Preface
Acknowledgements
Part One Understanding Reliability Parameters and
Costs
1 The history of reliability and safety technology 1
1.1 FAILURE DATA 1
1.2 HAZARDOUS FAILURES 4
1.3 RELIABILITY AND RISK PREDICTION 5
1.4 ACHIEVING RELIABILITY AND SAFETY-INTEGRITY 6
1.5 THE RAMS-CYCLE 7
1.6 CONTRACTUAL PRESSURES 9
2 Understanding terms and jargon
2.1 DEFINING FAILURE AND FAILURE MODES
2.2 FAILURE RATE AND MEAN TIME BETWEEN FAILURES 12
2.3 INTERRELATIONSHIPS OF TERMS 14
2.4 THE BATHTUB DISTRIBUTION 16
2.5 DOWN TIME AND REPAIR TIME 17
2.6 AVAILABILITY 20
2.7 HAZARD AND RISK-RELATED TERMS 20
2.8 CHOOSING THE APPROPRIATE PARAMETER 21
EXERCISES 22
3 A cost-effective approach to quality, reliability and safety
3.1 THE COST OF QUALITY
3.2 RELIABILITY AND COST 26
3.3 COSTS AND SAFETY 29
Part Two Interpreting Failure Rates
4 Realistic failure rates and prediction confidence
4.1 DATA ACCURACY
4.2 SOURCES OF DATA 37
4.3 DATA RANGES 41
4.4 CONFIDENCE LIMITS OF PREDICTION 44
4.5 OVERALL CONCLUSIONS 46
5 Interpreting data and demonstrating reliability
5.1 THE FOUR CASES
5.2 INFERENCE AND CONFIDENCE LEVELS
5.3 THE CHI-SQUARE TEST 49
5.4 DOUBLE-SIDED CONFIDENCE LIMITS 50
5.5 SUMMARIZING THE CHI-SQUARE TEST 51
5.6 RELIABILITY DEMONSTRATION 52
5.7 SEQUENTIAL TESTING 56
5.8 SETTING UP DEMONSTRATION TESTS 57
EXERCISES 57
6 Variable failure rates and probability plotting
6.1 THE WEIBULL DISTRIBUTION
6.2 USING THE WEIBULL METHOD 60
6.3 MORE COMPLEX CASES OF THE WEIBULL DISTRIBUTION 67
6.4 CONTINUOUS PROCESSES 68
EXERCISES 69
Part Three Predicting Reliability and Risk
7 Essential reliability theory
7.1 WHY PREDICT RAMS?
7.2 PROBABILITY THEORY
7.3 RELIABILITY OF SERIES SYSTEMS 76
7.4 REDUNDANCY RULES 77
7.5 GENERAL FEATURES OF REDUNDANCY 83
EXERCISES 86
8 Methods of modelling
8.1 BLOCK DIAGRAM AND MARKOV ANALYSIS
8.2 COMMON CAUSE (DEPENDENT) FAILURE 98
8.3 FAULT TREE ANALYSIS 103
8.4 EVENT TREE DIAGRAMS 110
9 Quantifying the reliability models
9.1 THE RELIABILITY PREDICTION METHOD
9.2 ALLOWING FOR DIAGNOSTIC INTERVALS 115
9.3 FMEA (FAILURE MODE AND EFFECT ANALYSIS) 117
9.4 HUMAN FACTORS 118
9.5 SIMULATION 123
9.6 COMPARING PREDICTIONS WITH TARGETS 126
EXERCISES 127
10 Risk assessment (QRA)
10.1 FREQUENCY AND CONSEQUENCE
10.2 PERCEPTION OF RISK AND ALARP 129
10.3 HAZARD IDENTIFICATION 130
10.4 FACTORS TO QUANTIFY 135
Part Four Achieving Reliability and Maintainability
11 Design and assurance techniques
11.1 SPECIFYING AND ALLOCATING THE REQUIREMENT
11.2 STRESS ANALYSIS 145
11.3 ENVIRONMENTAL STRESS PROTECTION 148
11.4 FAILURE MECHANISMS 148
11.5 COMPLEXITY AND PARTS 150
11.6 BURN-IN AND SCREENING 153
11.7 MAINTENANCE STRATEGIES 154
12 Design review and test
12.1 REVIEW TECHNIQUES
12.2 CATEGORIES OF TESTING 156
12.3 RELIABILITY GROWTH MODELLING 160
EXERCISES 163
13 Field data collection and feedback
13.1 REASONS FOR DATA COLLECTION
13.2 INFORMATION AND DIFFICULTIES
13.3 TIMES TO FAILURE 165
13.4 SPREADSHEETS AND DATABASES 166
13.5 BEST PRACTICE AND RECOMMENDATIONS 168
13.6 ANALYSIS AND PRESENTATION OF RESULTS 169
13.7 EXAMPLES OF FAILURE REPORT FORMS 170
14 Factors influencing down time
14.1 KEY DESIGN AREAS
14.2 MAINTENANCE STRATEGIES AND HANDBOOKS 180
15 Predicting and demonstrating repair times
15.1 PREDICTION METHODS
15.2 DEMONSTRATION PLANS 201
16 Quantified reliability centred maintenance
16.1 WHAT IS QRCM?
16.2 THE QRCM DECISION PROCESS 206
16.3 OPTIMUM REPLACEMENT (DISCARD) 207
16.4 OPTIMUM SPARES 209
16.4 OPTIMUM PROOF-TEST 210
16.6 CONDITION MONITORING 211
17 Software quality/reliability
17.1 PROGRAMMABLE DEVICES
17.2 SOFTWARE FAILURES 214
17.3 SOFTWARE FAILURE MODELLING 215
17.4 SOFTWARE QUALITY ASSURANCE 217
17.5 MODERN/FORMAL METHODS 223
17.6 SOFTWARE CHECKLISTS 226
Part Five Legal, Management and Safety
Considerations
18 Project management
18.1 SETTING OBJECTIVES AND SPECIFICATIONS
18.2 PLANNING, FEASIBILITY AND ALLOCATION 234
18.3 PROGRAMME ACTIVITIES 234
18.4 RESPONSIBILITIES 237
18.5 STANDARDS AND GUIDANCE DOCUMENTS 237
19 Contract clauses and their pitfalls
19.1 ESSENTIAL AREAS
19.2 OTHER AREAS 241
19.3 PITFALLS 242
19.4 PENALTIES 244
19.5 SUBCONTRACTED RELIABILITY ASSESSMENTS 246
19.6 EXAMPLE 247
20 Product liability and safety legislation
20.1 THE GENERAL SITUATION
20.2 STRICT LIABILITY 249
20.3 THE CONSUMER PROTECTION ACT 1987 250
20.4 HEALTH AND SAFETY AT WORK ACT 1974 251
20.5 INSURANCE AND PRODUCT RECALL 252
21 Major incident legislation
21.1 HISTORY OF MAJOR INCIDENTS
21.2 DEVELOPMENT OF MAJOR INCIDENT LEGISLATION 255
21.3 CIMAH SAFETY REPORTS 256
21.4 OFFSHORE SAFETY CASES 259
21.5 PROBLEM AREAS 261
21.6 THE COMAH DIRECTIVE (1999) 262
22 Integrity of safety-related systems
22.1 SAFETY-RELATED OR SAFETY-CRITICAL?
22.2 SAFETY-INTEGRITY LEVELS (SILs) 264
22.3 PROGRAMMABLE ELECTRONIC SYSTEMS (PESs) 266
22.4 CURRENT GUIDANCE 268
22.5 ACCREDITATION AND CONFORMITY OF ASSESSMENT 272
23 A case study: The Datamet Project
23.1 INTRODUCTION
23.2 THE DATAMET CONCEPT
23.3 FORMATION OF THE PROJECT GROUP 277
23.4 RELIABILITY REQUIREMENTS 278
23.5 FIRST DESIGN REVIEW 279
23.6 DESIGN AND DEVELOPMENT 281
23.7 SYNDICATE STUDY 282
23.8 HINTS 282
Appendix 1 Glossary
A1 TERMS RELATED TO FAILURE
A2 RELIABILITY TERMS 285
A3 MAINTAINABILITY TERMS 286
A4 TERMS ASSOCIATED WITH SOFTWARE 287
A5 TERMS RELATED TO SAFETY 289
A6 MISCELLANEOUS TERMS 290
Appendix 2 Percentage points of the Chi- square
distribution
Appendix 3 Microelectronics failure rates
Appendix 4 General failure rates
Appendix 5 Failure mode percentages
Appendix 6 Human error rates
Appendix 7 Fatality rates
Appendix 8 Answers to exercises
Appendix 9 Bibliography
BOOKS
OTHER PUBLICATIONS
STANDARDS AND GUIDELINES
JOURNALS
Appendix 10 Scoring criteria for BETAPLUS
common cause model
1 CHECKLIST AND SCORING FOR EQUIPMENT
CONTAINING PROGRAMMABLE ELECTRONICS
2 CHECKLIST AND SCORING FOR
NON-PROGRAMMABLE EQUIPMENT
Appendix 11 Example of HAZOP
EQUIPMENT DETAILS
HAZOP WORKSHEETS
POTENTIAL CONSEQUENCES
Appendix 12 HAZID checklist
Index
Preface
After three editions Reliability, Maintainability in Perspective became Reliability, Main-
tainability and Risk and has now, after just 20 years, reached its 6th edition. In such a fast
moving subject, the time has come, yet again, to expand and update the material particularly
with the results of my recent studies into common cause failure and into the correlation between
predicted and achieved field reliability.
The techniques which are explained apply to both reliability and safety engineering and are
also applied to optimizing maintenance strategies. The collection of techniques concerned with
reliability, availability, maintainability and safety are often referred to as RAMS.
A single defect can easily cost £100 in diagnosis and repair if it is detected early in production
whereas the same defect in the field may well cost £1000 to rectify. If it transpires that the failure
is a design fault then the cost of redesign, documentation and retest may well be in tens or even
hundreds of thousands of pounds. This book emphasizes the importance of using reliability
techniques to discover and remove potential failures early in the design cycle. Compared with
such losses the cost of these activities is easily justified.
It is the combination of reliability and maintainability which dictates the proportion of time
that any item is available for use or, for that matter, is operating in a safe state. The key
parameters are failure rate and down time, both of which determine the failure costs. As a result,
techniques for optimizing maintenance intervals and spares holdings have become popular since
they lead to major cost savings.
‘RAMS’ clauses in contracts, and in invitations to tender, are now commonplace. In defence,
telecommunications, oil and gas, and aerospace these requirements have been specified for
many years. More recently the transport, medical and consumer industries have followed suit.
Furthermore, recent legislation in the liability and safety areas provides further motivation for
this type of assessment. Much of the activity in this area is the result of European standards and
these are described where relevant.
Software tools have been in use for RAMS assessments for many years and only the simplest
of calculations are performed manually. This sixth edition mentions a number of such packages.
Not only are computers of use in carrying out reliability analysis but are, themselves, the subject
of concern. The application of programmable devices in control equipment, and in particular
safety-related equipment, has widened dramatically since the mid-1980s. The reliability/quality
of the software and the ways in which it could cause failures and hazards is of considerable
interest. Chapters 17 and 22 cover this area.
Quantifying the predicted RAMS, although important in pinpointing areas for redesign,
does not of itself create more reliable, safer or more easily repaired equipment. Too often, the
author has to discourage efforts to refine the ‘accuracy’ of a reliability prediction when an
order of magnitude assessment would have been adequate. In any engineering discipline the
ability to recognize the degree of accuracy required is of the essence. It happens that RAMS
parameters are of wide tolerance and thus judgements must be made on the basis of one- or,
at best, two-figure accuracy. Benefit is only obtained from the judgement and subsequent
follow-up action, not from refining the calculation.
A feature of the last four editions has been the data ranges in Appendices 3 and 4. These were
current for the fourth edition but the full ‘up to date’ database is available in FARADIP.THREE
(see last 4 pages of the book).
DJS
xii Preface
Acknowledgements
I would particularly like to thank the following friends and colleagues for their help and
encouragement:
Peter Joyce for his considerable help with the section on Markov modelling;
‘Sam’ Samuel for his very thorough comments and assistance on a number of chapters.
I would also like to thank:
The British Standards Institution for permission to reproduce the lightning map of the UK
from BS 6651;
The Institution of Gas Engineers for permission to make use of examples from their guidance
document (SR/24, Risk Assessment Techniques).
ITT Europe for permission to reproduce their failure report form and the US Department of
Defense for permission to quote from MIL Handbooks.
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