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EP-4741791-A2 - SUBMARINE POWER CABLE WITH CURVATURE MONITORING CAPABILITY

EP4741791A2EP 4741791 A2EP4741791 A2EP 4741791A2EP-4741791-A2

Abstract

A multi-phase submarine power cable (1) comprising: a plurality of power cores (3, 5, 7) arranged in a stranded configuration, and a curvature sensor (15) comprising: an elastic elongated member (15a), and a plurality of Fibre Bragg Grating, FBG, fibres (15b), each FBG fibre (15b) extending axially along the elongated member (15a) at a radial distance from the centre of the elongated member (15a); wherein the elongated member (15a) extends between the stranded power cores (3, 5, 7) along a central axis of the multi-phase submarine power cable (1), wherein the elongated member (15a) is arranged in an interstice between the power cores (3, 5, 7) at the centre of the multi-phase submarine power cable (1), and wherein due to its bend stiffness and elasticity, the elongated member (15a) follows the curvature variations of the multi-phase submarine power cable (1).

Inventors

  • TYRBERG, ANDREAS
  • HOLMBERG, Patrik
  • ALTHINI, Petrus

Assignees

  • NKT HV Cables AB

Dates

Publication Date
20260513
Application Date
20201029

Claims (15)

  1. A multi-phase submarine power cable (1) comprising: - a plurality of power cores (3, 5, 7) arranged in a stranded configuration, and - a curvature sensor (15) comprising: an elastic elongated member (15a), and a plurality of Fibre Bragg Grating, FBG, fibres (15b), each FBG fibre (15b) extending axially along the elongated member (15a) at a radial distance (r) from the centre (C) of the elongated member (15a); wherein the elongated member (15a) extends between the stranded power cores (3, 5, 7) along a central axis of the multi-phase submarine power cable (1), wherein the elongated member (15a) is arranged in an interstice between the power cores (3, 5, 7) at the centre of the multi-phase submarine power cable (1), and wherein due to its bend stiffness and elasticity, the elongated member (15a) follows the curvature variations of the multi-phase submarine power cable (1).
  2. The multi-phase submarine power cable (1) as claimed in claim 1, wherein the elongated member (15a) comprises a plurality of channels (15c), and wherein the FBG fibres (15b) are fixedly arranged in a respective channel (15c).
  3. The multi-phase submarine power cable (1) as claimed in claim 1 or 2, wherein the plurality of FBG fibres (15b) are at least three FBG fibres (15b).
  4. The multi-phase submarine power cable (1) as claimed in any of the preceding claims, wherein the FBG fibres (15b) are distributed in the circumferential direction of the elongated member (15a).
  5. The multi-phase submarine power cable (1) as claimed in any of the preceding claims, wherein the elongated member (15a) is made of a composite material or a thermoplastic material.
  6. The multi-phase submarine power cable (1) as claimed in any of the preceding claims, wherein the elongated member (15a) has a bending stiffness of at least 0.3 Nm 2 , such as at least 1 Nm 2 , such as at least 1.5 Nm 2 , such as at least 2 Nm 2 , such as at least 2.5 Nm 2 , such as at least 3 Nm 2 .
  7. The multi-phase submarine power cable (1) as claimed in any of the preceding claims, wherein the elongated member (15a) has a circular cross-section.
  8. The multi-phase submarine power cable (1) as claimed in any of the preceding claims, wherein the elongated member (15a) is a rod or a tube.
  9. The multi-phase submarine power cable (1) as claimed in any of the preceding claims, wherein the multi-phase submarine power cable (1) is a dynamic submarine power cable or a static submarine power cable.
  10. A method of preparing a multi-phase submarine power cable (1) for operation, the method comprising: a) providing a multi-phase submarine power cable (1) comprising a plurality of power cores (3, 5, 7) arranged in a stranded configuration, the multi-phase submarine power cable (1) having an open end, b) providing a curvature sensor (15) comprising an elastic elongated member (15a), a plurality of Fibre Bragg Grating, FBG, fibres (15b) extending axially along the elongated member (15a) at a radial distance (r) from the centre (C) of the elongated member (15a), and c) pushing the curvature sensor (15) from the open end of the multi-phase submarine power cable (1) in between the stranded power cores (3, 5, 7) and along a central axis of the multi-phase submarine power cable (1).
  11. The method as claimed in claim 10, comprising attaching an end portion of the multi-phase submarine power cable (1) to a hang-off (23), the end portion being provided with the open end, wherein step c) is carried out after the multi-phase submarine power cable (1) has been attached to the hang-off (23).
  12. The method as claimed in claim 11, wherein the curvature sensor (15) is pushed to a section of the multi-phase submarine power cable (1) that is arranged in a bend stiffener or a bellmouth.
  13. The method as claimed in any of claims 10-12, wherein the elongated member (15a) comprises a plurality of channels (15c), and wherein the FBG fibres (15b) are arranged in a respective channel (15c).
  14. The method as claimed in any of claims 10-13, wherein the plurality of FBG fibres (15b) are at least three FBG fibres (15b).
  15. The method as claimed in any of claims 10-14, wherein the FBG fibres (15b) are distributed in the circumferential direction of the elongated member (15a).

Description

TECHNICAL FIELD The present disclosure generally relates to submarine power cables. BACKGROUND Submarine power cables that undergo repeated, dynamic, bending variations are subjected to fatigue stress that may cause fatigue damage. Fatigue stress can occur during offshore installation or repairs when the submarine power cable is suspended from a marine vessel to the seabed, during operation of dynamic submarine power cables suspended from floating platforms, and during operation of static submarine power cables over free-spans, for instance from the seabed to the J-tube at a stationary platform such as a wind turbine platform. The bending variations of the cable results from wave induced movements of the floating platform or vessel or by hydrodynamic loads induced by waves and currents on the suspended cable. EP3483579 discloses a method for fatigue-monitoring of a submarine cable during off-shore operations such as installation and repairs on a marine vessel. Inclination sensors or strain gauges are mounted onto the submarine cable in the region of the vessel chute and the measurements are used to calculate the accumulated fatigue damage using an S-N fatigue curve and the Palmgren-Miner linear damage hypothesis during the off-shore operations. EP3483579 is directed to off-shore operations monitoring and does not enable fatigue-monitoring during operation of an installed submarine power cable. WO2010/136062 discloses an electric cable with a strain sensor embedded in a strain-transferring filler. The strain sensor extends longitudinally along the cable and includes a strain optical fibre arranged within a bending neutral region of the electrical cable. WO2010/136062 discloses a method to measure tensile strain, i.e. axial elongation, of the electrical power cores and does not enable measurement of the cable curvature variations. SUMMARY In view of the above, a general object of the present disclosure is to provide multi-phase submarine power cable that solves or at least mitigates the problems of the prior art. Another object is to provide a method of preparing a multi-phase submarine power cable for operation. There is hence according to a first aspect of the present disclosure provided a multi-phase submarine power cable according to claim 1. The elongated member acts as a distancing member from the centre of the multi-phase submarine power cable for the FBG fibres. The local curvature C of the elongated member is determined by C=ε/r, where ε is the strain in the FBG fibre and r is the radial distance from the centre of the elongated member to the centre of an FBG fibre. In this way, the local curvature of the elongated member and thus of the multi-phase submarine power cable can be determined. The strain that the FBG fibres are subjected to can be calculated based on electromagnetic waves reflected in the gratings of the FBG fibres. Due to its bend stiffness and elasticity, the elongated member follows the curvature variations of the multi-phase submarine power cable. The interstice or cavity between the power cores is typically not perfectly round as it is formed between the stranded power cores. The elongated member will however due to its bend stiffness and elasticity adapt to the shape of the cavity along the axial direction of the interstice or cavity and the elongated member will therefore contact the power cores in a plurality of axial locations. This adaptation to the cavity shape in the axial direction is similar to mathematical curve fitting using a polynomial. The radial distance of the FBG fibres ensure that the curvature of the elongated member can be determined. The curvature of the multi-phase submarine power cable can thus be determined. The curvature resolution becomes very high, for example less than 0.001 m-1. Moreover, the sampling frequency of the curvature variations can be as high as 10-20 Hz. The elongated member is provided with the FBG fibres. The elongated member is arranged in an interstice between the power cores at the centre of the multi-phase submarine power cable. The FBG fibres are preferably only provided in a monitored length of the multi-phase submarine power cable. The FBG fibres may transition into optical fibres without Bragg gratings as they extend towards an open end of the multi-phase submarine power cable. The FBG fibres may be spliced with the optical fibres without Bragg gratings. The optical fibres may extend from the multi-phase submarine power cable through the open end of the cable. The optical fibres extending from the open end may be connected to a monitoring system. The monitoring system may be configured to transmit electromagnetic waves into the optical fibres towards the FBG fibres. The monitoring system may be configured to detect reflected electromagnetic waves from the FBG fibres in the optical fibres. The monitoring system may be configured to calculate a curvature of the elongated member based on reflected electromagnetic waves from the FBG fib