Updated: May 20, 2022
Guided Wave Ultrasonic Testing of Encased Pipeline and Spacer Selection
The performance of guided wave ultrasonic testing (GWUT) or long-range ultrasonic testing (LRUT) is generally well understood in aboveground and underground inspection applications . At this time, the expected inspection range for uncoated and coated pipe are also well understood for GWUT testing of above and underground piping systems . A niche GWUT testing market are pipelines that travel under crossings which may be broadly categorized as under rivers, roads, and railroad right of ways  and range in distance from 20 to 300 feet. This article discusses the implications of caser selection on the guided wave testing of encased pipeline crossings.
Figure 1: Guided wave sensor installed on steel pipeline.
Context: Guided Wave Ultrasonic Testing (GWUT) and 49 CFR Part 192
If transmission pipelines transverse through areas of high consequence it is required to comply with U.S. Department of Transportation (DOT) Pipeline and Hazardous Materials Safety Administration (PHMSA) Integrity Management (IM) protocols and outlined in 49 CFR Part 192 . 49 CFR § 192.921 - How is the baseline assessment to be conducted? [5 ] calls out inspections using an internal inspection tool, pressure testing, spike pressure testing, excavation and direct in-situ examination, guided wave ultrasonic testing, and direct assessment as acceptable methods. Due to economics of using guided waves compared to these other methods, GWUT is a logical solution should it provide the desired performance.
49 CFR Appendix F to Part 192 outlines the criteria for conducting integrity assessments using guided wave ultrasonic testing (GWUT). There are many variables that influence the two primary performance metrics of guided wave testing; GWUT sensitivity and GWUT range. Broadly, these may be defined as the smallest detectable cross-sectional area (CSA) loss and at what distance this sensitivity may be achieved from the GWUT sensor. For existing pipelines, variables such as coating, spacer type, and product are fixed and as result may disqualify GWUT as an effective NDT solution. However, new pipeline crossings may be designed and tested to optimize the influence of these variables on the GWUT performance. This article demonstrates how casing selection is critical to GWUT performance of encased pipe traveling under crossings.
Quick Summary of Guided Wave Ultrasonic Testing (GWUT) Performance Specifications
When ultrasonic waves encounter a change in pipe wall thickness, whether an increase or a decrease, a proportion of the ultrasound is reflected back to the transducer. These reflected signals provide a mechanism for the detection of discontinuities. In the case of a pipe feature such as a girth weld, the increase in thickness is symmetrical around the pipe, so that the advancing circular wave front is reflected uniformly.
The reflections are displayed as rectified signals in an amplitude vs. distance 'A-scan' display, similar to that used in conventional ultrasonic inspections, but with a time-base range measured in many feet rather than inches. Girth welds in the pipe produce dominant signals in the A-scan and act as important markers, used to calculate the amount of attenuation in each individual pipe. The rate of attenuation is calculated and expressed as Time Corrected Gain (TCG) with the units of dB/ft.
The GWUT technique detects changes in cylindrical cross-sectional area. Due to limitations of a quantitative calibration method, the GWUT technique generally provides a qualitative measure of %CSA. The qualitative measurement is quantified with the use of Ultrasonic Thickness Testing (UTT) at the distance seen on the GWUT software.
GWUT Casing Spacers and GWUT Performance
Two commonly used and highly regarded casing spacers are the fiberglass sleeve model and the High Density Poly Ethylene (HDPE) model. The pre-stressed fiberglass sleave product is installed using a methylmethacrylate based adhesive with high lap shear strength ~ 1,250 – 1,500 psi (8.6 MPa – 10.3 MPa) per ASTM D1002. The HDPE model is applied by first installing a double-sided tape to the desired area around the pipe, and then wrapping the spacer around the tape and then ratcheting to a desired tension. The fiberglass sleeve model is very tightly adhered using the high lap strength epoxy. This condition effectively damps out the guided wave signal. As a result, after travelling through multiple fiberglass sleeve spacers the GWUT signal strength is insufficient to return a signal from 5-10% CSA loss with sufficient signal-to-noise ratio. In addition, the fiberglass sleeves provide relatively strong reflections from both the near and far sleeve edges. Example GWUT data from these sleeves in Figure 2 shows multiple reflections that in general complicate the interpretation of the ultrasonic data. Simulated CSA losses were inserted into the pipe a various locations but are not clearly observable above the coherent noise level.
The combination of the double-sided tape and ratcheted tension of the HDPE spacer allows detection of 5-10% cross-sectional area loss at excellent signal-to-noise ratio as shown in Figure 3. The GWUT travelled through one double HDPE spacer and 2 additional single HDPE spacers before reaching simulated CSA loss close to the end of the pipe. Reflections from the HDPE spaces do not appear at noticeable signal-to-noise levels.
Figure 2: Guided wave ultrasonic testing (GWUT) data from a 20-foot pipe section with multiple fiberglass sleeve spacers installed.
Figure 3: Guided wave ultrasonic testing (GWUT) data from a 20-foot pipe section with multiple HDPE sleeve spacers installed.
While the encased pipeline owner and engineering services company duo may not be able to optimize GWUT performance on existing pipelines, there are excellent opportunities for future collaboration between the asset owner, engineering services company and spacer manufacturers to design and test casing spacer that optimize GWUT performance on the proposed system and to significantly reduce the inspection cost over the asset lifetime.
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4. 49 CFR Part 192 - TRANSPORTATION OF NATURAL AND OTHER GAS BY PIPELINE: MINIMUM FEDERAL SAFETY STANDARDS
5. 49 CFR § 192.921 - How is the baseline assessment to be conducted?
6. 49 CFR Appendix F to Part 192 - Criteria for Conducting Integrity Assessments Using Guided Wave Ultrasonic Testing (GWUT).