Pipelines play a crucial role in transporting liquids and gases over long distances. Numerous sectors, from the food and beverage to the pharmaceutical industry, rely on these structures as an integral part of their production process.
Perhaps no other industry relies on piping networks more than the oil and gas industry. The U.S. alone consists of over 2.4 million miles of energy pipeline, making it the owner of the largest piping network in the world.
However, most of these pipelines are buried, making them prime candidates for lack of maintenance and neglect. Corrosion-related defects such as cracking can lead to numerous failures, including leaks, loss of containment and decreased structural integrity.
In extreme cases, cracks can also be the cause of catastrophic explosions. (Related reading: The El Paso Natural Gas Company Pipeline Explosion.)
While there are several methods available for detecting cracking defects in pipelines, ultrasonic testing techniques are perhaps the most efficient. As such, this technology should form an integral part of an asset owner’s quality control and management processes.
In this article we will take a more in-depth look at ultrasonic technology and describe its role in preventing pipeline failure.
Ultrasonic Technology Explained
Ultrasonic is used to describe sound waves that possess frequencies higher than the audible limit of human hearing. These sound waves are used to produce data that providers can analyze and the operator can then use this insight to make more informed decisions.
As the waves travel through various media (liquids, solids, gases), portions are reflected back to a receiver as the waves encounters different boundaries.
This underlying principle forms the basis for ultrasonic flaw detection (UFD) in pipelines.
A typical ultrasonic testing unit consists of a pulser/receiver, ultrasonic transducer and a display device. The pulser/receiver is an electronic device capable of generating high-voltage electrical pulses. The voltage from the pulser powers the transducer to create high-frequency ultrasonic wave energy.
The sound produced by the transducer propagates through different boundaries, with some of it reflecting back to the receiver when a boundary or an anomaly (such as a crack) is encountered. The reflected sound wave is then converted to an electrical signal by the transducer and shown on a display screen.
In addition to flaws, ultrasonic testing can also reveal other crucial information, such as size, orientation, thickness and other critical features.
How Is Ultrasonic Technology Used During Crack Inspection?
Ultrasonic testing of cracks in pipelines is conducted using two main methods: straight beam testing and angle beam testing.
Straight beam testing is typically used to identify cracks or other defects that are parallel to the surface of the object. This technique is also ideal for the identification of voids and other porous locations. Straight beam testing operates on the principle that the sound energy will continue to propagate through the medium until it encounters a boundary, such as a crack.
During this ultrasonic testing method, the operator couples the transducer to the test piece, sending ultrasonic waves through the structure. The operator then identifies and sorts the echoes returning from the back wall of the pipe from those arriving ahead of the back wall.
Any significant signal echoes that arrive before the back wall indicate that a crack or another significant discontinuity is present at the tested location.
Angle beam testing, as its name implies, is used to locate cracks and discontinuities that are tilted with respect to the longitudinal axis of the pipeline. This method is crucial because defects that are not parallel to the surface are usually undetected by straight beam testing due to their orientation to the sound wave.
Such angled cracks and defects typically occur in welds and structural metal components due to the presence of undue stresses. During this technique, a common angle beam (wedge) transducer or an immersion transducer is angled at the appropriate inclination to emit the sound wave into the pipeline.
The Benefits of Using Ultrasonic Crack Inspections for Pipes
In the oil and gas industry, where pipeline failures can give rise to catastrophic events, timely and effective ultrasonic inspections can mean the difference between life and death. Leak detection and prevention can also help avoid millions of dollars in product losses. (Learn more in the article "Use of Non-intrusive Inspection in Onshore Gas Facilities.")
One of the main benefits of ultrasonic inspections is the speed and versatility it possesses over other inspection techniques. Some of the other advantages of ultrasonic inspections include:
- Sensitivities to surface and subsurface discontinuities allow the inspections to detect the early onset of more significant structural issues.
- The depth of penetration of ultrasonic sound waves surpasses that of other nondestructive testing (NDT) methods, such as magnetic flux inspection.
- Some ultrasonic testing techniques require only single-side access, thus making this method ideal for pipelines that may be difficult to access.
- Minimal part preparation is required; therefore, ultrasonic testing units can be quickly mobilized and deployed.
- Ultrasonic testing equipment provides almost instantaneous results.
- Other vital information, such as thickness, dimensional measurements, shape variations, etc., can also be obtained.
- Automated imaging systems can be used to produce detailed visual images of pipeline flaws.
Limitations of Ultrasonic Testing
While ultrasonic testing is effective at detecting the early onset of cracking and other structural defects, it is essential for asset owners to understand the limitations of this technology to ensure that the best results are obtained.
Some common limitations of ultrasonic testing include:
- The pipeline must be accessible to transmit the ultrasonic sound waves.
- This technique requires more skill and training than some other testing methods.
- Materials that are rough, very small or very thin can be difficult to inspect or produce erroneous results.
- Testing equipment requires frequent calibration in accordance with suitable reference standards.
- Cast iron pipes (or pipes constructed from other rough materials) can be difficult to inspect due to low sound transmission or high signal to noise ratio.
- The proper method (straight beam or angle beam testing) needs to be employed to ensure that all pipe defects are captured.
The benefits of ultrasonic testing make it an essential addition to a piping network’s quality management program. This technology can detect existing cracks and can even identify the early onset of defect formation. These characteristics allow asset owners to take immediate preventative measures to mitigate the effects of pipeline failures.
However, as with any testing method, it is essential to understand the limitations of the technology to guarantee the accuracy of test results and the effectiveness of remedial strategies.