Lecture 3 - Engineering Risk Assessment

Question 1

How many stages does risk assessment have?

Answer 1

Has three stages:
* Risk identification
* Risk analysis/ assessment
* Risk evaluation

Question 2

What is the basis of risk analysis?

Answer 2

• The choice of analysis method is influenced by the
• Context, Objectives, Available resources
• At a strategic level, broad categories may be identified and analyzed to provide an organizational risk profile to identify important issues for which management systems need to be established.
• At a project level, managers need to identify and prioritize the specific risks that threaten the objectives they need to achieve,
• At an equipment level, asset owners need to identify and analyze risks that threaten the ability of the equipment to fulfill its function.

Question 3

What are some approaches to risk analysis?

Answer 3

• There are three classes of approach to risk analysis
• Qualitative
• Semi-quantitative (semi-QRA)
• Quantitative (QRA)
  It is inappropriate to assume that quantitative is superior to qualitative analysis. It is most appropriate to ensure the best approach to fit the situation at hand.

Question 4

What is Qualitative analysis?

Answer 4

• Qualitative analysis is any method of analysis which uses description rather than numerical means to define a level of risk
• Used when:
• Where quantitative precision is not required
• To perform an initial screening of risks
• Where the level of risk does not justify the time or resources needed to do a numerical analysis
• Where the numerical data are not available or
  inadequate for a more quantitative analysis

Question 5

What is Semi-Qualitative analysis?

Answer 5

• In semi -quantitative analysis, numbers are used to provide a relative measure (e.g. Risk Priority Number RPN) of the likelihood and consequence of one event against others.
• Numbers should not be used to give an appearance of a level of precision which does not exist
• The measurement scales for each component need not be linear.
• Often text is used as a ‘descriptor’ for the axes ie, likelihood descriptions of ‘remote’ or ‘frequent’ but these descriptors are tied to estimated values, as shown on the following slide.

Question 6

What is Quantitative analysis?

Answer 6

• The level of risk can be calculated using a number of quantitative methods in situations where the consequences and likelihood of occurrence can be estimated numerically.
• The context and assumptions that underlie the quantitative techniques should be clearly stated and understood.

Question 7

What are the generic steps in risk assessment?

Answer 7

1. Select a specific hazardous sequence or scenario (i.e. chain of events) – this is called hazard identification.
2. Estimate the likelihood or frequency of the event or scenario, Fi. The likelihood of an event or scenario (i) may be expressed as the probability of occurrence of the event. Alternatively, a frequency per year may be used. Estimates can be generated from past events, available historical data, industry standards, and from judgements based on experts’ belief of the situation.
3. Estimate the consequence of the event or scenario, Ci. This may be assessed in the form of health, economic, environmental and/or safety impact i.e. using number of fatalities, $ loss etc.
4. The total expected risk value maybe expressed as 𝑅 = ∑𝑖 𝐹𝑖 𝐶𝑖
5. Risk matrices must be used to visualise results. It is vital to understand where each risk is in terms of high consequence, low frequency for example.
6. Estimation of uncertainty is central to interpreting risk models

Question 8

What are some methods used in likelihood estimation, risk assessment
and/or barrier identification and their acronyms? - Function based

Answer 8

• FMEA/ FMECA Failure modes & effects analysis/ Failure modes, effects & criticality analysis– starts with the item and function.
  Identifies potential failures & impacts and prioritize them on a numeric
  scale called Risk Priority Number (RPN)
  1. DFMEA-PFMEA-SFMEA Design, Process and Software FMEA
  2. A semi-QRA Used in design and for maintenance strategy
  determination.
  —-
  FSA Functional safety assessment – applies to safety instrumented systems (SIS) to determine if probability of failure meets tolerable levels. Does not consider consequence beyond the failure event.

Question 9

What are some methods used in likelihood estimation, risk assessment
and/or barrier identification and their acronyms? - Scenario based logic models

Answer 9

• FTA Fault-tree analysis – work back from top event to identify how it occurs. Uses logic symbols (AND,OR) to visualise sequences and can also be used to determine failure probabilities. Can identify common cause failures.
  1. Used in design and for troubleshooting
  —–
  ETA Event-tree analysis – work forward from event to identify subsequent events. Uses Yes and No logic. ETA is used in conjunction with FTA for quantitative risk assessment (QRA)

Question 10

What are some methods used in likelihood estimation, risk assessment
and/or barrier identification and their acronyms? - Scenario based risk identification and control processes

Answer 10

• HAZID (Hazard identification) – General risk identification tool. Uses scenarios to identify hazards and assess each for likelihood and consequence.
  1. Used in early phases of project life cycles.
  ——
• LOPA – Layer of Protection analysis – used to determine probability of failure on demand for controls of individual high consequence events. LOPA focuses on a single event with a nominated consequence to assess the adequacy of the Independent Protection Layers (IPLs)
  1. Used in design in high hazard industries.