Post on 19-Dec-2015
Reliability Risk Assessment
Ray Barlog, PE
Cornerstone Electrical Consultants, Inc.
“Service Measured to the Standard”
March 24, 2015
Safety and Reliability
• Both deal with uncertainty, aim to reduce undesired outcomes
• Safety mostly concerned with avoiding harm to humans
• Reliability most often concerned with reducing economic losses - $$
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Risk
- An event that has a negative consequence and has a probability of occurring (not an opportunity)
• Risk = Likelihood x Consequence
• Reliability Risk = Failure Probability x $$ Impact
• Reliability Risks are often not constant across time
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Risk
Do We Want To…………
Eliminate Risk?
Reduce Risk?
Manage Risk?
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Risk Management Process
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Identify
Analyze
EvaluateRespond
Control
Assessment
Risk Assessment
The process of identifying, analyzing, and evaluating, and prioritizing risks
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Some Reliability Risk Assessment Methods
Bow Tie AnalysisRAM ModelingStochastic Life Cycle Cost Concept FMEA
Event Tree Analysis
Layer of Protection Analysis
Markov Analysis
Functional FMEAProcess FMEA
Equipment FMEA
Expected Value FMEA
Fault Tree Analysis
Qualitative Fault Tree
What If Analysis
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3 Reliability RA Tools
• Functional Failure Mode and Effects Analysis
• Bow Tie Analysis
• Reliability, Availability, Maintainability (RAM) Modeling
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FMEA
• Probably the most common reliability risk assessment tool
• Structured method
• Best using team with diverse backgrounds
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FMEA• Came from Military Procedure MIL-P-1629, Procedures for
Performing a Failure Mode, Effects and Criticality Analysis, dated November 9, 1949.
• FMEA used and improved by NASA in the 1960's to improve and verify reliability of space program hardware.
• Mil-Std-1629A used in the military and by commercial
• Used in the Nuclear Power Industry for evaluating design risks
• SAE J1739 - an FMEA standard used in the auto industry
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FMEA-asks the questions
• What is the intended function?
• How does it fail? ( failure mode )
• How often do we expect the failure to occur?
• How severe are the effects?
• What are the potential causes of the failure?
• How likely is the onset of failure to be detected?Cornerstone Electrical Consultants, Inc.
Common Example
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Objective: Determine the most critical risk and its cause(s) for this boiler feed water system.
Common Example
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If 2 pumps fail, both boilers trip
Common Example
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P-1 P-2 P-3
Risk Rating Factors
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RATING
DEGREE OF
SEVERITY
QualitativeFAILURE
RATE (_/yr)DetectionCertainty
1 Less than $50K
Likelihood of occurrence is remote 1.00E-06
Almost certain that the potential failure will be found or prevented before producing an economic loss
100%
2 $50k to $100k
Low failure rate with supporting documentation
1.00E-05
Current controls may or may not detect impending failure
50%
3 $100k to $500k
Low failure rate without supporting documentation
1.00E-04
Current controls probably will not detect the potential failure 0%
4 $500k to $1mm
Occasional failures 1.00E-03
5 $1mm to $5mm
Medium Failure Rate 1.00E-02
6 $5mm to $10mm
Moderately High Failure Rate
1.00E-01
7 $10mm to $100mm
High Failure Rate 1
OCCURRENCE DETECTION
FMEA Worksheet
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SubsystemFunction of Subsystem
Potential Failure Mode
O C C
Potential CausesS E V
Potential Failure EffectsD E T
R P N
Current Controls
Recommended Actions
Action Owner
42oo3 Pumps Fail Simultaneously due to seal failure
4
Boilers trip, Production Loss of $100k per day x 5 days plus $50k pump repair cost, Total $550k loss
2 32
Manual Condition Monitoring for vibration
Consider continuous vibration monitoring
Joe Engineer
31 Pump fails and auto-start for standby fails
3
Boilers trip, Production Loss of $100k per day x 5 days plus $60k repair cost. Total $560k loss
3 27Periodic Testing of Auto-Start
None NA
4Loss of Station Service Bus B
5
Boilers trip, Production Loss of $100k per day x 15 days plus $100k repair cost. Total $1.6mm loss
2 40Periodic ultrasonic corona testing
None NA
3Pump 1 fails and Station Service bus B fails
5
Boilers trip, Production Loss of $100k per day x 15 days plus $50k repair cost. Total $1.55mm loss
2 30Periodic ultrasonic corona testing
None NA
Loss of ALL feed water flow
Deliver feedwater to boilers at 2mmpph rate
Boiler Feed Pump System
Bow Tie Analysis
A simple graphical tool that shows the link between potential causes, preventive and mitigating controls, and consequences of a risk event• Shows at a glance how risks are managed• Can be purely qualitative or semi-
quantitative
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Reason’s Swiss Cheese
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Generalized Bow TieT
hrea
ts o
r C
ause
s
Con
sequ
ence
s
Cause 1
Cause 2
Cause 3
Cause 4
TOP EVENT
BarriersMitigations
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Example Risk Matrix
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A <$50k
B $50 to $500k
C $500k - $5mm
D $5mm-$50mm
E $50mm - $100mm
1 ( 1/yr)
2 (1/10yr)
3 (.001)/yr
4 (.0001)/yr
5 (.00001/yr)
Freq per Year or
Likelihood
Financial Consequence Severity
Bow Tie-Common Example
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_ _ _ _
2 Medium 1 Weak 3 Strong 3 Strong
_ _ _ _
2 Medium 2 Medium 1 Weak 1 Weak
_ _ _
2 Medium 2 Medium 2 Medium
_
2 Medium
THREATS or CAUSES
BARRIERS / PREVENTIVE CONTROLS MITIGATIVE CONTROLS
Large Production Downtime Losses $550k-$5mm
CONSEQUENCES
2oo3 pumps fail due to seal failures F=1
Pump redundancy
Robust shaft and bearing design
Burner Trip System
3 Element BFW Control System
Boiler Tubes Damaged $10mm
TOP EVENTInadequate
BFW Flow to Boilers
One pump fails and auto-start fails F=1
Periodic testing of auto-start
Operator response
Quick Pump Repairs
Spares Stocking Strategy
Planned Repairs Prior to Major Damage
Significant Pump Repair Costs >$100k
Station Service Bus B Failure F=2
Corona testing to detect onset of failure
P-2 or P-3 fails and SS Bus A fails F=3
Corona testing to detect onset of failure
Use of Predictive Maintenance Techniques
RAM Model• RAM: Reliability, Availability, Maintainability
• Reliability: Probability of surviving a given time interval without failure under given conditions
• Availability: Average % time a system is in a state to perform a function
• Maintainability: Probability of completion of a maintenance task in a given time interval
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RAM Model
• A graphical and mathematical representation of system operation, dependency, and performance
• Most quantitative of the three methods presented
• Requires failure data, repair time data, and system operating logic
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RAM Model Building Block
• Series
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RAM Model Building Block
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RAM Model-Example
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RAM Model-Typical Input
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RAM Model Results
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System Mean Availability 99.986%, +/- 0.052%
Average Annual Production Losses 2.457 mmLb/yr
Average Annual Production Losses $5,120/yr
Average Outage Duration 160.6 Hrs
Longest Duration Outage 372 Hrs
Shortest Duration Outage 0.34 Hrs
Results of 1000 Simulations, 20 Years in Length
System Life Cycle Performance Summary
RAM Model Results
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Pros / Cons - FMEA
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• Structured, Thorough
• Easy to Learn
• Uses Group Knowledge
• Requires no special software
• Excellent for evaluating designs early in the process
• Tedious, Time Consuming
• Requires robust risk matrix
• Doesn't handle redundancy or multiple failures well
• Doesn't handle dependencies well
• Doesn’t handle increasing failure rates well
Pros / Cons – Bow Tie
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• Excellent risk management communication tool
• Easy to learn and interpret
• Uses group knowledge to develop
• Fairly quick to develop
• Quantifying risk requires modification
• Requires robust risk matrix
• Software recommended for good documentation
• Becomes complex with large systems
Pros / Cons – RAM Model
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• Quantifies risks for prioritization
• Estimates risks over time
• Handles dependencies, redundancy, special ops rules
• Evaluating “What Ifs” can be done quickly
• Can be labor and $$ intensive for large systems
• Not easily understood by person not trained
• Requires special analyst skills for model building
• Quality of model depends on quality of data
Final Thoughts
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• There is NO one best or universal method.
• Use the simplest method that can help you meet the objective of your assessment with the minimum investment of time and resources.
• Risk assessment alone is valueless- risks must be managed and that takes action.
What are your questions?
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