Hazards Flashcards
What are HAZOPs?
A qualitative technique whose purpose is to identify:
1. All possible deviations from the designs expected operation
2. Hazards associated with these deviations
What is the primary concern of system safety within the safety life cycle?
The management of hazards, including their identification, evaluation, and elimination or control.
Define “hazard” in the context of system safety.
A hazard is a state of the system that could lead to an accident with potential to harm people or the environment.
What is “risk” in the safety life cycle?
A: Risk is the combination of hazard severity (likelihood of the hazard occurring) and hazard exposure (likelihood of the hazard leading to an accident).
What is the purpose of the safety life cycle?
A: To provide a framework for hazard management alongside the product or software lifecycle.
What are the six stages of the safety life cycle?
A: 1) Hazard identification, 2) Risk assessment, 3) Risk reduction, 4) Safety requirements definition, 5) Safety requirements verification, and 6) Safety case provision.
What does it mean to switch from “thinking backwards” to “thinking forwards” in hazard management?
A: It means shifting from analyzing what went wrong to anticipating what could go wrong and working to prevent or minimize it.
What are the four stages of hazard management?
A: 1) Hazard identification, 2) Hazard causal analysis, 3) Hazard resolution and control, and 4) Hazard verification.
Which techniques are used in the first stage of hazard management, hazard identification?
A: Checklists, hazard indexes, event trees, and HAZOPS.
What are key goals of hazard identification?
A: To identify hazards that could lead to accidents, assess their potential effects, and categorize them by severity.
Which techniques are commonly used in hazard causal analysis?
A: Reliability block diagrams (RBDs), failure modes and effects analysis (FMEA), and fault trees.
What is the main purpose of hazard causal analysis?
A: To evaluate causal factors of hazards and understand which accidents they could lead to.
What are the main activities in the hazard resolution and control stage?
A: Identifying control or elimination methods for hazards, setting design criteria, and implementing safety devices and procedures.
What is the goal of hazard verification?
A: To ensure that hazards have been reduced, controlled, or eliminated and to assess the impact of any changes on system safety.
Why might hazard verification require returning to the analysis phase?
A: If operational experience or proposed changes indicate new hazards, reintroduction of resolved hazards, or an increase in the severity of unresolved hazards.
Why is no single technique sufficient for hazard management?
A: Because different stages of hazard management require different tools and methods to address the complexity and variety of hazards.
What is a primary requirement for effective brainstorming in hazard analysis?
A: Assembling a team with sufficient expertise and background knowledge.
How are checklists used in hazard analysis?
A: By listing hazards or design features to systematically confirm safety measures, either through yes/no questions or open-ended questions for deeper analysis.
What are two main advantages of using checklists in hazard analysis? And Disadvantages?
Adv: They capture existing safety knowledge for reuse and can guide thinking about potential hazards in the system.
Dis: They may lead to over-reliance, can be too lengthy and hard to use, and might create a false sense of security if completed superficially.
What is an event tree, and what does it represent in hazard analysis?
An event tree maps out possible outcomes from an initiating event, showing the sequences of events that could occur and whether each component succeeds or fails.
In an event tree, what do the branches under each component indicate?
Each branch represents two possible outcomes: whether the component operates or fails.
In safety, what is failure rate (lambda)?
Of a device/component is it the number of failures in a given period. Measures as failures per unit time.
Manufacturers provide it for in the product day
In safety: what is mean time between failures MTBF (1/lambda)
1/failure rate.
The mean period of time of operation taken for 1 failure to occur.
What is reliability block diagrams RBD?
Shows which subsystems contribute to a hazard
Aims to limit analysis to necessary parts
What is reliability block diagrams RBD process?
- Construct a block diagram for the system
- Define the system failure modes
- Connect blocks identifies in step 1 into “success paths”
- Analyse RBD to identify blocks that contribute to failure modes identified in step 2
In safety: RBD (reliability block diagrams) used to model what happens when:
- Components work (reliability)
- Components fail (unreliability)
What is the system failure rate?
The sum of individual component failure rates λs
In safety: define reliability R(t)
Probability of a device functioning correctly
Over a given period t
Under a given set of operating conditions
In safety: define unreliability Q(t)
Probability of a device failing to function correctly
Over a given period of time
In safety, for evaluating RBD’s, what is the relationship between reliability and unreliability?
Q(t) = 1 - R(t)
In safety: in RBD representation, explain reliability in series.
In a series system for it to be successful all components have to be working correctly.
Reliability = product of each component’s reliability
R(t) = R1(t) * R2(t) …
In safety: in RBD representation, explain reliability in parallel.
Diagram represents success if one component works.
Reliability is calculated indirectly using Q(t) =1 - R(t)
Safety: how to calculate the reliability of a parallel system?
Multiply reliabilities
For n parallel components with reliabilities R1(t) … Rn(t)
then for the system: Q(t) = (1 - R1(t)) * (1 - R2(t)) *…(1 - Rn(t))
6 stages of safety lifecycle
IAREVS: identify, assess, reduce, establish, validate, safety case
6 stages of system lifecycle
System conception
Requirements
Design and development
Deployment
Operation and monitoring
Retirement
Safety life cycle
Hazard identification
Risk analysis
Risk reduction
System requirements development
Verification
Documentation
