TWI Industrial Member Report Summary 722/2001
Can we afford to overlook risk-based inspection of high integrity structures?
John Wintle believes the answer is a resounding 'no' ...
Several industries operating high integrity structures and equipment with safety or financial dependence are considering planning in-service inspection on the basis of the information gained from an analysis of the risk of failure. Risk based inspection (RBI) is a structured approach to planning inspection, but many mechanical engineers responsible for the integrity of industrial plant currently only have a limited knowledge of what it entails. The objective of this report is to help TWI members from all industries and regulatory bodies understand the principles of risk based inspection and identify the essential elements required for its implementation.
The risk of failure is the combination of the probability and consequences of its occurrence. Inspection provides more information about the risk of failure caused by structural deficiencies and the report discusses the effect that inspection can have on the risk. The fundamentals of industrial risk analysis are summarised, and the key elements of the process of risk based inspection identified.
Risk based inspection uses an analysis of the risk of failure for the development of the inspection plan. The risk analysis identifies the credible types and causes of structural failure and assesses the rate of degradation in relation to future fitness-for-service. The report highlights the benefits from using a team and the value that TWI's experts can bring to the process.
The report describes the qualitative and quantitative approaches to risk analysis that are being developed for implementation of RBI, and highlights some of the difficulties that may limit its applicability. The process of RBI is illustrated by a case study using the approach within the TWI software RISKWISE TM. Appendices to the report describe the background and approaches to RBI taken by the American Petroleum Institute's Base Resource Document for RBI (API 581). Although intended for application at oil refineries, API 581 illustrates how risk based inspection is being addressed by a particular industry.
RBI requires the application of risk based principles to the development of the inspection plan. The information generated by the risk analysis is used to formulate what, where, when and how to inspect. Feedback from inspection into the risk analysis and future planning is highlighted as an essential part of the process.
The report discusses the development of RBI from the research by the American Society of Mechanical Engineers (ASME) on risk based methods in the late 1980s and subsequently by sectors of US industry. Interest by industry in the UK and Europe is growing, and regulators are responding to developments to make sure that safety is not compromised. Guidelines are being developed to ensure consistent standards for risk based inspection, but there is a need for comparative studies of the different approaches to identify best and common practice.
The review concludes that when properly implemented, RBI is a good process for developing an optimised scheme of inspection. RBI has the potential to assist many industries to understand and manage the risks of failure better. In some cases this may be a means to reduce or re-target plant inspections, but where there is uncertainty, RBI may indicate a need for increasing inspection.