MAGNETIC FIELD GRADIENT APPARATUS AND METHOD FOR DETECTING PIPE LINE CORROSION

• US 3,745,452 A
• Filed: 02/23/1971
• Issued: 07/10/1973
• Est. Priority Date: 02/23/1971
• Status: Expired due to Term

• Current Assignee: Donald R. White, John D. Osburn
• Original Assignee: Donald R. White, John D. Osburn
• Inventors: Osburn, John D., White, Donald R.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US05/118,033
• Time in Patent Office: 868 Days
• Field of Search: 324/52,54,3,67,43
• US Class Current: 324/559
• CPC Class Codes: G01N 17/02 Electrochemical measuring systems for weathering, corrosion or corrosion-protection measurement

Abstract

Electrical current patterns indicative of either the cathodic protection or the existence of external corrosion of a subsurface pipeline are detected by measuring the magnetic fields produced by electrical current flowing respectively into or exiting from the buried pipeline at the situs of a bare region of the pipeline at which corrosion may take place.

Claims

1. The method of surveying subsurface pipe to locate a discontinuity which may be present in a protective covering on the pipe by detecting electrical current flowing through such a discontinuity, which comprises the steps of:

a. orienting at least a pair of axially directional magnetic field sensors above the surface with their sensing axes essentially vertically disposed and with one sensor located essentially in alignment above the pipeline and the other sensor displaced laterally thereof whereby said one sensor lies essentially outside the magnetic field associated with holiday currents which may be flowing to and from the pipeline while said other sensor may pass through such magnetic fields;

b. translating the sensors longitudinally with respect to the subsurface pipe while maintaining the orientation of step (a); and

c. detecting only those magnetic field gradients occurring between said sensors during the translation of step (b) which are due to the presence of magnetic fields associated with electrical currents flowing generally perpendicular to said pipeline.

2. The method of detecting electrical current patterns indicative of externally corroded regions of subsurface pipeline having cathodic current impressed thereon, which comprises the steps of:

a. orienting at least a pair of axially directional magnetic field sensors above the surface with theiR sensing axes essentially vertically disposed and with one sensor located essentially in alignment above the pipeline and the other sensor displaced laterally thereof whereby said one sensor lies essentially outside the magnetic field associated with holiday currents which may be flowing to and from the pipeline while said other sensor may pass through such magnetic fields;

b. translating said sensors along the direction of the pipeline while maintaining the orientation of step (a); and

c. detecting only those magnetic field gradients occurring between said sensors during the translation of step (b) which are due to the presence of magnetic fields associated with electrical currents flowing generally perpendicular to said pipeline.

3. The method according to claim 2 including the steps of:

a. orienting a second pair of axially directional magnetic field sensors above the surface with their sensing axes essentially vertically disposed and in trailing relation to respective ones of the first pair of sensors;

b. translating said second pair of sensors along the direction of the pipeline while maintaining the orientation of step (a);

c. detecting only those magnetic field gradients occurring between said second pair of sensors which are due to the presence of magnetic fields associated with electrical currents flowing generally perpendicular to said pipeline; and

combining the detected gradients of the two pairs of sensors in time-coincident relation.

4. Apparatus for translation along a path generally parallel to the axis of and to detect electrical current patterns indicative of corroded regions of a subsurface pipeline, comprising in combination:

a pair of magnetic field sensors disposed in laterally spaced relation to each other with respect to said path, each sensor having a sensitive axis and the sensitive axes of the two sensors being disposed generally vertically;

first feedback means connecting the output of one of said sensors as inputs to both of said sensors for nulling variations in magnetic field sensed by said one sensor which are below a first selected frequency, said first frequency being sufficiently high as to obviate effects of gross magnetic fields such as earth'"'"''"'"'s magnetic field due to movement of the sensors; and

second feedback means connecting the output of the other of said sensors as an input to said other sensor for nulling variations in magnetic field sensed by said other sensor which are below a selected second frequency, said second frequency being lower than said first frequency by an amount sufficient to produce an output from said other sensor indicative of variations in the difference between the magnetic field gradients sensed by said two sensors due to movement of said other sensor through magnetic fields produced by electrical currents flowing generally perpendicular to the path of movement of said sensors.

5. Apparatus as defined in claim 4 including means for translating said sensor along a path generally parallel to but above a subsurface pipeline with said one sensor disposed generally above the axis of the pipeline and said other sensor disposed laterally with respect to the pipeline.

6. Apparatus for translation along a path generally parallel to the axis of and to detect electrical current patterns indicative of corroded regions of a subsurface pipeline, comprising in combination:

a pair of magnetic field sensors disposed in longitudinally spaced relation to each other with respect to said path, each sensor having a sensitive axis and the sensitive axes of the two sensors being generally vertically disposed;

means for translating said sensors along said path in laterally displaced position with respect to the pipeline and at a selected speed;

means connected to the output of the leading sensor for delaying such output in accord with the spacing between sensors and said selected speed so as to bring the outputs of the two sensors into coincidence;

meaNs for combining the outputs of said sensors to detect magnetic field variations produced by movement of said sensors through magnetic fields associated with electrical currents flowing generally perpendicular to said path;

a second pair of sensors mounted on said means for translating, one sensor of said second pair being disposed with its sensitive axis generally vertical and located laterally of said leading sensor and the other sensor of said second pair being disposed with its sensitive axis generally vertical and located laterally of the trailing sensor of the first pair of sensors;

first feedback means connecting the output of said one sensor as inputs to said one sensor and said leading sensor for nulling variations in magnetic field sensed by said one sensor which are below a selected first frequency, said first frequency being sufficiently high as to obviate effects of gross magnetic fields such as earth'"'"''"'"'s magnetic field due to movement of said one and said leading sensors;

second feedback means connecting the output of said leading sensor as an input to such leading sensor for nulling variations in magnetic field sensed by said leading sensor which are below a selected second frequency, said second frequency being lower than said first frequency by an amount sufficient to produce an output from said leading sensor indicative in variations in the difference between the magnetic field gradients sensed by said leading sensor and said one sensor due to movements of said leading sensor through magnetic fields produced by electrical currents flowing generally perpendicular to said path;

third feedback means connecting the output of said other sensor as inputs to said other sensor and said trailing sensor for nulling variations in magnetic field sensed by said other sensor which are below said first frequency;

fourth feedback means connecting the output of said trailing sensor as an input to such trailing sensor for nulling variations in magnetic field sensed by said trailing sensor which are below said second frequency; and

said second pair of sensors being oriented to move generally vertically above the axis of the pipeline.

7. Apparatus according to claim 6 including a third pair of sensors mounted on said means for translating, said third pair of sensors having their sensitive axes generally vertically disposed and being disposed in longitudinally spaced relation to each with respect to said path in minor image relation to said leading and trailing sensors relative to said second pair of sensors;

said first feedback means also being connected as an input to the leading sensor of said third pair and said third feedback means also being connected as an input to the trailing sensor of said third pair;

fifth feedback means connecting the output of said leading sensor of the third pair as an input to such leading sensor for nulling variations in magnetic field sensed thereby which are below said second frequency;

sixth feedback means connecting the output of the trailing sensor of said third pair as an input to such trailing sensor for nulling variations in magnetic field sensed thereby which are below said second frequency;

means connected to the output of said leading sensor of said third pair for delaying such output in accord with the spacing between the sensors of said third pair and said selected speed so as to bring the outputs of said third pair of sensors into coincidence; and

means for combining the outputs of the sensors of said third pair to detect magnetic field variations produced by movement of said third pair of sensors through magnetic fields associated with electrical currents flowing generally perpendicular to said path.