Delivering Integrated, Sustainable,
Water Resources Solutions
What is Engineering Risk and Reliability?
Why We Use It?
Robert C. Patev
NAD Regional Technical Specialist
(978) 318-8394
“ Building Strong “
1
Delivering Integrated, Sustainable,
Water Resources Solutions
Risk Based vs. Traditional
Analysis Methods
• Traditional
(Deterministic)
• What went wrong or
what is broken?
• Why did it go wrong or
break?
•
•
•
•
• Risk Based
(Probabilistic)
What went wrong or
what is broken?
Why did it go wrong or
break?
How likely is it to occur?
What are the
consequences?
“ Building Strong “
2
Delivering Integrated, Sustainable,
Water Resources Solutions
Traditional Methods
• Describe the condition of an existing facility
• Parameters describing the condition are
assigned single values
• Uncertainty is implicitly included in selecting
values
• Engineer’s experience, judgment and
conservatism are reflected qualitatively
• Component performance is compared to a
standard -- usually one for new components
“ Building Strong “
3
Delivering Integrated, Sustainable,
Water Resources Solutions
Risk Based Methods
• Describe the condition of an existing facility
• Parameters describing the condition are
selected as random variables
• Uncertainty is explicitly included, as probability
distributions, in selecting values
• Engineer’s experience, judgment and
conservatism are quantified
• Component performance is related to the
consequences of unsatisfactory system
performance.
“ Building Strong “
4
Delivering Integrated, Sustainable,
Water Resources Solutions
What is Risk?
“ Building Strong “
5
Delivering Integrated, Sustainable,
Water Resources Solutions
What is Risk?
• Insurance industry: risk = consequence
Consequence
High Risk
Low Risk
“ Building Strong “
6
Delivering Integrated, Sustainable,
Water Resources Solutions
What is Risk?
• Public health: risk = incidence (frequency or
probability)
High Risk
Low Risk
Probability
“ Building Strong “
7
Delivering Integrated, Sustainable,
Water Resources Solutions
What is Risk?
• Management & engineering:
Risk = probability * consequence
Consequence
High Risk
Low Risk
Probability
“ Building Strong “
8
Delivering Integrated, Sustainable,
Water Resources Solutions
What is Risk?
“ Building Strong “
9
Delivering Integrated, Sustainable,
Water Resources Solutions
What is Risk?
• Risk is a measure of the probability and
consequence of uncertain future events
• Risk includes
– Potential for gain (opportunities)
– Exposure to losses (hazards)
• Risk may be expressed mathematically as the
product of the probability of an adverse event,
and the economic or life threatening
consequences
– Probabilities and event trees are determined by engineers
and consequences are estimated by economists
“ Building Strong “
10
Delivering Integrated, Sustainable,
Water Resources Solutions
Why Use Risk?
• Risk is the common basis for comparing different types of
engineering problems such as:
– Gates subjected to corrosion and fatigue do not operate
correctly, fail and cause navigation delays
– Levee fails by breaching and causes property damages
during flood event
– Dam fails during earthquake causing release of pool
and damage to property downstream
– Breakwater fails during Northeaster and navigation to
harbor is shut down until dredging and breakwater
repairs can be made
“ Building Strong “
11
Delivering Integrated, Sustainable,
Water Resources Solutions
Where We Use Risk?
• Across all USACE Business Lines
–
–
–
–
Navigation
Hydropower
Flood Damage Reduction (Dams and Levees)
Coastal Storm Damage Reduction
• Use in USACE Engineering
• Major Rehabilitation since 1991
• Navigation System-wide Studies
• Hurricane Protection System Studies
– IPET Hurricane Katrina Performance Evaluation
• Dam Safety
• Levee Safety
• Asset Management
“ Building Strong “
12
Delivering Integrated, Sustainable,
Water Resources Solutions
Risk Based Method
• Combines:
– Probabilistic analyses
– Statistical data
– Engineering judgment
– Facility performance
Engineering Reliability
• Goal is to assists the Risk Informed Decision Making
(RIDM) Process:
– Adds valuable information to decision-making
process for justifying rehabilitation or other funding
by integrating engineering reliability with economic
and life consequences
“ Building Strong “
13
Delivering Integrated, Sustainable,
Water Resources Solutions
What is Reliability?
• Definition: Reliability is a measure of a facility or
component to perform its intended function under
specified operating conditions for a given period of time
• P(f) + P(s) = 1
• P(f) is the probability of failure (failures / event)
• P(s) is the probability of survival (successes / event)
• So the probability that satisfactory performance will occur
is called the reliability (R) or:
R = 1 - p(f) = 1- p(u)
Where p(u) is the probability of unsatisfactory performance (PUP)
“ Building Strong “
14
Delivering Integrated, Sustainable,
Water Resources Solutions
Probability of Unsatisfactory Performance or
Probability of Failure
P(u) = ???
P(f) = ???
“ Building Strong “
15
Delivering Integrated, Sustainable,
Water Resources Solutions
Determination of Performance
Probabilities, P(f) or P(u)
•
•
•
•
Historical Rates of Occurrence
Survivorship Curves
Analytical Methods
Expert Opinion Elicitation
“ Building Strong “
Delivering Integrated, Sustainable,
Water Resources Solutions
Basic ERA Framework
• Evaluate Structure or Component
Performance (PUP)
– Determine Current Performance Level
– Estimate Future Performance
– Estimate Repair Alternatives
• Predict Consequences of Unsatisfactory
Performance for Evaluated Planning
Alternatives
• Develop Consequence Event Trees
Consistent with Unsatisfactory Performance
Limit State
“ Building Strong “
17
Delivering Integrated, Sustainable,
Water Resources Solutions
Common Problems with ERA
• Reliability Side
–
–
–
–
–
Reliability models are not calibrated to experience
Hazard function analysis is not used for time-dependent conditions
Inappropriate or incorrect use of expert elicitation
Post repair reliability is not adjusted in the life-cycle analyses
The “Fix-as-fails” base case and the advance maintenance
alternatives are not consistently evaluated for reliability
• Consequences Side
– Only worst case scenarios are used in the event trees.
Consequences are grossly overstated….”The sky is falling!”
– Event trees are not complete or consistent with the limit states
– Unreliable alternatives are mistakenly recommended as viable
solutions
“ Building Strong “
18
Delivering Integrated, Sustainable,
Water Resources Solutions
ERA Guidance
• New Engineering Circular – EC1110-2-6062
– Entitled “Risk and Reliability Engineering for Major Rehabilitation
Studies”
– Single source covers risk and reliability applications for all
disciplines (structural, geotechnical, mechanical and electrical and
hydropower)
– Covers engineering reliability integrated with modeling of economic
consequences
– However, does not cover Dam Safety or Levee Safety ERA
“ Building Strong “
19