Guided wave testing
Encyclopedia
Guided Wave testing is one of latest methods in the field of
non-destructive evaluation
. The method
employs mechanical stress waves that propagate along an elongated
structure while guided by its boundaries. This allows the
waves to travel a long distance with little loss in energy. Nowadays, GWT is widely used to inspect and screen many
engineering structures, particularly for the inspection
of metallic pipelines
around the world. In
some cases, hundreds of meters can be inspected from a single
location. There are also some applications for inspecting
rail tracks, rods and metal plate structures.
Although Guided wave testing is also commonly known as Guided Wave
Ultrasonic Testing (GWUT) or Long Range Ultrasonic Testing (LRUT),
it is fundamentally very different to
conventional ultrasonic testing. Guided wave
testing uses very low ultrasonic
frequencies compared
to those used in conventional UT, typically between 10~100kHz.
Higher frequencies can be used in some cases, but detection range is significantly reduced. In addition, the underlying physics of guided waves is more
complex than bulk waves. Much of the theoretical background has
been addressed in a separate article
. In this
article, the practical aspect of GWT will be discussed.
traced back to as early as the 1920s, mainly inspired by the field
of seismology. Since then, there has been an increased effort on
the analytical study of guided wave propagation in cylindrical
structures. It was only in the early 1990s that guided wave testing was
considered as a practical method for the
non-destructive testing
of engineering
structures. Today, GWT is applied as an integrated health
monitoring program in the oil, gas and chemical industries.
guided wave modes that exist for a pipe geometry, and they can be
generally grouped into three families, namely the torsional,
longitudinal and flexural modes. The acoustic properties of these
wave modes are a function of the pipe geometry, the material and the
frequency. Predicting these
properties of the wave modes often relies on heavy mathematical modeling which are typically
presented in graphical plots called Dispersion
curves.
In Guided Wave Testing of pipelines, an array of low frequency
transducers is attached around the circumference of the pipe to generate
an axially symmetric wave that propagate along the pipe in both the forward and backward
directions of the transducer array. The Torsional wave mode is most commonly
used, although there is limited use of the longitudinal mode. The equipment operates in a pulse-echo
configuration where the array of transducers is used for both the
excitation and detection of the signals.
At location where there is a change of cross-section or a change in
local stiffness of the pipe, an echo is generated. Based on the
arrival time of the echoes, and the predicted speed of the wave mode at a
particular frequency, the distance of a feature in relation to the
position of the transducer array can be accurately calculated. GWT
uses a system of distance amplitude curves (DAC) to correct for
attenuation and amplitude drops when estimating the cross-section
change (CSC) from a reflection at a certain distance. The DACs are
usually calibrated against a series of echoes with known signal
amplitude such as weld echoes.
Once the DAC levels are set, the signal amplitude correlates well to the CSC of a defect. GWT does not measure the
remaining wall thickness directly, but it is possible to group the
defect severity in several categories. One method of doing this is
to exploit the mode conversion phenomenon of the excitation signal
where some energy of the axially symmetric wave mode is converted to
the flexural modes at a pipe feature. The
amount of mode conversion provides an accurate estimate of the
circumferential extent of the defect, and together with the CSC,
operators could establish the severity category.
A typical result of GWT is displayed in an A-scan style with the
reflection amplitude against the distance from the transducer array position.
In the past few years, some advanced systems have started to offer
C-scan type results where the orientation of each feature can
be easily interpreted. This has shown to be extremely useful when
inspecting large size pipelines.
non-destructive evaluation
Nondestructive testing
Nondestructive testing or Non-destructive testing is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage....
. The method
employs mechanical stress waves that propagate along an elongated
structure while guided by its boundaries. This allows the
waves to travel a long distance with little loss in energy. Nowadays, GWT is widely used to inspect and screen many
engineering structures, particularly for the inspection
of metallic pipelines
Pipeline transport
Pipeline transport is the transportation of goods through a pipe. Most commonly, liquids and gases are sent, but pneumatic tubes that transport solid capsules using compressed air are also used....
around the world. In
some cases, hundreds of meters can be inspected from a single
location. There are also some applications for inspecting
rail tracks, rods and metal plate structures.
Although Guided wave testing is also commonly known as Guided Wave
Ultrasonic Testing (GWUT) or Long Range Ultrasonic Testing (LRUT),
it is fundamentally very different to
conventional ultrasonic testing. Guided wave
testing uses very low ultrasonic
Ultrasound
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...
frequencies compared
to those used in conventional UT, typically between 10~100kHz.
Higher frequencies can be used in some cases, but detection range is significantly reduced. In addition, the underlying physics of guided waves is more
complex than bulk waves. Much of the theoretical background has
been addressed in a separate article
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...
. In this
article, the practical aspect of GWT will be discussed.
History
The study of guided waves propagating in a structure can betraced back to as early as the 1920s, mainly inspired by the field
of seismology. Since then, there has been an increased effort on
the analytical study of guided wave propagation in cylindrical
structures. It was only in the early 1990s that guided wave testing was
considered as a practical method for the
non-destructive testing
Nondestructive testing
Nondestructive testing or Non-destructive testing is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage....
of engineering
structures. Today, GWT is applied as an integrated health
monitoring program in the oil, gas and chemical industries.
How it works (Pipeline Inspections)
Unlike conventional ultrasonics, there are an infinite number ofguided wave modes that exist for a pipe geometry, and they can be
generally grouped into three families, namely the torsional,
longitudinal and flexural modes. The acoustic properties of these
wave modes are a function of the pipe geometry, the material and the
frequency. Predicting these
properties of the wave modes often relies on heavy mathematical modeling which are typically
presented in graphical plots called Dispersion
Dispersion relation
In physics and electrical engineering, dispersion most often refers to frequency-dependent effects in wave propagation. Note, however, that there are several other uses of the word "dispersion" in the physical sciences....
curves.
In Guided Wave Testing of pipelines, an array of low frequency
transducers is attached around the circumference of the pipe to generate
an axially symmetric wave that propagate along the pipe in both the forward and backward
directions of the transducer array. The Torsional wave mode is most commonly
used, although there is limited use of the longitudinal mode. The equipment operates in a pulse-echo
configuration where the array of transducers is used for both the
excitation and detection of the signals.
At location where there is a change of cross-section or a change in
local stiffness of the pipe, an echo is generated. Based on the
arrival time of the echoes, and the predicted speed of the wave mode at a
particular frequency, the distance of a feature in relation to the
position of the transducer array can be accurately calculated. GWT
uses a system of distance amplitude curves (DAC) to correct for
attenuation and amplitude drops when estimating the cross-section
change (CSC) from a reflection at a certain distance. The DACs are
usually calibrated against a series of echoes with known signal
amplitude such as weld echoes.
Once the DAC levels are set, the signal amplitude correlates well to the CSC of a defect. GWT does not measure the
remaining wall thickness directly, but it is possible to group the
defect severity in several categories. One method of doing this is
to exploit the mode conversion phenomenon of the excitation signal
where some energy of the axially symmetric wave mode is converted to
the flexural modes at a pipe feature. The
amount of mode conversion provides an accurate estimate of the
circumferential extent of the defect, and together with the CSC,
operators could establish the severity category.
A typical result of GWT is displayed in an A-scan style with the
reflection amplitude against the distance from the transducer array position.
In the past few years, some advanced systems have started to offer
C-scan type results where the orientation of each feature can
be easily interpreted. This has shown to be extremely useful when
inspecting large size pipelines.
Advantages
- 100% Volumetric coverage - full coverage within the inspection range.
- Long range inspection - potential to achieve hundreds of meters of inspection range.
- Limited access - insulated line with minimal insulation removal, corrosion under supports without need for lifting, inspection at elevated locations with minimal need for scaffolding, and inspection of road crossings and buried pipes.
- Data is fully recorded.
- Fully automated data collection protocols.
Disadvantages
- Interpretation of data is highly operator dependent.
- Difficult to find small pitting defects.
- Not very effective at inspecting areas close to accessories.
- Needs good procedure