Detection of Defects and Corrosion Under Pipe Supports

Corrosion and cracking present problems for every industry that relies on pipelines to transport liquid and gas products. There are many different kinds of techniques and equipment used for examining pipelines. These vary based on specific needs, access restrictions, and environmental conditions. In the last few years, electromagnetic acoustic transducer (EMAT) has come to prominence for its ability to quickly scan exposed pipelines for corrosion, cracks, and other defects using medium-range guided waves and without the requirement for liquid couplants. Recent sensor and technique developments have now enhanced these capabilities to reliably inspect inaccessible or difficult to access areas for corrosion, such as the undersides of pipe supports and pipes with heavy coatings.

Axial and Circumferential Scanning with MRUT

Long range ultrasonic testing (LRUT) uses encircling rings to generate guided waves that can detect moderate defects over long distances on relatively clean pipes. One of the main disadvantages of this technique is that it has an uninspectable dead zone of approximately 1-2 meters in front of the transducer, which makes it inappropriate for small distances and where valves, connections, or other components are present.

EMAT-generated MRUT lamb waves have been used for axial scanning using attenuation techniques for several years. This technique permits fast scanning of exposed pipe with great detection capabilities. However, its use in reflection mode (circumferential scanning) for inspecting inaccessible areas has been limited because lamb waves can leak into coatings and supports, which restricts its practical use in the field.

[picture goes here] Figure 1. Circumferential scanning using attenuation vs Axial Scanning using reflection.

A new patent-pending scanner and technique that introduces the ability to use shear horizontal (SH) guided waves for MRUT circumferential scanning has been developed.

SH waves provide several benefits for defect and corrosion detection:

  • They are less-dispersive than Lamb waves, which allows for clearer signal interpretation.
  • They are more consistent through materials and less susceptible to mode conversion.
  • Higher frequency ranges can be used to provide improved axial and lateral resolution.
  • They have minimum out-of-plane motion and can travel greater distances with minimum attenuation on coated pipes and on full pipes.

While the circumferential scanner provides obvious benefits, it does have limitations. The scanner has a dead zone of 10-15cm (4” to 6”) which prevents inspection in very close proximity to the area of interest and the temperature range for the scanner is currently rated up to 200°C (392°F). Moreover, by inspecting closer to the defects, the detection and resolution can be 10 or more times greater than with LRUT.

[picture goes here]

Equipment

The equipment used for this technique includes:

  • PowerBox H, High-Power EMAT instrument.
  • MRUT Software for Circumferential Scanning.
  • Scanner with built-in coil and encoder (for OD > 4” or 100mm).
  • Magnetostrictive strip material
  • Magnetizer

The technique uses a thin magnetostrictive strip material, which is either temporarily or permanently wrapped around the circumference of the pipe. The material can be applied to the pipe with an adhesive epoxy or using a strip tensioner device. The strip is then magnetized and the scanner is moved along the strip and around the pipe. Magnetic wheels facilitate scanning on ferromagnetic pipes, although the technique is valid for any type of pipeline material, including plastic.

Inspection data can be recorded and saved for analysis and reporting after the job is completed. If the magnetostrictive strip is permanently applied, it only needs to be magnetized again before performing the inspection, so multiple locations can be inspected quickly on a scheduled basis.

This method can complement LRUT, providing greater detection and resolution on shorter lengths of pipe or on restricted-access areas where the deployment of LRUT rings is not feasible.

[picture goes here] Figure 3. Pipe scanning model and sample data from an EMAT circumferential scanner.

Conclusion

With the addition to the SH circumferential scanner, EMAT has increased its range of applications from axial scanning on exposed pipe, to circumferential scanning for inspection under support and other inaccessible areas.

Used as a complement to other techniques or as a standalone application, the MRUT circumferential scanner provides a new EMAT solution for the inspectors’ toolbox.

[picture goes here]