November 2018

Features

INSIDE JOB: Phased Array Ultrasound Can Deliver Better Weld Inspections

By Dominic Giguere, Zetec

Pipeline weld inspections are one of the most challenging – and common – types of nondestructive testing (NDT) applications in the oil and gas industry. Joining materials with different metallurgical properties, grain alignments, acoustic characteristics, cylindrical geometries, and other attributes can produce minute flaws and defects that are difficult to identify or detect even with sophisticated probes and instruments.

“The results of weld inspections can have a big effect on decisions about uptime, code compliance, and safety,” said Chad Ivy, regional advanced operations manager for Versa Integrity Group, which is based in Houston.

Ultrasonic testing (UT) helps clients make those important decisions, Ivy said.

“Having a clean, clear digital picture on the screen, in the moment, lets clients see what’s going on with their assets,” Ivy said. “We’re able to put inspection data into a 3-D imaging format with a level of resolution that’s so clear and detailed that clients can recognize flaws without the understanding of ultrasonics that we have as technicians. The image goes up on a big display and we walk them through it. It changes the way we communicate inspection results with clients.”

Beyond Radiography

For years, inspectors used industrial radiography (RT) to examine welded joints and heat-affected zones in pipelines. But RT has significant limitations. It’s insensitive to mis-oriented planar defects. It provides no immediate feedback to the welders, and it’s disruptive to productivity at the job site. There are obvious safety and regulatory concerns as well.

Codes, standards, and technology have evolved to allow the use of ultrasonic testing in lieu of RT. “Today’s advanced UT instruments, software, and probes can capture and analyze cracks, corrosion, changes in thickness or metallurgical structure, and other distortions efficiently and accurately, creating a digital record you can integrate into a history of inspection data,” Ivy said.

You don’t have to be an expert in ultrasound but if you’re responsible for pipeline maintenance or compliance, it does help to understand how and why NDT technicians use it to conduct weld inspections. In particular, two terms you should know are phased array UT and time-of-flight diffraction (TOFD).

ABCs of Ultrasonics

Ultrasonic testing uses pulses of high-frequency sound energy to detect surface and subsurface cracks and other defects. These pulses come from a transducer or probe, which a technician manually moves over the surface of the component under inspection.

The probe emits ultrasonic waves into the material at precise intervals and a set angle. A UT instrument continually calculates the time it takes for these sound waves to return to the probe. When the waves encounter a defect, they’re disrupted; the time it takes for that energy to reflect back to the probe is analyzed by the instrument and presented as a graphic on the instrument’s screen for the technician to review.

Phased array UT is a more advanced application of ultrasonics that packs multiple individual elements (typically from 16 to 64) into a single probe. By exciting each element in a controlled manner, a phased array UT instrument can produce a precise beam shape and render 2-D and 3-D views of the area that’s being inspected. The probability of detection (POD) is considerably greater and you get a more accurate characterization and visualization of flaws and other defects.

Probe alignment is critical to a proper weld inspection, and curved geometry, long seams, irregular surfaces, dissimilar metals, and tough work environments add to the challenge. In many cases technicians will use a weld crawler – a wheeled frame – to hold the probes in position as they guide them over the weld. The more advanced weld crawlers have magnetic wheels, motors, onboard cameras to stream a live image to the ultrasound instrument display and can be oriented for circumferential or axial welds.

Time-of-Flight Diffraction

Another advantage to phased array UT is that it allows the use TOFD, a highly reliable NDT technique for flaw detection and sizing in pipeline weld inspections. It can also be used for weld overlays and the heat-affected zones of other components, as well as pressure vessels and storage tanks.

In a TOFD system, a pair of ultrasonic probes are situated on opposite sides of a weld. One probe acts a transmitter, emitting an ultrasonic pulse into the material; the other is a receiver. Instead of measuring only for the high amplitude sound waves that reflect off of the back of the component, TOFD calculates the response time of low amplitude waves that are diffracted by the tips of cracks. It can precisely determine the depth and shape of a defect, as well as the thickness of the pipeline wall, based on the sound wave’s time of flight.

TOFD also has a high degree of repeatability. Because of this, changes in flaws can be recorded and compared over time, creating a valuable history of inspection data.

Useful Combination

In combination, phased array UT and TOFD can detect all welding flaw types and provide reliable through-wall sizing capability in one inspection. The inspector can use a two-sided phased array UT examination with standard shear wave probes to detect planar and surface-breaking flaws, while the TOFD technique can locate embedded flaws and offers accurate through-wall sizing performance.

Using phased array UT and TOFD together can also increase the productivity of the inspection crew simply by reducing the number of scans and manipulations that need to be done.

The choice of inspection instrument and scanner or probe can make it easier for the technician to apply the two different ultrasonic techniques, including duplicating the mechanical references (start position on the scan and offset from the weld center line) and calibration. In order to maximize the efficiency of the inspection crew, the scanner used for the simultaneous use of phased array UT and TOFD needs to adapt to as many inspection configurations as possible.

Although materials and welding techniques evolve, fatigue cracks and corrosion will continue to be threats to pipeline reliability. A better understanding of NDT testing and ultrasonics can provide next-level insight into the integrity of your assets, the quality of your inspections, and help you make informed decisions about maintenance and uptime. P&GJ

Author: Dominic Giguere is product manager for UT Portable Solutions at Zetec, which develops ultrasonic and other NDT solutions for oil and gas and other industries.