Ultrasonic testing (UT) covers a range of non-destructive testing methods that use the propagation of ultrasonic waves to test an object, structure or material. It is used on a variety of materials including metals and alloys, concrete, composites and even wood, with varying degrees of resolution.
UT uses an ultrasound transducer that is connected to a diagnostic machine before being passed over the area to be inspected. A couplant such as gel, oil, or water is typically used to separate the transducer from the test object, although this is not required with an electromagnetic acoustic transducer (EMAT) system.
The ultrasound waveform can either be reflected or attenuated:
- Reflection: Also known as ‘pulse-echo,’ with reflection mode, the transducer both sends and receives the ultrasonic waves as they are reflected back to the device. This reflection occurs when the ultrasound meets an interface, like the further side of an object or an imperfection. By assessing the intensity (or amplitude) of the reflection as well as the time it took for the signal to reflect back, it is possible to work out the severity and location of an imperfection
- Attentuation: Also known as ‘through-transmission,’ this type of system uses separate devices to send and receive the ultrasonic waves. The transmitter sends ultrasound through a surface and a receiver on the other side of the material measures the amount of ultrasound that reaches it. The amount of sound that is transmitted is reduced by the presence of imperfections. This technique is greatly improved by the use of a couplant, which prevents the transmitter and receiver from separating from the surfaces and causing a loss in ultrasonic wave energy.
Ultrasonic testing is used in a wide range of industries, from aerospace and automotive to the aluminium and steel, construction, manufacturing, and transport sectors.
Long range ultrasonic testing, also called LRUT or long range UT, differs from conventional ultrasonic testing in a number of ways.
Firstly, it does not require the use of a couplant between the test surface and the transducers, which speeds up the testing process. In addition, while conventional UT can only check for defects in the area under and adjacent to the transducer, LRUT can test large areas of material from a single test point.
TWI, in partnership with Imperial College London, pioneered LRUT for the in-service surveying of pipelines and piping for metal loss in the 1990s. This culminated in the creation of Teletest®, the first commercially-available guided wave testing system, in 1998. Since then, the process has continued to be developed with upgraded testing systems.
To perform LRUT, you need some transducers, a low frequency flaw detector, a computer or device containing the required software to control the system and, in the case of attenuated LRUT, a pulser receiver unit. We will focus on pipeline LRUT for this process description, but the process is similar for testing other objects, although the transducer array would be configured differently.
A transducer ring is fixed around the pipe to be tested. The transducers are uniformly-spaced so that the low frequency guided waves propagate symmetrically along the pipe’s axis. These low frequency waves (15-85kHz) deliver 100% coverage of the pipe wall from a single location, even on sleeved or buried pipes, including areas such as at clamps. The waves propagate axially along the pipe from both sides of the transducer ring until their energy is attenuated or dissipates.
When the waves detect corrosion, erosion metal loss and other discontinuities, laminar waves are reflected back to the transducer, which can pick up changes in wall thickness and the time taken for the reflected signals to return, showing the location of the defect.
There are several factors that can impact the sensitivity and reduce the interpretation distance of long range ultrasonic testing, including:
- Size of Defect: If a large proportion of the area is lost to a defect like corrosion (a combination of defect depth and circumferential extent), it can affect the interpretable range due to excessive noise
- Axial Extent of Defect: LRUT is less sensitive to picking up axial defects, although longer defects create a stronger signal than short ones
- Pipe Features: Features such as butt welds, bends, branches, and attachments will be recognised as discontinuities by the guided waves, so will affect the signals
- Material Thickness: The effectiveness of the guided waves can be reduced by excessively thick materials and large diameter pipes
- Coatings: Some coatings, such as bitumen, will affect the ultrasound’s rate of attenuation, reducing the effective test range, this includes viscous liquid deposits on either the internal or external pipewalls
- Signal-to-Noise Ratio: There needs to be a minimum ultrasound signal-to-noise ratio in order to maintain the required sensitivity to defects
Although the interpretation distance of LRUT can be affected by a range of factors, as shown above, inspection range is typically set in relation to size of the defect being sought (via sensitivity) and the attenuation of the sound.
Standard LRUT has been shown to be effective in detecting loss of cross section (LCS) and circumferential corrosion and erosion on pipe diameters ranging from 38mm (1.5”) to 1200mm (48”), with a typical sensitivity of around 2.5-3% of the total cross sectional area being tested. Although this can be reduced in certain circumstances, such as where there are large outer diameter pipe or material thicknesses. ASTM E2775 sets out guidelines for LRUT on carbon or low alloy steel pipes ranging in size from 2-48” and with wall thicknesses of 3.8-25.4mm.
Testing can be performed on in-service pipelines operating between 0°C and 70°C, although testing can be performed at higher temperatures and down to -30°C with the application of exceptional risk management plans.
The usual interpretable scanning range from a single test location is up to ± 90 metres (295 feet) in each direction (180 metres / 590 feet bidirectionally) for above ground or encased pipelines.
Long range ultrasonic testing is mostly associated with testing pipelines as it can be used on structures with limited or localised access as well as avoiding unnecessary excavation, coating removal or the need to install scaffolding. It is also used for pipe sections that are inaccessible and unpiggable, reducing costs while examining 100% of the pipe wall.
However, pipes are not the only structures that are tested with LRUT, which has found uses in a range of different industries including aerospace and rail.
LRUT technology has developed significantly over the decades since TWI released the first commercially-available product. Modern equipment can create longitudinal, torsional and flexural guided waves and phased array technologies mean that the ultrasound can be focused to analyse the circumferential responses of potential defects.
Newer transducer systems with inflatable collars can inspect pipe sizes of up to 78”and at temperatures as high as 240°C.
Although metal loss of just 3% of the pipe wall cross section can be located with LRUT, the signal amplitude is usually used at 9% to ensure truer results. Blind trials showed that defects with an area loss of over 4.5% can be located across a range of pipe sizes and conditions.
LRUT differs from conventional ultrasonic testing as the ultrasonic waves are able to propagate over long distances rather than just in the area close to and beneath the transducers themselves. This provides time and cost savings when testing larger structures.
This greater range is possible because the different resonances along the boundary of the structure act as a guide for the waves. These guided waves can detect flaws metres away from the transducer, and also offer smaller detect detection at lower frequencies than with conventional ultrasonic testing.
LRUT offers a range of benefits for inspection personnel although there are also a number of limitations that should be considered when using this technique.
Advantages:
- Capable of in-service inspection, preventing production losses and downtimes
- Highly productive inspection capable of examining up to 180 metres of structure per hour, reducing overall onsite inspection times
- Able to measure 90 metres either side of the transducers, to examine 180 metres from a single test location
- Allows for the examination of 100% of a circumferential pipe wall from the test location
- Only need to remove coatings or insulation to attach the transducers, reducing inspection times
- No need to grind or sand blast the pipeline surface under examination
- Can examine non-piggable pipelines without excavation
- No need for couplants as used in conventional ultrasonic tests
- Able to detect weld root erosion faster than with conventional test methods
- Able to detect corrosion in hardtoreach areas, such as at pipe supports
Limitations:
- Unable to measure the precise minimum wall thickness, which would require techniques such as phased array ultrasonic testing (PAUT)
- Unable to detect isolated pits
- Has difficulty in testing cryogenic pipelines and condensation may freeze the transducer to the pipe surface in exposed locations
- Wet soil, silt and bitumen coatings can affect sound attenuation, making inspection difficult
- Joint configurations can make areas around fillet welds hard to assess
- Unable to examine pipe diameters below 1.5 inches and requires a minimum pipe length of 5 metres for cost effective use of LRUT
- Conditions and factors such as pipeline layouts, viscous liquid deposits on pipe walls, severe corrosion across the whole pipeline, the type of material used, and excessive thickness or structure diameters can reduce the interpretation distance
Long range ultrasonic testing, also called guided wave ultrasonic testing and even guided wave ultrasonic long range ultrasonic testing, is non-destructive testing method that offers rapid screening of large structures.
Although it is used in industries such as rail and aerospace, it is widely used to inspect pipelines in the oil and gas industry as it can be used where pipes or tubes are inaccessible for other inspection method, such as when they are unpiggable, buried, encased in an insulation sleeve, or in difficult to reach, elevated positions.
This testing method offers 100% pipe wall coverage from a single test point and reduces the costs associated with gaining access and the removal of insulation or coatings (where present).