Hazards

Question 1

What are HAZOPs?

Answer 1

A qualitative technique whose purpose is to identify:
1. All possible deviations from the designs expected operation
2. Hazards associated with these deviations

Question 2

What is the primary concern of system safety within the safety life cycle?

Answer 2

The management of hazards, including their identification, evaluation, and elimination or control.

Question 3

Define “hazard” in the context of system safety.

Answer 3

A hazard is a state of the system that could lead to an accident with potential to harm people or the environment.

Question 4

What is “risk” in the safety life cycle?

Answer 4

A: Risk is the combination of hazard severity (likelihood of the hazard occurring) and hazard exposure (likelihood of the hazard leading to an accident).

Question 5

What is the purpose of the safety life cycle?

Answer 5

A: To provide a framework for hazard management alongside the product or software lifecycle.

Question 6

What are the six stages of the safety life cycle?

Answer 6

A: 1) Hazard identification, 2) Risk assessment, 3) Risk reduction, 4) Safety requirements definition, 5) Safety requirements verification, and 6) Safety case provision.

Question 7

What does it mean to switch from “thinking backwards” to “thinking forwards” in hazard management?

Answer 7

A: It means shifting from analyzing what went wrong to anticipating what could go wrong and working to prevent or minimize it.

Question 8

What are the four stages of hazard management?

Answer 8

A: 1) Hazard identification, 2) Hazard causal analysis, 3) Hazard resolution and control, and 4) Hazard verification.

Question 9

Which techniques are used in the first stage of hazard management, hazard identification?

Answer 9

A: Checklists, hazard indexes, event trees, and HAZOPS.

Question 10

What are key goals of hazard identification?

Answer 10

A: To identify hazards that could lead to accidents, assess their potential effects, and categorize them by severity.

Question 11

Which techniques are commonly used in hazard causal analysis?

Answer 11

A: Reliability block diagrams (RBDs), failure modes and effects analysis (FMEA), and fault trees.

Question 12

What is the main purpose of hazard causal analysis?

Answer 12

A: To evaluate causal factors of hazards and understand which accidents they could lead to.

Question 13

What are the main activities in the hazard resolution and control stage?

Answer 13

A: Identifying control or elimination methods for hazards, setting design criteria, and implementing safety devices and procedures.

Question 14

What is the goal of hazard verification?

Answer 14

A: To ensure that hazards have been reduced, controlled, or eliminated and to assess the impact of any changes on system safety.

Question 15

Why might hazard verification require returning to the analysis phase?

Answer 15

A: If operational experience or proposed changes indicate new hazards, reintroduction of resolved hazards, or an increase in the severity of unresolved hazards.

Question 16

Why is no single technique sufficient for hazard management?

Answer 16

A: Because different stages of hazard management require different tools and methods to address the complexity and variety of hazards.

Question 17

What is a primary requirement for effective brainstorming in hazard analysis?

Answer 17

A: Assembling a team with sufficient expertise and background knowledge.

Question 18

How are checklists used in hazard analysis?

Answer 18

A: By listing hazards or design features to systematically confirm safety measures, either through yes/no questions or open-ended questions for deeper analysis.

Question 19

What are two main advantages of using checklists in hazard analysis? And Disadvantages?

Answer 19

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.

Question 20

What is an event tree, and what does it represent in hazard analysis?

Answer 20

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.

Question 21

In an event tree, what do the branches under each component indicate?

Answer 21

Each branch represents two possible outcomes: whether the component operates or fails.

Question 22

In safety, what is failure rate (lambda)?

Answer 22

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

Question 23

In safety: what is mean time between failures MTBF (1/lambda)

Answer 23

1/failure rate.

The mean period of time of operation taken for 1 failure to occur.

Question 24

What is reliability block diagrams RBD?

Answer 24

Shows which subsystems contribute to a hazard

Aims to limit analysis to necessary parts

Question 25

What is reliability block diagrams RBD process?

Answer 25

1. Construct a block diagram for the system
2. Define the system failure modes
3. Connect blocks identifies in step 1 into “success paths”
4. Analyse RBD to identify blocks that contribute to failure modes identified in step 2

Question 26

In safety: RBD (reliability block diagrams) used to model what happens when:

Answer 26

1. Components work (reliability)
2. Components fail (unreliability)

Question 27

What is the system failure rate?

Answer 27

The sum of individual component failure rates λs

Question 28

In safety: define reliability R(t)

Answer 28

Probability of a device functioning correctly
Over a given period t
Under a given set of operating conditions

Question 29

In safety: define unreliability Q(t)

Answer 29

Probability of a device failing to function correctly
Over a given period of time

Question 30

In safety, for evaluating RBD’s, what is the relationship between reliability and unreliability?

Answer 30

Q(t) = 1 - R(t)

Question 31

In safety: in RBD representation, explain reliability in series.

Answer 31

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) …

Question 32

In safety: in RBD representation, explain reliability in parallel.

Answer 32

Diagram represents success if one component works.

Reliability is calculated indirectly using Q(t) =1 - R(t)

Question 33

Safety: how to calculate the reliability of a parallel system?

Answer 33

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))