Real Consequences
How do we ensure that protective systems can actually deliver a level of risk that is acceptable to our business? Real Consequences shows how modern risk management techniques can ensure that protective systems deliver the level of risk that their designers intended. It covers both practical issues and the mathematical background needed to analyse systems.
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Real Consequences
Preface
Please read the preface to Real Consequences before the remainder of the book. It includes important statements about the responsibility for use of the materials presented.
Section 1: Principles
1
Hidden failures, Real Consequences
This chapter draws out lessons for the maintenance and design of protective systems from high profile accidents and disasters in which they have been implicated.
2
Hidden Functions
After introducing basic hidden function concepts, this section looks at some of the subtleties of hidden and evident functions and tries to answer an apparently simple question: when is a function evident, and when is it hidden?
3
Managing Hidden Failures
Any management policy that focuses on the effects of a hidden failure is doomed, simply because there are no effects to manage. This chapter examines the factors that affect the selection of maintenance tasks for any failure, and emphasises those that are relevant for hidden failures.
4
Failure-Finding Basics
This chapter builds the foundations that you will need to apply failure-finding and other failure management policies to real equipment.
5
The Basis of Decision-Making
The frequency of a scheduled test can be driven by risk or by cost. This chapter shows how to apply these criteria to real systems, and demonstrates how simple availability targets can be misleading.
6
Tolerable Risk
Far from being fixed by design, the risk of a multiple failure can be managed by increasing the availability of the protective system, or by reducing the demand rate. Developing the concept of tolerable risk, the one-time friend ALARP (As Low As Reasonably Practicable) is put to the sword.
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7
Failure-Finding Tasks
What could go wrong with a periodic test? This chapter looks at the practical issues surrounding invasive maintenance checks, and at the unfortunate psychology of task reporting.
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Section 2: Failure-Finding Task Intervals for Simple Systems
8
Availability
The availability of a protective system is a single number that sums up its effectiveness. This chapter introduces simple availability calculations and explores the relationship between availability and failure rate.
9
Risk
This chapter derives test intervals for simple systems from the key criterion: the maximum tolerable risk of a multiple failure.
10
Economic
Some multiple failures have no safety or environmental consequences, but they have a direct monetary impact on the organisation. Find out here how to balance the risked cost of multiple failure against the cost of checking and to determine an optimum task interval.
11
Parallel and Redundant Systems
Designers use redundant systems to deliver higher availability than could be achieved by a single protective device. This chapter expands the model and enables you to calculate the availability of more complex systems.
12
Data
Developing a model of a protective system is often far easier than finding the right data. This chapter discusses data sources and offers suggestions for working with weak and uncertain information.
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13
Imperfect Testing
So far, the models that we have used assume that failure-finding tasks are perfect: a protective device is never passed as working when in reality it has failed. Most tests disturb the equipment in some way, and this chapter expands the model to deal with imperfect tests and testers.
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14
Practical Analysis Guidance
Moving from theory to practical application, this chapter discusses the problems and pitfalls encountered in trying to calculate failure-finding intervals for real systems.
15
Failure Management Options
Failure-finding is not always the right management policy for protective systems. This chapter examines other possibilities and outlines the criteria that are used to select the right option.
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16
Other Factors
This chapter is a final round-up of other factors that need to be considered as part of the analysis of protective systems.
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Section 3: Failure-Finding Task Intervals for Complex Systems
17
Multi-Level Systems
The analysis presented in the first two sections has focussed on single-level systems. This chapter expands the model further to include multi-level systems, such as protection that includes both an alarm and trip.
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18
Complex Systems
This chapter discusses the analysis of more complex systems through techniques such as fault tree analysis.
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Section 4: Advanced Topics
19
Fault-Tolerant Systems
Parallel redundancy can improve system availability, but at the expense of an increased rate of false alarms and trips. Voting systems try to strike a balance by improving the effectiveness of protective systems while at the same time providing a defence against unwanted shutdowns.
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20
Mission-Based Analysis
Manufacturing and process industries are usually concerned about the availability of a protective system, but applications in aerospace and defence sectors often express risk in terms of mission success. This chapter develops models based on the probability of mission success (or failure) rather than an average long-term availability.
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21
Common Cause and Common Mode Failure
However well designed a protective system may be, and however much care is taken over its management, common mode and common cause failures can rip through multiple layers of protection.
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22
Other Topics
This chapter discusses the relationship between failure-finding and other topics such as HAZOP, risk-based inspection (RBI) and safety integrity levels (SIL).
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23
Examples for Solution
This chapter contains a selection of examples for solution based on the material from the first three sections of the book.
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24
Solutions
Suggested solutions to the examples in the previous chapter.
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25
Mathematical Annex
Contains full mathematical derivations of the main results used in sections 1 to 3.
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26
Equation Summary and Reference
This chapter collects together all the main results from sections 1-3, with an explanation of the terms used in each equation.
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