Layers of protection analysis

Layer of protection analysis (LOPA) is a simplified form of event tree analysis. Instead of analyzing all accident scenarios, LOPA selects a few specific scenarios as representative, or boundary, cases. LOPA uses order-of-magnitLide estimates, rather than specific data, for the frequency of initiating events and for the probability the various layers of protection will fail on demand. In many cases, the simplified results of a LOPA provide sufficient input for deciding whether additional protection is necessary to reduce the likelihood of a given accident type. LOPAs typically require only a small fraction of the effort required for detailed event tree or fault tree analysis. 

Frequency Phase 3 Use Branch Point Estimates to Develop a Ere-quency Estimate for the Accident Scenarios. The analysis team may choose to assign frequency values for initiating events and probability values for the branch points of the event trees without drawing fault tree models. These estimates are based on discussions with operating personnel, review of industrial equipment failure databases, and review of human reliability studies. This allows the team to provide initial estimates of scenario frequency and avoids the effort of the detailed analysis (Frequency Phase 4). In many cases, characterizing a few dominant accident scenarios in a layer of protection analysis will provide adequate frequency information. 

Using Layer of Protection Analysis for Estimating Chemical Process Risk (Final Draft), American Institute of Chemical Engineers, New York, NY, 2000. 

Risk is the product of the probability of a release, thepjpbability of exposure, and the consequences of the exposure. Risk is usually described graphically, as shown in Figure 11-15. All companies decide their levels of acceptable risk and unacceptable risk. The actual risk of a process or plant is usually determined using quantitative risk analysis (QRA) or a layer of protection analysis (LOPA). Other methods are sometimes used however, ORA and LOPA are the methods that are most commonly used. In both methods the frequency of the release is determined using a combination of event trees, fault trees, or an appropriate adaptation. 

CCPS, Layer of Protection Analysis Simplified Process Risk Assessment, D. A. Crowl, ed. (New York Center for Chemical Process Safety, AICHE, 2001) (in press). 

Layer-of-protection analysis (LOPA) A method, based on event tree analysis, of evaluating the effectiveness of independent protection layers in reducing the likelihood or severity of an undesired event. 

Layers-of-protection analysis (LOPA) is a semiquantitative methodology for analyzing and assessing risk. It is typically applied after a qualitative hazards analysis has been completed, which provides the LOPA team with a listing of hazard scenarios with associated safeguards for consideration. LOPA uses simplified methods to characterize the process risk based on the frequency of occurrence and consequence severity of potential hazard scenarios. The process risk is compared to the owner/operator risk criteria. When the process risk exceeds the risk criteria, protection layers are identified that reduce the process risk to the risk criteria. 

General References Guidelines for Hazard Evaluation Procedures, Second Edition with Worked Examples, American Institute of Chemical Engineers, New York, 1992 Layer of Protection Analysis A Simplified Risk Assessment Approach, American Institute of Chemical Engineers, New York, 2001 ISA TR84.00.02, Safety Instrumented Functions (SIF)—Safety Integrity Level (SIL) Evaluation Techniques, Instrumentation, Systems, and Automation Society, N.C., 2002. 

In 1993, the Center for Chemical Process Safety (CCPS) published Guidelines for Safe Automation of Chemical Processes (referred to henceforth as Safe Automation). Safe Automation provides guidelines for the application of automation systems used to control and shut down chemical and petrochemical processes. The popularity of one of the hazard and risk analysis methods presented in Safe Automation led to the publication of the 2001 Concept Series book from CCPS, Layer of Protection Analysis A Simplified Risk Assessment Approach. This method builds upon traditional process hazards analysis techniques. It uses a semiquantitative approach to define the required performance for each identified protective system. 

Local Emergency Planning Committee Layer of Protection Analysis Methyl Isocyanate Minimum Ignition Energy Management of Change Material Safety Data Sheet Not Applicable... 

Many methods have been developed that are suitable for assessing risks associated with the operation of facilities involving chemical reactivity hazards. The more commonly used methods are summarized in Table 4.9. They differ in their applicability, level of effort, and how systematic they are in identifying accident scenarios. All of the methods except layer of protection analysis (LOPA) may be applied qualitatively, and all except checklist reviews may be performed in at least a semiquantitative manner. CCPS (1992a) is a basic source of information on each of these methods. 

Fault tree analysis (FTA) and event tree analysis (ETA) are the methods most commonly applied quantitatively. Since they only address the likelihood of undesired events, these methods are often combined with consequence severity calculations in a quantitative risk analysis, as described by CCPS (1999b). Layer of protection analysis (LOPA) uses a semiquantitative, order-of-magnitude approach. It is documented with worked examples in CCPS (2001b). 

Layer of Protection Analysis (LOPA) Scenario- based Order-of- magnitude By preidentified scenario Processes likely to require independent protection layers, such as safety instrumented systems, to meet predefined risk criteria Dependent on comprehensiveness of scenario list identified by other method(s) Higher... 

Once the precautions that are needed to control exposures are determined, the next step is to compare the level of controls needed against what controls are already in place. There frequently are multiple control strategies that will minimize the potential for exposure. The determination of which control strategy is the best to use is determined by the hierarchy of control principle. Our particular approach to managing risk is to use this hierarchy to evaluate possible risk management approaches for a specific project in conjunction with a modified layer of protection analysis (LOPA) approach.5... 

The information included in this chapter is based on contributions over several years from many professionals in Pfizer s Occupational Hygiene Community of Practice, as well as from the professionals in Occupational Toxicology. In addition, we would like to acknowledge the contributions of Jorge Marzari, PhD, CSP in the application of the Layer of Protection Analysis approach to our risk assessment process. Much of the data used for the case studies was generated by Mike Bums and Jeff Kaminski of OccuHealth, Inc. Thanks to Flow Sciences Inc. for the photograph (Fig. 16.2) used in this chapter to illustrate experiences. 

Center for Chemical Process Safety. 2001. Layer of protection analysis—Simplified process risk assessment. New York American Institute of Chemical Engineers. 

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