Structure monitoring

• US 10,145,821 B2
• Filed: 07/17/2013
• Issued: 12/04/2018
• Est. Priority Date: 07/17/2012
• Status: Active Grant

• Current Assignee: Chevron USA Incorporated (Chevron Corp.), Silixa Limited
• Original Assignee: Chevron USA Incorporated (Chevron Corp.), Silixa Limited
• Inventors: Farhadiroushan, Mahmoud, Finfer, Daniel, Kamil, Yousif, Kutlik, Roy Lester
• Primary Examiner(s): Amara, Mohamed K
• Application Number:
  US13/944,469
• Publication Number:
  US 20140022530A1
• Time in Patent Office: 1,966 Days
• Field of Search: 356 355, 356 32, 356477, 356520, 356480, 356450, 356 731, 356484, 73800, 73802, 385 12, 385 13, 25022714, 25022719
• CPC Class Codes: E21B 47/007 Measuring stresses in a pipe string or casing for locating blocked portions of pipes E21B47/09
• CPC Class Codes: E21B 47/095 by detecting an acoustic anomalies, e.g. using mud-pressure pulses
• CPC Class Codes: G01D 5/35383 using multiple sensor devices using multiplexing techniques
• CPC Class Codes: G01H 9/004 using fibre optic sensors light guides per se G02B6/00, acousto-optical devices specially adapted for gating or modulating in optical wave guides G02F1/125
• CPC Class Codes: G01N 29/2418 using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics photoacoustic cells G01N21/1702; measuring characteristics of vibrations by using radiation-sensitive means G01H9/00; acousto-optical conversion techniques for short-range imaging G01S15/8965; sound-producing devices using laser bundle G10K15/046
• CPC Class Codes: G01S 5/18 using ultrasonic, sonic, or infrasonic waves
• CPC Class Codes: G01S 5/186 Determination of attitude using inertial means G01C9/00; control of attitude G05D1/49
• CPC Class Codes: G01S 5/26 Position of receiver fixed by co-ordinating a plurality of position lines defined by path-difference measurements G01S5/28 takes precedence
• CPC Class Codes: G01S 5/30 Determining absolute distances from a plurality of spaced points of known location

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.

Claims

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.