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Defect Assessment Using Conformable Array Data
Project Number

The goal of this project is to develop an affordable, easy-to-use technology for making accurate measurements of external pipeline corrosion in the field, based on the application of eddy current technology in an effort to help to maintain the nation's natural gas transmission infrastructure.


Southwest Research Institute (SwRI) – project management and research product
Clock Spring Company – commercialization partner

San Antonio, Texas 78238


When external surface corrosion in gas transmission pipelines has been detected by an in-line inspection (ILI), the common procedure is to unearth the pipe and examine the corroded area to determine its dimensions. These dimensions are used as inputs for assessment algorithms that calculate the safe operating pressure of the pipe in its corroded condition. A common way to acquire the dimensional data is to construct a measurement grid on the pipe surface and make physical measurements of the corrosion depth at nodes of the grid. This process is time-consuming, requires that the pipe be quite clean, and is subject to human error. The objective of this project was to develop an affordable, rugged, easy-to-use technique for making measurements of external corrosion in a typical, dirty field environment. A previous project had demonstrated that eddy current coils could be used for measuring the depth of corrosion pits. This project focused on the design, fabrication, and testing of a larger and more versatile eddy current coil array for mapping corrosion areas.

The eddy current approach is based on the principle that a coil of wire carrying an alternating current will generate a magnetic field and if brought close to a conducting surface, will induce eddy currents to flow as to oppose the magnetic field. The magnitude and phase of these “secondary currents” are influenced by the geometry of the arrangement and the conductivity and permeability of the material. Eddy current systems use coil pairs, with one serving as an exciter and the other as a receiver. The coupling between the two coils is affected by the spacing between the coils and conducting surface. When the coil is close to the surface, the coupling is strong; as the coil is moved farther away, the coupling is reduced. This response enables an eddy current system to be used to measure the distance between the probe and the tested surface and thus, to measure the depth of corrosion pits.


A low cost-effective and efficient method of determining the extent of pipeline corrosion damage could be a very effective tool for the gas industry. Current methods are either extremely time- consuming and inherently inaccurate (hand measurement) or very complex and costly (laser evaluation systems). This method provides a means to quickly and accurately evaluate damage and automatically evaluate pipe condition or pipe life based on accepted evaluation criteria, providing a real time tool for repair/replacement decisions. The ability to perform this function in real time allows gas companies to more effectively maintain the pipeline infrastructure, enhancing overall safety, integrity and reliability of gas delivery through the network.

Conformable array on pipe specimen
Conformable array on pipe specimen


Accomplishments (most recent listed first)
  • Designed and fabricated a new and larger eddy current array with 256 coil pairs,
  • Designed and fabricated interface circuitry to drive the coils and receive signals from them,
  • Developed data collection and analysis algorithms to acquire, analyze and display the data needed for input into assessment algorithms,
  • Validated the performance of the eddy current array approach on natural corrosion in a field test using the prototype array, and
  • Documented the working design specifications for the commercialization partner to use in developing a production design.
Close-up of single coil pair
Close-up of single coil pair

In the previous project, a small array (75 mm square with 64 coil pairs) was designed and built to prove the principle of using an eddy current array to map corrosion pitting. The new and larger array (150 mm square with 256 coil pairs) was also built to be flexible enough to wrap around a corrosion patch on a pipe with stiff boards at the edges to carry the interface circuitry. State-of-the-art printed circuit techniques were used to make a multilayer flexible board with sufficient interconnects to drive the coils and receive signals from them. The primary data display was designed to be a color map of corrosion depth.

Map of corrosion depth produced by conformable array evaluation of a 12 by 24 inch specimen
Map of corrosion depth produced by conformable array evaluation of a 12 by 24 inch specimen

 After successful completion of the laboratory work the first field measurement was made on a 24-inch-diameter corrosion test specimen containing two external corrosion defects. The conformable array system included the conformable array PC board, 10 meters of connecting cable is provided between the control station and the pipe. This will allow the operator to position the system controls outside the bell hole in a field setup. For this test, a strap and resilient pad to hold the array onto the pipe surface, and a laptop computer to operate the system and store the scan data. Following a successful test of the system, a second test was arranged on a corroded pipe specimen belonging to the BP Company in Houston. This pipe had a 12-inch (305-mm) by 24-inch (610-mm) corroded area that was scanned by the array. To facilitate the alignment of the array, a transparent template was made with holes for marking index marks onto the pipe surface to position the array for multiple scans. Comparison between an RTD laser scan of the same area with the results of the scan by the conformable array showed a close correspondence, evidence that the array could accurately record corrosion depth. The ease of use and accuracy of the system developed significantly enhances the ability to efficiently and effectively evaluate the extent of corrosion damage and actually feeds data automatically into pipe life algorithms (R-STRENG or B-31G) for determination of remaining pipe strength in the area of concern.

Current Status

This project has been completed. Commercialization has been assigned to the co-funding partner of the project, Clock Spring Company of Houston, Texas, a company that provides corrosion remediation services to the pipeline industry with their Clock Spring composite banding. The array may be offered as a saleable product, or its application as a service to the pipeline industry.

Project Start
Project End
DOE Contribution


Performer Contribution


Contact Information

NETL – Richard Baker ( or 304-285-4714)
SwRI – Alfred E. Crouch (210-522-3157)

Additional Information

Final Report [PDF] December 2003