June 2015, Vol. 242, No. 6


External Corrosion Mapping, Anomaly Assessment Using Eddy Currents

By Taylor Spathias, Clock Spring Company, Houston, TX

Evaluating pipeline anomalies remains a vital task in the pipeline industry. Properly identifying, assessing, and repairing defects is crucial in ensuring public and environmental safety. Recent technological advancements have automated this process and drastically increased its overall efficiency.

New methods of external pipeline assessment technologies seamlessly measure, record, and assess defects according to various standards and regulations. The implementation of eddy currents in the industrial sector are proven and established. The versatility of their applications includes electromagnetic braking, structural assessment, and proximity sensing.

This versatility stems from fundamental eddy current operation principles. Eddy currents are circular and closed looped currents induced within conductive material, or conductors, when in the presence of fluctuating magnetic fields.

According to Lenz’s Law, which describes the direction of induced currents in the presence of varying fields, the induced currents oppose the change in applied magnetic field and possess a unique field of its own.

The magnitude of the induced current and field is proportional to the strength of the applied magnetic field at any given moment. These induced fields will attempt to negate the source field in an effort to keep the overall field zero. Thus, the induced eddy currents and their associated magnetic field exert a counteracting force upon the source of the initial magnetic field.

Eddy current measurement uses the principle of electromagnetic induction to detect the distance between a transducer, a current carrying coil, and the nearest conductive surface. This method of creating eddy currents can be created in two ways: by moving the conductor through a uniform magnetic field or by holding the conductor stationary and varying the magnetic field around it.

The latter is used when targeting localized clusters of pipeline anomalies to allow for stationary defect assessments. Therefore we must vary the magnetic field around the conductor, or pipe surface. This is accomplished via an alternating current source, commonly found in a personal computer or laptop. The alternating current within the primary sending coil generates time-varying magnetic fields, which then interact with the surface of the pipe and generate eddy currents.

Variations in the phase or magnitude of these eddy currents are monitored using a second coil, termed the “receiver coil.” Any discontinuities of these measurements are attributed to surface imperfections such as metal loss due to galvanic corrosion. Since these measurements are the cornerstone for all data interpretation, it is vital that they be as precise as possible.
The two key parameters with respect to pipeline anomaly assessment are measured maximum depth (i.e. wall loss) and longitudinal extent of the corroded area. Regulations, codes, and standards provide formulas to assist with calculating the total metal loss via defect shape assumptions. Thus, to optimize the sensitivity of the readings, the diameter of each transducer must be ideal. If the coil diameter is too large, it will provide accurate measurements, but will not be precise when reading narrow pits, most commonly associated with galvanic corrosion attributed to coating flaws.

If the coil diameter is too small, it will provide precise measurements but will lack in-depth accuracy due to limitations posed by its reduced magnetic field strength. The correct size coil will allow for both accurate geometric profile readings and precise depth measurements.

Once the measurements have been verified, the pipeline integrity engineer is able to calculate whether the pipe is suitable for continued operation, or requires reduction in operating pressure and subsequent repair, dependent on the resulting maximum allowable operating pressure (MAOP). The three most common practices for calculating the adjusted MAOP are: ASME B31.G, Modified ASME B31.G, and effective area method.

An engineer in possession of an instrument that is capable of assessing and virtually modeling an anomaly, recording the findings, and performing all three MAOP calculations, would be immensely more efficient than one who must do all three tasks manually. Clock Spring Company has developed a device that does just this – the Eddy Current Conformable Array.

The Eddy Current Array (ECA) is a flexible circuit board imprinted with a two-dimensional array of transducers which is powered via a laptop USB-connection. The board contains a total of 512 transducers, with a primary “sending” transducer and a secondary “receiving” transducer printed atop one another; resulting in 256 pairs of coils designed to scan a 6-inch by 6-inch area. The diameter of each coil was specifically chosen to be 0.375-inches to allow for optimal data collection.

Upon operation, the receiving coil monitors the changing levels of inductance resulting from the interaction of the varying magnetic field and the adjacent pipe surface. The amount of inductance recorded is dependent upon the current carrying conductor’s proximity to the nearest conductive surface, the pipe wall. Non-uniform inductance over a constant conductive material can be attributed to inconsistent distances between inductors and the work surface. Each scan requires approximately three seconds to record data, which is then interpreted by the ECA software.

The software seamlessly outputs a two- and three-dimensional schematic of the scanned area, identifies the location of the deepest pit, and calculates the adjusted MAOP based on standardized anomaly interaction protocol. The integrity engineer is then able to modify individual pit measurements, if desired, and adjust the anomaly interaction procedures.

The ECA is limited to anomalies without the presence of cracks, gouges, or other potential stress concentrators.

Author: Taylor Spathias has a degree in industrial engineering from Texas A&M University in College Station.

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