Guided waves Robert Ernst About Guided Ultrasonics Ltd
Guided waves Robert Ernst
About Guided Ultrasonics Ltd. ● Developer and manufacturer of the Wavemaker pipe screening system ● NOT a service provider ● Resource for support, training, and consultancy ● Their mission is to innovate and develop Guided Waves NDT methods ● Company formed in 1999 uniting world experts in guided waves
Typical applications ● Pipe racks ● Insulated pipe ● Pipe with restricted access ● Road crossings ● Submerged pipe ● Buried pipe
Guided Waves Basics
Propagation along, not through, a structure Transducer Conventional Structure Region of inspected structure Transducer Guided waves Structure Region of inspected structure ● The pipe walls form a guide for ultrasonic waves, which directs them down the length of the pipe
Standard UT vs Guided Waves Standard UT Guided Waves ● high frequency ● low frequency ● short wavelength ● long wavelength ● sensitive to small defects at high frequencies ● sensitive to “small” defects even at low frequencies ● point measurement ● rapid screening
Key advantages ● In service inspection ● Rapid 100% coverage ● Limited access required
The torsional mode ● Torsional (twisting of the pipe)
At every change in cross section there is a reflection of the guided waves The amplitude of the guided wave depends on ● The cross section and the cross section al change ● The distance from the transducer ring ● The transducer ring used The grey section represents the cross-sectional area
Change in cross-sectional area ● Method is equally sensitive to defects at any depth and wall position ● Method is sensitive to changes in cross section (increase or decrease) ● Reflection from welds and flanges are used as a reference ● Amplitude of reflection is scaled with distance
For example: a weld Incoming wave (100% of energy) Reflected wave Transmitted wave (20 % of energy) (80 % of energy)
At each reflection the transmitted energy gets weakened Incoming wave (100 % of energy) 20% 80% 16% 64% 13% 51%
These effects appear as an amplitude decay ● The reflected amplitude from distant features will be smaller than for close features ● DAC curves are used to compensate for attenuation ● DAC curves are used as the references
Symmectric reflectors ● When they reflect from a symmetric feature (such as a weld), they are reflected as a symmetric wave which appears as a black trace ● The reflected wave is presented as a black line on the screen Pipe Symmetric Reflector
Asymmetric Reflectors ● When the wave is reflected by a asymmetric reflector, then the reflected wave will have a symmetric fraction (torsional mode) and an asymmetric fraction (flexural mode) ● The flexural mode is represented as a red line on the result graphic ● The amplitude of the red line compared to the black line gives the degree of asymmetry Pipe Asymmetric Reflector
Examples to the symmetry ● Pure BLACK lines represent symmetric features - Uniform around the circumference ● RED lines represent non-symmetric features - Varies around the circumference Asymmetric feature Symmetric feature (Weld) #12403
Determination of wall thickness reduction ● The remaining wall thickness can never directly be measured by using guided waves. Guided waves should always be considered as a screening tool. ● The relation of the red to the black curve gives a hint about the severityof the feature. ● The features will be graded according to the amplitude of the black and red lines. ● It is always necessary to follow up any features with complementary methods. Most often conventional UT or visual inspection is used.
Wavemaker G 3, System Components
Guided waves Typical Performance
Detection Threshold ● Typically minimum detectable defect is 5% cross sectional change ● If pipe is in good general condition defects down to 1% have been detected ● A change in the cross section of 1% in a 3” pipe corresponds to a defect with a width of 5 mm and a depth of about half the wall thickness
Diagnostic Range ● In ideal conditions 200 m of pipe can be screened in each direction from a single test location ● Typically range on above ground pipe is 50 m in each direction ● For buried pipes 20 m in each direction is more typical unless the pipe is sleeved
Unrolled Pipe Example Defect at 150° (close to bottom) Defect at 50° (close to top)
Limiting factors ● The general condition of the pipe influences the detection threshold and the diagnostic range ● Some coatings or coverings are reducing the diagnostic range, e. g. earth or bitumen wrapping ● High external noise such as vibrations from compressors reduce the performance of the technique
Effect of the pipe content ● Gas – No effect ● Liquids - No effect for low viscous media ● Sludge or high viscous media - High viscous media can shorten the diagnostic range drastically by attenuating the signals
References ● Bayernoil, Neustadt an der Donau, Germany ● Statoil, Oslo, Norway ● Neste Oil, Finland ● Statoil, Norrköping, Sweden ● ST 1 Raffinerie, Göteborg, Sweden ● Dyna. Mate, Södertälje, Sweden ● Umeå Hafen, Umeå, Sweden ● E-on, Halmstad, Sweden ● STS, Norrköping, Sweden ● Luleå Energi, Luleå, Sweden
Thanks!
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