measurements comprises subtracting the parameter sensitive to stress determined from one of the first and second measurements from the parameter sensitive to stress determined from the other measurement.
  5. 5. The method according to any one of the preceding claims, wherein the first measurement and the second measurement each comprise a plurality of measurements each made at a different frequency of the alternating magnetic field.
  6. 6. The method according to any one of the preceding claims comprising: subjecting each ligament of a plurality of ligaments to the first magnetic field and obtaining the first measurement for each ligament; subjecting each ligament of a plurality of ligaments to the second magnetic field and obtaining the second measurement for each ligament; and comparing, for each ligament, the first and second measurements; evaluating stress in each ligament from the comparison of the first and second measurements.
  7. 7. The method according to claim 6, wherein both the first and second measurements for one ligament are obtained prior to obtaining the first and second measurements for a subsequent ligament.
  8. 8. The method according to claim 6 or 7, comprising detecting if any of the ligaments are broken or differently loaded based on variation in comparison of first and second measurements for the different ligaments.
  9. 9. The method according to any one of the preceding claims comprising: providing an apparatus, the apparatus comprising: a first electromagnet core defining two spaced part poles and a first drive coil wound around the first electromagnet core; an alternating current source configured to supply alternating current to the first drive coil to generate the alternating magnetic field; means for selectively producing a static magnetic field; and sensors configured to sense magnetic flux density; and aligning the apparatus with a location by the outer surface of the pipe; wherein subjecting a ligament to a first magnetic field comprises supplying a current to the first drive coil from the alternating current source; and wherein subjecting a ligament to the second magnetic field comprises supplying a current to the first drive coil from the alternating current source and producing the static magnetic field from the means for selectively producing a static magnetic field.
  10. 10. The method according to claim 9, wherein the means for selectively producing a static magnetic field comprises a second drive coil and a constant current source configured to selectively supply a constant current to the second drive coil; and wherein subjecting the ligament to the second magnetic field comprises supplying a current to the first drive coil from the alternating current source and supplying a current to the second drive coil from the constant current source.
  11. 11. The method according to claim 9, wherein the means for selectively producing a static magnetic field comprises a magnetic switchable device; and wherein subjecting the ligament to the second magnetic field comprises supplying a current to the first drive coil from the alternating current source and switching on the magnetic switchable device.
  12. 12. The method according to claim 9, wherein the means for selectively producing a static magnetic field comprises a constant current source configured to selectively supply a constant current to the first drive coil; and wherein subjecting the ligament to the second magnetic field comprises supplying a current to the first drive coil from the alternating current source and from the constant current source.
  13. 13. The method according to any one of claim 9 to 12, when dependent on claim 6, wherein repeating the first and second measurements for each of the plurality of ligaments comprises aligning the apparatus with different locations on the outer surface of the pipe.
  14. 14. An apparatus for monitoring stress in steel ligaments within a flexible pipe, the apparatus comprising: a first electromagnet core defining two spaced part poles and a first drive coil wound around the first electromagnet core; 5 an alternating current source configured to supply alternating current to the first drive coil to generate an alternating magnetic field; a static magnetic field production means configured to selectively produce a static magnetic field that overlaps with the alternating magnetic field; and sensors configured to sense magnetic flux density in response to the alternating 10 and static magnetic fields.
  15. 15. An apparatus according to claim 14, wherein the static magnetic field production means comprises at least one second drive coil and a constant current source configured to selectively supply a constant current to the second drive coil. 15
  16. 16. An apparatus according to claim 15, wherein the at least one second drive coil is wound around the first electromagnet core.
  17. 17. An apparatus according to claim 16, wherein the first electromagnet core comprises 20 two legs, each leg defining one of the spaced apart poles; and wherein each leg comprises a plurality of fingers and at least one second drive coil is wound around each of the plurality of fingers.
  18. 18. The method according to claim 15, wherein the static magnetic field production means 25 comprises a second electromagnet core defining two spaced part poles and wherein the at least one second drive coil is wound around the at least one second electromagnet core.
  19. 19. The method according to claim 15, wherein the static magnetic field production means comprises two second electromagnet cores and wherein at least one second drive coil is 30 wound around each of the second electromagnet cores, and optionally wherein the two second electromagnet cores are arranged so that a pole of the two second electromagnet cores is adjacent to a corner of the poles of the first electromagnetic core.
  20. 20. The method according to claim 18 or 19, wherein the first electromagnet core resides between the poles of the second electromagnet core or cores and/or wherein the first electromagnet core and the second electromagnet core or cores are orientated such that a direction of the alternating magnetic field is perpendicular to the direction of the static magnetic field.

Description

A METHOD AND APPARATUS FOR MONITORING STRESS IN STEEL LIGAMENTS WITHIN A FLEXIBLE PIPE The present invention relates to a method and apparatus for monitoring stress in steel ligaments within a flexible pipe. In particular, but not exclusively, the present invention relates to a method and apparatus for monitoring stress in steel ligaments within a flexible pipe to evaluate a change in stress in a ligament and, optionally, a broken ligament. Traditionally flexible pipes are utilised to transport production fluids, such as oil and/or gas and/or water, from one location to another. Flexible pipe is particularly useful in connecting a sub-sea location (which may be deep underwater) to a sea level location. The pipe may have an internal diameter of typically up to around 0.6 metres (e.g. diameters may range from 0.05 m up to 0.6 m). Flexible pipe is generally formed as an assembly of a flexible pipe body and one or more end fittings. The pipe body is typically formed as a combination of layered materials that form a pressure-containing conduit. The pipe structure allows large deflections without causing bending stresses that impair the pipe’s functionality over its lifetime. The pipe body is generally built up as a combined structure including polymer, and/or metallic, and/or composite layers. For example, a pipe body may include polymer and metal layers, or polymer and composite layers, or polymer, metal and composite layers. In many known flexible pipe designs, the pipe body includes one or more tensile armour layers. The primary loading on such a layer is tension. In high pressure applications, such as in deep and ultra deep-water environments, the tensile armour layer experiences high tension loads from a combination of the internal pressure end cap load and the selfsupported weight of the flexible pipe. This can cause failure in the flexible pipe since such conditions are experienced over prolonged periods of time. Failure can lead to significant quantities of the production fluids (such as oil and/or gas) carried by the pipe leaking into the surrounding environment. In known designs, the tensile armour layer may comprise a plurality of helically wound steel ligaments or wires. Failure in the flexible pipe may occur when one or more of the ligaments fails (i.e. breaks). However, these ligaments cannot be observed directly because they are generally enclosed by other layers within the flexible pipe. Prior patent document WO 2014/096817 A1 describes a method of and apparatus for inspecting flexible risers to warn of failures in a tensile armour layer thereby mitigating the rise of catastrophic failure of a flexible pipe. The method includes using an electromagnetic probe to determine a parameter sensitive to stress in ligaments in a tensile armour layer and using variation of the parameter between different ligaments to detect if any ligaments have broken. The described method may be applied to a flexible pipe in situ. WO 2014/096817 A1 describes that measurements of the parameter sensitive to stress may be taken with different stresses in the riser by changing the pressure of the fluids within the riser. Taking measurements at two different pressures can assist in distinguishing between residual stresses and applied stresses in the ligaments. In some cases, the residual stresses may vary significantly between the different ligaments in each tensile armour layer which may complicate the detection of whether any ligaments have broken. An externally applied stress in the riser should change the stresses in all the wires or ligaments, but if a wire or ligament is broken then its stress will not be significantly altered by changes in the externally applied stress. As such, the presence of a broken wire or ligament may be detected from the observation that the measured stress in that broken wire or ligament is not altered by a change in the externally applied stress. However, changing the pressure in a riser is less than ideal as it requires the interruption of the transportation of production fluids through the riser. As such, the requirement to alter pressure within the flexible riser is onerous on the operator. In addition, some risers are often operated at low pressures and/or have limits of the safe operating pressures so there may be little scope to vary this pressure sufficiently to affect a detectable change in applied stress. Therefore, an improved method of inspecting and monitoring stress is a flexible pipe is desirable. The present invention provides an improved method of and apparatus for monitoring stress in steel ligaments within a flexible pipe. According to an aspect of the present invention there is provided a method for monitoring stress in steel ligaments within a flexible pipe. The method comprising: subjecting a ligament to a first magnetic field and obtaining a first measurement indicative of a magnetic flux density near an outer surface of the pipe adjacent to the ligament; subjecting the