Guided waves Robert Ernst Slide 1

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 Slide 2

Typical applications Pipe racks Insulated pipe Pipe with restricted access Road crossings Submerged pipe Buried pipe Slide 3

Guided Waves Basics Slide 4

Propagation along, not through, a structure Transducer Structure Region of inspected structure Conventional Guided waves The pipe walls form a guide for ultrasonic waves, which directs them down the length of the pipe Slide 5

Reflection from a Feature (such as corrosion) When the guided wave hits a change in cross section, it reflects back toward the transducer Slide 6

Standard UT vs Guided Waves high frequency short wavelength sensitive to small defects at high frequencies point measurement Guided Waves low frequency long wavelength sensitive to “small” defects even at low frequencies rapid screening Slide 7

Key advantages In service inspection Rapid 100% coverage Limited access required Slide 8

The torsional mode Torsional (twisting of the pipe) Slide 9

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 Slide 10

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 Slide 11

For example: a weld Incoming wave (100% of energy) Reflected wave Transmitted wave (80 % of energy) Slide 12

At each reflection the transmitted energy gets weakened Incoming wave (100 % of energy) 20% 80% 16% 64% 13% 51% Slide 13

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 Slide 14

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 Slide 15

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 Slide 16

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 Slide 17

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. Slide 18

Wavemaker G3, System Components Slide 19

Guided waves Typical Performance Slide 20

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 Slide 21

Diagnostic Range In ideal conditions 200m of pipe can be screened in each direction from a single test location Typically range on above ground pipe is 50m in each direction For buried pipes 20m in each direction is more typical unless the pipe is sleeved Slide 22

Unrolled Pipe Example Defect at 150° (close to bottom) Defect at 50° (close to top) Slide 23

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 Slide 24

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 Slide 25

References Bayernoil, Neustadt an der Donau, Germany Statoil, Oslo, Norway Neste Oil, Finland Statoil, Norrköping, Sweden ST1 Raffinerie, Göteborg, Sweden DynaMate, Södertälje, Sweden Umeå Hafen, Umeå, Sweden E-on, Halmstad, Sweden STS, Norrköping, Sweden Luleå Energi, Luleå, Sweden Slide 26

Thanks! Slide 27