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1. Introduction

The use of AUT to test pipeline girth welds during pipeline

construction has increased substantially around the world since

the 1980's when Applus RTD introduced the worldwide first AUT

system (the "RTD Rotoscan"). This was partly driven by the

progress in flaw sizing and recording which made it possible to

use so-called rejection/acceptance criteria for weld defects based

on fracture mechanics, instead of good workmanship criteria as

common practice in radiography.

The development and actual use of such modern Engineering

Acceptance (ECA) Criteria, first in Canada and later also in other

countries, supported the introduction of AUT into the industry.

RTD, after having been the first company to offer these services

(cf. Fig. 1), was soon followed by other providers. Another

acceleration factor was the introduction of transit-distance C-scan

mapping in 1992 which enabled the system to cope with most

existing ultrasonic procedures and acceptance criteria, because ofits capability to visualize the weld geometry such as the root

penetration and weld cap reinforcement in order to eliminate false

call rate and to a certain extent, quantify volumetric defects.

Moreover, the integrated Time-of-Flight Diffraction (ToFD)

function, together with the amplitude-based, software-aided,

sizing capability provides accurate, state-of-the-art data on defect

through-thickness height.

The latest RTD Rotoscan (Fig. 2) makes also use of Phased

Array technology which gives the system advanced possibilities

and flexibility to accurately detect and size imperfections with

different orientations and locations in the girth welds. In addition,

all RTD Rotoscan systems make use of specifically designed

detection and sizing algorithms allowing not only compliance

with code and customer requirements but also to achieve a very

high Probability Of Detection (POD), together with a low false-

call rate (FCR).

The RTD Rotoscan is also optimized for the automated

inspection of pipeline girth welds consisting of anisotropic

welding materials (e.g. austenitic) in combination with a CRA

layer (Corrosion Resistant Alloy). Such austenitic girth welds

cannot be inspected by conventional ultrasonics, and have to be

examined by non-conventional angle beam compression wave /

creep wave technique. The present paper will briefly address this

type of inspections.

Despite the lack of international standards, ApplusRTD's

[ 27 p. 251s-256s (2009)]

by Cesar Buque

, Jan van der Ent

, Niels Prtzgen

, Marcel Blinde

, Tjibbe Bouma

, ISHIDA Tomoyoshi

Mechanized or Automated Ultrasonic Testing of pipeline girth welds is now in common use in the on - and offshore industry. Automated

ultrasonic testing (AUT) is globally seen as more than just an alternative to the standard radiographic inspection technique not only because it

does not poses safety hazards but also because it is faster, more reliable and has better detection capabilities of critical Lack of Fusion defects in

pipeline girth welds. One of the reasons to use the AUT technique is due to its possibility to use acceptance criteria which are based upon ECA

(Engineering Critical Assessment) instead of the so-called "Good Workmanship". Usually the AUT systems are mounted on a band strapped

around the pipe. From the weld, ultrasonic data is collected from which the defect sizes and positions can be determined by experienced

operators using dedicated software algorithms. This paper discusses the RTD Rotoscan system, which is the first worldwide AUT inspection

system for new construction pipeline girth welds. The principles of AUT, the conventional RTD Rotoscan as well as the RTD phased array

Rotoscan and its advantages in comparison with conventional (multi probe) AUT are discussed. Furthermore challenges regarding the use of

AUT on Austenitic welding having a corrosion-resistant alloy layer are presented. This paper describes also latest improvements made on AUT

during the last years in order to optimize inspection philosophy and minimize the system's "Uncertainties". In addition a brand new method,

RTD IWEX, is briefly described that allows the detection and sizing of weld imperfections in 3D.

Key Words: AUT, girth weld, pipeline, Phased array, CRA, weld inspection, austenitic weld

*Received on 13 June 2008

** Applus RTD Group, 3046 NC Rotterdam, The

Netherlands

*** Applus RTD IPS, 3046 NC Rotterdam, The Netherlands

**** Applus RTD Singapore, Singapore

***** Applus RTD KK, Tokyo, Japan

Fig. 1 World wide first AUT system for girth welds: RTD

Rotoscan in 1959.

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system has been qualified in various countries for different

individual customers and for a variety of weld processes, pipe

diameters, and wall thicknesses, and has demonstrated its on- and

offshore capabilities even under harsh field conditions. World-

wide applications proved that, contrary to expectations, ultrasonic

inspection does not lead to higher weld repair rates than

radiography. Between early 1989 and the present, over 30,000km

of pipeline (representing over 2,000,000 welds) have been

inspected with the RTD Rotoscan alone.

Development work continues to increase sizing accuracy

even further. Applus RTD just finalized the design and proto type

of a matrix phased array system with 1024 channels. The

advantages of such a system are going to be presented in this

paper. Applus RTD's latest innovation ("RTD IWEX") is the

future of AUT. This technology allows a much higher detection

and sizing accuracy without the use of reference blocks (also so-

called calibration blocks). The principles of this revolutionary

technology are going to be briefly discussed in this paper.

2. Conventional AUT of Girth welds

In AUT applications an automated scanning system carrying

a set of ultrasonic transducers at both sides of the weld is rapidly

rotated in a controlled manner for volumetric weld inspection in

one single scan around the circumference.

The transducer arrangement is designed on the basis of the

weld bevel configuration, in such a way that all defects that can be

expected are detected with maximum probability and with

minimum false call rate. To achieve this, the weld thickness is

traditionally divided into small zones of typically 2-3 mm in

vertical height (Fig. 3). In this zonal discrimination concept each

single zone is covered by an optimized separate transducer or

transducer combination. For more specific details on this subject

the reader is referred to1).

Applus RTD, well known as the pioneer in the AUT market

for girth weld inspection, introduced and patented the zonal

concept already in 1952. Since then this concept has now become

the world standard and has been described in a number of codes

and standards.

The amplitude in a channel corresponding with a particular

zone, indicates a possible defect in that zone.

For each zone on both sides of the weld a different transducer

is needed in different configurations such as pulse echo, tandem

and ToFD. The number of transducers required for the inspection

of a weld is related to the wall thickness, weld bevel configuration

and the number of inspection zones and in some cases it can be

easily be more than twelve.

The inspection system is often calibrated on flat bottom

holes/and or notches, whereby each zone has a corresponding flat

bottom hole. The echo caused by a flat bottom hole is set at a pre-

determined value. Calibrating such a system is time consuming,

because each transducer must be optimized, both its exact location

with respect to the weld and its sensitivity.

For optimum defect sizing, the ultrasonic beams are set such

that they hit an embedded defect perpendicularly, thus allowing

maximum reflection. AUT of girth welds generally assumes that

imperfections/defects are have the same orientation as the weld

bevel orientation, which is not always the case.

Therefore, in conventional AUT systems each part of the

weld is interrogated by a matched angle beam for the particular

zone and weld preparation it is intended for, offering best possible

sizing and ensuring a maximum POD.

A known potential problem in mechanized ultrasonic

inspection is that, especially in the root region, it is difficult to seethe difference between geometry and defect signals. In RTD

Rotoscan this is solved by digitization of signals from the root,

whereby these signals are displayed in the form of a C-scan map..

This enables the use of the geometry of the weld as a reference in

interpretation, just like one does in radiographic inspection.

However it has to be emphasized that in case of small root

penetration or no penetration at all it is still very difficult to

distinguish between geometry and defect signals in the root

region.

The same goes for detection and quantification of porosity.

Since porosity gives mostly only minor reflections, the only

accurate way to achieve a reliable detection of porosity is by

Automated Ultrasonic Testing of Pipeline Grith Welds252s

Fig. 2 The most recent RTD Phased Array Rotoscan for AUT on

pipeline girth welds. This unit was first introduced to the

market in August 2008. The phased array probes emulate

the multi-zone inspection approach as required by most

codes and standards.

Fig. 3 Schematic illustration of a girth weld divided into four zones.

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means of pattern recognition. The use of C-scan mapping,

supported by ToFD, makes this possible (Fig. 4).

The use of ToFD technique is important. ToFD is used as a

"safety net" in detection, and also for sizing and positioning of

defects in through thickness direction. Some codes already

require the use of ToFD in mechanized inspection of pipelinegirth welds, in addition to pulse echo technique.

The use of computers for data acquisition and storage not

only enables a coherent display of signals, highly comprehensible

for the operator, but also opens the way to automatic

interpretation. The signals from the weld are in this case directly

compared to the requirements of the applicable code or standard

in terms of defect amplitude, length and through thickness height.

Such automatic interpretation software can take over a lot of the

routine work from the operator, thus avoiding subjective errors.

In the case of RTD Rotoscan the software also allows for

automatic generation of site reports, thus avoiding typing errors

and increasing feedback speed.

Nowadays RTD Rotoscan is used efficiently in a number of

onshore and offshore pipe construction applications. Specific

applications for AUT systems like Rotoscan are:

* Cross-country and lay barge projects

* Riser and riser bundle manufacture

* Steel catenary risers (SCR's)

* Pipe with CRA layer

* Different types of steels including austenitic steels and 9%

Nickel steels

* etc.

With an inspection cycle time of usually a few minutes,(

cycle time consist of calibration ; inspection of weld and transport

time in case of on-shore application) the conventional RTD

Rotoscan is of proven value to the offshore and onshore industry

as the NDT activities are removed from the critical path, saving

e.g. barge days. Furthermore the industry benefits greatly from

the instantaneous availability of real-time inspection data, as

immediate feedback to welding crews improves field monitoring

of the welding process, consequently reducing overall repair rates.

3. Phased AUT on Girth Welds

3.1 What is Phased Array

As it can be deducted from the Fig. 5 Phased array

technology is a technical solution for generating and receiving

ultrasound using an ultrasonic transducer consisting of an array of

elements that can be individually pulsed simultaneously or one

after other following a certain pre-determined sequence. Most

details of ultrasonic inspection remain unchanged. For example

for an ultrasonic inspection the ultrasonic transducer is selected

according to the desired ultrasonic beam angle, focal distance and

scan pattern for that specific inspection. Therefore very often

different sets of transducers are required making the inspection

costly. In the case of phased array technology a large variety of

inspection parameters (beam angle, focal distance and scanpattern) can be achieved using the same transducer and by

applying time delays in the pulsing sequence. The time delays

values are calculated using time-of-flight from the focal spot, and

the scan assembled from individual "Focal Laws". Also,

modifying a prepared set-up is quick in comparison with

physically adjusting conventional transducers.

3.2 Advantages of Phased Array technology in AUT

The use of array technology in girth weld AUT offers many

advantages and potentials when compared with conventional

AUT. For example, in conventional AUT of pipeline girth welds,

a new transducer set has to be composed in hardware for each new

application (i.e. combination of wall thickness and weld

geometry). In case of phased array this job preparation is reduced

to optimization of the probe system's main beam characteristics

(beam angles, beam widths and focal points) by means of

software settings (Fig. 5).

When phased array technology is used rather than fixed

transducers, the beam characteristics can be controlled and thus

optimized with one system and performed by one linear phased

27 2009 2 253s

Fig. 4 Screenshot with a typical view on a situation where

porosity and lack of fusion were detected. In this

particular figure ultrasonic indications caused by porosity

are clearly visible.

Fig. 5 Phased array simulation. Using phased array system the

only changes needed on the transducer setup are made by

software settings. Beam angle, beam spread, beam width

and beam directivity can thus be easily programmed for

different jobs without physically changing the transducer

setup.

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array transducer, without changing the hardware. When the linear

phased array contains enough elements, configurations for

multiple element transducers such as tandem transducers can be

formed in this way. The flexibility offers operational benefits and

reduction in preparation and inspection time.

Calibration of a phased array system can be done withsoftware settings. The linear phased array is placed on a fixed

distance from the centre line of the weld. The signals can be

optimized without physically moving the transducer, which is the

case in a conventional system. Preparing and calibrating the

phased array system for the weld inspection, therefore, consumes

less time than in the case of a conventional system. Furthermore,

rather than applying a large number of individual probes a phased

array system only requires one probe at either side of the weld. In

addition to the two phased array transducers, two probes for the

TOFD function are necessary. Since ToFD usually uses a higher

frequency than pulse-echo does, ApplusRTD Rotoscan has the

ability to use separate ToFD probes rather than use the phased

array probes for ToFD. Dependent on the application in some

cases a set of two extra probes is added to detect transverse

defects.

3.3 Phased Array technology in AUT of girth welds

Using linear phased arrays, the number of transducers can be

drastically reduced. With one pair of linear phased array

transducers, all configurations required can be made to inspect all

zones from both sides. Instead of a separate transducer, a

different set of delay times is used for each configuration. A

normal pulse echo configuration will be obtained when a group of

active elements is used for both transmitting and receiving. An

active group of elements (typically up to 32) can be multiplexed

along an entire array (with typically up to 128 elements) enabling

the index point of the beam to be shifted. A tandem configuration

is formed when the reflected beam from one active group is

received by another active group elsewhere on the array. The

receiving group may also partially overlap the transmitting group,

thus eliminating limitations caused by physical crystal dimensions

in conventional transducers.

The PRF (pulse repetition frequency) of a PA system is

identical to that of the conventional systems. This ensures the

same high measuring point density for a full inspection sequence

(cycle), maintaining the same scanning speed. During a cycle of

sequences all probe functions as previously programmed are

activated to provide full volumetric weld cross section coverage.

4. AUT on pipelines with Corrosion-Resistant Alloy layer

4.1 Preliminary considerations

Pipelines with corrosion-resistant alloy layers inside have

generally austenitic welds. Ultrasonic inspection of such welds is

difficult and requires special attention from AUT245). Main

reasons for that are the coarse grain structure of austenitic welds

and the strong dependence of the sound velocity on the

crystallographic grain orientation which in combination result in

strong ultrasonic reflections (Fig. 6).Compression waves suffer significantly less from these

phenomena than shear waves (Fig. 6) which inevitable leads to the

use of focused angle beam compression and creep wave probes

instead of shear wave probes.

The AUT inspection set-up for austenitic welds is similar to

the standard inspection configuration for ferritic girth welds in

that the wall thickness is divided into a number of depth zones.

The inspection philosophy however is different as the selected

probes are not dictated by the weld bevel configuration (as for

AUT of ferritic girth welds), but are designed to minimize

reflections out of the anisotropic weld structure or interface

between weld and parent metal that could interfere with the

inspection result interpretation.

Instead of the traditional shear wave, dual crystal focused

angle beam compression waves and creep wave probes are used to

enable the full penetration of ultrasonic waves through the weld

volume, not hampered by the structure of the involved dissimilar

metals and austenitic coarse grain size structure.

4.2 AUT data on CRA: data evaluation and presentation

For the ultrasonic inspection of austenitic welds and CRA

layers, having dissimilar interface(s) and anisotropic material

structure (compare Fig. 7 and Fig. 8), the standard AUT pulse

echo presentation is not adequate to present all the essential

features required for the correct interpretation of the AUT CRA

inspection channels 2) . As illustrated in Fig. 9 the used

compression wave has to pass the primary and coincidence

interface between the carbon steel and the austenitic weld

material, identified with the green and red borders. In order to

identify the Primary and Coincidence interface position, the RTD


Fig. 6 Typical ultrasonic reflections as caused by the grain

morphology when inspecting austenitic weld using

conventional shear waves (top image) and when using

compression waves (bottom image).

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Rotoscan result presentation has been enhanced with an image

format being referred to as "All channels mapping" presentation.

In this context "All channels mapping" has to be understood as

stacked C-scans in color, with each c-scan originating from a

certain transducer focused to a certain area of the weld

configuration.The "all-channels mapping" facility within the AUT system

software is an essential feature to visualize the inspection result on

the screen and to be able to correlate / evaluate the indications

present at the primary and /or coincidence interface within the

weld volume to increase the confidence level. This technique is

called "primary/coincidence technique".

With the function "all channel mapping" all ultrasonic wave

form information (A-screen dynamic range 0-100% FSH) derivedfrom inspection gates, are digitized with a high sampling

frequency. Recorded amplitude and corresponding transit

distance are converted into a color pattern. In this presentation a

coherent display is obtained showing the defect position(s) and

geometrical features (e.g. noise out from the interface) in relation

to the weld centerline. For more details on this the reader is

referred to van der Ent et al.2).

5. AUT on pipelines with Corrosion-Resistant Alloy layer

It is well-known that all welded structures will always

contain different types of imperfections. It is also known that not

all flaws necessarily affect structural integrity or service

performance of a welded structure, in this case newly welded

pipelines. These facts are implicitly recognized by most welding

fabrication codes which based on different so-called Engineering

Critical Assessment criteria (ECA) specifying the level of

acceptance of imperfections in new girth welds.

Quantitative characterization of weld integrity based on ECA

considerations has conclusively proven that knowledge on

accuracy with which weld imperfections can be sized, particularly

in terms of defect height and defect orientation is of paramount

importance2). To achieve the required sizing accuracy new

inspection concepts combined with new physical algorithms are

needed. Two of the most prominent methods being close to

market introduction are presented in the paragraphs below.

5.1 The RTD Matrix Phased Array

The development of the newest generation of RTD Rotoscan,

the RTD Matrix Phased Array Rotoscan, has just been finalized

and is now entering the validation phase in the USA (Fig. 10).

This solution, consisting of 1024 PA channels, has the major

advantage to focus and to steer the beam in two different

directions, allowing a volumetric insonification of the

imperfections and/or defects. A girth weld can thus be inspected

using a variety of probe angles and hence accurate sizing of tilted

and skewed defects is possible and significantly improved.

5.2 Future generation AUT: quantitative 3D imaging of

defects

To date, the performance of all ultrasonic technologies used

in the oil & Gas industry depend on the quality of the so-called

calibration blocks, and because of that the interpretation of the

27 2009 2 255s

Fig. 8 Light microscopic macro showing a ferritic girth weld in

carbon steel pipeline. In comparison with Fig. 7 it can be

seen that in this case the grain structure is much finer.

Fig. 9 The CRA inspection technique makes use of specially

designed compression wave probes to avoid reflections

from the anisotropic welding structure. As can be seen

from the figure the compression wave has to pass the

primary and coincidence (opposite) interface between the

carbon steel and the austenitic weld material, identified

with the green and red borders.

Fig. 7 Light microscopic image showing an austenitic girth weld

in CRA (metallurgical bonded) clad pipeline (austenitic

welding material having coarse grains).

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results achieved is not straight forward, and special interpretation

skills and special training (Level III) are needed in order to be

able to read and interpret the results.

Furthermore the most used ultrasonic technologies in use

have strong limitations. For example testing using pulse echo

technique and ToFD has have been proven to sufficiently reliableto assess weld integrity. However, quantitative defect

characterization with pulse-echo remains challenging because the

signal amplitude caused by the reflection at the defect is very

dependent of defect orientation. TOFD has good sizing

capabilities, but only very limited capabilities in flaw

characterization. Data display is not straightforward and require

operator skill and experience. A better and more reliable

ultrasonic inspection would be achieved if a methodology would

be used that allows direct imaging of defects. Such a technology

is RTD IWEX (Inverse Wavefield EXtrapolation). The physical

basis of this new imaging process is the Rayleigh II integral for

back propagation which gives the possibility to extrapolate a wave

field from known values at a certain surface to any location in

space. The principles of IWEX are discussed in detail in3).

The potential of IWEX for ultrasonic testing of steel

components has been demonstrated by several examples by which

2D and 3D images of embedded and surface defects were made

(Fig. 11). The investigations have shown that location, shape,

orientation and height of the defect are imaged with high

accuracy. The interpretation of the results is straightforward,

making the use of reference blocks superfluous3).

6. Conclusions

After many years of persistent pioneering and marketing by

ApplusRTD, AUT of pipeline girth welds is now offered by a

number of NDT providers as a worldwide service and has reached

a high level of professionalism. Primary key factors in the

ultimate success of pipeline AUT became its capability to

discriminate between defects and geometry indications, to detect

and quantify porosity and to use ECA criteria. These three factors

(including mapping of weld geometry such as root penetration and

cap reinforcement) helped to avoid false calls, thus reducing the

number of unnecessary repairs. AUT practices, now used by all

AUT providers, are still based on the zonal concept patented by

ApplusRTD in 1952. Developments in pipeline AUT are still inprogress. Defect detection, characterization and sizing will, in the

future, be further enhanced by efficient use of array technologies,

like IWEX technology.

Acknowledgments

The authors would like to acknowledge the pre-work

performed by Jan De Raad, Frits Dijkstra and few others which

ensured that this paper could be made in the required quality.

References

1) A. De Sterke. NDT, 74 conference, Lacenster, (1974).

2) J.van der Ent. 12th Asia pacific conference (2006).

3) N. Prtzgen. PhD Thesis. TU Delft, Netherlands (2007).

4) F. H. Dijkstra and Jan de Raad, Duplex Conference in Maastricht

(NL), Manuscript 106 (1997).

5) Dutch patent registrated under nr: 1024726, PCT/NL2004/000874,

(2005).

Email contact:

* Dr. Cesar Buque, email: [email protected]

**** Tomoyoshi Ishida; email: [email protected]


Fig. 10 The RTD Matrix Phased Array unit consisting of 1024

channels (top image). The image below illustrates the

flexibility that can be achieved while using Matrix

Phased Array technology. The matrix-like arrangement of

PA elements enables the operator to steer side ways and

focus the beam in the lateral direction

Fig. 11 Example of the imaging and sizing accuracy that can be

achieved with RTD IWEX. The three bore holes on the

left sample have a 0.5 mm diameter. The center of the

holes are separated 1mm and 1.5 mm respectively. On the

IWEX image (right side) all three bore holes are clearly

visible3).

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