STRUCTURE MONITORING

• US 20140025319A1
• 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 11,656,204 B2
• Time in Patent Office: Days
• US Class Current: 702/56
• 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 monitoring the position of 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 a known part of the structure, the method comprising:

using the optical fiber as a distributed acoustic sensor to detect, at a plurality of acoustic sensor positions along the fibre, acoustic signals emitted by a plurality of acoustic sources deployed at known positions in an area in which the structure to be monitored is located;

calculating relative positions of a plurality of the acoustic sensors in dependence on the detected acoustic signals from the acoustic sources; and

from the calculated positions of the sensors along the fibre, determining a shape, or shape and position, of the structure in dependence on the known relationship between the fiber and the structure.

2. A method according to claim 1, wherein the relative position of one of the acoustic sensors on the fiber is determined in dependence upon the relative position determined for one or more others of the sensors.

3. A method according to claim 2, wherein the relative position found for one of the acoustic sensors on the fiber is checked to determine whether it is within an allowable distance of the position previously found for another of the acoustic sensors on the fibre, given the known length of fiber between the respective positions of the two acoustic sensors on the fiber.

4. A method according to claim 1, wherein the relative position of one of the sensors is determined in dependence on the time taken for respective signals from one or more of the acoustic sources of known position to reach the sensor.

5. A method according to claim 1, wherein the calculating further comprises:

forming a plurality of subsets of the acoustic sensors, a subset of sensors comprising a virtual line array of acoustic sensors, and

determining direction from a subset of sensors to an acoustic source of known position in dependence on a phase delay of receipt of an acoustic signal from the acoustic source across the acoustic sensors of the array.

6. A method according to claim 5, wherein the subsets of sensors overlap, such that any one acoustic sensor is a member of more than one subset.

7. A method according to claim 5, wherein the position of a subset is determined by detecting direction to n acoustic sources, where there are n degrees of freedom of the sensors forming the subset.

8. A method according to claim 1, and further 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.

9. A method according to claim 8, and further comprising frequency filtering the signal detected by the optical fibre distributed acoustic sensor into low frequencies and high frequencies, wherein the low frequencies (<

100 Hz) are used for vibration detection, and the high frequencies (>

1 kHz) are used for position monitoring.

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

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

13. A method according to claim 1, and further comprising repeatedly determining the shape, or shape and position, of the structure so as to track changes in shape and/or movement of the structure with respect to time.

14. A method according to claim 13, and further comprising undertaking fatigue monitoring or analysis of the structure in dependence on the tracked changes in shape and/or movement of the structure.

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

15. A system for monitoring the position of a structure, 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 a known part of the structure, the sensor system further comprising a processor arranged to perform the following;

i) use the optical fiber as a distributed acoustic sensor to detect, at a plurality of acoustic sensor positions along the fibre, acoustic signals emitted by a plurality of acoustic sources deployed at known positions in an area in which the structure to be monitored is located;

ii) calculate relative positions of a plurality of the acoustic sensors in dependence on the detected acoustic signals from the acoustic sources; and

iii) from the calculated positions of the sensors along the fibre, determine a shape, or shape and position, of the structure in dependence on the known relationship between the fiber and the structure.

16. A system according to claim 15, wherein the relative position of one of the acoustic sensors on the fiber is determined in dependence upon the relative position determined for one or more others of the sensors.

17. A system according to claim 16, wherein the relative position found for one of the acoustic sensors on the fiber is checked to determine whether it is within an allowable distance of the position previously found for another of the acoustic sensors on the fibre, given the known length of fiber between the respective positions of the two acoustic sensors on the fiber.

18. A system according to claim 15, wherein the relative position of one of the sensors is determined in dependence on the time taken for respective signals from one or more of the acoustic sources of known position to reach the sensor.

19. A system according to claim 15, wherein the calculating further comprises:

forming a plurality of subsets of the acoustic sensors, a subset of sensors comprising a virtual line array of acoustic sensors, and

determining direction from a subset of sensors to an acoustic source of known position in dependence on a phase delay of receipt of an acoustic signal from the acoustic source across the acoustic sensors of the array.

20. A system according to claim 15, and 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.