Third-Generation EMAT Tool Enhanced For Finding SCC And Disbonded Coating In Dry Gas Pipelines

This article looks at third-generation enhancements to electro-magnetic acoustic transducer (EMAT) inline inspection technology for dry gas pipelines. The enhancements have been derived from five years of operational experience by GE Oil & Gas-PII.

Since GE’s initial product launch of the EMAT tool, several hardware and analysis enhancements have resulted from the evaluation by engineers of more than a 1,000 km of EMAT tool pipeline inspections and 207 field confirmations of findings. These enhancements are now incorporated into GE’s new 30-inch, third-generation EMAT ILI tool.

Until 2002, conventional ultrasonic technologies were used for crack detection in liquid-filled pipelines. The application of the traditional piezoelectric transducers for inline inspection requires a liquid couplant for ultrasonic energy and signal transfer into the pipe wall. This requirement for a liquid couplant limits the cost-effective use of such tools for gas-filled pipelines. Filling the gas pipeline with a liquid for inspection or by batching the inspection tool in a liquid slug is--from a technical and operational perspective--not always feasible.

Electro Magnetic Acoustic Transducers (EMAT) are dry-coupled sensors. GE launched an inline inspection tool with EMAT sensors for 36-inch pipelines in 2002. Locating most of the key crack features identified earlier by the benchmark ultrasound crack detection tool, the initial launch inspection was considered a success. However, further inspections encompassed a larger sample set of the pipeline population with its broader variety in pipe conditions and environment which uncovered certain limitations in the design from achieving the expectations set by the historical performance of the ultrasound crack detection technology. The 36-inch EMAT tool has now completed over 1,000 km of inspections with operators in the U.S. and Canada. Results of these inspections have been confirmed with more than 200 field verifications.

This initial launch of 36-inch EMAT tool has demonstrated the significant potential of the EMAT technology for use in inspection of gas pipelines without a liquid coupling and for location of a wider range of defects with better accuracy than the niche technologies of Transverse Flux Induction (TFI) and wheel-coupled ultrasound tool, respectively. By overcoming the shortcomings of this first generation, EMAT seeks to provide a financially superior option to hydrostatic testing.

The latest generation of EMAT crack-detection tool is a new design which incorporates lessons learned from operational experience. The tool delivers an enhanced set of performance requirements which were quantified up front by pipeline operators.

Table 1: Performance comparison of EMAT tool GEN I and GEN III.

EMAT Principle

The conventional ultrasound generation technique utilizes piezoelectric transducers in which ultrasound wave pulses are generated by a crystal element and fed into the pipe wall via a coupling liquid. In contrast, Electro Magnetic Acoustic Transducers (EMAT) are dry-coupled. For transmission into the pipe wall, an alternating current in a wire induces an eddy current in the metal surface. When this is combined with a static magnetic field, a force is produced which causes the steel metal grid to oscillate, thus launching a guided ultrasonic sound wave in the pipe wall.