TWI Industrial Member Report Summary 750/2002
J B Wintle, R M Sanderson and C R A Schneider
Plant operators and regulators are increasingly interested in maximising the value of information from inspection of structures by non-destructive testing (NDT). This report critically appraises the value of repeat and sample inspections and inspection updating in simple statistical terms. It considers the benefits and practical limitations of these approaches, and ways in which inspection strategies may be optimised.
Repeat and sample inspections often occur in inspection plans yet their value and limitations require better understanding. The assumptions underlying the simple statistical analysis of the probability of detection of flaws are often not realised in practice. A greater appreciation of the factors that can make repeat and sample inspections more effective will be of benefit to practical inspection planning.
There is increasing interest in quantifying the increased confidence provided by inspection when this appears to confirm prior estimates of the condition of a plant. This is called inspection updating. Whilst the principles of inspection updating are presented in various publications, there is a lack of published examples of how these principles may be implemented in practice.
The report is for TWI Members in industries that have a need to inspect structures, whether for safety and/or economic reasons. It will be of use to engineers and managers involved with industrial inspection and risk assessment, and it will also be of interest to the relevant regulatory authorities. The methods given in this report should assist duty holders obtain more value from inspections and reduce costs by eliminating inspections of little value.
This report aims to present and appraise repeat and sample inspection in simple statistical terms, and to illustrate the principles of inspection updating.
Specific technical objectives are to:
- Examine critically the factors determining the probability of detection of a flaw in repeat inspections and the results of a simple statistical approach.
- Present a statistical method for determining the probability of detection of randomly distributed flaws in sample inspections and to determine the practical limitations of such a method.
- Demonstrate, by means of a worked example, how inspection updating can be used to increase confidence in the assessed corrosion rate of a structure.