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Pipeline Inspection with Guided Waves


Corrosion is a leading cause of pipeline failure. Inspecting and monitoring pipelines are essential for maintaining line integrity and preventing leaks during the transport of oil, gas, and other chemicals.  Because it is time consuming and expensive to inspect a pipeline in its entirety using localized inspection techniques such as conventional UT, guided wave technology is now commonly used as a complementary screening tool to improve corrosion detection in various structures. With guided wave, low-frequency waves are sent along pipe; these waves propagate over long distances, covering 100% of the pipe wall thickness from a single inspection position. Guided wave testing is used to identify areas of concern, which are then locally scanned with conventional UT or phased array to size the indication, ensuring that the rest of the line is free of corrosion.

Detection and sizing

In this example, corrosion was detected with the Olympus UltraWave LRT guided wave system, and the indications were confirmed and sized with the Olympus 38DL PLUS thickness gage. The first step in a guided wave inspection is to attach the collar, containing an array of low-frequency transducers, around the pipe and acquire an axisymmetric scan (A-scan) over a predefined range of frequencies.

Pipe inspection with the Olympus UltraWave LRT.
Pipe inspection with the Olympus UltraWave LRT.

The F-scan color map, which contains all acquired A-scans, helps the user determine the most sensitive frequency for further analysis. The F-scan is also used to help characterize defects based on their behavior over the range of frequency and identify signals caused by mode conversion or echoes that can arise from other features in the line.

An F-scan map of a pipeline being inspected.
An F-scan map of a pipeline being inspected.

Once the user selects the ideal A-scan, a distance amplitude correction curve is set up using the weld’s peaks; indications can then be categorized based on their amplitude. Indications are illustrated below. From yellow to red, the classification indicates an estimated severity of the indication with its distance on the pipe’s length. For this inspection, the purple dots refer to a special tool that was connected to the pipe while the other indications are real corrosion. Every indication was confirmed and sized with the 38DL PLUS thickness gage.

Analysis graph showing indications, classified by severity from yellow to red, and marks on the pipe where the indications were characterized using the 38DLP.
Analysis graph (above) showing indications (classified by severity from yellow to red)
and marks on the pipe (below) where the indications were characterized using the 38DLP.

Additional focusing tools are available to estimate the circumferential extent and position of the indications. Below is the unrolled pipe view, also called synthetic focusing, where the indications are in line with channel 9, corresponding to the top area of the pipe. Operators can also use the active focusing function where concentrated energy is induced in the pipe at a specific distance.

A synthetic color map of the unrolled pipe view.
A synthetic color map of the unrolled pipe view.

Benefits of using guided wave as a screening tool for corrosion detection

  • Improve productivity with long range coverage and rapid screening
  • Ensure 100% coverage of the inspected zone
  • In-service inspection; no production shutdown required
  • Scan pipes with limited access such as buried, coated, and insulated pipes
  • Reduce operating costs 

Equipment used

  • UltraWave LRT system
  • Laptop computer with advanced UltraWave LRT software
  • Inspection collar for 12 in. OD pipe
  • 38DL PLUS thickness gage


The Olympus UltraWave LRT enabled users to speed up the inspection of multiple pipelines at this site. Areas of concern were confirmed and sized with conventional UT, improving productivity when compared to using UT to inspect the entire pipe length. Guided wave can also be used to inspect other types of pipe, including coated, buried, vertical, and limited access lines, reducing operating costs. The maximum pipe length that can be inspected with the UltraWave LRT is 91 meters (300 feet) from each side of the collar for a total of 182 meters (600 feet). The maximum value will vary for every application. Factors such as coating, level of corrosion, insulation, and type of soil can attenuate the guided wave signal and reduce the diagnostic length. Below is a table (Table 1) with typical average test ranges for different pipe configurations. The value may vary when multiple factors are combined, and a test shot should be always taken to determine the exact distance of inspection.  No matter the type of pipeline, guided wave testing helps users quickly inspect long portions of pipe for maximum productivity.

Typical test range for different test pipe configurations 

The typical test range (in meters and feet) of guided wave testing under various test conditions.

Test conditionTypical range of test
Clean, straight pipe 50–182 164–600
Clean, wool insulated pipe 40–175 131–574
Minor corrosion 20–50 65–164
Significant corrosion 15–30 50–98
Kevlar wrapped 30–182 98–600
Spun epoxy coating 30–50 98–164
Well-packed earth 15–30 49–98
Thin, hard bitumen tape 2.5 mm 5–25 16–82
Thick, soft bitumen tape > 2.5 mm 2–8 6–26
Well-bounded concrete wall 1–2 3–6
Grout-lined pipe 10–30 32–98
Loosely bonded concrete wall 2–8 6–26

Olympus IMS


UltraWave LRT

The UltraWave LRT uses guided wave technology and excites low-frequency ultrasonic waves that travel lengthwise over tens of meters along a pipe, detecting wall thickness variations. The system is ideal to screen in-service pipes and pipelines, and to inspect limited access pipes from a single inspection position. It includes advanced software, acquisition unit, touch screen laptop, and compact probes with bands and bladders.
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