Reliability Engineering

Fred Schenkelbergfms@fmsreliability.com

FAULT TREES & BLOCK DIAGRAMSDay 2 Session 1

Objectives

• Recognize vulnerabilities in a system and reduce risk

• Fault Tree Analysis and Block Diagrams• Using simulation to improve decision making• Using simulation options (policies, pools &

resources)• Combining life distributions with life stress

relationships

Discovery of Risks

• What could go wrong?

• What has gone wrong?

• Analysis

• Testing

Analysis

• FMEA

• What if

• Modeling

• Expert opinion

Hazard Analysis

• Plot severity and occurrence

• Local policy on priorities

Likelihood of Occurrence

Seve

rity

Identification in not enough

• Experiments

• Design

• Constrain

• Mitigate

Discussion & Questions

Fault Tree Analysis

Deductive analysis process

FTA Construction

1. Define the fault condition, and write down the top level failure.

2. Using technical information and professional judgments, determine the possible reasons for the failure to occur.

3. Continue to break down each element with additional gates to lower levels.

4. Finalize and review the complete diagram. The chain can only be terminated in a basic fault: human, hardware or software.

5. If possible, evaluate the probability of occurrence for each of the lowest level elements and calculate the statistical probabilities from the bottom up.

What is Fault Tree Analysis, by Simha Pilot, Quality Progress, March 2002

Reliability Block Diagram

• Reliability Block Diagrams (RBDs) are graphical representations of the components of the system and how they are reliability-wise related

--Reliasoft.com

RBD Construction

• Series • Parallel

A

B

A B

Discussion & Questions

Simulation

• Build RBD

• Run simulation– 100 days– 100 cycles– What if analysis

Blocksim Simulation

• An airline does annual inspections of vests

• Replace faulty vests only

• Vests fail with Weibull distribution – beta 2.55 and eta 6.89 years

• Should we change the inspection interval?

Blocksim Simulation

Discussion & Questions

Policies

• Rules to govern repair and maintenance tasks

• Inspections• Crews• PM and CM tasks

Block Diagram

Which relays are most important availability wise?

Importance Plot

Spares planning

Discussion & Questions

Monty Carlo Methods

• Failure mechanism models

• Empirical models

• Use stress and environment models

Monte Carlo

1. Define a domain of possible inputs.

2. Generate inputs randomly from a probability distribution over domain.

3. Perform a deterministic computation on the inputs.

4. Aggregate the results.Area of Circle = π / 4Area of Square

Example

• 30 known failure mechanisms

• Distribution of use and environmental stresses

• Models to predict failures

Outputs

Time

Prob

abili

ty o

f Fai

lure

Discussion & Questions

Summary

• Recognize vulnerabilities in a system and reduce risk

• Fault Tree Analysis and Block Diagrams

• Using simulation to improve decision making

• Using simulation options (policies, pools & resources)

• Combining life distributions with life stress relationships

Fault Trees &Block Diagrams