Transcript Safety Engineering - دانشکده مهندسی صنایع

Fault Tree
Analysis
Fault Tree Analysis
• Used in both reliability engineering and
system safety engineering
• Developed in 1961 for US ICBM program
• Guide published in 1981
• Used in almost every engineering discipline
• Not a model of all system or component
failures
Applying Fault
Tree Analysis
• Postulate top event (fault)
• Branch down listing faults in the system
that must occur for the top event to occur
• Consider sequential and parallel or
combinations of faults
• Use Boolean algebra to quantify fault tree
with event probabilities
• Determine probability of top event
Fault Tree Logic
• Use logic gates to show how top
event occurs
• Higher gates are the outputs from
lower gates in the tree
• Top event is output of all the input
faults or events that occur
Terms
• Faults and failures
• System and subsystem faults
• Primary and secondary failure
• Command fault
Fault Tree Symbols
Primary Event Symbols
Basic Event
Conditioning Event
Gate Symbols
AND
OR
Exclusive OR
Undeveloped Event
External Event
Priority AND
Inhibit
Intermediate Event Symbol Transfer Symbols
Transfer IN
Transfer OUT
Fault Tree Symbols
Primary Event Symbols
Basic Event
Conditioning Event
Undeveloped Event
External Event
Fault Tree Symbols
Gate Symbols
AND
OR
Exclusive OR
Priority AND
Inhibit
Fault Tree Symbols
Intermediate Event Symbol
Transfer Symbols
Transfer IN
Transfer OUT
Union
A
No Current
A=B + C
A=B Union C
B OR C must occur
for event A to occur
B
C
Switch A
Open
Battery B
0 Volts
Intersection
D
E
5mA Current
in System
Over-heated
Wire
D=E * F
D= E Intersection F
E AND F must occur
for D to occur
F
Power Applied
t >1ms
Fault Tree Quantification
• Fault tree analysis - is not a quantitative
analysis but can be quantified
• How to
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Draw fault tree and derive Boolean equations
Generate probability estimates
Assign estimates to events
Combine probabilities to determine top event
Fault Tree Example
Outlet Valve
Relay
K1
K2
Switch S1
Pressure
Switch
S
Relay
Timer
Relay
Motor
Pump
Pressure
Tank
Common Mistakes in
Fault Trees
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Inputs with small probabilities
Passive components
Does quantified tree make sense
Don’t fault tree everything
Careful with Boolean expressions
Independent Vs dependent failure modes
Ensure top event is high priority
FMECA, Human Factors,
and Software Safety
Non-Safety Tools
• Failure Modes, Effects, and
Criticality Analysis
• Human Factors Analysis
• Software Safety Analysis
FMEA
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Reliability engineering tool
Originated in 1960s
OSHA recognized
Limitation - failure does not have to
occur for a hazard to be present in
system
• Used to investigate how a particular
failure can come about
FMEA Process
• Define system &
analysis scope
• Construct block
diagrams
• Assess each block for
effect on system
• List ways that
components can fail
• Assess failure effects
for each failure mode
• Identify single point
failures
• Determine corrective
actions
• Document results on
worksheet
System Breakdown
Subsystem 1
Subsystem 2
Subsystem 3
Assembly 1 a
Assembly 1 b
Assembly 1 c
Subassembly 1c.1
Subassembly 1c.2
Subassembly 1c.3
Component 1c.3.1
Component 1c.3.2
Component 1c.3.3
Total System
Part 1c.3.3.a
Part 1c.3.3.b
Part 1c.3.3.c
FMEA Worksheet
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Component #, name, function
Failure modes
Mission phase
Failure effects locally
Failure propagation to the next level
Single point failure
Risk failure class
Controls, recommendations
Failure Modes
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Premature operation
Failure to operate on time
Intermittent operation
Failure to cease operation on time
Loss of output or failure during operation
Degraded output or operational capability
Unique failure conditions
Failure Modes, Effects,
& Criticality Analysis
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Virtually same as FMEA
Identifies criticality of components
Emphasizes probability of failure
Criticality components
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Failure effect probability
Failure mode ratio
Part failure rate
Operating time
Human Factors Safety Analysis
• Many different techniques
• Human element must be considered in
engineering design
• The merging of three fields:
– Human factors
– Ergonomics
– Human reliability
Performance & Human Error
• Why do people make mistakes?
• Combination of causes - internal/external
• Performance shaping factors (factors
that influence how people act)
– External PSF
– Internal PSF
– Stressor PSF
Human Error
• Out of tolerance action within human/machine
system
• Mismatch of task and person
• Significant contributor to many accidents
• False assumptions
– Human error is inevitable
– People are careless
• More complex systems must be less dependent
on how well people operate them
Human Error Categories
• Omission - leaving out a task
• Commission
– Selection error
– Error of sequence
– Time error
– Qualitative error
HF Safety Analysis
The Process
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Describe system goals and functions
List & analyze related human operations
Analyze human errors
Screen errors & select
Quantify errors & affect on system
Recommend changes to reduce impact of
human error
Software Safety
• Newest member of system safety field
• Software controls millions of systems
• Treat software like any system component
– Determine the hazards
– If software is involved in hazard - deal with it
• Common tools
– Software Hazard Analysis
– Software Fault Tree Analysis
– Software Failure Modes & Effects
Software Facts
• Software is not a hazard
• Software doesn’t fail
• Health monitoring of software only assures it
performs as intended
• Every line of code cannot be reviewed
• Fault tolerant is not the same as safe
• Shutting down a computer may aggravate a an
already dangerous situation
Software Safety Analysis (SSA)
Flow Process
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Software Requirements Development
Top-level System Hazards Analysis
Detailed Design Hazard Analysis
Code Hazard Analysis
Software Safety Testing
Software User Interface Analysis
Software Change Analysis
SSA
Required when software is used to:
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Identify a hazard
Control a hazard
Verify a control is in place
Provide safety-critical information or
safety related system status
• Recovery from a hazardous condition
Safety Tool Categories
• Software safety requirements analysis
– Flowdown analysis
– Criticality analysis
• Architectural design analysis
• Detailed design analysis
– Soft tree analysis
– Petri-Net
• Code analysis
Software Testing
• Software testing
• System safety testing
• Software changes
• IV &V organization
Other Techniques
MORT
• Qualitative tool used in 1970s
• Merges safety mgt & safety engineering
• Analyses mgt policy in relation to RA and
hazard analysis process
• Uses a predefined graphical tree
• Analyze from top event down
• Too large and doesn’t tailor well to
smaller problem
Energy Trace Barrier
Analysis (ETBA)
• Qualitative tool for hazard analysis
• Developed as part of MORT
• Traces energy flow into, through, & out of
system
• Four typical energy sources
• Energy transfer points & barriers analyzed
• Advantages
ETBA Procedure
• Examine system / identify energy sources
• Trace each energy source through system
• Identify vulnerable targets to energy
• Identify all barriers in energy path
• Determine if controls are adequate
Sneak Circuit Analysis
• Standardized by Boeing in 1967
• Formal analysis of all paths that a
process could take
• Find sneak paths, timing, or procedures
that could yield an undesired effect
• Review engineer drawings, translate, &
identify patterns
• Disadvantages
Cause-Consequence Analysis
• Uses symbolic logic trees
• Determine accident or failure scenario
that challenges the system
• Develop a bottom-up analysis
• Failure probabilities calculated
• Consequences identified from top event
• Consequence may have variety of
outcomes
Dispersion Modeling
• Quantitative tool for environmental and
system safety engineering
• Used in chemical process plants, can
determine seriousness of chemical release
• Internationally recognized model CAMEO
• Features of the system
• Advantages
Test Safety
• Not an analysis technique
• Assures safe environment during testing
• Must integrate system safety process into
test process
• Three layers of test environment
• Safety analysis needed at each level
• Test readiness review
Comparing Techniques
• Complex Vs simple
• Apply to different phases of system life
cycle
• Quantitative Vs qualitative
• Expense
• Time and personnel requirements
• Some are more accepted in certain
industries
Selecting A Technique
• All techniques are good analyses
• Consider advantages and disadvantages
• Select technique most suited to the
problem, industry, or desired outcome
• Ask yourself a few questions
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What’s the purpose?
What is the desired result?
Does it fit your company and achieve goals?
What are your resources and time available?
Data Sources
and Training
Data Reliability
• Start with company historical data
• Analyses only as good as the data that is
used
• Caution about misunderstanding data
• Quantifiable data is not always the best
• Always cite sources and assumptions
Data Limits
• Most failure data is generic
• Break large items into smaller parts
• Data may not consider environmental
changes
• Use expert judgement to convert generic
data into realistic values
Government Data Banks
• Government Printing Office
– Books from DoD, NASA, EPA, & OSHA
• Government-Industry Data Exchange
Program
– Army, Navy, FAA, Dept of Labor, Dept of
Energy, National institute of Standards and
Technology
• Databases of other countries
Industry Data Banks
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Corporations
Insurance companies
Electronics Industries Associations
Consumer Product Commission
System Safety Society
Material Safety Data Sheets
Creating Your Own Databank
• Collect data on system
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Design
Assessments
Hazard identification
Compliance verification
• Make the data easily accessible and
consolidated in one place
• Computers and new software make
collection easier
Data Bank
Systems Info
System Safety Data
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Hazardous materials
MSDS
System design info
Safety critical systems
Best design practices
Testing history
Failure history
Safety analyses
Accident histories
Safety Standards
Identified hazards
Causes of hazards
Proven hazard controls
Hazard consequences
Hazard tracking system
Safety Training
• Twofold approach
– Employee training
– Emergency response
• Types of training
– Initial training
– Refresher training
– New training for changes
Employee Training
• Training needs assessment
• Purpose of training
• Assess current operations
• Review hazard analysis data
• Develop and implement training
• Record training
Emergency Preparedness and
Response Training
• Train all personnel affected by possible
emergency
• Training subjects
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Evacuation procedures
Shutdown of equipment
Firefighting and first aid
Crowd control and panic prevention
• Conduct exercises
Certification for
Hazardous Operations
• Determine personnel that require
training
• Certification program elements
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Certification examination
Physical examination
Classroom and hands-on training
Test of safe working practices
Recertification schedule
Safety Awareness
• Highlight safety in organization
• Positive incentives
• Establish safety representatives in each area
• Conduct meetings to discuss safety program
• Safety reps should be trained in workplace
safety inspections and program monitoring
Accident Reporting,
Investigation,
and Documentation
Reporting the Accident
• Accident reporting without retribution
• Posting of reportable accidents
• New-employee briefing
• Management involvement
Setting Up a Closed-Loop
Reporting System
• Pre-accident plan
• Report within 24 hours
– Pass data up the chain
– Initiate board
– Capture perishable information
• Investigate all accidents
Forming a Board
• Company policy
– Accident classification
– Standing list of board candidates
• Selecting the Board members
– Various backgrounds
– Voting members and advisors
• Board responsibilities
Conducting the Investigation
• Preparing for investigation
• Gathering evidence and information
• Analyzing the data
• Discussion of analysis and conclusions
• Recommendations
Investigation Report
• Abstract of report
• Analysis results
• Summary of F & R
• Conclusions
• Procedure used
• Detailed F & R
• Background
• Minority reports
• Sequence of events
• Appendixes
• Analysis
methodology
Accident Documentation
• Investigation Report
– Retained with supporting documents
– Corrective action implemented
– Available for future safety analysis
• Retain the records
• Public release of information
Risk Assessment
What is Risk?
• Severity of consequences of an accident
times the probability of occurrence
• Risk perception may vary from actual risk
• Risk: realization of unwanted, negative
consequences of an event (Rowe)
• Risk: summation of three elements
– Event scenario
– Probability of occurrence
– Consequence
Risk Perception
Factors concerning perception of risk
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Voluntary Vs nonvoluntary
Chronic Vs catastrophic
Dreaded Vs common
Fatal Vs nonfatal
Known Vs unknown risk
Immediate or delayed danger
Control over technology
Risk Assessment Methodology
Formal process of calculating risk and
making a decision on how to react
1 Define objectives
5 Quantify scenarios
2 Define system
6 Consequences
3 Develop scenarios
7 Risk evaluation
4 Develop event trees
8 Risk management
Risk Assessment Methodology
Step 1
Step 2
Define
Objectives
Step 5
Quantify
Scenarios
Define
System
Step 6
Consequences
Determination
Step 3
Develop
Scenarios
Step 7
Risk
Evaluation
Step 4
Develop
Event Trees
Step 8
Risk
Management
Identifying Risk in a System
• Risk identified through analysis techniques
• Use several techniques
• Construct fault tree
• Use analysis tools to focus on which
component is the trigger
Risk Communication
• Communicating with public
– Acknowledge the community
– Do not imply irrationality or ignorance
• Methods to promote communication
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Community participation
Approach “group” appropriately
Consultation with community
Involve community in negotiations
Be open with information
Risk Evaluation
A Probabilistic Approach
• Quantifying risk through probability of
failure
• Hard to quantify probability of some events
• Understand the data, the sources, & the
limitations
• Follow rules of probability
Risk Analysis Model
• Developing accident scenarios & initiating
event
• Event Trees
• Consequences determination
• Uncertainty
• Risk evaluation - Risk profiles
Calculating Safety Costs
• Tracking data costs
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System downtime (lost productivity)
Equipment damage and replacement
Accident clean-up
Personnel injuries and death
• Expected value
• Cost-benefit analysis