The objective of this methodology is to describe a method by which a risk based inspection programs could be established. The document outlines methods for evaluating probability and consequence of failure, performing an assessment of the risk level, and concluding on the appropriate actions that can be taken to manage that risk at a level acceptable for Client.
The reasons for selecting a risk based approach for inspection programs are:
- To focus inspection efforts on items where the safety, economic or environmental risks are identified as being high, whilst correspondingly reducing the effort applied to low risk systems.
- To ensure that the overall installation risk does not exceed Client’s risk acceptance criteria.
- To identify the optimal inspection and monitoring methods according to the identified degradation mechanisms.
The risk based inspection assessment should assess all relevant degradation mechanisms; however client should make themselves aware of any special circumstances that are relevant to any individual facility or installation.
Risk based inspection methods and applications are described in a number of documents published by Standards Australia (AS/NZS), American Petroleum Institute (API), American Society of Mechanical Engineers (ASME) and Det Norske Veritas (DNV).
There are also a number of design codes that cover inspection and operations of pressurized piping, vessels and heat exchangers, where these should be referenced as required. Other codes and practices have also been developed in relation to fitness for service and remaining life assessment such as the API 679 code. These codes and practices may be used to justify continued service even when damage is found during inspection.
The risk based inspection methodology described in this document is appropriate where sufficient in-service knowledge has been gained to be confident that the facility or installation is within the region of the ‘bath tub’ curve. That is, it excludes design verification and the infant/mortality phase following commissioning.
This risk based inspection methodology does not cover damage modes which would normally be expected to be detected by routine plant walk-around such as atmospheric corrosion of carbon steel. It does not cover damage modes which are initiated by a specific significant event as for example structural overload by a cyclone or liquid metal embrittlement following a fire.
The probability of other failures is not expected to be affected or mitigated by inspection, and as such, it should be addressed through the Reliability Centered Maintenance (RCM) approach.