Guided wave testing uses transducers to generate ultrasonic acoustic waves that propagate along the structure being tested. In the case of pipes, a ring of transducers is used around the pipe’s circumference to produce waves which are ‘guided’ by the boundaries of the pipe wall as they travel out from the transducer ring.
These guided waves propagating locate changes in the cross-section of the structure caused by cracking, corrosion or erosion. These defects create an echo in the waves as they pass over them. By measuring the speed of the wave and the time that the echo arrives it is possible to locate the position of defects tens of metres away from the transducers.
Because the waves travel along the structure with very little loss in energy, it is possible to test large areas quickly. This is different from regular ultrasonic testing, where only the area beneath the transducer is tested.
Due to the nature of guided waves, this testing method allows for in-service inspection in areas that would otherwise be difficult to access.
Guided waves reflect from cross-sectional area changes, like cracks or corrosion defects, as they propagate along a structure. These ultrasonic waves, which are confined and guided by the boundaries of the structure itself, can be used to locate and estimate the size of a defect by assessing the arrival time and the amplitude of the ultrasonic signal.
Although GWT inspections can be customised, they tend to have a low operating frequency of 5 to 250 kHz. This low frequency reduces the attenuation for long range inspections as well as helping to create non-dispersive guided waves.
Transducers are designed and positioned so that the correct ultrasonic wave modes are transmitted for the specific scan application along the structure.
Guided wave ultrasonic testing is widely used for the rapid inspection of pipelines and piping systems. The long-range ultrasonics used in GWT means that hard-to-reach piping sections can also be assessed both internally and externally without a need for access. This means that it is particularly useful for inspecting unpiggable pipes and those that are resting on supports that make them prone to point-of-contact or touchpoint corrosion (TPC).
While GWT tends to be used for pipeline inspection, it can be used for other structures, such as railway tracks and even aeroplane skins.
Modern guided wave systems, sensors and software boast increased computational power that is able to manage signal processing, focusing algorithms and dispersion curve solutions, meaning that users can complete inspections without needing in-depth knowledge of guided wave mechanics.
These technologies solve challenges associated with the guided wave modes in a structure as well as the changing guided wave velocities and frequencies related to the structure’s geometry, wall thickness and other factors.
Ultrasonic guided waves are able to propagate long distances, meaning that they offer great potential for cost and time savings when inspecting large structures. The inspection range for GWT is much larger than that for conventional ultrasonic testing, which is only able to test areas close to or beneath the transducer.
This greater range is because the structure acts as a waveguide by exploiting the different resonances between the structure’s boundaries.
Not only can guided waves detect flaws hundreds of feet away from the transducer, but they can also detect small defects at low frequencies compared to conventional ultrasonic testing.
GWT offers a number of benefits, including:
- Affords precise and safe inspection and data collection
- Fast inspections over large areas from a single probe position
- Can provide in-service inspections with minimal surface preparation
- Able to inspect areas with limited access as well as isolated structures
- Capable of inspecting unpiggable pipelines as well as those that are buried, coated or underwater
- Offers 100% coverage throughout the thickness of the structure
- Inspection unaffected by fluid type being carried in a pipeline
- Minimal insulation removal required on insulated pipes
- Able to pinpoint and characterise damage location, depth and length
- Can determine point of contact corrosion without needing to lift the structure, also helping to avoid environmental damage as a result of potential pipe leaks
Despite the many advantages of GWT, there are also some challenges that need to be considered when using this inspection method.
The main difficulty is in determining the maximum reliable inspection distance. This is dependent on the strength of the torsional or longitudinal waves generated by the system along with the presence and type of insulation, the depth at which a structure or pipe is buried, the nature of the soil around the structure, the level of compaction, and the size of the flaws being detected.
A lot of importance has been placed on the amplitude (or size) of the guided waves, but this alone will not provide accurate measurements in all conditions. For a more reliable measurement, the signal-to-noise ratio (SNR), which is the difference between the desired signal and the unwanted noise, is preferable. Should the amplitude of the noise reach that of the level signal, you will lose accuracy due to distortion and can even lose the level signal itself. However, many of today’s advanced GWT systems can account for such unwanted impedance discontinuities.
Long-range ultrasonic guided wave testing has shown to be able to detect corrosion and other defects in even hard-to-reach areas across large distances. This has led it to be used for the inspection of pipelines, railways and other large structures.
Fast, safe and reliable, GWT has seen great developments since the mid-1990s, with TWI pioneering advances in the technology in answer to a need from the oil and petrochemical industries to detect corrosion under insulation in pipework.
Guided wave technology is now widely used as an accepted method for inspecting pipes in inaccessible areas such as road and river crossings, power plants, risers, offshore topside pipeworks, jetty lines and refinery pipeworks.
Although it is still primarily used to assess pipelines, GWT can be used in other industries too, including rail, where it has been used to inspect large distances of railway.