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Guidelines for Enabling Conditions and Conditional Modifiers in Layer of Protection Analysis Ed. by CCPS

  • Erscheinungsdatum: 25.11.2013
  • Verlag: Wiley-AIChE
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Guidelines for Enabling Conditions and Conditional Modifiers in Layer of Protection Analysis

The initial Layer of protection analysis (LOPA) book published in 2001 set the rules and approaches for using LOPA as an intermediate method between purely qualitative hazards evaluation/analysis and more quantitative analysis methods. Basic LOPA provides an order-of-magnitude risk estimate of risk with fairly reproducible results. LOPA results are considered critical in determining safety integrity level for design of safety instrumented systems. This guideline clarifies key concepts and reinforces the limitations and the requirements of LOPA. The main scope of the guideline is to provide examples of CMs and ECs and to provide concrete guidance on the protocols that must be followed to use these concepts. The book presents a brief overview of Layer of Protection Analysis (LOPA) and its variations, and summarizes terminology used for evaluating scenarios in the context of a typical incident sequence. It defines and illustrates the most common types of ECs and CMs and shows how they interrelate to risk criteria as well as their application to other methods. Since 1985, the Center for Chemical Process Safety (CCPS) has been the world leader in developing and disseminating information on process safety management and technology. CCPS, an industry technology alliance of the American Institute of Chemical Engineers (AIChE), has published over 80 books in its process safety guidelines and process safety concepts series, and over 100 training modules through its Safety in Chemical Engineering Education (SACHE) series.


    Format: ePUB
    Kopierschutz: AdobeDRM
    Seitenzahl: 136
    Erscheinungsdatum: 25.11.2013
    Sprache: Englisch
    ISBN: 9781118777947
    Verlag: Wiley-AIChE
    Größe: 2275 kBytes
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Guidelines for Enabling Conditions and Conditional Modifiers in Layer of Protection Analysis

Chapter 1


The Guidelines in this book characterize when and how to apply enabling conditions and conditional modifiers to Layer of Protection Analyses (LOPAs). A LOPA may have consequences and risk criteria expressed in final endpoint (impact) terms such as fatalities or environmental damage, and include conditional modifiers such as probability of fatality associated with a material or energy release. It may also take into account probabilities called enabling conditions that sometimes apply to scenario initiating events. One way to differentiate these two factors is that enabling conditions are associated with the part of an incident sequence leading up to a release of hazardous material or energy, whereas conditional modifiers are probabilities generally associated with the post-release part of an incident sequence.

As discussed in Chapter 4, these enabling conditions and conditional modifiers may be used with other scenario risk analysis methods such as Quantitative Risk Analyses and HAZOP/LOPA Studies. However, the main focus in this text will be their use in the context of LOPAs.

An overview of Layer of Protection Analysis is given in Section 1.1, followed by some pertinent variations in Section 1.2. Section 1.3 discusses the appropriate application of enabling conditions and conditional modifiers. These decisions on use of conditional modifiers are closely related to a company's risk criteria endpoints used when determining loss event impacts, as discussed in Section 1.4.
1.1 LOPA Overview

This section, taken in large part from Center for Chemical Process Safety's Guidelines for Hazard Evaluation Procedures, Third Edition (CCPS 2008) 1 , provides a brief summary of the methodology for conducting Layer of Protection Analyses as described in the CCPS Concept Book Layer of Protection Analysis: Simplified Process Risk Assessment (CCPS 2001), with minor updates. Experienced LOPA practitioners and users may want to advance to Section 1.2, Pertinent LOPA Variations.

Layer of Protection Analysis is a simplified form of quantitative risk analysis that uses order-of-magnitude categories for initiating event frequency, consequence severity and probability of failure of independent protection layers (IPLs) to analyze and assess the risk of one or more incident scenarios. LOPA can be useful in the process development, process design, operational, maintenance, modification and decommissioning life cycle phases.
Purpose of LOPA

LOPA was developed to help answer questions such as:

What layers of protection are needed to meet our risk goals?
How much risk reduction does each layer provide or need to provide?
LOPA can help answer these questions with less time and effort than a full quantitative risk analysis (QRA), although there are instances when use of a complete QRA may be warranted.

LOPA is an order-of-magnitude type of quantitative method (sometimes termed "semi-quantitative") that builds on qualitative hazard evaluations such as HAZOP Studies. By analyzing selected scenarios in detail, effective application of LOPA can determine whether the risk posed by each analyzed scenario has been reduced to meet a specified risk goal. However, if the analyst or team can make a reasonable risk decision using only qualitative methods, then LOPA may not be warranted. Qualitative hazard evaluation methods such as HAZOP Studies are intended to identify a comprehensive set of incident scenarios and qualitatively analyze those scenarios for the adequacy of safeguards. LOPA is generally used to analyze a subset of incident scenarios.

At times, simple order-of-magnitude LOPAs can be expanded with greater rigor by implementing aspects of a more complete QRA. The use of en

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