Method and apparatus for estimating the condition of a coating on an underground pipeline

• US 20100117622A1
• Filed: 11/10/2008
• Published: 05/13/2010
• Est. Priority Date: 11/10/2008
• Status: Active Grant

• Current Assignee: Saudi Arabian Oil Company (Government of Saudi Arabia)
• Original Assignee: Saudi Arabian Oil Company (Government of Saudi Arabia)
• Inventors: Miller, Scott Downing, Davis, Thomas James, Perez, Jaime Paunlagui
• Granted Patent: US 7,880,484 B2
• Time in Patent Office: Days
• US Class Current: 324/71.100
• CPC Class Codes: G01N 17/02 Electrochemical measuring systems for weathering, corrosion or corrosion-protection measurement

Abstract

Methods are provided for reducing interference from stray currents in buried pipelines/metal structures during MEIS testing or other current-sensing applications in the pipeline. Methods are also provided for measuring bulk complex electrical impedance between a buried pipe and the soil, thereby rendering an indication of the quality of the anti-corrosive coating. Methods are also provided for measuring the complex propagation constant of AC voltages propagating along an attenuative pipeline. This information is useful for assessing the general condition of the anti-corrosive coating involved, or to enhance MEIS inspection of the pipeline. Methods are also provided for enhancements to MEIS testing, including (a) canceling magnetometer offset effects associated with the Earth'"'"'s magnetic field after the magnetometer is positioned for measurement, (b) implementing a separate sensing connection to the pipe so as to avoid interference from voltage loss in the pipe feed-line connection, (c) providing a power amplifier to excite the pipe with large-amplitude signals.

Claims

1. A method for estimating a condition of a coating on a pipeline buried in soil using a magnetometer, comprising:

conducting a magnetically-detected electrochemical impedance spectroscopy (MEIS) test over a plurality of locations along one or more segments of the buried pipeline; and

compensating for undesirable interfering and/or stray current signals flowing through at least one of the plurality of locations along a segment of the pipeline by providing at least one sinusoidal signal having a predominant amplitude and frequency to an output signal from the magnetometer.

2. The method of claim 1, wherein the step of conducting a MEIS test comprises the steps of:

providing a voltage between a selected segment of the pipeline and the soil at a plurality of test frequencies; and

measuring on-pipe current at a respective pipe-to-soil junction for each of a plurality of locations along the segment of the pipeline using the plurality of test frequencies, wherein said interfering and/or stray current signals are canceled or suppressed from the measured currents at each location.

3. The method of claim 2, wherein the step of conducting a MEIS test comprises the steps of:

determining complex impedances of the respective pipeline-to-soil junction for any segment of the pipeline using the plurality of test frequencies, said complex impedances being used for estimating the condition of the coating on the respective segment; and

plotting pipe-to-soil impedance results at the plurality of frequencies as a graphical representation.

4. The method of claim 3, wherein the compensating step further comprises the steps of:

receiving an on-pipe test current signal and interfering current signal from a magnetometer;

passing the on-pipe current signal and the interfering current signal through a first band pass filter for enhancing the predominant frequency component of the interfering current signal. sending the output of the first band pass filter to a phase-lock loop for generating phase-locked square wave at the predominant frequency;

performing a square-to-sine wave conversion of the phase-locked square wave at the predominant frequency;

inverting the converted sine wave at the predominant frequency; and

summing the inverted sine wave with the interfering current signal from the magnetometer to cancel the interfering current signal and pass the desired current signal through for performing the determining step.

5. The method of claim 4, wherein the predominant frequency is about 60 Hz or a harmonic of about 60 Hz.

6. The method of claim 4, further comprising the step of adjusting weighting of the inverted sine wave prior to performing the summing step so as to maximize suppression of the interference signal.

7. The method of claim 3, wherein the providing a sinusoidal signal step comprises providing a phase-lock loop for generating the sinusoidal signal at a predominant frequency to cancel or suppress the interfering current signal.

8. The method of claim 1, wherein said estimating the condition of the coating includes identifying at least one of normal bonding, disbonding, holidays and micro-cracking in the coating of the pipeline.

9. The method of claim 1, wherein the measuring step comprises the further step of canceling magnetometer offset effects associated with the Earth'"'"'s magnetic field after positioning a magnetometer proximate the pipe segment for measurement.

10. The method of claim 1, further comprising the step of switching between operation modes of a dual-mode potentiostat for performing a calibration step and performing the MEIS test.

11. The method of claim 1, wherein the interfering current signals are power line ground-return current signals.

12. The method of claim 1, wherein the compensating step further comprises:

measuring a first on-pipe interfering current signal component and test current signal component from said segment using a first magnetometer;

measuring a second interfering current signal component from an adjacent structure; and

combining the first and second interfering current signal components to produce only the on-pipe current signal component for the determining step.

13. The method of claim 12, wherein the combining step comprises:

phase shifting and weight adjusting the first interfering current signal component to provide an equal but opposite interference signal component with respect to the second interfering current signal component;

passing the phase shifted first interfering current signal component to a combiner; and

passing the second interfering current signal component directly to a combiner for combination with the first interfering current signal component.

14. Apparatus for estimating a condition of a coating on a pipeline buried in soil using a magnetometer, comprising:

means for conducting a magnetically-detected electrochemical impedance spectroscopy (MEIS) test over a plurality of locations along one or more segments of the buried pipeline; and

means for compensating for undesirable interfering and/or stray current signals flowing through at least one of the plurality of locations along a segment of the pipeline by providing at least one sinusoidal signal having a predominant amplitude and frequency to an output signal from the magnetometer.

15. The apparatus of claim 14, wherein the means for conducting a MEIS test comprises:

means for providing a voltage between a selected segment of the pipeline and the soil at a plurality of test frequencies; and

means for measuring an on-pipe current at a respective pipe-to-soil junction for each of a plurality of locations along the segment of the pipeline using the plurality of test frequencies, wherein said interfering and/or stray current signals are canceled or suppressed from the measured currents at each location.

16. The apparatus of claim 15, wherein the means for providing a voltage includes a power amplifier coupled between a ground-return electrode and a first end of the pipe via a feed line, said power amplifier being driven by an a/c voltage signal to excite the pipe with large-amplitude signals.

17. The apparatus of claim 16, further comprising a sense line for sensing the actual voltage at the first end of the pipe independently of interfering voltage drops in the feed line, said actual voltage being fed to a differential amplifier for providing an output signal representing the pipe-to-soil voltage.

18. The apparatus of claim 15, wherein the means for measuring comprises:

a magnetometer for measuring a magnetic field produced by current in said pipeline segment and providing the output signal from the magnetometer to the means for compensating, the output signal including an on-structure current signal component and the interfering current signal component at the predetermined frequency.

19. The apparatus of claim 18, wherein the means for compensating comprises:

a phase-locked loop (PPL) circuit coupled to the output of the magnetometer; and

a combiner for adding an inverted sinusoidal signal produced by the PPL to the interfering current signal component, wherein a modified output signal from the magnetometer excludes the interfering current signal component.

20. The apparatus of claim 18, further comprising a means for weighting the output signal from the PPL prior to being combined with the interfering current signal component at the combiner.

21. The method of claim 15, wherein the means for measuring further comprises means for canceling the magnetometer offset effects associated with the Earth'"'"'s magnetic field after the magnetometer is put into position for measurement.

22. Method for estimating a condition of a coating on an underground metal structure using bulk structure-to-soil spectroscopy, comprising:

providing an AC voltage between a ground-return electrode and an injection end point of the structure via a feed line at a plurality of test frequencies;

positioning a reference electrode proximate said structure;

positioning a magnetometer adjacent to the feed line to sense all currents being provided to the structure;

calibrating the magnetometer at a predetermined number of the test frequencies;

receiving a first voltage potential value representing a difference between the voltage at the injection end point and the reference electrode;

receiving a second voltage potential representing a difference between a voltage sensed by the magnetometer and the reference electrode; and

computing, from the first voltage potential and the magnetometer-measured currents, a first net impedance between the structure and soil at the plurality of frequencies, thereby defining a first impedance spectrum;

computing, from the second voltage potential and magnetometer measured currents, a second net impedance between the structure and the ground-return electrode at the plurality of frequencies, thereby defining a second impedance spectrum;

determining the difference between the first and second net impedances and/or first or second impedance spectra, said differences providing indicia of the earthing resistance of the ground-return electrode; and

analyzing the first and second impedance spectra to assess the condition of the coating on the structure under test.

23. The method of claim 22, wherein said estimating a condition of a coating on an underground metal structure comprises:

estimating a condition of a coating on a pipeline buried in soil.

24. The method of claim 23, further comprising:

receiving a third voltage value representing a difference between the voltage at a next or down-pipe electrical access point on the structure; and

computing attenuation and phase shift metrics of the third voltage values relative to the injection end point at each test frequency, wherein said metrics include a propagation constant.

25. The method of claim 24, further comprising estimating, from the metrics a coating condition of the structure as between adjacent electrical access points.

26. The method of claim 24, further comprising determining, from the metrics, actual pipe-to-reference electrode voltage or voltage spectrum at any test point located between adjacent electrical access points on the pipe.

27. The method of claim 26, further comprising conducting MEIS testing at any of the test points between the adjacent electrical access points.