Training 35 13:30

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Fundamentals of Risk Assessment Risk,
Reliability and Safety Engineering Chuck Goode,
CSP
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Objectives

• The objectives of the presentation are to
  discuss
• Risk
• The elements of a risk assessment
• Hazard analysis techniques
• Consequence analysis including effects
• Frequency analysis and data requirements
• How risk is developed and displayed
• Mitigation, cost-benefit and sensitivity analysis

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Section 1Introduction to Risk
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Risk Concepts
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Risk

• Risk equals frequency (F) times consequence (C),
  often expressed as
• R F x C

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Risk How People See it
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100 Safe?

• Absence of risk is hardly possible and very
  expensive

Cost of safeguards
Cost of accidents
Degree of Safety
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Section 2Risk Assessment Process
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Risk Assessment

• Process of
• Evaluating the consequences
• Determining the likelihood
• Calculating the risk

Risk Assessment is NOT a decision making tool
Risk Management IS a decision making tool
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Risk Assessment

• Qualitative
• Less time and cost involved
• Less experience required
• Answer is less sophisticated
• Risk ranking matrix and LOPA are examples

• Quantitative
• More time and cost involved
• Experience analyst required
• Answer is more useful for decision making
• Only use after qualitative method does not give
  enough information to make decision

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When/Why do Risk Assessment?

• Regulatory, such as Safety Case
• Decision-making tool
• Cost
• Need better answer
• Internal standard
• Good business practice

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Study Objective

• What event or scenario are you concerned about?
• Fire and explosion with offsite consequence
• Toxic releases
• Study boundaries
• What decision is management being asked to make?
• What increased risk is there if the shut down
  postponed for one-year?
• What mitigation measure provides the best risk
  reduction?
• Is it justified spend 2 MM to solve that problem?
• You need acceptance criteria before starting!

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The Basic Steps of a QRA
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Overview of Risk Assessment Methods
Hazard Identification Methods
Frequency Assessment Methods
Risk Evaluation Methods
Consequence Assessment Methods
Literature search What-if review Safety
audit Walk-through Checklist Brainstorming HAZOP F
MEA
Historical records Fault tree analysis Event tree
analysis FMEA Human reliabilityanalysis Common
causefailure analysis External eventsanalysis
Source term models Atmospheric dispersion
models Blast and thermal radiation models Aquatic
transport models Effect models Mitigation models
Risk Matrix F-N curve Risk profile Risk
isopleth Risk density curve Risk Index
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Section 3Hazard Identification
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The Basic Steps of a QRA
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Hazard Identification
Listed in order of complexity (time and cost)

• Checklist
• What-if?
• HAZOP
• Failure Modes and Effects Analysis (FMEA)

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Section 4Consequence Analysis
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The Basic Steps of a QRA
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Consequence Analysis Process
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Consequence Analysis Involves Estimating the

• Source term, based on the release scenario of
  interest
• Transport of material and the release of energy
• Fires
• Explosions
• Dispersion
• Effects on targets of interest
• Toxic effects (exposure and dose)
• Thermal radiation effects
• Overpressure effects
• Overall safety and economic impacts

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All models are wrong, but some are useful.

• George Box, Statistician

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Computer Models

• Spreadsheet models
• Simple consequence models
• CFD models
• Integrated models reduce number crunching

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Section 5 Fire Modeling
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Fires

• Pool
• Jet
• Fireball
• Flash Fire
• BLEVE

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Failure Descriptions

• Catastrophic vessel failure
• Vessel leak/rupture
• Largest connected line shears off
• Relief valve release
• Smaller equipment leak

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Mitigation Systems

• Passive
• Dikes
• Quench tanks
• Active
• Isolation valves
• Depressurizing systems
• Deluge/scrubbing spray systems

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Section 6Explosion Modeling
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Vapor Cloud Explosions (VCEs)

• The development of a VCE can be described in a
  four-step process
• Release of a flammable vapor, gas, or mist
• Mixing of the released material with ambient air
  to form a cloud that is in the flammable range
  within at least part of its volume
• Sufficient confinement and congestion
• Ignition of the flammable mixture

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Section 7Dispersion Modeling
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Key Source Term Parameters in Dispersion Modeling

• Selection and application of an appropriate
  atmospheric dispersion model generally requires
  that the analyst determine the following key
  parameters
• Atmospheric stability
• Wind speed
• Wind direction
• Persistence

• Surface roughness
• Mixing height
• Ambient temperature
• Ground temperature
• Gas density

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Models Available

• Range from simple to complex
• Free to costly
• Model validation is important
• Most good models require a knowledgeable person
  who works with model on regular basis

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Section 8Consequence Effects
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Consequence Effects

• What is the impact on personnel, equipment and
  the environment if the consequence reaches that
  point
• Level of Concern
• Heat radiation
• Overpressure
• ERPG for toxics, IDLH
• Use of look-up tables

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Personnel Injury Data from Explosion Effects

• Lees (1996) presents the following injury data
  for direct and indirect blast effects

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Section 9Frequency Analysis
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The Basic Steps of a QRA
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Frequency Analysis

• Determine the likelihood of the scenario
  occurring
• It generally is a series of events that need to
  occur for the ultimate event to happen
• Data is important at this step
• Some times this step can be skipped if the
  consequences are deemed so bad, the likelihood is
  not really important

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The Fault Tree, The Event, and the Event Tree
Event Tree
Fault Tree
The Event

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Fault Tree Analysis

• What is it?
• When do you use it?
• How do you use it?

T
AND
E
A
OR
B
C
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Fault Tree Analysis Limitations

• No formalized way to ensure that human factors
  are evaluated correctly
• Quantitative results depend on the quality of the
  data used (sensitivity analysis can be used to
  test data)
• An experienced and knowledgeable analyst is
  required

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Sample Event Tree
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Sources of Data

• Generic industry data
• E P Forum
• Nuclear industry
• CCPS book
• CCPS work group
• Plant specific data is best
• Human error analysis
• Engineering judgment when no data exists

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Section 10Assess Risk
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The Basic Steps of a QRA
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Risk Calculation

• Sum risk for all scenarios
• Display risk number for individual, all personnel
  and offsite
• Risk can be shown for operations, areas, day,
  night, etc.

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Relative vs. Absolute
Relative What to do first?
Absolute When to stop?
W H I C H I S B E T T E R ?
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Risk Matrix
T tolerable M marginal U
unacceptable
High
Medium to High
Low to Medium
Low
High
Medium to High
Low to Medium
Low
Consequence Severity
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Use of the Risk Ranking Matrix
Severity
Severity
High
Low
High
Likelihood
Low
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Individual vs. Societal Risk

• Individual Risk Risk to a person in the vicinity
  of a hazardous facility
• Societal Risk Risk to a defined population (e.g.,
  employees, residential neighbors, citizens) that
  may suffer multiple casualties as the result of a
  hazardous event

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Societal Risk
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Individual Risk Isopleth
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The Basic Steps of a QRA
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Acceptability of Risk

• Company establishes their criteria of risk
  tolerance
• If risk low, consider options and continue
• If risk is too high, consider options for
  reducing risk
• Proposed measures must reduce total risk
• Determine risk reduction for each proposed
  measure
• Rank remedial measures on basis of risk reduction
  and cost (Cost Benefit Analysis)

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ALARP

• Objective is to show that the hazards have been
  reduced to be as low as reasonably practical
• Often proven using risk assessment
• May require frequency only
• May require consequence only
• Most often requires risk (frequency x consequence)

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ALARP
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Risk Mitigation

• How to evaluate the effects of implementation of
  recommendations and new safeguards
• If a risk can not be eliminated, it needs to be
  brought to an acceptable level
• Acceptable level may be defined by risk ranking
  matrix or company policies
• Individual and societal risk

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Documentation
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Documentation

• Complete so that it can be understood in 5 25
  years
• Legal review ??
• List assumptions
• List data references
• Electronic files of all calculations

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keys to success

• Clear definition of the purpose
• Clear specification of requirements, including
  presentation of results
• Up-to-date process and facility information
• An experienced analyst
• Close involvement of company personnel
  knowledgeable in design, operations, and
  maintenance
• Careful documentation of assumptions and final
  results

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Understanding Risk