amplitude sizing

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Amplitude Sizing and Mechanised Ultrasonic InspectionUsing Linear Scanning

E.A. Ginzel

January 2000

Abstract:

There has been over two decades in the development of mechanised ultrasonic inspection techniques using

zonal discrimination used with acceptance criteria based on fracture mechanics. Recent reports of the

“accuracy” of amplitude sizing have been greatly exaggerated. An explanation why AVG (DGS) style

sizing is not appropriate for mechanised ultrasonics is provided. The use of unfocused probes for zonal

discrimination is cautioned against and a demonstration of the superiority of smaller beams sizes is

provided. The need to provide sizing tolerances as the true measure of a sizing system is recommended.

Introduction:

Mechanised ultrasonic inspection of girth welds is now commonplace in the pipeline industry. In the

1980’s it was shown that the zonal discrimination technique allowed a convenient method for applying

acceptance criteria based on fracture mechanics. This form of acceptance criteria considers the metal

properties, the expected strains and the remaining wall thickness in the component. Dividing the

component wall thickness into zones, an estimate of flaw height is made by looking for ultrasonic signalsover a threshold. Each “zone” is examined using a dedicated ultrasonic beam directed at a fixed position.

“Sizing” a flaw’s vertical extent by this system has normally been conservative. This conservatism results

from a flaw that is less than the zone height, but still providing a signal over threshold, being assigned a full

zone height. Attempts to improve the accuracy of these assessment have long used amplitude information

from adjacent zones but this has been an effort to reduce rejections due to two probes seeing the same flaw

in a the region shared by the probe beam overlap. This condition is shown in Figure 1.

Figure 1 Flaw position with respect to Probe beam position and overlap

Zone 1

Zone 2Zone 3Zone 4

Flaw in Zone 2 Flaw straddlesZones 2&3Zone 3 Beam

Zone 2 Beam



In a recent report1

the subject of amplitude sizing was raised, again. Upon closer review of the details of this paper and others that expound this concept as a precision sizing technique, it can be seen that many of

the assumptions and conclusions made are incorrect or misleading. This paper will attempt to point these

out in an understandable fashion and propose limitations to consider.

Amplitude sizing techniques are commonly used in manual ultrasonic inspection Standards. Amplitudedrop methods

2whereby a signal is compared to distance-amplitude-correction (DAC) curves are more

prevalent in North America while amplitude ratio (AVG) methods3 are more prevalent in Europe.

There is some rationale behind these methods. The DAC system relates echo dynamics of the signal to

probe position and relates these back to a known reflector (typically a side drilled hole). The AVG system

devised by Krautkramer in 1958 (see4) is a family of distance-amplitude curves. These were originally

computed for the case of compressional waves incident upon small, smooth, planar (disc) reflectors, and

relating echo height to the size of the disc, its distance from the probe, and the intensity of the back echo.

The curves have since been adapted for use with flat-bottomed-hole calibration blocks and shear wave

probes.

Gross et al have proposed a variation of the AVG principles to size vertical extent of planar (non-fusion)

defects using a linear scanning technique whereby an array of probes is used in fixed positions from the

weld. In an earlier paper the writer5

proposed a similar technique for porosity assessment. However,

subsequent experience has indicated that the variability in porosity reflection is far to great to allow anAVG style of treatment of porosity.

In his paper Gross stated, “To measure the height of a flaw with an ultrasonic beam, that beam has to be

larger than the flaw height and provide “over-trace” on either side of the flaw.” This seems to derive as an

“adaptation” of a statement made in Ultrasonic Testing of Materials (Krautkramer 3rd

English edition). On

page 91 of this text it states; “The echo wave produced by a circular disc reflector can be analysed most

conveniently if we first consider a small circular disc placed on the axis at a great distance from the

radiator.” Udo Schlengermann6

(page 47), states that the substitution of a “reference reflector” for a real

reflector is valid if the disc shaped reflector is ”lying on the acoustic axis of the sound beam”. He further

states that, “This is the case for all discontinuities which represent small reflectors, i.e. reflectors which do

not extend beyond the sound beam in any direction.”

The assumptions made by the AVG system are inappropriate to extrapolate to the application of mechanised UT with probes in fixed positions for two important reasons:

1. The AVG sizing is applicable ONLY to on-axis flaws and this can only be done using manual

UT (mechanised UT detecting off-axis flaws does not satisfy the on-axis pre-requisite)

2. The requirement that this applies to reflectors which do not extend beyond the sound beam in

ANY direction can only be valid for short flaws, not the long flaws putatively duplicated by

notches (proposed by Gross et al.)

To extrapolate AVG area sizing to a vertical extent sizing ignores the effects of integrated sound pressures

from laterally located reflectors as cautioned by Schlengermann7. Even in DAC sizing systems it is now

1B. Gross, T. Connelly, H. van Dijk and A. Gilroy-Scott, “Flaw sizing using mechanised ultrasonic inspection on

pipeline girth welds”, International Conference on Advances in Welding Technology in Galveston, Texas, October

26-28.

2 ASME, Boiler and Pressure Vessel Code, American Society for Mechanical Engineers, New York, USA.

3 DIN 54 127 part 1, Calibration of ultrasonic flaw detection equipment and echo height evaluation

4J. Krautkramer & H. Krautkramer, Ultrasonic testing of materials, Springer Verlag, 3rd English edition 1983

5 E. Ginzel. R. Ginzel. B. Gross, M. Hoff, P. Manuel, Developments in Ultrasonic Inspection for Total Inspection of Pipeline Girth Welds, 8th Symposium on Pipeline Research, Houston, 19936 U. Schlengermann, Krautkramer-Branson Booklet, 1985,

7 U. Schlengermann, Krautkramer-Branson Booklet, page 47, 1985



commonly recognised that for flaws less than the beam size the dimensions obtained by probe displacement

are nothing more than the measurement of the beam size8.

The amplitude relationship between a target and a beam is based on the ratio of the target area to the beam

area. The AVG (Abstand/Verstarken/Grosse in German) or DGS (in English this is Distance/Gain/Size)

was developed to simplify the sizing of a defect, but this is the relative size of the flaw (the S in the DGSsystem). In the development of the DGS system the flaw is assumed to be detected well into the far zone

(Krautkramer 3rd English edition page 93). As well, the flaw size is assumed to be fully encompassed by

the beam and lying on the beam axis.

These three items are clearly not met in the mechanised technique using fixed probe positions (as in the

zonal discrimination inspection method). E.g.

1. For all work in the zonal inspection technique, we are working very near the focal point (at

the near zone boundary if using unfocused probes).

2. For the small beam profile available with well focused probes, the flaw sizes are typically

about as high as the beam and usually longer than the beam is wide. For the unfocused

probes, the beam may be higher than the flaw dimension vertical extent but the flaw (typically

nonfusion) is longer than the beam is wide.

3. Attempting to use an area relationship for an off axis long flaw is again outside of the pre-

requisites laid out for the ideal sizing. The AVG approach uses a ratio of areas which required

the flaw be on the beam axis.

Mechanics of Mechanised Linear Scanning and Zones

Contrary to raster scanning, which attempts to provide a volume coverage inspection using a similar back-

and-forth probe motion as manual inspections, linear scans fix the probe exit point with respect to the weld

centreline. See Figures 2 and 3 for differences. By comparison, the raster scan is much slower. Motor

logic for the change between steps requires a slowing down of the probe to a stop before the raster step

occurs. Subsequently the raster step is made and then the motors speeds up to full scanning speed and

slows again as the end of the scan sweep is neared.

Figure 2 Traditional raster scan in a mechanised set-up

Data Collec tion

StepRaster Step - No

Data Collection

A linear scan moves the probe parallel to the long axis of the weld. Data collection is done on the scan

parallel to the weld. The raster step may not be required if multiple probes are used, or if the probes used

provide the coverage required (e.g. TOFD and limited pulse-echo coverage).

8 R. Murphy, Ultrasonic Defect Sizing Using Decibel Drop Methods, Atomic Energy Control Board, March 1987



Figure 3 Linear scan for increased data collection speed

Data Collec tion

StepRaster Step - No

Data Collec tion

Linear scans are used in the zonal discrimination technique discussed earlier.9,

10

Amplitude sizing by an AVG principle using linear scanning would require all flaws be detected well into

the far zone on the beam axis by divergent beams, and these flaws would always be smaller than the beam

detecting them. Clearly this is not likely to occur.

Simply considering areas of flaws compared to the beam area is both difficult and not altogether accurate.

For the tandem arrangement of probes typically used in linear scanning of narrow gap welds, further

considerations must be made. Figure 4 shows a typical configuration for a tandem probe.

Figure 4 Tandem probe arrangement

Figure 4 shows the centre ray of the tandem configuration for a 14.2mm wall using a 45° transmitter (front)

and a 55° receiver (rear). This is appropriate for a 5° bevel face when looking for nonfusion.

Again, Krautkramer cautions us (Ultrasonic Testing of Materials, page 99) about the use of tandem probes

and the effects of oblique incidence. For example, if we use a 55° transmitter instead of a 45° as illustrated

in Figure 3, the incident angle on the planar flaw would be 30° and the reflection coefficient would bereduced to about 60% of that for the 40° incidence provided by the 45° probe.

9E.A.Ginzel and M. Hoff, Further Developments in UT Inspection of Pipeline Girth Welds Ultrasonic

Online Journal, Herford, Germany, Dec. 1995, http://www.ultrasonic.de 10

E.A.Ginzel P.den Boer and M. Hoff, Application of Mechanized UT Inspection to Manually Welded

Pipeline Girth Welds, Ultrasonic Online Journal, Herford, Germany, March 1997,http://www.ultrasonic.de



After many years of development it has been shown that focused probes have provided a higher signal-to-

noise ratio for flaw detection and are less subject to false calls associated with reflections from surface

geometries. In spite of this proven history, there are still some systems using standard flat element

divergent probes. This might be rationalised by cost of the probes and the concept that these are needed for

AVG sizing.

Figure 5 shows a view of the ideal beam for a flat element 5Mhz 9mm diameter on the same tandem set-up

in Figure 4. The upper half of the image shows the view from an end-on position with the beam conic

section projected onto the notch. The lower half of the image shows the proportion of the through wall

dimension occupied by the divergent beam.



Figure 5 Standard Divergent Beam for a 5MHz 9mm diameter probe

Using a standard divergent beam calculation for the 6dB drop

= −

DSin

λγ 51.0

1

6

one can estimate the beam dimension. Using the 9mm diameter 5MHz probe the –6dB half angle of

divergence is 3°. At 39 mm of equivalent sound path to the target illustrated, this represents 2mm of off-

axis displacement or 4mm of beam diameter. When corrected for the 40° angle of incidence the projected

vertical extent is 5.2mm. This is very close to the near zone when a 10mm refracting wedge is used and

Huygen modelling indicates a projected height of 5mm to 8mm may occur in this region for the –6dBboundary.

When focused probe modelling is done using RayTracing a misconception results that a small beam with a

spot size smaller than 2mm diameter is achievable as shown in Figure 6.

Figure 6 RayTracing of a focused beam

A more realistic representation is seen using a Huygen wavelet modelling as in Figure 7 where a 12.5mm

diameter 5MHz probe is focused with a radius of curvature of 80mm. This indicates that the minimum –

6dB beam size possible is on the order of 3.5mm diameter, which when projected at 40° shows a vertical

extent of about 4.6mm. This is only a slightly better spot size than the unfocused 9mm probe. However,

lobes that exist when using an unfocused element are more wide spread compared to the focused probe,

thereby focusing improves the signal-to-noise ratio.



Figure 7 Wavelet model representation of the focused transmitted beam in Figure 6

This beam sizing background can now be used to consider amplitude sizing.

A drop in amplitude from a target in a beam can be caused by several variables:

1. Relative vertical position with respect to the beam axis

2. Relative lateral position with respect to the beam axis

3. Relative area of the target with respect to the beam area (a change in dimension inany direction).

4. Relative reflectivity of the target (texture of the surface) with respect to a machined

target5. Relative angle of the reflecting face of the target with respect to the beam axis

6. Distance of the target from the probe (relative to the near zone or focal spot)

If the premise is made that the flaw detected is a simple nonfusion on the weld bevel, then items 5 and 6

can reasonably assumed to be known. For a nonfusion type defect the texture of the target interface is

always assumed to be smooth but fracture images clearly show this is not entirely true. The actual vertical

position of the flaw may vary due to its actual position in the test piece or it may vary “virtually” as a result

of small changes in the probe position with respect to the centre reference line of the weld if the probes are

not exactly where they are calibrated to be.

Since position and area of the defect with respect to the beam axis have identical effects on the signalamplitude, no definitive judgement can be made regarding these parameters separately when using a

mechanised set up with a linear scan.

Area Calculations

If all the signal evaluated in a linear scan originated from on-axis defects, a simple area relationship may be

feasible for a given beam area. When used as a tandem transmitter the beam projects an ellipse in the plane

of the fusion face.

Area of an ellipse is calculated from:

4 Dd A π= where D is the length of the major axis and d the length of the minor axis.

For an ideal spot using the 3.5mm minor axis proposed as the best lateral extent of the spot, and 5mm as the

major axis the spot area is 13.7mm2. For the more likely 8mm vertical extent of the major axis, the area of

the unfocused spot is 22 mm2.

For an ideal spot using the 2mm diameter proposed for a well focused beam, 2.0mm would be the minoraxis for the best lateral extent of the spot and 3.5mm the projected major axis, giving an area of 5.5mm

2.



We can then compare notch and flat bottom hole areas as they would appear in the beam.

Notches:

• Area of a 1mm high notch across the spot is approximately 1mm x 3.5mm = 3.5 mm2

• Area of a 2mm high notch across the spot is approximately 2mm x 3.5mm = 7.0mm2

• Area of a 3mm high notch across the spot is approximately 3mm x 3.5mm = 10.5 mm

2

Holes:

• Area of a 1mm diameter FBH is 0.8 mm2

• Area of a 1.5mm diameter FBH is 1.8 mm2



For on-axis assessments, a reasonable linear correlation between area and amplitude is seen (as verified by

Gross et al). However, when the same reflector is positioned off axis, amplitude also drops. Movement of

a point reflector in the beam is in fact the most commonly used method of characterising beam shape (e.g.

ASTM E-1065, BS 4331, AS-2083 and ASME Section V). In these codes and standards the target used is a

side drilled hole for contact testing probes. The probe movement related to the amplitude drop indicates

the beam dimension. Military Standards and ASTM (MIL STD 2154 and ASTM E-127) have used the areaamplitude relationships for area estimates, but these are for on axis targets and compression mode only.

Since moving a target position relative to the beam axis can have the same effect as changing the target

area for an on-axis target, i.e. alter the amplitude, separating the effects becomes a troublesome issue when

making assumptions of defect size based on amplitude. Efforts to apportion amplitudes from adjacentzones using divergent beams encounter a problem when a flaw changes dimensions along its lateral extent

as well as changing its vertical position in the weld.

Each time a calibration is run in a linear scan using the zonal discrimination technique an assessment of

beam size is made. These calibration blocks use FBHs located at equally spaced intervals on a weld bevel

profile to indicate the zones. By knowing the vertical separation of the targets and by observing the relative

amplitudes one can get an estimate of beam size.

Figure 8 represents the beams from an ideal unfocused probe (left) and an ideal focused probe (right) as

they would appear when projected onto the plane of the flat bottom holes for the set-up shown in Figure 4.

The circles represent the flat bottom hole targets. The main target is centred in the beam (grey shaded area

indicates ideal –6dB boundary) and the “adjacent zone” target is located 2.8mm up from the main target (aswould be for the configuration in Figure 4).

• On the left is the ellipse formed by the “standard” probe (3.5mm x 8mm) Note: for this

representation the more precise Huygen interference model is used instead of the simple half-angle

divergence which gives the 5.2mm calculated earlier.

• On the right is the ellipse formed by a spot focused probe (2.5mm x 4.8mm)



Figure 8 Calculated beam shapes as per modelling

When we combine the ideal model with actual scan results from calibrations an even greater area is

projected as indicated in Figure 9. This is established from the –6dB envelope amplitude and length of the

travel in the lateral direction for the width of the beam and the amplitude of the adjacent zone target.

Figure 9 Most probable beam shapes as per echo response from known 2mm diameter

reflectors (unfocused 9mm diameter probe on left and focused probe on right)

Again, using the 2mm diameter FBH images separated by the 2.8mm vertical displacement, area

representations of actual beams can be represented based on measured lateral beam widths from the two

types of beam. The typical standard probe (flat) has a lateral beam extent of 6 to 7mm as measured off

typical calibration charts (based on 6dB drop from peak of the 2mm diameter FBH). Similarly measured, a

typical focused probe provides a 4 to 5mm beam width. It is interesting to note that using the –20dB

envelope, the adjacent FBH centred 2.8mm above the on-axis target would be just about totally

encompassed by the –20dB envelope for the focused beam. These modelled results compare well with scan

results. Gross et al reported that the unfocused probe used in their experiment reported a 50% “over-trace”

with the adjacent zone target and the focused probe they used had a 20% over-trace. This would indicate

that the 2mm diameter FBH used in this calibration was a reasonable approximation of a point reflector.

Had the beam been smaller and/or the target larger, the amplitude differences may not have been as easy to

relate to calculated beam sizes. This follows as the area ratio (beam to target) more closely approximates

unity.



Alternative Solution

Instead of measuring spot size using a point reflector, an infinite reflector might be used. Moving a probe

across a plate until the amplitude drops to 50% of maximum is the middle of the beam (half the beam on

the plate and half off). At the point where the amplitude is half that of when half the probe was on the plateis the 6dB drop from the centre of the beam. This is 3dB up or down from the midpoint amplitude of the

full drop from maximum to minimum.

In Figure 10 two illustrations are made. Signal amplitude is in the vertical scale and probe position with

respect to a target is indicated on the horizontal scale. The experiment was done using an immersion probe

12.7mm diameter and 5Mhz nominal frequency. The left image shows the signals from an aluminium plate

set to near full screen height and then the probe is moved horizontally until the beam no longer interacts

with the plate (zero amplitude). An FFT on the unfocused probe indicated an actual frequency of 6.5MHz

putting the near zone at about 180mm waterpath. Modelling indicated that the spot size would be about

3.4mm at this point. The measured displacement using the 3dB difference from the midpoint gives this a

3.8mm 6dB spot size. For comparison of techniques the unfocused probe was evaluated using a steel rod

1.8mm diameter. Using the 6dB drop at the same waterpath, a 6dB beam width of 4.6mm was established.

Figure 10 Beam size comparisons using delta 3dB from midpoint

Unfocused beam at 190mm waterpath (3.8mm

spot size using infinite reflector principle.Standard 6dB drop from the reflection off a rod

indicates 4.6mm at 190mm

If a beam is relatively small with respect to the target, the centre of the beam can be located in the target

most of the time. As the target dimension changes such that the beam “edges” miss the target, the

amplitude drops from its maximum. If the target is long and the height reduces, then the drop in amplitudecan be attributed to a height reduction or a shift of the probe with respect to the flaw centre as shown in

Figure 11.

The ellipse in Figure 11 represents a beam 7mm wide with a 2.5mm 6dB vertical projection. Whencompared against a 3mm high notch, the beam is seen to be totally covering the notch. When the notch is

made smaller (1.5mm high in top right of Figure 11) the beam interaction with the target is reduced. The

same effect occurs for the offset of the target or beam with respect to one another as in the lower example

in Figure 11.



Figure 11 Amplitude reduction due to change in height or probe position for small spot size

7mm x 2.5mm focused beam on notch-type targets. Change in target size upper right and change

in probe or notch position lower right, result in reduction in signal amplitude.

Production of good quality spot sizes (small vertical extent) is possible using standard single element

probes, but use of phased arrays for this purpose is even more effective. In a recent experiment a 15mmthick weld section was divided into 29 different zones using a phased array system11 (see Figure 12). When

using a standard calibration configuration this provided a very small (-14dB to –20dB) overlap between the

2mm diameter FBHs centred in the traditional locations. Each of the 4 fill zones was sampled at 5-6

different vertical positions. With sub-zones less than the weld pass height, it is possible to see variations in

both flaw vertical position due to wandering of the weld puddle and total vertical extent.

Figure 12 Phased Array amplitude responses using “sub-zones” (courtesy RD Tech)

In Figure 12 the left side shows a C-scan presentation of the root notch and 4 Fil l targets (Root notch on the

Top Right and the 2mm diameter FBH Fill 4, on the bottom left). Late arrivals in the Fill 1 gate show over-trace with the Root Notch and similarly the Root channels show over-trace on Fill 1. However, when the

first arrivals in the gate are plotted against a weld profile (on the right side of the image), the overlaid target

positions show how no areas are missed and the targets are correctly positioned. This presentation makes it

is easy to see how flaw position and extent would be more readily discerned using “sub-zones”. Any

movement of a flaw “between” the main calibration target centres is seen as a shift in the affected “sub-

zones”.

11 P. Piché & M. Moles, personal correspondence, RD Tech, Québec, Canada, 2000-01-10



When a –14dB beam overlap between two zones is seen on a calibration run some have expressed concerns

for “missing coverage” between zones when using well focused probes. However, this concern is not well

founded when a system is calibrated on a FBH. Gross et al reported that a difference of 6dB existed

between the response from a 2mm FBH and a 2mm high flat notch. It was not explained if they used a

focused or unfocused probe for this determination but 4 to 8dB would be typical, depending on the probequality and beam size. Therefore, if a system is calibrated on a FBH, there is a built in over-sensit ivity to

longer flaws with the same vertical extent. For example, using a 12.7mm diameter 7MHz probe focused

with a 60mm radius of curvature, the –6dB spot size is about 2.2mm diameter at the focal point. But if

scanning is done so that a 2mm diameter FBH is brought to 80% screen height then sensitivity to longer

reflectors (e.g. a notch) results in a signal about 6dB greater. Using the standard 40% evaluation threshold,

this is equivalent to examining for the longer flaws down to the –12dB level. For a 12.7mm diameter

7MHz focused probe the –12dB envelope at the focal point increases to 3.0mm (a 36% increase in vertical

extent compared to the –6dB coverage).

In practice, a small diameter spot size calibrated on a 2mm FBH would be operated in the region around the

+3dB from the probe centre response as indicated in Figure 10. This follows from the fact that the ratio of

areas for the FBH and spot size of a well-focused probe is greater than 50%. Such a probe, when

encountering an infinite reflector (like the “backwall in an AVG diagram) would reflect nearly all of the

available pressure back to the receiver. If the flaw is longer than the beam width then the 6dB extra

sensitivity that exists between the FBH response and the long notch response for the focused probe, wouldplace the amplitude response from the flaw at a point comparable to nearly 3dB under the maximum

possible response for a 100% return of sound pressure. This implies that over-sizing by 12dB is not

possible for small focal spots, but might easily occur for unfocused probes. Attempts to compensate for

this oversizing when using unfocused probes by using logarithmic amplifiers and signal characteristics

from adjacent zones is of limited value considering the relatively large areas integrated in the beams.

Use of a small spot size beam for vertical extent estimations is also subject to off axis and area effects on

signal amplitude. However, providing the spot size can stay small compared to the flaw height, the effects

of off axis variations can be better monitored by the presence of signals in the adjacent zones. When the

vertical zones are closely matched to spot sizes (and both are small), signals with amplitudes over an

agreed level and confined to a single zone, may reasonably be assumed to have variations in amplitude due

to variations in flaw area restricted to the zone height of that probe. When a small flaw moves between two

zones such that it is off axis for both probes detecting it, the apportioning of amplitudes minus the overlapseen on calibrations may provide a means of preventing excessive over-sizing. However, this option must

also be used with caution as the effects of flaw vertical position need not vary amplitude in exactly the

same way that variation in probe stand-off would. This is especially true when using tandem probe

techniques where the stand-offs of both the transmitter and the receiver vary with the vertical position

being covered.

Compared to large beams from unfocused probes, the use of more and smaller beams is a more effective

method of estimating flaw extent and position in the vertical plane when using mechanised linear scanning.

Summary

In summary, amplitude relationships used to indicate vertical extent of a flaw are subject to many

limitations. The principles involved in published (AVG) amplitude sizing are based on area ratios, not the

dimension in a single axis. Amplitude variation calculations are based on reflections off ideal reflectors

(usually with perpendicular incidence) for reflectors that are on the centre of the beam axis and well into

the far field of the beam, and for reflectors that have a small area compared to the beam cross-sectional area

at the point of reflection.



Since these prerequisite parameters do not exist for the mechanised UT used in the zonal discrimination

method, all efforts to “accurately” size flaws in the vertical extent using amplitude are prone to errors.

These errors can be minimised by using focal spots very closely matching the zone height. Assessment of

vertical extent can be improved by observing zonal interaction of signals from zones with smaller height

using beams with similarly small spot sizes. Vertical assessments are further improved using sub-zones

feasible when using phased arrays.

Concerns for “missed coverage” between zones when using small focal spots can be reduced when

calibration is performed on FBHs instead of notches. This follows because increased sensitivity results to

long. off-axis reflectors (more typical of nonfusion) as compared to the point reflector sensitivity provided

by the FBH calibration target .

For the purposes of fracture mechanics fitness-for-purpose, sizing estimates are made using the information

ascertained from the zone amplitudes. However, even with the observance of interaction of signals

between zones, amplitude sizing will always require that reasonable tolerances be used due to the variety of

sources of amplitude differences. In a paper by Kopp et al12

, sectioned results showed that amplitude based

evaluations were often close to sectioned flaw sizes but variations of 0.5 to 1.5mm could occur. In the

more recent efforts reported by Gross et al, their results indicated an average error in amplitude sizing using

unfocused probes to be around 0.5mm also but even so, some items in the report showed over-sizing by

3.3mm and 2.6mm (Appendix C & F2 of the report) and they often used a range of 3mm (3.0-6.0mm) to

allow for possible variations. Low repair rates claimed by the technique expounded by Gross et al aretherefore not a result of improved sizing but in fact a result of reducing the evaluation level by 12 dB as

compared to previous projects.

Suggestions by some, that present sizing “accuracies” exist that are small percentages of a millimetre, are

unrealistic. This would make the putative “accuracy” 10 to 50 times greater than the tolerances established

by statistics.

When designing an acceptance criteria this statistical deviation must be allowed for as reported by Førli13

.

Comparing the reports by Kopp et al and Gross et al, the tolerances for unfocused probes are greater than

for focused probes. Projects conforming to DNV requirements will probably need a statistical analysis

involving both PoD and tolerance determinations14

. This will put the results of unfocused probes at a

distinct disadvantage.

12 F. Kopp, G. Perkins, B. Laing, G. Prentice, S. Springmann, D. Stevens, Automatic welding, ultrasonic inspection

used on J-Lay project, Offshore Pipeline Technology, April 1998

13 O. Forli, Automated ultrasonic testing during offshore pipelaying, Acceptance criteria and Qualification, DNV, Oslo,

Norway, 1999

14 O. Forli, personal correspondence, 2000-01-14



References

1. B. Gross, T. Connelly, H. van Dijk and A. Gilroy-Scott, “Flaw sizing using mechanised ultrasonic

inspection on pipeline girth welds”, International Conference on Advances in WeldingTechnology in Galveston, Texas, October 26-28.

2. ASME, Boiler and Pressure Vessel Code, American Society for Mechanical Engineers, NewYork, USA.

3. DIN 54 127 part 1, Calibration of ultrasonic flaw detection equipment and echo height evaluation

4. J. Krautkramer & H. Krautkramer, Ultrasonic testing of materials, Springer Verlag, 3rd

English

edition 1983

5. E. Ginzel. R. Ginzel. B. Gross, M. Hoff, P. Manuel, Developments in Ultrasonic Inspection for

Total Inspection of Pipeline Girth Welds, 8th Symposium on Pipeline Research, Houston, 1993

6. U. Schlengermann, Krautkramer-Branson Booklet, 1985,

7. R. Murphy, Ultrasonic Defect Sizing Using Decibel Drop Methods, Atomic Energy Control Board, March

1987

8. E.A.Ginzel and M. Hoff, Further Developments in UT Inspection of Pipeline Girth Welds

Ultrasonic Online Journal, Herford, Germany, Dec. 1995, http://www.ultrasonic.de 9. E.A.Ginzel P.den Boer and M. Hoff, Application of Mechanized UT Inspection to Manually

Welded Pipeline Girth Welds, Ultrasonic Online Journal, Herford, Germany, March 1997,http://www.ultrasonic.de

10. P. Piché & M. Moles, personal correspondence, RD Tech, Québec, Canada, 2000-01-10

11. F. Kopp, G. Perkins, B. Laing, G. Prentice, S. Springmann, D. Stevens, Automatic welding,

ultrasonic inspection used on J-Lay project, Offshore PipelineTechnology, April 1998

12. O. Førli, Automated ultrasonic testing during offshore pipelaying, Acceptance criteria and

Qualification, DNV, Oslo, Norway, 1999

13. O. Førli, personal correspondence, 2000-01-14

amplitude sizing

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