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Process Piping Mechanical Integrity and Guided Wave Ultrasonic Testing - GWUT

A Mechanical Integrity (MI) program is an important element of the OSHA Process Safety Management standard which was established to prevent or minimize the consequences of catastrophic releases of toxic, reactive, flammable, or explosive chemicals which may result in toxic, fire or explosion hazards.  An integral part of a mechanical integrity program per the OSHA standard is Inspection and testing.  Section (j) Mechanical integrity [1] requires that testing be performed on process piping, that the non-destructive testing procedures followed recognized engineering practices, the frequency at which the non-destructive testing takes place and the  non-destructive testing documentation satisfy minimum reporting requirements.  This article describes the role that guided wave ultrasonic testing (GWUT) or long range ultrasonic testing (LRUT) can play in a process piping mechanical integrity program.

Guided Wave Ultrasonic Testing of Process Pipelines

GWUT technology of today can trace its origins back to work at the Pennsylvania State University [2], Imperial College [3], and the Southwest Research Institute [4] over 3 decades ago. Since then, guided wave pipeline inspection has become a proven technique to inspect underground process piping, elevated piping, and insulated piping.  The principal motivation for using guided waves to inspect chemical or refinery plant piping is to assess the condition of difficult, or costly, to access piping. Or, to screen more pipe that would otherwise be inspected with conventional ultrasonic thickness testing.   Consider the thousand plus feet of insulated process piping shown below and the applicable non-destructive testing methods, safe access to the pipe via boom lift or scaffolding, and insulation removal.   Corrosion under insulation (CUI) occurs when water, moisture or condensation contamination  attacks the piping system outside-diameter (OD).  CUI can often be concentrated over a relatively small area due to localized deterioration of insulation.  As result, the large majority of the equipment may be in good condition and small sample area inspection such as ultrasonic thickness testing (UTT) via insulation ports may infer incorrect process pipe condition. 

There are several non-destructive testing technologies available to assess insulated process piping including guided wave ultrasonic testing, automated radiographic testing, and pulsed eddy current each with their own inherent advantages and disadvantages.  The advantages of GWUT include inspection of extended lengths from a single sensor position.  The maximum range will depend on pipe material properties, coating, product, number of welds, and other pipe attachments/supports.   The disadvantages include some limitations on accurate inspection data correlation to actual loss in pipe wall thickness.  Secondary non-destructive testing like ultrasonic thickness testing is required to quantify metal loss accurately.   



Guided Wave Ultrasonic Testing Compliance with Mechanical Integrity Programs

Guided wave testing complies will all aspects of a mechanical integrity program [1] for process piping.  In general, it can be effectively applied to stainless and carbon steel piping, above and below ground piping, insulated piping and heated lines using dedicated high temperature sensors.  GWUT equipment and inspection guidelines are published in ASTM and BS standards [10,11].  The ASTM practice is intended for use with tubular carbon steel or low-alloy steel products with a nominal pipe size (NPS) of 2 to 48 inches. GWUT technician’s minimum required non-destructive testing training and on-the-job hours are defined in SNT-TC-1A [12] and ISO-9712 [13].  In the former for example, options are available for GWUT Level 1 and 2 technician levels.  The minimum classroom training for each level is 40 hours.  GWUT certification requires a minimum of 240 hours of practical experience setting guided wave testing up, acquiring GWUT data, and compiling reports.  GWUT is compatible with the frequency at which pipeline should be inspected in accordance with API-570, state, or federal codes and regulations.

Guided Wave Ultrasonic Testing Data and Reporting

GWUT data acquisition and analysis is very similar to conventional longitudinal and shear wave ultrasonic testing where A-scan, or amplitude scan, is the fundamental data point.  GWUT example data is shown below.  By nature of the GWUT transducer, guided waves are bi-directional and, therefore, guided waves are generated in the upstream and downstream directions by the transducer array. The transducer and pulsing sequence are configured to focus torsional guided waves upstream while cancelling the wave travelling in the downstream direction.  Similarly, the sequence is reversed to focus the torsional wave downstream while cancelling the upstream direction. 

The result is ultrasonic data centered about the sensor position (0) with the upstream and downstream directions to the right and left, respectively.  The GWUT vertical amplitude scale is displayed in traditional amplitude units and correlated to percent cross-sectional area (% CSA) loss or thickness loss.  Guided wave sensitivity is established comparably to standard straight or shear wave ultrasonic testing.  In conventional ultrasonics, sensitivity is established using a side drilled hole or notch of known dimensions.  Similarly, the guided wave sensitivity is established by correlating the change in pipe cross-sectional area at circumferential welds.  GWUT time compensated gain (TCG) or distance amplitude correction (DAC) are also performed similarly to standard ultrasonic testing.  GWUT using two or more weld reflections to establish the TCG parameters. 

These GWUT analyses steps are compiled in the above example data showing an upstream and downstream range of 70 and 30 feet, respectively.  Consider first, the large reflection from welds at approximately 15% reflection amplitude.  Using the TCG math, these reflections are all very similar in amplitude.  Some variation in GWUT amplitude from girth welds is expected and observed.  Of particular interest here are the multiple small reflections observed between 20 and 60 feet upstream.  These indications were attributed to a series of generalized and pitting corrosion areas. Follow-up visual and ultrasonic thickness testing were recommended at these locations to identify the minimum wall thickness, true corrosion rates, and pipeline remaining life. 


Summarizing the Cost Benefit of Guided Wave Pipeline Testing

A pipeline integrity program requires input from non-destructive testing methods including visual inspection, ultrasonic thickness testing, automated radiographic testing, pulsed eddy current and guided wave ultrasonic testing.  The latter three are considered advanced NDT techniques and come at a premium NDT personnel and equipment cost.  GWUT can offer a cost benefit plus risk enhanced mitigation under the right inspection scenarios.   While ultrasonic thickness testing will always be a more accurate gage of metal loss, the risk that the minimum wall thickness is left unidentified in an insulated pipeline at select inspection ports is high.  While GWUT is less accurate in terms of quantifying metal loss, the area screened is enhanced by two or more orders of magnitude and risk of decreasing the risk of not identifying the minimum all thickness in the piping system.



2.    American Petroleum Institute (API) Recommended Practice 574 (RP 574), Inspection Practices for Piping System Component.

3.  API 570 Piping Inspection Code: In-service Inspection, Rating, Repair, and Alteration of Piping Systems

4. Barshinger, J., Rose, J. L. and Avioli, M. J. Jr., 2002, 'Guided Wave Resonance Tuning For Pipe Inspection', Journal of Pressure Vessel Technology 124, pp. 303 – 310

5.    M.J.S. LoweD.N. AlleyneP. Cawley, Defect detection in pipes using guided waves, Ultrasonics Volume 36, Issues 1–5, February 1998, Pages 147-154

6. H.Kwan, C.Dynes, “Long-range Guided Wave Inspection of Pipe Using the Magnetostrictive Sensor Technology – Feasibility of Defect Characterization”, Nondestructive evaluation of utilities and Pipelines II, International Society for Optical Engineering, SPIE, Vol.3400, 1998, pp.326-337

8. Burch S.F., HOIS-G-023 ( Guidelines for in-situ inspection of corrosion under insulation (CUI), 2021 Open Publication.

9. ASTM E2775-16 Standard Practice for Guided Wave Testing of Above Ground Steel Pipework Using Piezoelectric Effect Transduction

10. BS 9690-2:2011▹Non-destructive testing. Guided wave testing-Basic requirements for guided wave testing of pipes, pipelines and structural tubulars

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