The objective of this project is to determine the feasibility of using eddy current technology in an array of sensors for measuring the profile of a corroded area on the outside surface of a gas transmission pipeline.

Southwest Research Institute (SwRI) – project management and research products

Location:
San Antonio, Texas 78238

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, induce eddy current flow rather than a 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.

• Designed and built a small array (75 mm square with 64 coil pairs) to prove the principle of using an eddy current array to map corrosion pitting, and
• Demonstrated that eddy current coils could be used for accurately measuring the depth of corrosion pits.

This project showed that pairs of circular eddy current coils could be used to measure the depth of machined pits in a test plate. Depths of the simulated pits ranged from 1.09 mm to 12.19 mm and surface diameters from 6.35 mm to 25.40 mm. The pits were arranged in rows of constant diameter and increasing depth. The coil array was an 8-by-8 matrix of 64 sensing coils and two contact patterns for mounting dual-in-line sockets. The response from scans of the machined pits showed a very good correlation between actual pit depth and indicated pit depth, with only one outlier among the data points. 

This project was completed and a follow-on project was begun to further develop the conformable array technology (DE-FC26-02NT41644).

$65,713

$35,384

NETL – Richard Baker (richard.baker@netl.doe.gov or 304-285-4714) 
SwRI – Alfred E. Crouch (acrouch@swri.edu or 210-522-3157)

Final Report [PDF] November 2002