International Series in Industrial and Systems Engineering, Prentice Hall, Englewood Cliffs, NJ Carter CL 1978 The control and assurance of quality, reliability and safety.. Safety and R
Trang 168 Consider a steady-state solution to the availability Petri net model.
69 Explain complex systems theory
70 Discuss systems engineering and complex systems theory
71 Consider the application and significance of systems engineering in engineering design
72 Briefly discuss complexity in engineering design and its significance in systems engineering
73 Give a brief account of the functions of systems engineering analysis
74 Describe reliability block diagrams (RBDs) and availability block diagrams (ABDs), and indicate their fundamental differences
75 Consider effectiveness measures in systems engineering and their significance
in engineering design
76 Give a brief account of evaluating complexity in engineering design
77 Define complexity in systems design
78 Describe various system state definitions and evaluating complexity of the dif-ferent state definitions
79 Define complicatedness in systems design
80 Describe complexity in systems and complicatedness as a function of complex-ity in designing for complex but uncomplicated systems
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Trang 6Safety and Risk in Engineering Design
Abstract In this chapter, the introduction of new or modified systems into an
engi-neering process is considered, whereby safety with respect to risk and loss through accidents or incidents resulting from the complex integration of systems is pre-dicted, assessed and evaluated, to ensure that the design will have as minimum
a risk as is reasonably practicable Risk relates to a combination of the likelihood
of occurring hazards, and to the severity of their outcome or consequence Safety
in engineering design begins with identifying possible hazards that could occur, as well as the corresponding system states that could lead to an accident or incident
in the designed system This is determined through hazards analysis The initial
hazards analysis should begin at the earliest concept formation stages of systems design, and the information should be used to guide the emerging design with re-spect to safety requirements throughout the engineering design process Safety in
engineering design normally includes a causal analysis, which involves
identify-ing various cause-effect sequences of hazardous events that may combine to cause
the identified hazards Thereafter, a consequence analysis identifies the sequences
of events that could lead from a hazard to an accident or incident Working through
these phases of hazards and safety analysis, and iterating where appropriate, a safety case is prepared that relates to the assurance that the system is relatively safe
Haz-ards and safety analyses provide a comprehensive methodology for designing for safety Designing for safety includes risk reduction measures and involves conduct-ing risk mitigation strategies to, first, reduce the likelihood that a hazard could result
in an accident or incident and, second, to aim at reducing the severity of the likely event Because designing for safety strives for a significant level of confidence in the results of these strategies, and the need for an objective systems scrutiny from
a safety viewpoint, it typically involves systematic safety analysis with independent safety prediction, safety assessment, and safety evaluation during the schematic, preliminary and detail design phases respectively of the overall engineering design process
R.F Stapelberg, Handbook of Reliability, Availability, 529
Maintainability and Safety in Engineering Design, c Springer 2009
Trang 75.1 Introduction
The previous two chapters dealt with an analysis of engineering design that con-sidered prediction, assessment and evaluation of systems reliability and functional performance, and of systems availability and maintainability during engineering process operations In this chapter, the introduction of new or altered systems into
a complex engineering process environment is considered, whereby safety with re-spect to risk and loss through accidents or incidents resulting from the complex
integration of systems is predicted, assessed and evaluated, to ensure that the design will have as minimum a risk as is reasonably practicable Risk relates to the
combi-nation of the likelihood of occurring hazards, and to the severity of their outcome or
consequence An accident or incident may be viewed as an unintended event that re-sults in either a critical or non-critical loss, and may include events such as death or personal injury, and environmental or financial losses, according to a relative scale
of safety criticality.
Safety in engineering design starts by identifying the possible hazards of the
new system, which are system states that can lead to an accident or incident This
is typically conducted through a series of collaborative hazards analysis sessions,
during which keyword prompts and checklists are used to aid identification of haz-ardous system states Suitably qualified experts representing all the areas that are relevant to the system being designed must participate in these sessions Normally,
a causal analysis is then conducted, which involves identifying various cause-effect
sequences of hazardous events that may combine to cause the hazards already
iden-tified Thereafter, a consequence analysis is conducted, which identifies the next
sequences of events that could lead from a hazard to an accident or incident
Work-ing through these phases of analysis, and iteratWork-ing where appropriate, a safety case
is prepared, which relates to an assurance that the system is relatively safe This assurance is not a statement that the system is risk free—almost no system of any complexity can demonstrate this property Instead, risks are typically divided into three categories, and each category is treated slightly differently
The three categories of risks are the following:
• Intolerable risks:
These are risks that are not acceptable under any circumstances—for example, the hazardous exposure to process products of a system that have a high likeli-hood of affecting workers occupational safety and health The engineering design will need to include ways of removing such risks, or of drastically reducing their severity The safety case must show that no such risks remain in the system
• Tolerable risks:
These are risks that are considered acceptable provided they confer some benefit, and the risk has been reduced as much as was reasonably practicable The ‘ben-efit’ may be hard to measure objectively, especially in placing a cost value on accidents such as personal injury or death with respect to the cost of preventive measures A typical example is the consideration of tolerable risks in the case
of large construction projects of engineered installations during which accidents
Trang 8and incidents are inevitable The safety case would argue that there is a trade-off benefit of allowing certain risks at a given criticality level
• Negligible risks:
These are risks that are so small as to be insignificant, and no further precautions are considered necessary The safety case would only include negligible risks that merit attention, such as those previously considered to be relatively significant risks
Designing for safety entails definitive risk reduction measures and involves
conduct-ing or specifyconduct-ing mitigation strategies to, first, reduce the likelihood that a hazard will result in an accident or incident and, second, to aim at reducing the severity
of the likely event Because designing for safety strives for confidence in the re-sults of these strategies, and the need for an objective systems scrutiny from a safety viewpoint, it typically involves systematic safety analysis, with independent safety prediction, safety assessment, and safety evaluation audits dovetailing with the re-spective schematic, preliminary and detail design phases of the overall engineering design process Designing for safety tends to be both costly and time consuming because of the number of domain and other experts needed to determine those areas
of high safety risk in the total integrated engineering design, the wide range of fac-tors that need to be considered, and the implementation of additional safety control systems
Techniques that are to be added into this work must therefore be cost and time effective, whilst fitting within existing as well as new methodologies in determining the integrity of engineering design
Hazards and safety analyses provide a comprehensive methodology for design-ing for safety The initial hazards analysis should begin at the earliest concept
for-mation stages of systems design, and the inforfor-mation should be used to guide the emerging design with respect to safety requirements throughout the engineering de-sign process Later equipment hazards analysis information is used to evaluate the
integrity of the design and to make trade-off decisions The development of a safety intent specification supports both the evolution of systems design as well as system
safety analysis The design rationale for safety issues that are normally lost during the design’s development stages is preserved in a single, logically structured docu-ment (or electronic database) that is based upon fundadocu-mental principles of human problem solving Safety-related requirements and design constraints are traced from the highest systems levels, down through system design to component design and into hardware schematics and detail design specifications An important feature of the safety intent specification is that it integrates formal and informal design speci-fications
It is thus during the design stage of an engineering project when major improve-ments in safety and occupational health relating to construction, ramp-up and op-eration of an engineered installation can be achieved However, there are real chal-lenges involved in designing for safety in order to achieve the required step change
in a safe and healthy environment in the construction and operation of industrial process plant and facilities To date, there have been many factors that have limited improvements in this area, such as a lack of time and funding—besides the lack
Trang 9of communication, understanding and commitment The culture of a segmented en-gineering construction industry with its fragmented processes, along with the fact that many project clients are reticent in fully appreciating the significant added costs
of designing for safety, must be critically addressed in order to break through into
a new arena of safe working practices and performance In appreciation of the chal-lenges involved in designing for safety with the construction and operation of en-gineered installations, an agenda for change was developed at a major international conference on Designing for Safe and Healthy Construction, organised by the Euro-pean Construction Institute (ECI) and the Conseil International du Bâtiment (CIB)
in London in June 2000
These changes—in particular with respect to changes required of process engi-neering designs—included the following (ECI 2001):
• Recognising the fact that engineering designs will dictate, to a considerable
de-gree, the nature and extent of hazards that will pose a threat to worker safety and health, not only during construction but throughout the life cycle of the project
• Concentrating on significant complex risks that competent contractors would not
be expected to be aware of, rather than on easily identified residual risks
• Achieving better risk identification methods.
• Utilising different levels of risk assessment at different stages in the project.
• Concentrating on interfaces between systems where high risks occur.
• Developing a better awareness of safe working practices and ergonomics.
• Making occupational safety and health (OSH) a top priority in the design process.
• Considering OSH implications in the earlier part of the engineering design
pro-cess, such as safety predictions during the conceptual design phase
• Recognising duty of care in considering OSH requirements in engineering
de-signs, and its impact on construction activities
• Maximising the use of innovative techniques and methodology that reduces OSH
risk, such as pre-assembly and/or off-site manufacturing, and standardisation of equipment
• Using the appropriate CAD systems to schematically examine the project during
the preliminary design phase, to determine engineering design integrity
• Using intelligent computer automated methodology for determining the integrity
of engineering design through the application of automated continual design re-views throughout the engineering design process
• Applying safety constructability reviews that contribute towards addressing
con-struction worker safety in the design
• Maintaining communication feedback and risk data to reduce unplanned
con-struction work greater than required in the design
• Designing for safe access for maintenance personnel to restricted areas, including
access for routine and preventive maintenance and for installation of replacement equipment
• Including risk analysis not only for construction, commissioning, ramp-up and
operation but also for decommissioning or deconstructing of plant and facilities
Safety engineering has also received much attention from the defence industry for
several decades, particularly the US Department of Defence The first military safety
Trang 10document titled “System Safety Engineering for the Development of United States Air Force (USAF) Ballistic Missiles” was published in 1962 In 1963, the USAF published a document titled “Safety Engineering of Systems and Associated Sub-Systems and Equipment” (MIL-STD-38130 1963) This document was superseded
in 1969 by a document titled “Requirements for Safety Engineering of Systems and Associated Sub-Systems and Equipment” (MIL-STD-882), which has subsequently been updated in 1977 STD-882A), in 1984 STD-882B), in 1993 (MIL-STD-882C) and in 2000 (MIL-STD-882D)
Additional military safety documentation covering system safety includes the
fol-lowing handbooks:
• the US Army handbook ‘System safety design guide for army materiel’
(MIL-HDBK-764 1994),
• the US Air Force Systems Command handbook ‘System safety design handbook’
(AFSC DH 1-6 1967),
• the US National Aeronautics and Space Administration (NASA) handbook
‘Sys-tem safety handbook’ (NASA DHB-S-00 1999)
In any engineered installation, human factors are an important part of process
con-trol Therefore, an effective safety program cannot consider only the automated sys-tems hierarchy but must also consider the impact of human error on the system, and the effect of systems design on errors in human judgement and control
Increased automation in complex systems has led to changes in the human con-troller’s role, and to new types of technology-induced human error Such errors abound in records of major process engineering catastrophes In a detailed survey
of safety incidents in the US nuclear power industry (INPO 84-027 1984, 1985), it was revealed that of the roughly 1,000 identified root causes of incidents that were investigated, 51% were classified as “human performance problems”, and 74% of these (i.e 38% of all root causes) were “maintenance related”, this being broadly defined to include preventive and corrective maintenance, surveillance testing and modification work
The Three Mile Island nuclear power generator accident in 1979 demonstrated the significance of human error The accident was attributed to mechanical failure
and operator error Despite the fact that about half of the reactor core melted, the
containment building that housed the reactor prevented any release of radioactivity, and the reactor’s other protection systems also functioned as designed The emer-gency core cooling system would have prevented the accident but for the interven-tion of the operators Investigainterven-tions following the accident led to a new focus on the human factors in nuclear safety No major design changes were called for in nuclear reactors but controls and instrumentation were improved and operator training was overhauled
By way of contrast, the Chernobyl reactor in the Ukraine did not have a
contain-ment structure like those used in the West or in post-1980 Soviet designs The April
1986 disaster at the Chernobyl nuclear power plant was the result of major design deficiencies in the type of reactor, the violation of operating procedures and the ab-sence of a safety culture The accident destroyed the reactor, killed 31 people, 28 of