MINUTES OF THE MEETING
Microseminar - 'Automated UT and ToFD - Techniques. - Acceptance Criteria, Reliability, Cost-effectiveness, Human Factor and Qualification' (V-1142-99)
H. Wuestenberg, the chairman of Subcommission VB opened the session by an introduction. He pointed out why during the preparation phase of the microseminar the title has been changed. The planning of the seminar started with the problem of the acceptance criteria for automated UT. Because this is closely linked to the necessity of a better quantification of the defect height ToFD as a potential technique came into focus. The main reason is the actual interest which is world-wide in the application of ToFD and the advantages the technique has in order for sizing the defect height and in cost reduction as an automated UT-technique as well as the fact that some equipment manufacturers very aggressively have pushed the ToFD technique into the market. Therefore there is a tremendous need for detailed discussions in order to inform the scientific community as well as end-users about realistic estimates of the potentials.
O. Førli presented the document V-1144-99, 'Automated ultrasonic testing during offshore pipelaying, Acceptance criteria and Qualification'.
The offshore-industry is one of the main innovators of automated UT (Impulse Echo multiple-probe concept including focused Tandem and ToFD) which today is available to reasonable prizes of the instruments/systems and with satisfactory operating safety, accuracy and reliability for rapid testing. Online data accumulation, processing and documentation is possible.
The report tells the story why for girth weld inspection during pipelaying- after qualification, i.e. after the evaluation of the achievable testing sensitivity and the definition of the acceptance criteria for each individual application, means each relevant defect - automated UT can replace radiography.
90% POD for detection is required for allowed defect sizes with 95% confidence level but in any case better than the POD of the reference technique which is to replace; the sizing accuracy asks for a sizing error within the 95% or 99% quantile.
O. Førli discussed the paper V-1145-99, 'Draft Nordtest Guidelines for the Development of NDE Acceptance Criteria' he prepared with Knut O. Ronold.
The document is an excellent introduction for each which is interested to learn the methodology because of its definitions and description of the mathematical statistic behind.
The procedure takes into account the defect severity, i.e. of a defect of a given size leading to failure. In this case the system is based on fracture mechanics and fatigue mechanics. In the same way the optimisation can be made on the base of cost minimisation, taking into account as well as the inspection costs as well as the costs for repair and failure.
QA acceptance criteria can be defined to achieve at least a 50% PoR (Probability of Rejection) for severe defects compared with a described quality level.
M. Katoh gave two presentations to experiences with the application of ToFD in the Japanese industry.
The 1st paper was to heavy component applications, i.e. thick plates and pressure vessels and large steel structures of some km weld seam length for offshore airports.
In the case of thick plates a test piece with 26 different realistic weld defects are prepared.
At this thick component (350mm test block) a 50% POD was achieved for Radiography as well as for manual UT (using different angle-probes), only with ToFD a 98% POD was documented with standard deviation in sizing accuracy between 5mm (x,y) and 10 mm (z). Based on the trial results acceptance criteria were defined in accordance to ASME 2235.
K. Iwamoto gave a report to ToFD Boiler inservice inspection (V-1137-99), i.e. the application in the smaller wall thickness range and for the detection of thermal and corrosion fatigue cracks. ToFD has documented a higher accuracy compared with conventional UT and is not so sensitive against changes in crack orientation, i.e. is less dependent from a given angle of incidence. Limitations are for near-surface defects where ToFD has a dead zone.
H. Wuestenberg discusses especially the effects of crack inclination on the diffracted wave which can lead to a loss of the diffraction signal if the signal is not foreword-scattered into the receiver probe as well as influences of stress onto and roughness of the flanks of the crack.
U. Ewert gave a contribution (no paper with Commission V No.) to ToFD in discussion with conventional UT and radiography.
He explains the new standard CEN/TC 54 for pressurised equipment, where ToFD is introduced as a possible alternative approach to standard automated pulse echo UT.
The classical radiography approach mainly is limited by the capabilities of the human beings as inspectors but also by the limited range of available incident angles to optimal crack detection in a standard radiographic procedure. Therefore one needs approaches like Computer Tomography in order to overcome these limits. These today are on the way to become available.
The ToFD problem in the surface-near regime was especially discussed, where the defect echo is hidden by the direct surface echo and where as pulse echo clearly can detect the defect. One reasonable approach can be the combination of pulse echo and ToFD.
The chairman, H. Wuestenberg, started the session coming back to the election of a new chairman of Commission V and thanked the former chairman Tom Siewert in the name of the full audience and on behalf of the Subcommissions for his fruitful work all over the years.
Wuestenberg dispatched in the audience in addition to his Annual Report (V-1128-99) handouts of his overhead sheets (these papers should be named as attachment 1 and 2 of V-1128-99).
The Subcommission has had one intermediate meeting in Orlando (V-1138-99) and the WP for the Austenitic Weld Inspection had 3 meetings.
The NIST document (V-1133-99) concerning the problems with the IIW calibration blocks is dispatched; a 2nd paper is in preparation by the BAM.
The ToFD Technique was especially discussed in the morning microseminar (V-1142-99). Europe plans to organise a ToFD round robin action managed by the Joint Research Centre in Petten (Lemaitre).
Wuestenberg introduced for the audience again into the problems with the IIW calibration blocks, revealed by NIST in the US and then also approved by the BAM in Germany by checking different sets of calibration blocks used in service.
The Subcommission proposed special measurement techniques in order to re-qualify old blocks used in the service for their quality assured application which are listed in the above mentioned handouts but clearly again pointed out in these minutes.
UT Systems for Automated Inspections:
A German proposal for an new European standard was introduced in the Subcommission discussion. The guideline will give instructive and helpful information to users considering relevant requirements of clients and of existing inspection standards to
UT of near probe surface area:
Wuestenberg gave the information to this new German activity in CEN TC121.
Phased Array Probe/Equipment:
More and more these type of equipment and probes are in use and different instrument makers are very active to marketing the technique, not only for nuclear applications. The situation is that no appropriate standard exists and, because of the differences to conventional probes, there is a need for setting standards.
UT data formats:
Wuestenberg gave the information that research institutes as well as end-users (mainly utilities and service companies in the nuclear field) of automated examination using UT have found together in a Working Party in order to define a data format for automated NDT, but including also combined inspection with eddy currents.
Wuestenberg introduced the audience into the problems in order to define acceptance criteria by using the findings of UT. The problem is mainly reliability (see the papers of the microseminar) coming from a under-sampled multiple-angle insonification, and, which is in comparison with a x-ray Computed Tomography (360(-angular spectrum round about the defect) always 'incomplete' in the mathematical sense, i.e. only a region of interest reconstruction. Furthermore, not the defect density is imaged, but only its reflectivity which is also in most of the cases incomplete because of scattering and mode conversion of the waves and influences of residual and applied stresses. These facts all together result in an uncertainty of the NDT-data which complicate the definition of acceptance criteria with strict limited tolerances. However, there is a need for enhancement and for better understanding of the significance of the results in terms of sizing for fracture mechanical or similiar evaluation schemes
The Commission approved the resolution:
Document V-1145-99, 'Draft Guidelines for development of NDE acceptance Criteria is transmitted (Class B document of the Commission) to Commissions X, XIII and XV for information, and to promote grounds for a joint meeting or seminar related to Fitness-for-Service/Purpose assessment of welded constructions to further promote IIW activities in this area. The Commission V contact person is Olav Førli. email@example.com
D. Chaburkin presented the paper 'Assessment of Serviceability of Gas- and Oil-Pipeline Weldments by the Result of Diagnostic Examinations in Operation'. This document (XI-715/99) was also presented in Commission XI. The paper - for further discussion of the activity - has received the Commission V No V-1148-99.
The annual report of the chairman was approved by the audience.
The chairman G. Dobmann opened the session. The minutes of Commission V, July 22, 99 were approved.
Dobmann introduced the audience into the agenda of the Subcommission VE meeting (V-1129-99), he gave an overview about the intermediate meetings and their topics and introduced the relevant documents which presently are discussed.
Working party to eddy current inspection and modelling:
The WP-chairmanship since the dead of John Moulder in 98 is still vacant. The current work is to low and multiple-frequency applications with four objectives:
The chairman will present at least two documents to these activities during the Annual Assembly in Florence 2000.
Working party to fluorescent inspection media and thermal techniques (Peri, Marmigi):
Performance test with a proposed new thermographic approach (Keremzhanov, V-1086-97) to the online monitoring of electric resistance spot welding in a German industrial company.
Working party to the inspection of non-metallic Weldments, i.e. thermoplastic polyethylene materials (Khimchenko):
Planned is the organisation to an international conference in Moscow in spring 2000 and to publish a state of the art document describing the world-wide state of guidelines and standards.
Working party to the characterisation of residual stresses in Weldments (A. Doubov, G. Dobmann, N.V. Khimchenko):
There is a running round robin action in Russia comparing the magnetic memory approach with other competitive techniques. The activity should be opened to an IIW/European activity by applying an INTAS project, a special sponsoring of scientific and technological research in the new independent countries of the former USSR.
The annual report of the chairman was approved by the audience.
M. Katoh gave a presentation to the document (V-1134-99) 'FEM Study on the Influence of Air Gap and Specimen Thickness on the Detectability of Flaw in Yoke Method. This paper is a very basic contribution for better understanding magnetic leakage flux phenomena. The presented modelling results very clearly give an interpretation of experimentally investigated facts developed some 15 years ago in terms of distributed magnetic resistors and can contribute especially in the education of engineering students and inspection personnel.
The Commission approved the resolution:
Commission V proposes the document V-1134-99, M. Katoh, K. Nishio and T. Yamaguchi, FEM Study on the Influence of Air Gap and Specimen Thickness on the Detectability of Flaw in Yoke Method, for a publication (as a Class A document of the author) in Welding of the World.
At the end of the Subcommission meeting v. Hofe asked the audience whether the present title of Commission V
is still an appropriate name, regarding the fact that NDT & E is actually the primary task of the Commission's activities. Av. Hofe recommended a change. The Commission should discuss the topic again July 23, 99.