STRUCTURE MONITORING

• US 20140022530A1
• Filed: 07/17/2013
• Published: 01/23/2014
• 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
• Granted Patent: US 10,145,821 B2
• Time in Patent Office: Days
• US Class Current: 356/35.5
• 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 vibration detection in a structure using an optical fiber distributed acoustic sensor deployed in a known relationship with respect to the structure such that a known part of the optical fiber corresponds to and moves with a known part of the structure, the method comprising:

detecting backscattered light on the fiber, the backscatter being dependent on strain induced in the fibre due to mechanical strain in the structure to which the fibre relates caused by vibrations in the structure;

from the detected backscatter, processing a signal representative thereof to determine a frequency of oscillation of the vibrations in the structure.

2. A method according to claim 1, and further comprising frequency filtering the signal detected by the optical fibre DAS to obtain low frequencies, wherein the low frequencies are used for vibration detection.

3. A method according to claim 2, wherein the low frequencies comprise frequencies of less than 100 Hz.

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

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

6. A method according to claim 1, wherein the structure is a subsea riser.

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

8. A system for vibration detection in a structure, the system comprising:

an optical fiber distributed acoustic sensor system deployed in a known relationship with respect to the structure such that a known part of the optical fiber corresponds to and moves with a known part of the structure, the sensor system further comprising;

an interferometer arrangement arranged to detect backscattered light on the fiber, the backscatter being dependent on strain induced in the fibre due to mechanical strain in the structure to which the fibre relates caused by vibrations in the structure; and

a processor arranged, from the detected backscatter, to process a signal representative thereof to determine a frequency of oscillation of the vibrations in the structure.

9. A system according to claim 8, 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 vibration detection.

10. A system according to claim 9, wherein the low frequencies are frequencies less than 100 Hz.

11. A system according to claim 8, wherein the processor is further arranged to undertake passive acoustic monitoring.

12. A system according to claim 11, wherein the passive acoustic monitoring comprises detecting acoustic events having an energy greater than a predetermined energy threshold.

13. A system according to claim 8, wherein the structure is a subsea riser.

14. A system according to claim 8, wherein the vibrations are caused by mechanical vibration rather than acoustic vibration.

15. A computer readable storage medium storing a computer program or suite of computer programs so arranged such that when executed by a computer they cause the computer to operate to perform a method of vibration detection in a structure, the method using an optical fiber distributed acoustic sensor deployed in a known relationship with respect to the structure such that a known part of the optical fiber corresponds to and moves with a known part of the structure, the method comprising:

detecting backscattered light on the fiber, the backscatter being dependent on strain induced in the fibre due to mechanical strain in the structure to which the fibre relates caused by vibrations in the structure; and

from the detected backscatter, processing a signal representative thereof to determine a frequency of oscillation of the vibrations in the structure.

16. A computer readable storage medium according to claim 15, the method further comprising frequency filtering the signal detected by the optical fibre DAS to obtain low frequencies, wherein the low frequencies are used for vibration detection.

17. A computer readable storage medium according to claim 16, wherein the low frequencies comprise frequencies of less than 100 Hz.

18. A computer readable storage medium according to claim 15, wherein the method further comprises undertaking passive acoustic monitoring.

19. A computer readable storage medium according to claim 18, wherein the passive acoustic monitoring comprises detecting acoustic events having an energy greater than a predetermined energy threshold.

20. A computer readable storage medium according to claim 15, wherein the structure is a subsea riser.