Practical Applications based on Rigorous Experimentation

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Project objective

The legs of steel high voltage transmission towers, like most metal structures, start to corrode once they have been installed and buried in the ground. The rate of corrosion depends on the leg material, geometry, coating applied and environment in which they are placed. A reliable method to assess the condition of these buried structures is required.

Scope of work

The concept of ultrasonic (UT) determination of buried structures’ condition is based on the excitation of traveling acoustic waves in the leg. The equipment for inspection consists of the instrument (UT pulser-receiver) and UT angle contact transducer connected to the pulser-receiver by coaxial cable. An operator puts the transducer on the unburied part of the leg.

To provide a good acoustic contact between transducer and leg, some couplant, e.g. water, is used. The transducer excites the UT wave propagating along the leg length buried in the ground. This acoustic wave travels as a “guided” wave. The wave is termed “guided” because it travels along the medium (leg in the subject case) guided by the geometric boundaries of the medium.

Signals reflected from geometric irregularities in the structure are detected in the pulse-echo mode, which means that in this case, the wave reflected from a defect or corrosion area of the leg will return back to the transducer. Attenuation of such a wave in a metal is insignificant, and typically the achievable inspection range is a few meters. The received signal will then be processed in the pulser-receiver.

As a result, the location of the defect and potentially its size will be determined. Because of the long-inspection range and good sensitivity to defects, guided-wave inspection technology is very useful for quickly surveying a structure for defects, including those in remote locations that are difficult to access. 

Work performed

Samples of tower legs with simulated corrosion areas were used for these experiments.   Available transducers with different frequencies and diameters have been tested and various prisms for transducers, providing different incident angles for incoming UT waves, were manufactured and tested.

The type of traveling wave and optimum transducer, at which maximum sensitivity is achieved, has been determined. Results show that a contact angle probe, generating a surface wave propagating at a 90⁰ refraction angle (i.e. tangentially to leg surface), can reliably detect even small, shallow simulated “corrosion” pits (about ~10% of wall thickness and a few millimeters in size) located at 2-3’ from the transducer. Their location and dimensions are estimated by determining response position and its width and amplitude.

It can then be concluded that the guided wave technique can effectively detect and size very shallow and / or small flaws in tower legs.

Proven client benefits  

After determining location and size of the corrosion area, the condition of the buried part of tower leg can be assessed accurately. 

For more information, contact: Kevin Maynard, 416.207.6000 x5477, kevin.maynard@kinectrics.com

For participation inquiries, all interested parties should contact: CEATI, 1155 Metcalfe St. Suite 1120, Montreal, QC, H3B 2V6, 514.866.5377