Guided waves are ultrasonic waves that propagate along the length of a structure, guided by and confined in the geometric boundaries of the structure
4 Conventional UT measures the wall thickness at a spot, while GWT can identify locations of metal loss along a length of the pipe Weld Metal loss Flange Conventional Ultrasonic Test Weld Metal loss Guided Wave 100% Inspection Localized Inspection Conventional ultrasonic inspection provides a local thickness measurement GWUT Inspection provides detection of both internal and external corrosion The difference in concept between UT and GWT
Guided waves are bulk waves; therefore the entire volume of the pipe is inspected Frequencies used in guided wave inspection are much lower than conventional ultrasonic testing; therefore the wave lengths are much longer and are scattered instead of reflected from changes in the dimension of the wave guide The pipe acts as a wave guide, permitting the waves to travel long distances The waves can be introduced at a single location: When introduced with piezoelectric crystals an array of transducers are used. Coils of wire are used to create vibrations in the pipe via the magnetostrictive effect 5
An array of piezoelectric crystals are positioned in modules that typically hold two transducers each. The modules are spaced around the pipe under an air bladder which when pressurized forces the units against the surface. The individual crystals oscillate at the frequency at which they are excited and transmit the wave into the pipe. 7
8 Coils of insulated wire are wrapped around the pipe. An alternating current is passed through the coils, and an oscillating magnetic field is produced. Due to the magnetostrictive effect,this produces a wave in the pipe which can be amplified by using a nickel or cobalt strip bonded to the pipe under the coil.
9 Pressurized bladder containing the array of piezoelectric crystals Monitor Field electronics Umbilical cable connecting electronics to transducers
10 Weld Area of corrosion Example of graphical data display
11 Example of graphical data display Distance Amplitude Correction (DAC) Curves Weld Minor Anomaly Welds at two elbows
Can test long distances of pipe from a single access point Has developed into an effective screening tool useful in locating and ranking areas of corrosion; thereby minimizing the amount of follow-up inspection needed to determine the integrity of piping. Can be used on in-service pipelines Both internal and external corrosion can be identified Current commercial systems are packaged in a small number of durable components. The systems are easily transported and quickly setup in the field with preliminary results available at the time of the test 12
Complicated evaluation of data by highly trained operators is required because of the complex signals involved Dimensions of corrosion cannot be directly determined Significant corrosion can be missed, especially localized damage The scattered signal cannot be directly equated to a specific area or volume of loss due to a lack of an absolute calibration standard Many field conditions exist that limit the distances that can be effectively inspected and that cause artifacts which can complicate analysis. The tests identify change in cross-sectional area, and can miss corrosion that is general in nature, is in the configuration of grooves that pass under the array, or are too small to detect 13
various coating such as coal tar epoxies, asphalt-tar wraps, concrete, etc, plastic sleeves, particularly those with internal mastics wet insulation, particularly if ice is present rough internal or external surfaces direct buried pipe, particularly in situations where heavy or wet soil is encountered dense product, internal build up of solids, and situations with variable product flow system noise created by factors such as turbulent product flow or pumps temperature variations and gradients that can lead to changes in the wave velocity 14
This is a photograph of the corroded area which caused the leak in a buried 6 line. Along the line drawn, the cross-sectional area of the ½ walled pipe is approximately 9.62 square inches, while the area lost to corrosion through the hole is 0.5 square inches. this is a loss of approximately 5.2% of the cross-section. It would not been seen in a scan since the section was buried. However, if this line was above-ground and exposed the corrosion probably would have been noted as a minor anomaly 17
1. Guided wave offers valuable new inspection technology if its capabilities and limitations are kept in mind. 2. It is a SCREENING tool. Need to follow up with other NDT techniques to quantify / evaluate possible defects. 3. If used without other verification, GWT cannot provide the level of detail needed to ascertain the integrity of piping. 4. Main advantage is the ability to screen long sections of pipe to determine overall, general condition and locate areas that require more detailed examination. 19
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