Independent qualification of phased array inspection of fillet welds

TWI Bulletin, November/December 2002

Charles Schneider is a Principal Project Leader in the NDT Technology Section at TWI. He has worked in the fields of inspection reliability and theoretical modelling since 1985, holding posts within the Central Electricity Generating Board and its successor companies. Since joining TWI in 1997, his work for the nuclear industry has included inspection qualification, development of technical justifications, and the statistical analysis of inspection results.

Colin Bird is a Principal Consultant in the NDT Technology Section at TWI. He has worked in the field of NDT development of all the major NDT methods since 1979, holding posts within Rolls Royce & Associates and the Central Electricity Generating Board and its successor companies. Since joining TWI in 2000, he has specialised in ultrasonic applications, in particular the application and development of specialised phased array ultrasonic techniques.

Robin Shipp is an independent consultant, operating through his own company, Firecrest Consulting. He has been involved in NDT since 1980, holding posts with the South of Scotland Electricity Board, the Central Electricity Generating Board and Nuclear Electric. He is a Chartered Engineer and Fellow of the British Institute of NDT. For the past three years he has been a member of the qualification body for a number of ENIQ qualifications in BNFL Magnox Generation and has worked as a European expert advising the Lithuanian State Nuclear Regulator on the implementation of the ENIQ methodology. He is currently working as a consultant for the NDT section at TWI.

Magnox Generation has investigated whether it is feasible to carry out ultrasonic inspection of welds associated with boiler beam support brackets at various Magnox power stations. As Charles Schneider, Colin Bird, Robin Shipp and David Wood report, the welds are small, with a leg length typically of 6-15mm, and they could not be properly inspected using conventional ultrasonic probes.

A technique was therefore developed using a phased array system which allowed the ultrasonic beam to be focused electronically. This allowed the required spatial resolution, and the beam angle to be varied in order to direct the beam into the weld volume between the cap and root. Following the development work, it was decided to apply the technique to some boiler beam support bracket welds at Sizewell A power station.

This paper addresses the qualification of the inspection procedure, which TWI managed on behalf of Magnox Generation following the ENIQ methodology.

This qualification involved a novel technique with particular challenges and this paper describes the way in which these were overcome.

The comprehensive technical justification document was supported by practical trials of the procedure, equipment and individual data analysts. The qualification was completed successfully and confirmed that the procedure was capable of reliably detecting crack-like defects in the weld or heat affected zone (HAZ) greater than 1.5mm wide by 10mm long. It also confirmed that, subject to certain provisos, the defect width would not be underestimated by more than 1.5mm.

The boiler tube structures inside the boilers at Sizewell A are supported by beams which are themselves attached to the boiler shells by support brackets. These brackets are welded fabrications and volumetric inspection of the welds was considered desirable to check for possible in-service degradation. The bracket design is shown in Fig.1. It consists of a landing plate and two gusset plates and is welded to the boiler shell or, in some cases, to a pad which is welded to the boiler shell. The gusset to landing plate welds are un-penetrated fillet welds and the gusset to shell/pad welds are fully penetrated, as shown in Fig.2. The landing plate to shell/pad geometry is similar to the gusset plate to shell/pad geometry, except that the plate meets the shell/pad at right angles.

The plausible defects which the inspection must be capable of detecting are oxide jacking cracking, lack of fusion, hydrogen cracking and slag inclusions. The requirement was to provide highly reliable detection for crack-like defects greater than 1.5mm wide by 10mm long. The width is the dimension measured perpendicular to the weld direction on the face of the defect ie dimension W for the unpenetrated welds, and W1 and W2 for the fully penetrated welds, as shown on Fig.2. Defects were assumed to be tilted at any angle between the two fusion faces ie a, a1 and a2 on Fig.2, where p1 and p2 are the weld preparation angles on the fully penetrated welds. The inspection requirements were that the measured defect width should not underestimate the true defect width by more than 1.5mm.

A technique was developed for the inspection using an ultrasonic phased array system. ^[1] The inspection was required to be qualified according to the ENIQ methodology ^[2] and TWI was contracted to provide the independent qualification body required by ENIQ.

The use of the ENIQ methodology is well known and has been described in many papers. ^[3] The present paper will not therefore discuss the use of the ENIQ methodology in general but will instead concentrate on how this qualification dealt with the particular challenges presented by this inspection, namely:

• Use of a novel inspection technique.
• Lack of evidence from previous qualifications.
• Limited previous application of the technique for this component geometry.
• Lack of standards that could be cited
• Lack of evidence from computer modelling.
• Lack of recognised training and certification for the technique.
• Limited timescale.

Qualification

The qualification body consisted of three people all experienced in inspec-tion qualification. TWI chaired the qualification body and was supported by an independent consultant. The third member was an employee of Magnox Generation, who provided detailed knowledge of the procedures and their application and reported to the chairman for the purposes of this qualification. The qualification body discharged the usual ENIQ responsibilities of agreeing the input information, reviewing and endorsing the inspection procedure and technical justification, carrying out practical trials and issuing qualification certificates. This work was reported in a summary of technical evidence, which provides the primary reference to the extent and limitations of the qualification. The way the challenges of this inspection were addressed is described in the following sections.

Preparatory work

Because of the novel nature of the technique and the lack of experience in this area, extensive development work was carried out. As well as laboratory work, this included trials on actual plant initially using focused probes and later using the phased array system. At the start, members of the qualification body and a representative of the Nuclear Installations Inspectorate were given a training session, which described this preliminary work and included a demonstration of the phased array equipment. This helped to ensure that all those involved in the qualification were aware of the capabilities and limitations of the technique.

Inspection procedure

• With no established standards for equipment performance, the qualification body assessed the equipment checks to ensure that they provided adequate control of the beam characteristics, in particular that the focal laws required by the phased array equipment provided the required beam angles and focal lengths. In addition, the data quality checks were assessed to ensure that they were capable of ensuring that the procedure was capable of collecting data of adequate quality for subsequent analysis.
• Although the technique is novel, many features of the data analysis are similar to those of other automated systems, in particular the GUIDE system which has been used for many other inspections of UK nuclear plant. The qualification body checked that experience from these systems had been taken into account.
• There is no established personnel qualification for phased array inspections. The approach adopted in the procedure is, first, to invoke PCN qualification to ensure a basic understanding of ultrasonic inspection. Second, recognising similarities to other automated inspection systems, to invoke established qualifications for staff carrying out automated inspections on UK nuclear plant - the MIPS qualification for data collectors and the GUIDE qualification for data analysts. Finally, to provide training specific to the phased array system and to this procedure. The qualification body assessed these proposals to ensure that they provided adequate control of the inspection personnel and the qualification records for all inspection personnel to ensure that they met the required criteria.

Technical justification

In many cases it is possible to invoke the results of computer modelling in the technical justification. The development team made extensive efforts to identify a suitable model but, at the time the work was done, was not able to obtain one. The qualification body was satisfied that all reasonable attempts were made and that the technical justification was acceptable without that support.

Practical trials

The inspection is carried out with one inspector manipulating the phased array probe inside the boiler shell and a second operator operating the data collection equipment outside the boiler. The inspector inside the boiler has to work in conditions of difficult access, wearing protective clothing. In designing and invigilating the data collection open trial, the qualification body took particular note of:

• The need to simulate the access conditions.
• Checking for adequate communication between the inspectors inside and outside the boiler.
• Checking the effectiveness of the data quality checks.

For the data analysis blind trials, the candidates were presented with data obtained from a series of test blocks containing defects induced during welding. A macrograph of a section taken from a typical example is shown in Fig.3. For the trial, the candidates were assessed against the required criteria of detecting all defects meeting the required description and not undersizing defect width by more than 1.5mm. The blind trial was initially attempted by two of the three data analysts. Both met the detection criterion but failed to meet the sizing criterion on some defects. With the information available at that time, the qualification body could not identify if this resulted from the performance of the analysts, the capability of the procedure or poor characterisation of the defects in the test pieces (at that time, the value specified for defect implantation was taken as the 'true' value). This was resolved by providing additional training for the first two analysts, including the results from the blind trial on the third analyst and carrying out sectioning of selected defects. With this further information, the qualification body concluded that all three analysts met the required criteria.

Limited timescale

The data obtained during the outage was analysed in an interim form using the draft procedure and this work provided further information to help in writing the final drafts of both the procedure and the technical justification. The data analysis blind trial was carried out when the procedure and technical justification were finalised and the data was then re-analysed to provide the formal inspection results.

This retrospective approach was possible in this case (and is possible in most automated inspections) because the data collection and analysis parts of the inspection are largely independent. The qualification body did, however, check carefully that all parts of the qualification met the requirements in the final documents in all significant details. This process was documented and is available for audit.

Conclusions

This qualification has also demonstrated that it is possible to apply the ENIQ methodology in a cost effective and timely manner to an inspection using a novel technique and where there is little prior experience. Some of the factors needed to achieve this are:

• A preliminary programme of development work, including experience of application of the technique under realistic conditions.
• Training for the members of the qualification body, including the results of any preliminary work.
• Even if the technique is novel there may be features which are similar to other established techniques. Advantage should be taken of such experience.
• Similarly, the personnel qualification may be able to take advantage of other similar inspections. However, specific training in the technique and procedure is essential.
• If data collection and analysis can be separated, it may be possible to collect data from plant before the procedure and technical justification are complete, although this presents an economic risk of the inspection not being qualified retrospectively.

Acknowledgements

This paper is published by permission of BNFL Magnox Generation. The authors also acknowledge the assistance of the Dr K J Bowker and other members of the development team at BNFL Magnox Generation in the preparation of the paper.

References