Structural monitoring system using fiber optics

• US 4,654,520 A
• Filed: 03/18/1985
• Issued: 03/31/1987
• Est. Priority Date: 08/24/1981
• Status: Expired due to Term

• Current Assignee: G2 Systems
• Original Assignee: Richard W. Griffiths
• Inventors: Griffiths, Richard W.
• Primary Examiner(s): Westin, Edward P.
• Application Number:
  US06/712,889
• Time in Patent Office: 743 Days
• Field of Search: 250/227, 250/231 R, 250/231 P, 73/705, 73/800, 350/96.15, 350/96.16, 350/96.23, 350/96.29, 356/73.1, 356/32
• US Class Current: 250/227.14
• CPC Class Codes: G01B 11/18 using photoelastic elements
• CPC Class Codes: G01L 1/245 using microbending
• CPC Class Codes: G01M 11/086 Details about the embedment of the optical fiber within the DUT
• CPC Class Codes: G01M 3/047 with photo-electrical detection means, e.g. using optical fibres
• CPC Class Codes: G01P 3/36 Devices characterised by the use of optical means, e.g. using infrared, visible, or ultraviolet light G01P3/68 takes precedence; gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams G01C19/64

Abstract

An optical fiber is securely and continuously fastened to a structure such as a pipeline, offshore platform, bridge, building, dam or even a natural object. A light signal is passed into one end of the optical fiber. Any physical movement of the structure, or sectional movements along the optical fiber path, such as deflection, bending, displacement (changes in linear uniformity) or fracture of the structure will necessarily affect the optical fiber. As a consequence, detectable changes will occur in the "electro-optic signature" (for measurements made at the input end of the optical fiber) or in the light signal transmission (for measurements made at the opposite end of the optical fiber). Such measurements made at the input include reflections resulting from Rayleigh back-scattering and can be periodically or continuously made to determine the magnitude and location of the aforementioned physical movements of the structure. In addition, by utilizing additional optical fibers, the direction and rate, i.e., dynamics, of such physical movements can be determined. The optical fiber itself can be used as a communication link for the transmission of data and voice.

Claims

1. A method of monitoring a physical movement occurring at any point between spaced points on a structure including the steps of:

(a) attaching with attachment means at least one optical fiber continuously to said structure to extend from at least one of the spaced points to the other and thereby engage all points along the structure between said spaced points so that physical movement of the structure between said spaced points will result in physical movement of the optical fiber;

(b) passing a first light signal into said optical fiber; and

(c) detecting reflection changes in said first light signal resulting from said physical movement to thereby provide an indication that a physical movement has taken place and a indication of the location and magnitude of said movement.

2. The method of claim 1, including the steps of detecting and storing characteristics of said light signal passing through said optical fiber when said optical fiber is positioned to follow a precise, predetermined path prior to attaching the optical fiber to said structure;

and comparing the stored characteristics with detected changes in the light signal after the optical fiber has been attached to said structure in an identical path configuration, to thereby provide a measure of any subsequent deviations of the structure from said predetermined path.

3. A method of determining the locations of physical movement occurring along a continuum of points between spaced points on a structure, said method including the steps of:

(a) attaching with attachment means at least one optical fiber continuously to said structure to extend from at least one of the spaced points to the other and thereby engage all points along the structure between said spaced points so that physical movement of the structure between said spaced points will result in physical movement of said optical fiber;

(b) passing a first light signal into said optical fiber; and

(c) continuously detecting individual multiple light reflections in said fiber along the continuum of points between spaced points to identify points along the structure at which a physical movement has taken place and to provide an indication of the magnitude of all such movements.

4. An apparatus for monitoring a physical movement between spaced points on a structure including, in combination:

(a) at least one optical fiber;

(b) attachment means between said optical fiber and said structure for attaching said optical fiber continuously to said structure to extend from at least one of the spaced points to the other and thereby directly engage all points along the structure between said spaced points so that physical movement of the structure between said spaced points will result in physical movement of the optical fiber;

(c) means for passing a first light signal into said optical fiber;

(d) means for detecting and indicating reflection changes in said first light signal as a consequence of said physical movement; and

,

(e) means responsive to said reflection changes for indicating the location and magnitude of said physical movement.

5. An apparatus according to claim 4, including memory means for storing the light signal signature of the optical fiber when the optical fiber is positioned in a predetermined path prior to attachment to the structure;

and comparator means connected to receive the output of said memory and light signal changes after said optical fiber has been attached to said structure to follow an identical path so that subsequent movements of said structure away from said predetermined path are indicated when the light signal changes received differ from those stored in said memory.

6. An apparatus according to claim 4, in which said structure constitutes a pipe line.

7. An apparatus according to claim 4, in which said optical fiber includes a core and surrounding cladding;

a sheath surrounding the cladding; and

aggravating means in the form of a continuous particulate coating between said sheath and cladding continuously in intimate engagement with said cladding extending along the length of said optical fiber sheath and cladding continuously in intimate engagement with said cladding extending along the length of said optical fiber between said spaced points and responsive to said physical movement of said structure to effect a microbending in a portion of the optical fiber directly engaging the structure between said spaced points to thereby enhance said changes in said first light signal.

8. An apparatus according to claim 7, in which said means comprises a roughened surface simulating sand paper.

9. An apparatus for monitoring a physical movement between spaced points on a structure including, in combination:

(a) at least on optical fiber;

(b) means for attaching said optical fiber continuously to said structure to extend from at least one of the spaced points to the other and thereby engage all points along the structure between said spaced points;

(c) means for passing a first light signal into said optical fiber;

(d) means for detecting and indicating reflection changes in said first light signal as a consequence of said physical movement;

(e) means responsive to said reflection changes for indicating the location and magnitude of said physical movement;

(f) at least one additional optical fiber secured to said structure along portions of the structure spaced from said one optical fiber;

(g) means for passing a second light signal into said additional optical fiber;

(h) means for detecting changes in said second light signal resulting from said physical movement of the structure; and

(j) means for comparing the changes in said first light signal with the changes in said second light signal to thereby enable the direction of said physical movement to be determined.

10. An apparatus for monitoring a physical movement between spaced points on a structure including, in combination:

(a) at least one optical fiber;

(b) attachment means between said optical fiber and said structure for attaching said optical fiber continuously to said structure to extend from at least one of the spaced points to the other and thereby engage all points along the structure between said spaced points so that physical movement of said structure between said spaced points will result in physical movement of said optical fiber;

(c) means for passing a first light signal into said optical fiber;

(d) means for detecting and indicating reflection changes in said first light signal as a consequence of said physical movement; and

(e) means responsive to said reflection changes for indicating the location and magnitude of said physical movement;

(f) said optical fiber including a core and surrounding cladding, said cladding having an index of refraction different from the index of refraction of said core, said first light signal passing through said core being substantially retained in the core by internal reflections from the interface of said core and cladding so long as the index of refraction of said cladding is not changes;

a sheath surrounding said cladding, said sheath having a coating of fixed particulate material uniformly spread over its interior wall surface in intimate engagement with the outer surface of said cladding, said particulate material having a diameter of from 5 to 23 microns, said particulate material radially moving into and distorting the homogeneity of at least a portion of said cladding whenever said sheath, cladding and core are subject to tension as a result of said physical movement of said structure attached to said fiber, the distortion resulting in a change in the index of refraction of the cladding such that some light is permitted to escape from said core and cladding at the location of said physical movement thereby generating Rayleigh back scattering reflection changes in said light signal.

11. A method of monitoring a physical movement occurring at any point between spaced points on a structure including the steps of:

(a) attaching at least one optical fiber continuously to said structure to extend from at least one of the spaced points to the other and thereby engage all points along the structure between said spaced points;

(b) passing a first light signal into said optical fiber;

(c) detecting reflection changes in said first light signal resulting from said physical movement to thereby provide an indication that a physical movement has taken place and an indication of the location and magnitude of said movement;

(d) attaching at least one additional optical fiber to said structure extending along portions of the structure spaced from said one optical fiber;

(e) passing a second light signal into said additional optical fiber;

(f) detecting changes in said second light signal resulting from said physical movement of the structure; and

(g) comparing the changes in said first light signal with the changes in said second light signal to thereby enable the direction of said physical movement to be determined.