Guided Wave NDT: It gets a bad rap, but is it deserved?

Guided wave ultrasonic testing (GWT) has gotten a bad rap over the years, but is it truly deserved? Are there any advantages at all for this technology which could make it superior in certain use cases – or should it be avoided altogether? In this post we will explore why we think GWT has fallen out of favor and suggest where it could actually save a lot of time and money when used correctly.

First, a refresher

Guided Wave (also often referred to as GUL after the pioneers of the method, Guided Ultrasonics Limited) is a relatively recent NDT method in which a ring transducer array is attached around the circumference of a pipe to generate low frequency waves along the length in both directions. This ring array both emits the waves and records the echoes which indicate metal loss defects and their distance from the ring. The advantage of this method is that you can examine a large section of pipeline all at once without having to remove much insulation or excavate buried pipe. The effective range varies from about 10 meters to 35 meters depending on pipe condition, coatings, number or welds, and pipe contents.

So, what’s the issue?

In our opinion, Guided Wave was oversold as a fix-all magic bullet to the energy and chemical industries because you can screen a lot of pipe in one shot. Many inspection firms jumped on the bandwagon to offer GWT, but their technician training was rushed and/or inadequate leading to misuse. The end result was poor inspections and a collective eye-roll when you talk about GUL today. 

I was at a refinery recently and they had inspected a buried pipe that went under a roadway. There were high expectations and in the end the results were very poor. The customer felt that they had been “oversold” and under delivered. When we pulled the inspection apart there were a number of areas where proper application for the technology would have lead to a positive result, most notable using the correct frequency for a long distances. 

Errors like this contributed to a general distrust in the industry, which is unfortunate because it’s a fantastic tool when used where it can be successful.

Caveats and use cases for Guided Wave Ultrasonic Testing

First, GWT doesn’t stand alone and is not a “silver bullet.” You must understand the damage mechanism to apply the proper verification tool (UT, Visual, Creaform or Laser). You must understand the coatings and the weather conditions (high ambient temp alters coating viscosity which will impact readings. All of this is considered when setting expectations for the results. It’s not a “silver bullet” but it can provide invaluable information.

Second, bends and connections (45’s, 90’s, Flanges, T’s, Valves) must be considered. Clean 90’s can be shot successfully but the rest require another method.

Third, success with Guided Wave is very dependent on the individual technician’s skill. The attention paid to training AND experience are therefore the main differentiator between firms offering GWT. Experience shooting GUL combined with confirmed indications makes a quality GUL tech.

Where GUL shines as an NDT method:

• Allows for safe inspection of unpiggable pipes
• GUL is recognized as an inspection method per Pipeline and Hazardous Materials Administration (PHMS – https://www.phmsa.dot.gov) for cased pipe.
• Buried and sub-sea lines can also be inspected much easier
• Superior at detecting pipe rack point-of-contact corrosion
• Soil-to-air interfaces can be checked without costly and time-consuming soil removal
• Under-road crossings might otherwise require excavation and traffic disruption can be accomplished at tremendous time and cost savings 
• Heaters and boiler tubes can be checked much cheaper than using self-propelled pigs which can run in the six figures per inspection
• Helps achieve 100% coverage cost effectively – using traditional UT or RT your damage could be inches away from your inspection area and remain unidentified. GTW covers a wide area which is then confirmed with UT or RT.

There could be a large amount of pitting, corrosion, or moisture ingress undetected in your piping. Guided Wave provides a very cost-effective and rapid means of detecting issues that could lead to leakage, fire, explosion, or other disasters, but it has to be used right. We’d love a chance to show you how – please contact us to discuss your inspection needs and explore how GWT might be the perfect fit for your project.