|NDT.net - March 2003, Vol. 8 No.3|
The use of ultrasonics to test pipeline girth welds during pipeline construction has increased substantially around the world since equipment first became available in the 1980’s. This has been driven in part by a substantial improvement in the ultrasonic technology and in part by the safety and environmental benefits of avoiding the use of ionising radiation for radiography.
This paper describes work conducted at Advantica Technologies to support a Transco Pipeline construction programme. Two types of weld were studied. Samples of both were prepared containing a range of defects. Seven AUT contracting companies then tested the sample welds in blind trials. Their results were then compared with the best of several radiographic inspections. Some of the defects were then cut open for a definitive appraisal.
Particular attention was paid to the types of defect associated with mechanized welding such as copper pick-up, cold laps and other lack of fusion defects. Cracking, transverse to the direction of welding, was also simulated.
In general, results were obtained which were comparable to radiography in all areas except for isolated pores and inclusions. Copper cracking was generally misclassified and transverse defects were not detected reliably.
The novelty and complexity of the AUT data places a renewed demand on the skills of company representatives in terms of overseeing the application of AUT in the field. Measures, including training, need to be considered in order to guarantee the quality of contractor performance and this would best be addressed as an industry-wide activity. It is also important that users get the best economic benefit from the new technology and this means using the capability of ultrasonics to define the size of any defects that are found. This would be in order to leave small defects unrepaired where they are shown to be safe through a fitness-for-purpose analysis. The issues surrounding the accuracy of size evaluation and further work needed in this area is discussed.
Traditionally, pipeline girth welds made during pipeline construction in the UK have been manufactured using the Manual Metal Arc (MMA) welding process and inspected using 100% X-radiography. There is some degree of radiation hazard to the public and operators in using x-rays and Transco have considered whether this can be reduced or removed.
In 1981 work was conducted by British Gas to assess five ultrasonic scanner systems to replace x-rays but they were all found to have deficiencies, especially in discriminating different defect types. Since then there has been several technical developments so that, today, the use of automated ultrasonic testing (AUT) of girth welds is becoming increasingly accepted as a reliable and beneficial alternative to radiographic testing (RT). Confidence has been generated by an extensive list of pipeline construction projects where it has been used successfully. While the basic approach is similar from most suppliers, there are significant differences as well as new developments in response to the demands of the marketplace. It was decided to conduct a trial of several AUT systems to establish the capabilities. It was carried out by Advantica Technologies on behalf of Transco.
The early AUT systems used probes to focus beams of ultrasound onto zones along the weld fusion line. They were set at appropriate angles to detect reflected energy from any lack of fusion at the focal areas. A set of probes was therefore needed to cover the whole of the weld thickness, one set on each side. This is illustrated in figure.1.
|Fig 1: Schematic of pulse-echo reflection beam paths.|
Since then further probes have been added to inspect the volume of the weld, another pair to give diffracted data and two more pairs for transverse defect detection. The evolution of the equipment has thus led to a complex set of data requiring analysis. Further details of these elements can be found in references 1-3.
Nine butt welds were fabricated in 48 inch (1220mm) diameter, X80 grade, pipes using two mechanised welding systems. These welds contained a range of deliberately made defects. Seven AUT contracting companies were invited to bring their equipment to inspect the welds at a Transco site and report on the defects they found.
Each company inspected the welds independently, under the oversight of Advantica staff. Data collected was interpreted off-site following each test and the reports were then sent to Advantica for comparison against radiographic inspection, which was taken as the standard. Some of the defects were cut open to confirm their nature and size.
The objective was to establish the effectiveness of AUT as an adequate alternative to radiography for pipeline construction when using mechanised welding. It was also intended that the evaluation should prepare for the drafting of a specification for AUT use, which could then be applied to future construction projects.
In alphabetical order, the AUT companies who took part in the trials were: -
Several other companies were invited to take part in the trials but, for a variety of reasons, were unable to do so.
In order that the AUT contractors were able to set up their equipment effectively, a coupon of pipe material was sent to each one, to be made into calibration pieces, as specified in the ASTM standard (Ref.4.). This was modified to comply with the requirement to detect transverse defects. There is a considerable time and cost involved in this preparatory work.
Companies who offer mechanised welding services use various types of welding preparations for the profile of the ends of the pipe. There are two types in common use; the ‘CRC-type’ of preparation and the ‘J’-type, illustrated below:
a) The CRC-type weld has a root deposited from an internal head.
b) The J-prep is deposited from the outside against an internal copper backing ring|
Fig 2: Weld Profiles.
The different welding processes can result in some differences in the types of defect that can occur. Consequently, both types of weld were used for the trial.
Since the AUT systems are set up with the focussed beams of sound directed at different regions of the fusion faces of the weld (as described previously), it follows that the precise weld preparation profile must be specified to each prospective inspection contractor for the systems to be set up appropriately. This information was provided along with the calibration coupons to each company.
A wide range of defect types and condition was specified for specialised welding contractors to insert into the welds during their fabrication. These included both simple defects such as lack of fusion (LoF) both in the root and the sidewall (at and between different weld passes) as well as lack of penetration (LoP) at the root. Areas of porosity were required, as this was believed to be a feature that could be a problem for AUT detection. ‘Combination defects’ were also requested, so that some features would extend across more than one weld pass at, or near, the same circumferential position. The possibility of copper being picked up from the copper backing (as used in the J-preparation weld) was of some concern. It was intended to simulate the inclusion of copper, both as copper pickup from the backing bar and also where the welding head copper tip might make contact with the joint. Transverse cracking was also included in the specification. Such cracking could not readily be induced during welding so it was decided to machine narrow slots into the welds after fabrication. To be consistent with the API 5L specification (Ref.5.) for axial weld cracking in linepipe, the slot depths were made around 10% of wall thickness i.e. approximating to an N10 notch.
After being welded, the welds were inspected using panoramic X-radiography. These radiographs were analysed by four independent, qualified interpreters. They were also re-radiographed and digitised both for the record and as a further check. This degree of rigor was warranted since the RT results were to be used as the baseline to evaluate the reliability of the AUT results. The interpretation of the radiographs demonstrated that the welding companies had produced a good set of defects in both types of weld and the defects were well distributed around the weld circumferences. Additionally, two slots were ground externally, one through the weld cap of each type of weld and two more were ground internally, through the weld root of each type. The locations of these slots were randomly located away from other defects.
The distribution of the types of defect that were produced is illustrated in the table of fig.3. A plot of the RT lengths of the defects, also shown in figure 3, demonstrates that they were deliberately made to be short and so were more difficult to detect by the AUT. This had the added advantage of allowing a lot of defects to be included. Nearly 90 defects were used; eight inspections (from seven companies) thus gave a reasonable statistical set of data.
|Type of defect||Number|
|Lack of Fusion||44|
|Lack of Root Fusion/Pen.||17|
|Fig 3: The distribution of length and types of defect included in the tests.|
The nine welds amounted to 35 metres of weldment. This means there was a considerable length of non-defective weld. If any company had attempted to achieve good detection by over-reporting this would give spurious indications that would be evident. (In the event this didn’t happen; the spurious indications were consistently low).
The two pipe spools were located with good access and good facilities in terms of comfort – a dry working environment with light, power and water. Adequate time was given to each AUT company to ensure that they would not be pressured by time. This was because it was intended to test only the capability of the equipment and interpretation. A standard briefing was given to each company. The weld caps were covered with tape to prevent sight of the external defects and the pipe ends were covered to prevent access to see the insides. No accept/reject level was defined – all reflectors considered significant were to be reported.
The first analysis of results was to compare how the AUT indications lined up with the RT results. The accuracy of characterisation was evaluated subsequently on a subset of defects by metallographic section of all the defects in one weld, selected with a good selection of defect types.
The results confirmed that very good detection performance is available from AUT systems generically. All linear features longer than about 10 mm were detected by some of the systems tested, as well as most clusters of porosity above 10 mm. Overall the Lack-of-Fusion and Lack-of-Root-Fusion defects were detected in 94% and 90% of the cases, while porosity was detected only 75% of the time. The transverse defects were detected only in about 70% of the cases. Scattered or isolated features, including pores, cavities and inclusions (metallic and non-metallic), were generally not detected. Neither were geometric features, such as ‘low cap’, ‘underflush’ or ‘flush root’, reported.
It is generally considered that AUT is better than RT in detecting planar defects. This is confirmed by these results. The start of a defective area was shown to be detected earlier by the AUT in many of the defects that were sectioned (e.g. Figure 4 indicates that a Lack of Side Wall Fusion of even 0.5 mm through-wall height has been detected .)
|Fig 4: The LoF defect on the left fusion face, at 5 mm from the outer surface, was found by all but one AUT system. It was not visible at all on the radiographs. (Wall thickness = 16mm) (1 mm bar on enlargement.).|
Some variation between inspection companies was apparent, indicating a lack of consistency. This is illustrated in the table below.
|Defect Type||Missed by 1 or more inspections||Missed by 2 or more inspections||Missed by 3 or more inspections||Missed by 4 or more inspections|
|Lack of Fusion||38%||23%||13%||8%|
|Lack of Root Fusion/Penetration||46%||40%||27%||13%|
|Table 1: Breakdown of missed defects.|
While porosity is confirmed as problematic for defection, it is worth noting that half the inspections found 75% of these small areas of porosity.
The transverse defects were not detected well, especially the external slots.
The ‘other’ defects include geometric features (such as ‘low cap’, ‘underflush’ or ‘flush root’), which are not good ultrasonic reflectors, accounting for the low detection rate.
An important factor is likely to be the human operator’s individual interpretation of the data, when the complexity of the data involved is considered. In order to get good performance in field operations with AUT systems, this suggests that attention to the competence of the operator is essential. An on-the-job test of operator competence has been proposed for Transco operations, much as is done for welders.
Three of the eight inspections were conducted using phased array equipment. No trend was evident to suggest that either better or worse results were obtained with the phased arrays.
When considering the deficiencies of AUT, it should be remembered that RT is by no means a perfect inspection system to compare it with. However, the approach does allow a good breadth of coverage in the comparison.
The Detection of Copper.
Several areas of copper, inserted into the weld, were available for study. No pick-up of copper from the internal backing plate was evident.
The most easily detected area (called ‘craze cracks’ by RT) was detected by all the AUT systems, but was generally characterized as ‘porosity’. A section, (Figure 5), shows this form of cracking to be much like liquation cracking and is distributed throughout the weld so that its misclassification as porosity is understandable.
|Fig 5: Defects associated with copper in the weld.|
The second area of copper was only detected by RT as a small ‘transverse defect’; most of the feature was invisible on the radiographs, even on careful re-interpretation. When it was sectioned it proved to be more complex than expected and extended along the weld length (>12 mm ). Only one company detected it as a transverse feature, though two others also reported a ‘planar’ element. Generally it was reported as ‘porosity’.
Since these defects seemed to tax the interpreters’ capabilities, an explanation was sought. The two features were included into a set of other, real areas of porosity and the list fed back to each AUT contractor. They were asked if they could distinguish the two areas of copper-related defect and invited to say what, in the signals, made them think it might be due to copper. Generally, only one of the two areas was picked out, but three companies picked out neither area. More experience is clearly needed. This is important since the significant size (the accept/reject boundary) is very different for the two types of feature. Misclassifying a planar defect as volumetric might effectively be the same as missing it altogether, since it might not be investigated or repaired.
The transverse slots that were cut across the welds were generally detectable, but with some notable ‘misses’. Each of the internal slots was eventually detected across the board (one company did so only on a second attempt). The detection of the external slots was more variable. About half the AUT companies missed the external slot in the J-prep. weld but only two missed that in the CRC-type weld.
Each of the contractors set up their equipment with transverse slots in the calibration coupons but these are made of plane pipe with no weld cap or root bead. This geometry difference may account to some degree for the missed defects. A false cap may be worth including with the calibration or set-up block to reduce timing errors.
The detection of these defects is not covered in standards and an industry initiative to improve this aspect may be warranted.
Seam weld region defects
The region where the pipes’ seam welds run into the girth weld is also a region needing special attention. The ultrasonic properties of the seam weld material are unknown, so it cannot be assumed that the ultrasonic beams are appropriately targeted. In practice, any defect that extends into such an area might need to be further examined, manually, for any extension that would take it into the ‘reject’ class. Alternatively, such a defect could be assessed with an additional length added equal to the width of the pipe seam weld i.e. assume that the defect extends right across the seam weld area. This does not appear to be done generally and needs to be included in specifications. The location of the seam weld is generally visible on the TOFD trace.
Two size measures of a defect are significant – the length along the line of the weld and the length through the thickness of the wall i.e. its ‘height’. Only the first measure is available from radiography but both can be estimated from the ultrasonic data.
The length along the weld tends to be longer when measured ultrasonically. Measuring the through-wall height of a defect by ultrasonics is conventional, but is prone to errors. This is even truer of AUT systems, where only amplitude data at a fixed range are available, though Time Of Flight Diffraction (TOFD) probes are also now used. TOFD size measures have been proven to be good for buried and deep defects (Ref.6.) but are limited in accuracy when used on shallow, near-surface features. Conventional, zone-based sizing can give an error band in the range from 0-6mm. A comparison of amplitude based AUT size values with destructive measures (Ref.7.) has shown that these are generally, but not exclusively, conservative. The defects in this Advantica study have been analysed in a similar way and have led to a similar conclusion. Details of this will be included in a future publication.
An improved method, using the special options of phased array data, has been demonstrated on calibration defects (Ref.8.). An accuracy of better than 1 mm has been cited but it is unlikely to be as good for real defects. Neither is it yet a viable method of sizing defects in field operations where rapid processing is required.
When defect size values are put into ECA assessments a substantial conservatism becomes apparent. Real defects are rarely of the simple parabolic shape assumed by the ECA. The sensitivity of AUT may indicate a long tail either side of a short feature, but it will then be assessed as having the depth of the short section and the length of the whole indication, including the tails; it is therefore more likely to fail assessment and to be repaired, perhaps unnecessarily. The fracture mechanics method needs to be able to handle complex shaped defects to reduce such over-conservatism.
Following the positive results of this work the practical aspects of using AUT have been evaluated during construction of a new pipeline in Scotland in 2001. For the first 26 km of the line both RT and AUT were carried out and the results compared. Every indication noted on both records was entered onto a database, of a substantial size. This allows the impact of different inspection criteria to be studied. For example, with the same specification the repair rates using AUT or RT were shown to be comparable. If an engineering critical assessment (ECA) specification were to be used, however, the repair rate was shown to drop substantially, with the potential for significant cost savings.
A similarly low repair rate of 1% was reported (Ref.7.) for the Alliance pipeline construction, coupled with a very high productivity rate, which AUT can support. This has also been experience in a recent British Gas offshore pipe-lay, where even lower repair rates were achieved with an API1104 based ECA (Ref .9.)
The AUT systems on trial have produced inspection results that compare well with radiography, with good detection except for isolated pores and inclusions. Clustered porosity is frequently found, especially larger clusters. Copper cracking is generally misclassified when it is detected and so may be left unrepaired. Transverse defects are not detected reliably. Further work is needed to improve the detection of both copper and transverse defects. Some variations are apparent in the performance of different operators and companies and an on-the-job testing of operator competence has been proposed as this is such a crucial element of the operation.
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