Structure monitoring

Patent metadata

Field	Value
Publication number	US-10145821-B2
Application number	US-201313944469-A
Country	US
Kind code	B2
Filing date	Jul 17, 2013
Priority date	Jul 17, 2012
Publication date	Dec 4, 2018
Grant date	Dec 4, 2018

Abstract

A method and apparatus for monitoring a structure using an optical fiber based distributed acoustic sensor (DAS) extending along the length of the structure. The DAS is able to resolve a separate acoustic signal with a spatial resolution of 1 m along the length of the fibre, and hence is able to operate with an acoustic positioning system to determine the position of the riser with the same spatial resolution. In addition, the fiber can at the same time also detect much lower frequency mechanical vibrations in the riser, for example such as resonant mode vibrations induced by movement in the surrounding medium. By using vibration detection in combination with acoustic positioning then overall structure shape monitoring can be undertaken, which is useful for vortex induced vibration (VIV) visualisation, fatigue analysis, and a variety of other advanced purposes. The structure may be a sub-sea riser.

First claim

The invention claimed is:

1. A method of resonant vibration detection in a subsea riser tethered at opposite ends and a portion thereof free to move therebetween, the method using an optical fiber distributed acoustic sensor (DAS) having an optical fiber, the optical fiber being coupled to the subsea riser at at least two points in the portion such that a known part of the optical fiber is adjacent to, connected to and moves with a known part of the portion of the subsea riser that is free to move, the method comprising:
detecting backscattered light on the fiber as it moves with the subsea riser, the backscatter being dependent on strain induced in the fiber due to mechanical strain in the subsea riser caused by resonant vibrations occurring in the subsea riser as the portion between the two tethered ends moves;
from the detected backscatter, processing a signal representative thereof to determine a frequency of oscillation of the resonant vibrations of the subsea riser as the portion between the two tethered ends moves; and
frequency filtering the signal detected by the optical fiber DAS to obtain low frequencies, wherein the low frequencies are used for detecting the resonant vibrations of the subsea riser as the portion between the two tethered ends moves,
wherein the low frequencies comprise frequencies of less than 50 Hz.

2. A method according to claim 1 , wherein the low frequencies comprise frequencies of 1 Hz or less.

3. A method according to claim 1 , and further comprising undertaking passive acoustic monitoring.

4. A method according to claim 3 , wherein the passive acoustic monitoring comprises detecting acoustic events having an energy greater than a predetermined energy threshold.

5. A method according to claim 1 , wherein the vibrations are due to mechanical vibrations rather than acoustic vibrations.

6. The method according to claim 1 , wherein the low frequencies comprise frequencies of less than a few Hz.

7. A method according to claim 1 , wherein the optical fiber is coupled to the subsea riser using a plurality of clamps at regular spaced intervals along the length of the optical fiber.

8. A method according to claim 1 , wherein the optical fiber is coupled to the subsea riser using a plurality of clamps, wherein the fundamental mechanical resonance of the clamps is of a higher frequency than any resonant vibrations of the subsea riser.

9. A method according to claim 1 , wherein the optical fiber is coupled to the subsea riser using a plurality of clamps, wherein the inter-clamp spacing is smaller than the wavelength of the resonant vibrations.

10. A system for detecting the resonant vibrations of a subsea riser tethered at opposite ends and a portion thereof free to move therebetween, the system comprising:
an optical fiber distributed acoustic sensor (DAS) system having an optical fiber, the optical fiber being coupled to the subsea riser at at least two points in the portion such that a known part of the optical fiber is adjacent to, connected to and moves with a known part of the portion of the subsea riser that is free to move, the sensor system further comprising:
an interferometer arrangement arranged to detect backscattered light on the fiber as it moves with the subsea riser, the backscatter being dependent on strain induced in the fiber due to mechanical strain in the subsea riser caused by resonant vibrations occurring in the subsea riser as the portion between the two tethered ends moves; and
a processor arranged, from the detected backscatter, to process a signal representative thereof to determine a frequency of oscillation of the resonant vibrations of the subsea riser as the portion between the two tethered ends moves,
wherein the processor is further arranged to frequency filter the signal detected by the optical fibre DAS to determine low frequencies, wherein the low frequencies are used for detecting the resonant vibrations of the subsea riser as the portion between the two tethered ends move,
wherein the low frequencies are frequencies less than 50 Hz.

11. A system according to claim 10 , wherein the low frequencies are frequencies of 1 Hz or less.

12. A system according to claim 10 , wherein the processor is further arranged to undertake passive acoustic monitoring.

13. A system according to claim 12 , wherein the passive acoustic monitoring comprises detecting acoustic events having an energy greater than a predetermined energy threshold.

14. A system according to claim 10 , wherein the vibrations are caused by mechanical vibration rather than acoustic vibration.

15. The system according to claim 10 , wherein the low frequencies are frequencies less than a few Hz.

16. A system according to claim 10 , wherein the optical fiber is coupled to the subsea riser using a plurality of clamps at regular spaced intervals along the length of the optical fiber.

17. A system according to claim 10 , wherein the optical fiber is coupled to the subsea riser using a plurality of clamps, wherein the fundamental mechanical resonance of the clamps is of a higher frequency than any resonant vibrations of the subsea riser.

18. A system according to claim 10 , wherein the optical fiber is coupled to the subsea riser using a plurality of clamps, wherein the inter-clamp spacing is smaller than the wavelength of the resonant vibrations.

Assignees

• Silixa Ltd
• Chevron Usa Inc

Inventors

• Farhadiroushan Mahmoud
• Finfer Daniel
• Kamil Yousif
• Kutlik Roy Lester

Classifications

• E21B47/0224

  using seismic or acoustic means · CPC title

• E21B47/095

  by detecting an acoustic anomalies, e.g. using mud-pressure pulses · CPC title

• E21B47/007

  Measuring stresses in a pipe string or casing (for locating blocked portions of pipes E21B47/09) · CPC title

• G01S5/30 Primary

  Determining absolute distances from a plurality of spaced points of known location · CPC title

• G01S5/18

  using ultrasonic, sonic or infrasonic waves · CPC title