Various standards, codes and regulations in the downstream oil and gas industry require periodic testing of plant systems and components to ensure conditions and processes are safe and reliable.
Nondestructive testing is vital to determining safety and reliability of equipment. The data and information such testing provides helps determine the service life of components or materials within complex systems. Such techniques are performed at various times during service cycles. NDT minimizes downtime and provides the information operators need to conduct risk-based inspections and determine fitness for service.
Ultrasonic testing has become an indispensable nondestructive testing technique for the oil and gas industry. While some NDT methods are better than others in certain situations, the ultrasonic technique is particularly effective for testing for flaws in pressure vessels, and oil and gas pipelines, as well as piping corrosion and flaws in welds.
What is ultrasonic testing?
Ultrasonic testing makes use of sound to detect flaws and defects in materials. According to the International Atomic Energy Agency, ultrasonic testing can be used for metallic and nonmetallic materials. For nonmetallic materials, frequencies between 50 kHz and 100 kHz are used. For metal, frequencies between 0.5 MHz and 10 MHz are used most frequently.
Ultrasounds move at different velocities in different materials. However, the principle at work is that sounds continue to travel through materials until they hit a flaw. Then they are reflected back to the instrument that emits the sound, called a transducer. By comparing the sound that is emitted from the transducer to the sound that is reflected back from the flaw, operators can then determine the size and location of the flaw.
Transducers use piezoelectric materials that are able to convert sound waves into electrical pulses and vice versa. So when the transducer emits a sound wave, the sound will travel the entire length of the material being tested until it reaches the end and then it is reflected back.
Once the signal is reflected back to the transducer, the sound wave is converted to an electrical signal, which is then put on the ultrasonic display. If there is a flaw in the material being tested, a portion of the signal will be reflected back to the transducer while the remaining signal continues to the end of the object and then reflects back.
The comparison of the two signals allows operators to locate the flaw and see how big it is.
Results aren’t displayed in a visual format as they are with radiography. They are presented as amplitude vs time signals. So an experienced operator must interpret the results based on knowledge of the materials being tested and exact calibration of the testing equipment.
There are several different types of ultrasonic tests. One particularly effective method for piping or other large-volume test subjects is Long Range Ultrasonic Testing. According to Inspectioneering, an asset integrity management journal, transducer rings are fixed around a pipe and emit a series of low-frequency waves. The waves then travel along the entire length of the pipe, covering the whole wall.
“Ultrasonic testing is an extremely reliable and accurate form of testing.”
Advantages of UT
Inspectioneering noted that even though ultrasonic testing requires a significant amount of training for an operator to interpret results, it is an extremely reliable and accurate form of testing. Ultrasonic testing also can detect flaws on or beneath the surface.
TWI, an independent research and technology organization, noted that with ultrasonic testing, access to only one side of the component being tested is necessary. And thick items take no longer to test than thin items, which is particularly effective when testing for cracks or corrosion in piping.