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Risk-Informed Decision Making – FERC Perspective. Natural Hazards: Seismicity Events. David Lord, P.E., D2SI Dam Safety Risk team – Portland, Or. What is Risk-Informed Decision Making (RIDM). Decision-making, which has as an input the results of a risk assessment.

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Risk informed decision making ferc perspective

Risk-Informed Decision Making – FERC Perspective

Natural Hazards: Seismicity Events

David Lord, P.E., D2SI Dam Safety Risk team – Portland, Or


What is risk informed decision making ridm
What is Risk-Informed Decision Making (RIDM)

  • Decision-making, which has as an input the results of a risk assessment.

  • Risk information will play a key role in decisions related to dam safety but will not be the only information to influence the final decisions.

  • RIDM involves a balancing of benefits and risks while reducing risks as much as is practicable.


Risk

  • Risk = Probability of Failure x Consequences

  • Probability of Failure = Probability of Load x Adverse Structural Response Given Load (PF = PL x PAd )

  • Adverse Response includes all steps of event tree leading to failure.


From deterministic to probabilistic
From Deterministic to Probabilistic

  • Current standards-based program relied on conservative deterministic analyses.

  • Probable Maximum Flood (PMF) and Maximum Credible Earthquake (MCE) estimates

  • Structural analyses relied on these point estimates

  • Probabilistic Analyses give a curve of frequency estimates for the loading, not one particular point.


Probabilistic loadings
Probabilistic Loadings

  • Probabilistic loading has the benefit that different issues can be considered including:

    • Probable level of reservoir during earthquake

    • Range of inputs from far distance Cascadia Subduction Zone source to nearby infrequent local sources

  • Current standards-based current regulatory framework does not address how to use this information for decision making.


Evaluating frequency loading estimates
Evaluating Frequency Loading Estimates

  • Some form of fragility analyses are needed to evaluate how the dam performs during a range of frequencies.

  • If structure ok for a 1/5000 annual exceedance probability (AEP) event, but not ok for 1/10000 AEP, is this acceptable?

  • Reclamation and US Corps of Engineers have developed techniques for determining acceptability


Consequence risk potential life loss
Consequence Risk (Potential Life Loss)

  • The current hazard classification system does not distinguish between one only potential life loss (PLL) and 10000.

  • Risk consequences evaluate a range of PLL from 0, to 1, to many thousand.


Risk informed decision making ridm
Risk-Informed Decision Making (RIDM)

  • Decisions are made by using a chart that compares F (the likelihood of failure) to N (the PLL)

  • The fN (or FN) charts have a one to one relationship between frequency and PLL for societal risk




Why risk at the ferc1
Why Risk at the FERC?

  • Enables a better informed decision.

  • Enables a better understanding of the dam

  • Provides a better justification for decision-making

  • Makes dams safer.


Mid columbia prototype risk analyses
Mid-Columbia Prototype Risk Analyses

  • Six prototype sections

  • Workshop No. 1 held on May 22 and 23, 2012

    • Grant Co PUD: Wanapum Gate System

    • Developed a system model of gate system under seismic loading


Prototype risk analyses
Prototype Risk Analyses

  • Workshop No. 2 held on November 6 through 9, 2012

    • Chelan Co PUD: Chelan Surge Tank

    • Chelan: Rocky Reach Spillway Gate System

    • Grant: Priest Embankment (Liquefaction)


Prototype risk analyses1
Prototype Risk Analyses

  • Workshop No. 3 to be held in June 2013

    • Chelan: Rock Island North Abutment Wall

    • Grant: Wanapum Future Unit System - Anchors


Mid columbia risk analyses
Mid-Columbia Risk Analyses

  • The FERC likes the prototype studies for many reasons

  • RIDM can provide more understanding of particular issues as follows:

    • Combining the frequency of loading with consequences can provide a better perspective on the risk of a particular loading

    • Frequency aspects of loading event can be evaluated using risk, e.g., more likely loading versus more unlikely.


Mid columbia risk analyses1
Mid-Columbia Risk Analyses

  • RIDM provides more understanding (cont.)

    • Many MCE values are typically comparable to 2% in 50 year ground motions, an approximately 1/2500 AEP loading. (Note that not all MCEs are comparable to a 2% in 50 year ground motion.)

    • This calls into question whether an MCE is actually the maximum load that needs to be considered for dam evaluation.

    • Note that Wanapum Dam deterministic loading are well above the 1/10000 AEP loading.



Wanapum seismic loading
Wanapum Seismic Loading

  • Wanapum probabilistic results may be adequately conservative at a higher frequency earthquake, e.g., 1/5000 AEP, than the equivalent deterministic result.

  • Using the probabilistic loading would then result in less potential for remedial work.


Risk scalability
Risk Scalability

  • The prototype risk analysis technique is simplified and scalable.

  • Simplifying assumptions are made to determine if the dam system meets the selected tolerable risk guidance.

  • Process ends at any of four steps after acceptable performance at the lower step based on PLL.

  • This scaling technique means that only necessary detailed analyses are completed, and unnecessary steps are not completed.


Completeness
Completeness

  • Simplifying assumptions reduce the cost without sacrificing completeness.

    • A full risk quantitative risk analysis (QRA) of spillway gate systems can be quite costly.

    • The prototype captures the full system model without analyzing every detail reducing the cost.

    • If it is clear that the dam’s fragility is acceptable under the loading used, no further work is needed.

    • A full QRA could be completed if needed.


Systems model
Systems Model

  • Analysis of selected dam sections as systems rather than just individual components.

    • The full system model is described

    • The models develop for the Wanapum and Rocky Reach gate systems revealed very useful information about the various aspects of the spillway gate system

    • This information allows the dam owner to fix issues that had not been previously appreciated.

    • For example, anchorage of backup power systems and safety of dam operators and their continued ability to operate the spillway gates.


Usefulness of new risk analysis tool
Usefulness of New Risk Analysis Tool

  • Tool is a useful way to develop a result during the FERC’s integration of risk into its dam safety program.

  • Any necessary modifications to the tool can be made at a later date building upon the previous effort.

  • Refining the tool is part of the on-going work of the prototype development.


Potential issues
Potential Issues

  • Tool assumes seismic hazard uses half the risk space.

  • Future work includes the need to understand where the risk is in relation to the tolerable risk guideline including analyzing all of the PFMs.

  • We recognized that the FERC does not currently have a tolerable risk guideline which may vary from current guidance.


Tolerable risk
Tolerable Risk

  • Risks are tolerable if they are adequately managed and driven “as low as reasonably practicable” (ALARP).

  • An ALARP evaluation will be needed in the future to fully establish tolerability.


Conclusions
Conclusions

  • Prototype Risk Tool is a potentially useful tool

  • Risk-informed decision-making can provide more understanding of dam safety issues because it includes likelihood of failure and consequence.

  • Prototype Risk Tool is simplified and scalable.


Conclusions1
Conclusions

  • Reasonably complete analysis of complete dam systems can be done at relatively low cost.

  • Developing a system model is a very productive effort in understanding risks.

  • This technique is a useful tool.

  • Any potential changes and needed modifications can be made at a later date.



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