Risk Based vs. Traditional Analysis Methods

1. What is Engineering Risk and Reliability?Why We Use It?Robert C. PatevNAD Regional Technical Specialist(978) 318-8394

2. 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? Risk Based vs. TraditionalAnalysis Methods

3. 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

4. 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.

5. What is Risk?

6. What is Risk? • Insurance industry: risk = consequence High Risk Consequence Low Risk

7. What is Risk? • Public health: risk = incidence (frequency or probability) High Risk Low Risk Probability

8. What is Risk? • Management & engineering: Risk = probability * consequence High Risk Consequence Low Risk Probability

9. What is Risk?

10. 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

11. 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

12. 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

13. Risk Based Method • Combines: • Probabilistic analyses • Statistical data • Engineering judgment • Facility performance • 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 Engineering Reliability

14. 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)

15. Probability of Unsatisfactory Performance orProbability of Failure P(u) = ??? P(f) = ???

16. Determination of Performance Probabilities, P(f) or P(u) • Historical Rates of Occurrence • Survivorship Curves • Analytical Methods • Expert Opinion Elicitation

17. 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

18. 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

19. 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