Long distance guided wave ultrasound scan
Long-range ultrasound guided-wave ( english : long range ultrasonic testing (LRUT) or guided wave testing (GWT)) is a method of non-destructive material testing (NDT).
Process description
The process uses ultrasonic waves that propagate along an elongated component, being reflected on its surface and thus guided in the component. This allows the waves to travel a long distance with little energy loss. The method is increasingly used for the inspection and testing of pipelines and pipelines as well as other technical structures such as railroad tracks, masts and metal plates. Up to two lengths of 180 m of pipe can be examined from a single access point, both 180 m in one direction and 180 m in the other.
The procedure differs significantly from conventional ultrasonic testing. The frequency used in the inspection depends on the thickness of the component, however, guided waves typically use long wave ultrasonic frequencies in the range of 20 kHz up to 100 kHz, while conventional ultrasonic inspections typically use megahertz. In some cases higher frequencies are used, but the detection range is then significantly reduced. In addition, the underlying physics of guided waves is more complex than volumetric wave propagation.
Pipeline investigation
In contrast to conventional ultrasound examinations, there are an infinite number of guided wave modes for a pipe geometry, which can be grouped into three families, namely torsion, longitudinal and bending modes. The acoustic properties of these wave modes depend on the pipe geometry, the material and the frequency. In order to predict these properties of the wave modes, complex mathematical modeling is often required, which is typically represented in graphical representations (dispersion curves).
In long distance pipeline surveys, an array of low frequency transducers is placed around the circumference of the conduit to create an axially symmetric wave that propagates along the conduit in both the forward and reverse directions of the transducer array. The torsional wave mode is most commonly used and the longitudinal mode is rarely used. The device works in a pulse-echo configuration in which the transducer array is used for both excitation and acquisition of the signals.
An echo is generated at the point where the cross-section or the local rigidity of the pipeline changes. Based on the arrival time of the echoes and the predicted speed of the wave mode at a certain frequency, the distance of the feature in relation to the position of the transducer arrangement can be calculated precisely. Long-range ultrasound examination uses a system of distance amplitude curves to correct for attenuation and amplitude drops, and can thus estimate the change in cross-section at a given distance due to ultrasonic wave reflection. The range amplitude curves are typically calibrated using a series of echoes of known signal amplitude, such as weld echoes.
The method shows metal losses of 9% or more of the wall thickness, which are caused by erosion, corrosion or operational damage. Once the distance amplitude curve levels are set, the signal amplitude correlates well with the cross-sectional change of a defect. The remaining wall thickness is not measured directly by the examination unit, but it is possible to group the severity of the defects into several categories. One way to do this is to take advantage of the mode conversion phenomenon of the excitation signal, in which some axially symmetric wave mode energy is converted to the bending modes on a pipe feature. The amount of mode conversion provides an accurate estimate of the circumferential length of the defect, and along with the change in cross-section, operators could determine the severity category.
A typical result of a long range guided wave ultrasound exam is displayed in an A-scan with reflection amplitude versus distance from the location of the transducer array. In the last few years some advanced systems have started to deliver C-scans where the orientation of each feature can be easily interpreted. This has been found to be extremely useful when testing large pipelines.
Focusing the guided waves
The active focusing capacity can not only be taken over from the C-Scan, but also through long-distance ultrasound examination with guided waves using bending wave modes. This offers two main advantages. First, the signal-to-noise ratio (SNR) of a defective echo can be improved; second, it can be used as an additional tool to help distinguish between 'real' and 'false' readings. However, there are disadvantages associated with this technique. First, the location of the fault must be known before focusing can be applied. Second, the separate data set required for the active focusing technique can also significantly reduce the time and cost efficiency of long range guided-wave ultrasound examination.
Flexural wave modes have a sinusoidal variation in their displacement pattern around the circumference in integer values ranging from 1 to infinity. Active focusing involves the transmission of multiple flexural wave modes with time and amplitude corrections applied such that a circumferential node from each wave mode arrives at the same target position at the same target position and phase at the same time causing structural disturbances. At other circumferential positions, the circumferential nodes of the bending wave modes will arrive out of phase with one another and interfere destructively. By adjusting the excitation conditions, this focal point can be rotated around the pipe circumference.
The focusing technique can also be used to help distinguish between 'real' and false 'indications, where a' false 'indication is a received signal that does not directly correspond to the location of an error, e.g. B. by reverberation or incomplete cancellation of unwanted wave modes. If there is a 'wrong' display in the A-Scan, it is also reproduced in the C-Scan, since the same original data is used in the data processing. Since active focusing involves separate data acquisition, focusing on the position of a 'false' display will result in a negative result, while focusing on a 'true' display will result in a positive result. Therefore, the active focusing technique can help overcome the tendency towards 'false' displays.
properties
advantages
- Fast screening for degradation during operation (long-distance inspection)
- Potential for hundreds of meters of inspection range
- Detection of internal or external metal loss due to corrosion, erosion or mechanical damage
- Reduction of access costs for insulated lines with minimal removal of the insulation
- Examination of corrosion damage under supports and girders without lifting the pipe as well as on overpasses with minimal need for scaffolding
- Inspection of underground pipes, e.g. B. at crossroads, as well as under water, z. B. of drilling rig risers and oil loading facilities
- Data is fully recorded and stored in fully automated data acquisition logs
disadvantage
- The interpretation of the data depends heavily on the test personnel
- It is difficult to find small hole defects
- Not very effective at examining areas near junctions or accessories
- The development of application-specific adaptations of the test procedure is complex
- Elaborate equipment technology and training of the test staff
history
The study with guided waves propagating in a structure can be traced back to the 1920s, mainly inspired by seismology. Since then, the propagation of guided waves in cylindrical structures has been increasingly analyzed analytically.
Only since the early 1990s has long-distance ultrasound examination with guided waves been examined as a practicable method for the non-destructive testing of technical components. Since then, it has been developed into an industrially applicable test method for pipelines and pipelines , especially at The Welding Institute in Great Britain. It is now used worldwide to monitor structural integrity in the oil, gas and chemical industries.
Norms
- British Standards (BSI)
- BS 9690-1: 2011, Non-destructive testing. Guided wave testing. General guidance and principles
- BS 9690-2: 2011, Non-destructive testing. Guided wave testing. Basic requirements for guided wave testing of pipes, pipelines and structural tubulars
- ASTM International (ASTM)
- E2775 - 16, Standard Practice for Guided Wave Testing of Above Ground Steel Pipework Using Piezoelectric Effect Transduction
- E2929-13, Standard Practice for Guided Wave Testing of Above Ground Steel Piping with Magnetostrictive Transduction
Individual evidence
- ↑ a b c d Teletest FOCUS - long-distance ultrasound examination with guided waves. Retrieved December 5, 2018.
- ↑ SW Kallee: Excitation and propagation of guided ultrasonic waves in pipes by piezoelectric transducer arrangements. German translation of the English publication by X. Niu, KF Tee, HP Chen and HR Marques: Excitation and propagation of ultrasonic guided waves in pipes by piezoelectric transducer arrays. Retrieved December 5, 2018.