F-1

APPENDIX F—COMPARISON OF API AND ASME RISK-BASED INSPECTION

F.1 Summary

This appendix summarizes the differences and similaritiesbetween the API Risk-Based Inspection Base Resource Doc-ument (BRD) and the ASME documents. The ASME docu-ments reviewed were:

• Volume 1: General Document.• Volume 2: Part 1. Light Water Reactor (LWR) Nuclear

Power Plant Components.• Volume 3: Fossil Fuel Fired Electric Power Generating

Station Applications.

There are no philosophical differences between the APIand the ASME approaches to Risk-Based Inspection; how-ever, the final documents from the projects are notably differ-ent. The differences arise from the different scopes and goalsof the two projects. The ASME projects were research effortsto determine risk-based methods for developing guidelinesfor inspection. The API project was intended to developusable tools and methodologies that are understandable at aplant inspection level. The API project built upon the methodsoutlined in the ASME documents, but with considerable sim-plification where appropriate.

F.1.1 API RBI

The API BRD aims to be understandable and usable at theplant staff level. Application tools are needed (and are underdevelopment) to fully gain the benefit of risk based inspec-tion, since even with the use of simplified models, there is alarge database to be manipulated in a typical refinery orchemical plant. The BRD provides a good start to demon-strate the feasibility and value of the technology.

F.1.2 ASME RBI

The ASME effort aims to the highest levels of technicaldevelopment, since it is intended to be a research project.This approach provides much value to others who wish todevelop applications using these methods, however, the tech-nology as presented in the ASME documents is understand-able and usable only by integrated team of high levelspecialists. The ASME documents set high standards forfuture RBI development.

F.2 Scope

F.2.1 API RBI

The API BRD was intended to develop usable tools andmethodologies that are understandable at a plant inspectionlevel. The project attempted to identify the limitations of thetechniques used due to simplification of complex models,

while identifying opportunities for increased levels of sophis-tication where appropriate.

F.2.2 ASME RBI

The ASME projects were research efforts to determine riskbased methods for developing guidelines for inspection. Theydid not necessarily develop those guidelines. The ASMEapproach considers and includes all levels of complexity:

a. Technical.b. Component level.c. Fault/Event Tree analysis.d. Decision tree analysis.

F.3 Qualitative Risk-Based Inspection

Both the API and ASME documents use qualitative andquantitative approaches to Risk-Based Inspection, althoughnot necessarily in the same fashion. The ASME matrix isshown in Figure F-1.

F.3.1 API RBI

In the BRD, the qualitative approach is intended for use asa screening tool at the operating unit level. This will allow theuser to quickly focus on those areas of the plant that have thehighest contribution to risk. The approach is intended to beeasy to use:

a. Adds factors contributing to high risk.b. Subtracts factors contributing to risk management.

The results are presented in a 5

x

5 matrix of likelihood andconsequence. This approach can be extended to the equip-ment item level, and a current project is underway for thisdevelopment (Phase 2).

F.3.2 ASME RBI

The ASME approach to qualitative risk assessment can beextended to the component level if desired. In the ASMEapproach, “qualitative” means “judgmental”, i.e. based on theopinions of experts. Several methods for gleaning these opin-ions are presented:

a. FMEA (Failure Modes & Effects Analysis).b. HAZOP (Hazard & Operability Study).c. FTA (Fault Tree Analysis).d. MLD (Master Logic Diagram).e. What-if (Question sets).

Similar to the API approach, qualitative analysis results arepresented in a 5

x

5 matrix.

F-2 API 581

The API matrix is shown in Figure F-2. Note that theshaded risk categories are skewed to account for the effects ofrisk aversion in the face of high consequences.

F.4 Quantitative Risk-Based Inspection

F.4.1 LIKELIHOOD OF FAILURE

F.4.1.1 API RBI

The API BRD uses a database of “generic” failure frequen-cies to establish base failure rates (events/yr) of differenttypes of equipment common to the process industries. Thisapproach has the advantage of providing a starting point forthe application of RBI, but has the disadvantage that the data-base is not specific to any one type of industry. These“generic” frequencies are modified to account for variousdamage mechanisms using Probabilistic Structural Mechan-ics to evaluate the effect of varying degrees of damage on theprobability of failure. Simplified mechanistic models are usedto match the available data. The API approach uses a Baye-sian updating technique to account for the reduction in proba-

bility of failure due to inspection based on the effectiveness ofthe inspection technique at finding the damage before failure.

F.4.1.2 ASME RBI

The ASME approach is illustrated in the referenced docu-ments by the use of historical databases that are available forthe Power industries. This greatly simplifies the approach ifsuch data is available. The ASME documents also illustratethe use of Probabilistic Structural Mechanics (referred to asStructural Reliability and Risk Assessment, SRRA in theASME documents). The illustrations of these techniques inthe ASME documents in each case use the same demonstra-tion: fatigue crack growth evaluated via rigorous elastic plas-tic fracture mechanics. This illustration is used because thereare available models for crack growth, probability of detec-tion, and probabilistic evaluation of the impact of the damageon structural reliability (probability of failure). However, theASME approach does not address how to proceed in theabsence of such models and data, except to rely on expertjudgment of the POF in determined in a formal method.

Figure F-1—ASME Qualitative Risk Matrix

CONSEQUENCE CATEGORY

LIK

EL

IHO

OD

CA

TE

GO

RY

2

1

A B C D E

3

4

5

Low Risk

MediumRisk

Medium - HighRisk

High Risk

R

ISK

-B

ASED

I

NSPECTION

B

ASE

R

ESOURCE

D

OCUMENT

F-3

F.4.2 CONSEQUENCES

F.4.2.1 API RBI

The API BRD provides methods to quantify any of the fol-lowing types of consequences:

a. Flammable/Explosive.b. Toxicity.c. Environmental.d. Business Interruption.

The calculations are based on technical models of releasescenarios.

F.4.2.2 ASME RBI

The ASME approach uses various techniques for determi-nation of consequences. For LWR nuclear power plants, theconsequences are expressed as likelihood of core damage perevent. The actual modeling of release scenarios is notattempted in this case. For fossil-fuel-fired power plants(FFFPP), the consequences are taken directly from an indus-try database giving the cost of purchased replacement powerfor given failure events. The use of Fault Trees/Event Trees

for determination of consequences in the fossil fuel firedplant case is provided as a demonstration of the techniques,but is extremely complex.

F.4.3 QUANTITATIVE RISK ASSESSMENT

F.4.3.1 API RBI

The final results from the API BRD present the risk as oneor more of the following measures:

a. Business Interruption ($/yr).b. Equipment Damage (square feet/yr).c. Health Effects (square feet/yr).d. Environmental impact ($/yr).

F.4.3.2 ASME RBI

The final results from the ASME documents present therisk as one or more of the following measures:

a. Likelihood of Core Damage per year.b. Economic Loss (FFFPP) ($/yr).c. Casualties—FFFPP (Small—result of boiler rupture).

Figure F-2—API Qualitative Risk Matrix

CONSEQUENCE CATEGORY

LIK

EL

IHO

OD

CA

TE

GO

RY

2

1

A B C D E

3

4

5

Low Risk Medium Risk

Medium - HighRisk

High Risk

F-4 API 581

F.5 Conclusions

The ASME research studies present the groundwork neces-sary to develop Risk-Based Inspection Guidelines, but do notactually provide such guidelines. The API BRD project builds

upon the earlier ASME efforts to develop usable tools thatcan provide the benefits of Risk-Based Inspection with a rea-sonable expenditure of effort.