TWI Frequently asked questions
by John Wintle
The inspection of plant and machinery has traditionally been based on prescriptive industry practices, backed up by health & safety legislation. The type of equipment has determined inspection frequency, methods employed andlocations examined - with little consideration given to its age, specific duty or likely condition. Increased operational experience and a greater appreciation of the hazards are now leading some parts of industry to adopt a moreinformed approach to inspection planning, targeting the inspection required to reduce the risk as low as reasonably practicable.
Risk based inspection is the process of developing an inspection plan based on knowledge of the risk of failure of the equipment. The essential element is a risk analysis. This is the combination of an assessment of the likelihood(probability) of failure due to damage, deterioration or degradation with an assessment of the consequences of such failure.
The information gained from this process is used to identify (a) the type and rate of damage that may potentially be present and (b) the equipment or locations where failure would give rise to danger of different degrees. Higherrisk equipment may have active damage mechanisms or high consequences of failure, or a combination of the two. A suitable inspection scheme is then planned to increase confidence about the equipment's current and future condition,taking account of the potential damage mechanisms and the reliability of the inspection techniques used.
Risk based inspection may be applied in any industry, but there has been most interest from the power and petrochemical sectors. The UK Health and Safety Executive has published a report on best practice for risk based inspection asa part of plant integrity management.[1] In 2000, the European Commission launched the project RIMAP to develop risk based inspection and maintenance procedures for European Industry.[2] The American Petroleum Institute has published guidance for risk based inspection relevant to refineries.[3]
Industry sees RBI as a means of using inspection resources more effectively which can result in economic benefits from extended run lengths or from the use of advanced NDT or non-invasive schemes. Regulatory pressure will ensurethat the process of RBI is carried out rigorously so that non-prescriptive inspection decisions are based on adequate information and expertise. For some equipment with very high failure consequences, an approach to assuring safetybased only on RBI may not be tenable.
Further information
If you are a TWI Industrial Member and would like further information about RBI, please email TWI's Structural Integrity Department.
References
1. www.hse.gov.uk/research/crr_htm/2001/crr01363.htm
2. www.mpa-lifetech.de/RIMAP
3. API Recommended Practice 580 and Base Resource Document 581
