Monitoring subsea cables

Patent metadata

Field	Value
Publication number	US-11287336-B2
Application number	US-201816488467-A
Country	US
Kind code	B2
Filing date	Feb 9, 2018
Priority date	Feb 24, 2017
Publication date	Mar 29, 2022
Grant date	Mar 29, 2022

Abstract

This applications relates to methods and apparatus monitoring of subsea cables ( 100 ), for example subsea power or telecommunication cables, and in particular to determining a burial status for such cables. The method involves interrogating a sensing optical fibre ( 107 ) with an interrogator unit ( 108 ) to perform distributed fibre optic sensing. The sensing optical fibre ( 107 ) is deployed along the path of the subsea cable ( 100 ) and may, in some instances, form part of the subsea cable. A frequency spectrum of the respective measurement signal from at least one sensing portion of the sensing fibre ( 107 ), which corresponds to pressure variations (ΔP(t)) due to surface water waves ( 109 ) to determine a burial status. High frequency components of the pressure variation that are detected in any unburied section ( 106 ) are attenuated in buried sections ( 105 ).

First claim

The invention claimed is:

1. A method of monitoring a sub sea cable comprising:
performing distributed fibre optic sensing on a sensing optical fibre to provide a measurement signal from each of a plurality of longitudinal sensing portions of the sensing optical fibre, the sensing optical fibre being deployed along the path of the subsea cable; and
for at least one sensing portion, analysing a frequency spectrum of the respective measurement signal corresponding to pressure variations due to surface water waves to determine a burial status for that sensing portion.

2. A method as claimed in claim 1 wherein analysing the frequency spectrum of the measurement signal comprises determining an indication of relative attenuation of the measurement signal at different frequencies.

3. A method as claimed in claim 1 wherein analysing the frequency spectrum of the measurement signal comprises comparing the frequency spectrum to at least one reference frequency spectrum for a known burial status.

4. A method as claimed in claim 1 wherein analysing the frequency spectrum of the measurement signal comprises comparing the frequency spectrum for one sensing portion to a frequency spectrum for a different sensing portion to determine a relative burial status for the two sensing portions.

5. A method as claimed in claim 1 wherein analysing the frequency spectrum of the measurement signal comprises comparing the energy or amplitude of the measurement signal at a first frequency or within a first frequency band and the energy or amplitude of the measurement signal at a second different frequency or frequency band.

6. A method as claimed in claim 1 wherein analysing the frequency spectrum of the measurement signal comprises determining a value for a spectrum parameter corresponding to a ratio between signal energy at a first frequency or first frequency band and signal energy at a second frequency or second frequency band.

7. A method as claimed in claim 6 comprising comparing said spectrum parameter to at least one reference value for a known burial status.

8. A method as claimed in claim 6 comprising comparing said spectrum parameter to a corresponding spectrum parameter for the measurement signal for another different sensing portion and determining a relative burial status for the two sensing portions.

9. A method as claimed in claim 5 wherein the first frequency or first frequency band is a frequency or band of frequencies that is within a low frequency band and the second frequency or second frequency band is a frequency or range of frequencies that is within a high frequency band.

10. A method as claimed in claim 5 wherein the first frequency is 0.1 Hz or lower and the second frequency is 0.1 Hz or higher.

11. A method as claimed in claim 1 wherein the subsea cable is coupled to or forms part of a submarine power cable.

12. An apparatus for monitoring a subsea cable comprising:
a distributed fibre optic interrogator unit for interrogating a sensing optical fibre deployed along the path of the sub sea cable to provide a measurement signal from each of a plurality of longitudinal sensing portions of the sensing optical fibre; and
an analyser configured to analyse, for at least one sensing portion, a frequency spectrum of the respective measurement signal corresponding to pressure variations due to surface water waves to determine a burial status for that sensing portion.

13. An apparatus as claimed in claim 12 wherein the analyser is configured to determine an indication of relative attenuation of the measurement signal at different frequencies.

14. An apparatus as claimed in claim 12 wherein the analyser is configured to compare the frequency spectrum to at least one reference frequency spectrum for a known burial status.

15. An apparatus as claimed in claim 12 wherein the analyser is configured to compare the frequency spectrum for one sensing portion to a frequency spectrum for a different sensing portion to determine a relative burial status for the two sensing portions.

16. An apparatus as claimed in claim 12 wherein the analyser is configured to compare the energy or amplitude of the measurement signal at a first frequency or within a first frequency band and the energy or amplitude of the measurement signal at a second different frequency or frequency band.

17. An apparatus as claimed in claim 12 wherein the analyser is configured to determine a value for a spectrum parameter corresponding to a ratio between signal energy at a first frequency or first frequency band and signal energy at a second frequency or second frequency band.

18. An apparatus as claimed in claim 17 wherein the analyser is configured to compare said spectrum parameter to at least one reference value for a known burial status.

19. An apparatus as claimed in claim 12 comprising a sensing optical fibre connected to said distributed fibre optic interrogator unit.

20. An apparatus as claimed in claim 19 wherein said sensing fibre is coupled to or forms part of a subsea cable.

Assignees

• Optasense Holdings Ltd

Inventors

• Godfrey Alastair

Classifications

• H02G1/10

  in or under water · CPC title

• G01D5/35361

  using elastic backscattering to detect the measured quantity, e.g. using Rayleigh backscattering · CPC title

• G01L1/24 Primary

  by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis {using infrared, visible light, ultraviolet} · CPC title

• H02G9/02

  laid directly in or on the ground, river-bed or sea-bottom; Coverings therefor, e.g. tile · CPC title

• G01R31/58

  Testing of lines, cables or conductors (testing of electric windings G01R31/72) · CPC title