8. ACHIEVEMENT OF RELIABLE INSPECTION
8.4.6. Example Qualification Programme
An example Inspection Qualification Programme is outlined below:
a) Identification of possible defects of concern (development of a defect specification)
b) Preparation of a Technical Justification (identification of ‘worst-case’ defects) c) Fabrication of test pieces
d) Conduct of ‘open’ test piece trials e) Analysis of results
f) Conduct of ‘blind’ test piece trials g) Analysis of results
h) Issue of statement(s) of capability or qualification certificate(s) (if pass/fail criteria specified)
8.5. KEY NDT ISSUES IN THE CONTEXT OF RBI
Within the context of RBI, the performance and reliability of the NDT needs to be commensurate with the risk of failure of the components/equipment inspected.
High-risk equipment requires high NDT performance and reliability to be demonstrated. Equipment of lower risk needs to be inspected by NDT that is judged to be effective for its purpose.
A strategy used to achieve high performance and reliability is to apply the principles of diversity and redundancy when selecting NDT techniques and determining inspection procedures. The use of a number of complementary NDT techniques can significantly reduce the likelihood of missing flaws.
Human errors are a significant contributor to low NDT performance and reliability.
A strategy to reduce the possibility of human errors is to select automated, or semi-automated, NDT techniques. Techniques that provide a permanent record of inspection data should be favoured, since these allow the results to be independently assessed by more than one person.
Where manual NDT methods are necessary, NDT operators need to have appropriate training, qualification and experience. For high-risk situations, these aspects become critical and an independent review to demonstrate their adequacy is
Procedure Qualification Personnel Qualification
recommended. In particularly difficult and important inspections, it may be beneficial for more than one operator to carry out the same inspection.
NDT method/technique selection should be based on the capability to detect and assess the deterioration types anticipated/sought in the parts of interest. The Duty Holder and/or Competent Person should have evidence of this capability, together with knowledge of any significant limitations.
For established techniques, satisfactory evidence may be available through published literature. Additional confidence is provided by inspection procedures that are produced in accordance with national codes and standards. For newer or more specialised techniques, where the only available evidence may be capability data provided by the equipment supplier, an independent assessment of the capabilities and limitations may be necessary.
An important issue is whether the magnitude of the risk justifies the need for inspection qualification. In situations where the full process of qualification (requiring pass/fail criteria) is not considered necessary, the provision of capability statement(s) should be considered as a suitable alternative. In lower risk situations inspection qualification is not generally necessary.
Continuity of inspections and inspection data is important. A key part of the RBI process is the feedback of knowledge of plant condition into the inspection planning process. Thus, attention should be paid to how records of inspections carried out, and the results, are kept and archived.
8.6. SUMMARY OF MAIN POINTS
a) The performance and reliability of the inspection and NDT needs to be commensurate with the risk of failure of the components/equipment inspected.
b) Inspection procedures should be available that cover the whole range of plant components/weld geometries to be examined.
c) NDT methods/techniques must be selected that are appropriate for the detection and assessment of the types of deterioration anticipated/sought and the characteristics (e.g. location, orientation etc.) The size of flaw for which reliable detection is required may be based on existing acceptance standards or fitness-for-service calculations.
d) Personnel involved in inspection and NDT must be competent and have the appropriate training and qualifications for the tasks to be carried out.
e) Inspection equipment should be checked before use to ensure that it is functioning and calibrated correctly.
f) When high inspection performance and reliability is required, a number of complementary NDT techniques should be selected on the principles of diversity and redundancy.
g) For high-risk components, inspection qualification is beneficial in order to ensure high confidence in the inspection results.
h) Duty Holders should have evidence of the capability of NDT techniques by (in order of preference):
(i) Referring to independent published capability data, (ii) Carrying out their own capability assessment,
(iii) Obtaining capability statements from equipment manufacturers.
Capability assessment should be a requirement for new or specialised inspection techniques (particularly for non-invasive, long range, or remote techniques) where these are being used in situations where prior evidence and experience of capability is not available.
i) In order to enable an accurate assessment of component deterioration, the inspection results should be compatible with those from previous inspections.
This is important if the inspection technique being proposed differs significantly from the technique(s) used for previous inspections.
j) Inspection datums and co-ordinate systems marked on components being examined should be kept for future inspections.
k) Inspection results should be archived using an appropriate method.
8.7. REFERENCES FROM CHAPTER 8
8.1 ASNT Aerospace Committee: ‘Recommended Practice for a Demonstration of Non-destructive Evaluation (NDE) Reliability on Aircraft Production Parts’.
Materials Evaluation Vol. 40, August 1982, 922-932.
8.2 Rummel WD, Hardy GL and Cooper TD: ‘Applications of NDE Reliability to Systems’. Metals Handbook 9th Ed Vol. 17, ASM Int. 1989, 674-688.
8.3 Chapman RK: ‘CEGB Guidance Document on the Assessment of Defect Measurement Errors in the Ultrasonic NDT of Welds’. Proc 27th Ann Brit Conf NDT, Portsmouth, 1988, Brit Inst NDT, 173-185.
8.4 Chapman RK: ‘Guidance Document on the Assessment of Defect Measurement Errors in the Ultrasonic NDT of Welds’. Nuclear Electric Technology Division Report TIGT/REP/0031/93, 1993.
8.5 IIW/IIS-SST-1157-90: ‘Guidance on assessment of the fitness-for-purpose of welded structures’, Int. Inst. of Welding, 1990.
8.6 E C Thermie Project No.OG/112/95: ‘Reliability assessment for containers of hazardous material (RACH)’. Work performed by TWI, UCL, TSC, Bureau Veritas and sponsored by the European Commission, HSE, Shell, Marathon Oil, 1996-1999.
8.7 HSE Project: ‘Programme for the Assessment of NDT in Industry (PANI)’.
Work managed by AEA Technology. 1997-1999.
8.8 HSE Document: ‘Best Practice for the Procurement and Conduct of Non-Destructive Testing, Part 1: Manual Ultrasonic Inspection’. Document prepared by B A McGrath, Inspection Validation Centre, AEA Technology. 2000.
8.9 EUR 17299 EN, Second Issue (1997) - European Methodology for Qualification (document prepared by European Network for Inspection Qualification (ENIQ)).
8.10 98/711582 (1998) - Methodology for Qualification of Non-Destructive Tests (document prepared by Panel WEE/46/-/9 of BSI).
8.11 Mitsui Babcock Energy Ltd. Group Sponsored Project 6490: ‘Periodic inspection of vessels from the outside only’. 1995.
8.12 Mitsui Babcock Energy Ltd. Group Sponsored Project 6748: ‘Non invasive inspection within an asset risk management strategy’. 1996-1998.
8.13 Mitsui Babcock Energy Ltd. Group Sponsored Project 235: ‘Recommended practice for non-invasive inspections’. 1999-2001.
Fig. 8.1 Example of probability of detection (POD) versus flaw size data presentation
Fig. 8.2 Example of Receiver Operating Characteristic (ROC) data presentation
Fig. 8.3 Example of measured response versus actual flaw size data presentation
95th percentile
95th percentile
9. FEEDBACK FROM RISK BASED INSPECTION