Health and Safety Executive. Guidelines for use of statistics for analysis of sample inspection of corrosion

Prepared by TWI Limited. 2002y -30 pages. ISBN 0717625540
This document is intended to advise plant engineers and inspection personnel on methods for analysing and extrapolating inspections for large plant items including vessels, pipework and pipelines, taking into account the statistical nature of corrosion. The document is intended to introduce the methods of statistical analysis of corrosion inspection data. Before the methodology is included in standards, practical experience of industrial applications is needed to identify the most relevant distributions and statistical techniques.
Summary:
Leakages of hydrocarbons provide both a serious risk of fire and explosion, and a loss of plant availability. Corrosion has been shown to cause in the region of 15% of the leakage occurrences. Inspection is carried out, particularly for internal corrosion by means of non-destructive test methods which give values of the pipe or vessel wall thickness. Typically these methods only sample the overall area of a plant. There are risks associated with this. The sampling may lead to inaccurate estimates of corrosion rate, especially where pitting has occurred, or may not address the fact that a sample needs to be extrapolated over the whole area of plant in order to give a realistic estimate of the minimum wall thickness in that area.
Statistical methods to improve the estimation of corrosion rate or to estimate the minimum thickness over a larger area have been suggested for over 50 years, and have been applied in a few isolated cases. These methods, when combined with reliability methods, offer a potential for obtaining better information from inspections by further analysis of the data collected and can produce predictions of future probability of leakage. However widespread application is not common, largely because the use of statistics requires specialist knowledge, and no reference standards exist. These guidelines are intended to provide an introduction to the techniques and capabilities of the statistical methods with view to their wider application in industry.