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Pipeline Inspection with Guided Waves

Updated: Feb 27, 2020

TECHKNOWSERV specializes in the nondestructive testing of pipelines with long-range ultrasound (LRUT). Under ideal conditions, guided wave ultrasonic testing (GWUT) technology may be used to inspect up to 1000 feet (300 meters) of pipe from a single sensor position or up to 4000 feet (1.2 km) in one day. The actual inspection range depends on the type of pipe coating, degree of corrosion present in the pipeline, number of elbows, and whether or not the pipe is aboveground or underground. Guided wave testing (GWUT) technology may be applied to thin- and thick-walled pipe, with diameters ranging from 1” to 60”, rapidly and economically. Use reliable engineering data to assess exactly where your pipeline needs follow-up nondestructive testing such as long range ultrasound testing or visual inspection.

Benefits include:

››Rapid screening of long lengths of pipe

››100% of pipe wall is inspected

››Detection of corrosion in insulated and buried pipelines

How it Works

››The guided wave sensor is installed on the outer diameter (OD) of the pipeline.

››Long-range ultrasound is focused upstream and then downstream.

››Guided wave ultrasound is reflected back from pipeline corrosion.

››Corrosion under insulation is located with respect to the sensor location using the speed of sound in the pipeline tested.

››Corrosion severity is estimated in terms of cross-sectional area loss.

››Automatic report generation with reflector characterization and location.

Data and Reporting

Guided wave data is acquired and interpreted by professionally trained technicians and engineers with expertise in long-range ultrasound. Reflections are clearly identified as welds, corrosion, cross sectional area loss, pipe supports, repair clamps, elbows and other relevant pipeline features.

The Sensor and Instrumentation

TECHKNOWSERV uses magnetostrictive (MSS) sensing technology and instrumentation. The sensor is bonded to the pipe and consists of a proprietary cobalt alloy and conductors wrapped around the outer surface of the pipe. The sensor is unique because it generates ultrasound around the entire circumference of the pipeline. Competing guided wave pipeline inspection technology that use piezoelectric transducers, make contact only with a small fraction of the pipeline. Comparatively, the (MSS) sensing technology generates a very pure torsional guided wave. Sensitivity to corrosion is enhanced with (MSS) sensing technology. The instrumentation is portable and may operate on battery power for an entire day. The technology may be used on in-service and out-of-service pipelines.

Inspection Range

The pipeline inspection range depends on the condition of the pipeline, coatings, diameter and product inside the pipeline, and number of elbows. The best-case scenario is a straight section of pipeline where inspection ranges may exceed 1000 feet. The inspection range of underground pipelines may be limited to 60 feet depending on the inspected pipeline condition, moisture content of the soil, and pipeline product.

The guided wave inspection range may be estimated by providing information on the overall pipeline condition, condition and type of surface coating, number of elbows in the line, type of product inside the pipeline, and pipeline thickness and diameter. Actual inspection range is determined during initial testing on the target pipeline.


The MSS sensor generates a pure torsional guided wave in pipelines that is very sensitive to corrosion. Torsional guided waves are a powerful inspection tool since they attenuated less significantly by pipeline coatings and product. Inspection may be carried out at low frequencies—30 to 100 kHz—or at high frequency—100 to 250 kHz. Corrosion sensitivity is defined in terms of cross-sectional area (CSA) loss. The guided wave technology reliably detects CSA losses in the 10% range and can detect losses in the 3% range under ideal testing conditions.

Guided Wave Pipeline Testing Case Studies

Case Study # 1 – Guided Wave Inspection of Encased Piping

The following is an example of guided wave ultrasonic testing (GWUT) being used to perform a pipeline inspection on 8 meters (25 feet) of LNG pipeline pipe underneath a road crossing. The inspected pipe was encased in a larger diameter pipe. The entire 8 meters of pipe were inspected using a sensor placed approximately 30 cm, or 12 in., from the pipe encasement. Three large weld reflections are observed; W1 from the metal plate welded to the encasement and LNG pipe, W2 from the same reflector on the opposite side of the road, and W3 from the elbow weld located at 9 meters from the sensor on the opposite side of the road. Small CSA loss less than 5% is detected under the roadway between 1 and 3 meters from the sensor.

Figure 1. Guided wave sensors attached to LNG pipeline

Figure 2. Encased pipeline guided wave data

Case Study # 2 – Guided Wave Inspection of Buried Pipeline

The pipeline was installed in 1963 and has presently been determined to be unpiggable. Ten locations were identified by the client for guided wave inspection; these locations were considered to be High Consequence Areas on the line. The 10 locations were at road crossing and stream/swamp crossings. The line was 16” outside diameter with a wall thickness ranging from 0.5” – 0.615”. The approximate size of girth welds was 0.15” Tall x 0.5” Wide; this corresponds to approximately 14.5% CSA reflector at 16 kHz. The entire pipe is coated with a 0.1” – 0.5” thick coal tar enamel coating. The pipeline is buried in soil with high clay content at varying depths from 4’ – 16’ below grade.

Figure 3. Guided wave sensor placement

After a pipeline inspection was conducted using guided wave ultrasonic testing (GWUT) an indication was detected at – 14 feet and the pipe was excavated. A major coating failure and OD corrosion were discovered. Large reflections were observed at the ground entry points G1, G2, and G3. The range of inspection is very limited in each direction due to the soil and coating. Maximum range was estimated to be 7-10 meters in both directions.

Figure 4. Guided wave testing data for a buried pipeline

Case Study # 3 – Guided Wave Testing of Pipeline under Refinery Roadway

The following pipeline inspection was of 8 meters (25 feet) of crude oil buried pipeline. The pipe was encased in a larger diameter pipe. The location of the pipe supports, if any, were unknown. Pipe OD was 8”. The pipe was wrapped in bitumen coating. The sensor was installed approximately 1.5 meters (4.5 feet) above the 45 degree elbow buried under the roadway.

Figure 5. Guided wave testing sensor placement


The combination of the 45 degree elbow, bitumen coating, and buried condition made this underground pipeline very difficult to inspect using long range ultrasound (LRUT). The underground 45 degree elbow caused some initial energy loss begore the guided wave could get underground. Small reflections were observed from the 45 degree elbow weld on the opposite side of the roadway (EW2) and following weld EW3 above the roadway. Overall the signal-to-noise ratio of EW2 and EW3 are not strong due to attenuation. Some minor corrosion in the 2 to 6% CSA range is detected over the length of the underground pipeline. For future pipeline inspections, it is recommended to install sensors on both sides of the road below the 45 degree elbow weld directly on to the straight pipeline section below the roadway.

Unique Experience in Guided Wave Testing

TECHKNOWSERV is uniquely qualified to perform long range ultrasonic testing (LRUT) inspection of pipelines, rail track, bridge pile, and ship hulls. Our technical professionals are leaders in developing guided wave ultrasound inspection systems, procedures, training, and inspection services.

Our American Society for Nondestructive Testing (ASNT) certified inspectors have complementary training in fall protection, rope access, highway safety, railway safety, and confined space safety.

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