UT Principles.doc

SHAW / RTD HANDBOOK

ULTRASONIC EXAMINATION OF PIPELINE

GIRTH WELDS

Index

1. Definitions and formulas

2

2. Snellss law

43. Sound pressure values after reflection and refraction

54. Critical angles

65. Wave length calculations

76. Near field calculations

77. Velocities

88. Mode conversion for shear wave probes

99. Beam size measurements

1010. 6dB Beam profile chart

1111. Beam spread calculations

1212. Defect position calculations

1313. The difference between geometry and defects

1414. Displays used with mechanized Ultrasonics

1515. Porosity detection

1616. Hot pass probes

1717. Pitch / catch probes

1818. TOFD technique

1919. Crack detection in the root area

2020. Relationship between amplitude and dBs

2121. Calculations between different size flat bottom holes and notches 2222. Zonal sizing versus Amplitude sizing

2323. Gate start and minimum gate length of CRC / RMS welds 2424. Gate start and minimum gate length for Stick welds

2525 Working range Transducers

26

1.Definitions and Formulas GEOMETRIC RATIOS

Sin = side A,

Cos = side B,

Tan = side A

Side C

side C

side B

PYTHAGOREAN THEOREM: 2 2 2

22

A + B = C

or

C = A + B

LOGARITHM:10

0.3

Log 2 = 0.3

or

10 = 2

DECIMAL MULTIPLY UNITS: -12

3

pico = 10

kilo = 10

-9

6

nano = 10

mega = 10

-6

9

micro = 10

giga = 10

-3

12

milli = 10

tera = 10

-2

15

centi = 10

peta = 10

-1

18

deci = 10

exa = 10

2. Snells Law

Sin 1=Sin 2 SIN A = V1 C1

C2

SIN B V2angle of incidence

angle of reflection

Example:

70 degree probe is built for normal steel with velocity 3230 m / sec.

What angle will this probe be in a pipeline steel with a velocity of 3325 m / sec?

Sin 1=Sin 2

Sin 70

=Sin 2

C1

C2

3230

3325

o

2

=arcsin (sin 70 * 3325)

=75.3

3230

3. Sound Pressure Values after Reflection and Refraction

Reflection at the interface steel / air for an incident compression wave

Conclusion: Angle beam compression wave probes do not function past half skip.

Reflection at the interface steel / air for an incident shear wave.

Conclusion: angle beam shear wave probes do create mode conversion if the angle of

incident is < 33.2 degrees.

4. Critical Angles

o o

Wedge angle in Lucite ( = arcsin 2730 * sin90 = 27.3

5950

o o

In steel the angle ( = arcsin 3230 * sin90 = 32.9

5950

o o

Wedge angle in lucite ( = arcsin 2730 * sin90 = 57.7

3230

o

In steel the angle ( = 90

5. Wave Length Calculations

6. Near Field Calculations Round Crystals

2

2

N = Deff * f =0.94 * D *f= (mm)

4*c

4*c

Note: effective crystal diameter Deff = 0.97 X D

2

2

2

Deff = 0.972 * D = 0.94 * D

N = near field

D = diameter crystal

f = frequency

c = velocity

7. Velocities

MaterialCompression WaveShear wave

Steel59503230

Aluminum62503100

Lucite27301180

Rexolite2341?

Loten (PEI)2475?

Vespel2460?

Celasole2975?

Air330-

Oil1250-

Water1483-

Velocities in pipeline material vary between supplier.

The anistropic material used on pipelines has a significant effect on the pipe material velocity / transducers refracted angle.

These velocities are examples of those measured in pipe material from several different suppliers.

Pipe SupplierShear Wave

o

Velocity 45Shear Wave

o

Velocity 70

Ipsco3188 m/s3172 m/s

Berg3200 m/s3303 m/s

Campipe3250 m/s3295 m/s

Welland3141 m/s3208 m/s

In general, transducers with smaller angles of refraction (40 55) do not change much, compared to transducers with larger angles of refraction (60 75).

8. Mode Conversion for Shear Wave Probes

During examination with angle beam shear wave transducers mode changes will occur if the angle of incidence is smaller than 33 degrees.

o

o

32.9 = 1st critical angle (Compression wave 90 ).

o

90.0 = 2nd critical angle (Compression wave in wedge).

o

The 60 transducers shear wave converts into a compression wave mode @ a defect perpendicular to the pipe surface.

o

Angle of incidence = 30

o

The 70 transducers shear wave will have mode conversion also, but there is enough

o

energy left for the shear wave with a 20 angle of incidence.

For energy levels, see polar diagrams in Chapter 3.

9. Beam Size Measurements

Beam sizes for probes used on pipelines will be measured on the block sketched below.

The operator must determine the most sensitive side drilled hole to determine minimum attenuation setting.

The workable range (focal range) of the transducer is 6dB.

(normally 1/3 before focal spot & 2/3 after focal spot)

To determine the beam diameter the operator must record the probe movement between the 6 dB values of the targeted hole.

After connecting the probe movement values, the focal spot can be measured at the narrowest beam profile.

10. -6dB Beam Profile Chart

11. Beam Spread Calculations

Sin ( = k * (

( = angle of divergence

Deff.

k = factor

= wave length

Deff. = effective crystal diameter

- dB BorderK factor for round crystals

-30.37

-60.51

-120.70

-200.87

-240.93

-411.09

-(1.22

Note: Table for pulse / echo only

12. Defect Position Calculations

If we know S

If we know A

If we know d

d = S * cos(

d = A

A = d * tan(

tan(A = S * sin(

S = A

S = d

sin(

cos(

If we know S

If we know A

If we know d

d = 2T S * cos(

d = 2T * A

A = (2T d)

tan(

A = S * sin(

S = A

S = 2T d

sin(

cos(13. The Difference Between Geometry and Defects

The only reliable method to determine the difference between geometry indications and defect indications is to compare the stand-off distance with the weld centerline position.

If the stand-off measures past the weld centerline, the indication is most likely a reflection from geometry.

If the stand-off measures before the weld centerline, the indication must be a reflection from a defect.

14. Displays Used With Mechanized Ultrasonics

15. Porosity Detection

Porosity always has been the more difficult defect to detect with mechanized ultrasonic systems on pipelines.

The previous generation systems only had the strip chart display where the highest peak was displayed.

Porosity reflects ultrasound in many small amplitudes.

Raw data presentations used in most common used systems on pipelines display all these small amplitudes.

16. Hot Pass Probes

The sound beam of the Hot Pass probe reflects close to the Root Pass. If this Root Pass is wide or has a large geometry (internal Root pass), part of the beam of the Hot Pass probe may enter this Root Pass and reflect back to the Hot Pass probe at a sound path similar to the expected Hot Pass defect.

17. Pitch / Catch Probes

The Pitch / Catch technique used with mechanized systems to detect defects in the Fill Passes can also be used to detect centerline cracks or other defects in that area.

At the bottom of the pipe however, the probe will receive a response of the narrow Cap Pass, commonly seen on mechanized welded pipeline welds. This response is later in time, but the sound finds its way back to the receiver faster than the response of the Cap Pass normally seen in the Raw Data.

18. TOFD

The TOFD technique is used in mechanized ultrasonic systems on pipelines as a safety net.

It alarms the operator if a large crack is present in the weld, especially if the orientation is not perpendicular to any ultrasonic beams. It confirms also the defects detected with Pulse / Echo.

The defects detected with TOFD in pipeline welds are generated mostly through reflection and not through diffraction.

The weaker diffraction signals are very helpful when analyzing data.

19. Crack Detection in the Root Area

Crack detection in pipeline welds can best be done with ultrasound.

The technique responds very well to large planar defects.

Care must be taken with fixed probe position as in mechanized ultrasonic systems.

To receive a reflection from a crack like the crack in the picture below, 4 variables determine the height of the response.

Probe Angle

Probe Frequency

Crack Height

Crack Orientation

The parameters of the crack are unknown.

The best way to insure crack detection is to use 2 different probe angles (60 & 70) and / or two different frequencies (wide band).

20. Relationship Between Amplitudes and dBs

Definition: Change in amplitude dB ((dB) 20 log A1

A2

dBAmplitude Ratio

11.12

21.26

31.41

41.59

51.78

62

72.24

82.51

92.82

103.16

113.55

123.98

134.47

145

155.62

166.31

177.08

187.94

198.91

2010

2111.22

2212.59

2314.13

2415.85

2517.78

21. Calculations Between Different Size Flat Bottom Holes and Notches

Double the diameter results in 12 dB increase for flat bottom holes.40 log * D1

D2

Sensitivity: 2 mm FBH @ 80% FSH (F.B.H.s smaller than beam width)

Flat Bottom HoledB ValueFull Screen Height @ 0 dBFull Screen Height @ -12 dB

0.5-24 dB5%1.25%

1.0-12 dB20%5%

1.5-6 dB40%10%

2.00 dB80%20%

2.53 dB>100%30%

3.07 dB>100%45%

4.012 dB>100%80%

An increase of 1 mm for Notches, results in 6 dB sensitivity increase.Sensitivity: 2 mm FBH@ 80% FSH (notch longer than beam width)

Flat Bottom HoledB ValueFull Screen Height @ 0 dBFull Screen Height @ -12 dB

0.5 mm-12 dB40%10%

1.0 mm-6 dB80%20%

1.5 mm-3 dB>100%30%

2.0 mm0 dB>100%40%

2.5 mm2 dB>100%50%

3.0 mm4 dB>100%85%

22. Zonal Sizing Versus Amplitude Sizing

Defect sizing with zonal concept:

Probe 1: amplitude > 100% FSH = full zone = 3.0 mm

Probe 2: amplitude > 100% FSH = full zone = 3.0 mm

Total defect height = 3.0 + 3.0 = 6.0 mmDefect sizing using amplitude height:

Probe 1: amplitude > 100% FSH = -12 dB, 30% FSH = 1.5 mm

Probe 2: amplitude > 100% FSH = -12 dB, 30% FSH = 1.5 mm

Both probes show same signature, total defect height = 1.5 mm

Note: Full and even coverage through wall thickness in sensitivity is necessary to size defects correctly.

23. Gate Start and Minimum Gate Length of CRC / RMS Welds

TargetGate StartGate Length

Fill - 6-5 mm from Weld Prep.11.1






HP 2-5 mm from Weld Prep.9.5

HP 1-5 mm from Weld Prep.7.5

LCP-5 mm from Weld Prep.6.1

Root-5 mm from Weld Prep.6.5

24. Gate Start and Minimum Gate Length of Stick Welds

TargetGate StartGate Length









Root / HP-5 mm from Weld Prep.6.5

Root-5 mm from Weld Prep.6.5

25. Working Range Transducers

Element TypeFrequencyWedge DelayNear Field in SteelBeam Size (approx)

flat5.0 MHz7.0 mm9.6 mm1.6 mm


3/8 flat5.0 MHz10.0 mm25.3 mm3.0 mm

3/8 flat7.5 MHz10.0 mm41.8 mm3.0 mm



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