FAULT TREE ANALYSIS (FTA)

QUANTITATIVE RISK ANALYSIS• Some of the commonly used quantitative risk

assessment methods are;1. Fault tree analysis (FTA)2. Event tree analysis3. Failure Mode Effects Analysis (FMEA)4. Fault hazard analysis (FHA)5. Operational hazard analysis (OHA)6. Human reliability analysis (HRA)

FAULT TREE ANALYSIS (FTA)• 1st developed in the early 1960’s. since then

they have been readily adopted by a wide range of engineering disciplines as one of the primary methods of predicting system reliability and availability parameters.

• The Fault Tree Analysis (FTA) is an analytical technique that is use for:1. Reliability2. Maintainability3. Safety Analysis

• An FTA attempts to integrate all factors that effect the success or failure of a product into a single FTA Logic Diagram. Examples product:– Total loss of production– Explosion– Toxic emission– Safety system unavailable

• The symbols used in a single FTA Logic diagram are called Logic Gates and are similar to the symbols used by electronic circuit designers.

• The FTA Logic Diagram provides an elegant and precise method for defining the complex relationship that exists between the hardware, software and human components of a system.

• An FTA is a status driven analysis where the inputs to a Logic Gate represent the status of a part and other factor being included in the analysis.

• Other factors can include such things as training, tools, safety equipment, supervision etc.

• The output from a Logic Gate is a logic state that represents a condition that exists in the system. An event occurs when the output of a Gate changes state.

• If a part or other factor is functioning correctly, the state is TRUE. If the part or other factor is malfunctioning, the state is FALSE.

• When a logic statement is TRUE it is assigned a Boolean logic value of one (1). When a logic statement is FALSE it is assigned a Boolean logic value of zero (0).

• The FTA Logic Diagrams included in this analysis use the symbols listed in attached Logic Symbol Diagram. All of the rules for Boolean Algebra apply.

• An FTA is performed by systematically determining what happens to the system when the status of a part or other factor changes.

• The minimum criteria for success is that no single failure can cause injury or an undetected loss of control over the process. Where extreme hazards exist or when high value product is being processed, the criteria may be increased to require toleration of multiple failures.

• An FTA requires consideration of both positive and negative events.

• The logic tree segments leading to a Negative Event, such as an accident, defines all of the things that could go wrong to cause the negative event. Logic tree segments for negative events usually use more OR gates AND gates, expect for redundant safeguards.

• The logic segments leading to a positive event defines all of the things that must work together for the machine to operate or to complete a successful mission.

• Logic trees for positive events generally used more AND gates than OR gates, expect for redundancy.

• Maintenance troubleshooting trees are a good examples of logic trees for positives events. Inverting the output of a positive event converts it into a negative event.

• Besides OR and AND gates, there are other gates that are less used liked NOR and NAND gates.

FTA SYMBOLS• Two logic symbols or gates commonly used in

FTA are:OR symbol A C A + B = C

(Probability add) B

AND symbol A A x B = C

(Probability multiply) B C A . B = C

• Two logic symbols or gates less commonly used in FTA are:NOR symbol A A + B = C

(Probability add) B C

NAND symbol A A x B = C

(Probability multiply) B C or A . B = C

FTA ALGORITHM• The algorithm used will determine the probability

of system failure, an the importance of the event to the failure.

• Fault trees can be analysis using Monte Carlo simulation to find not only probability of a system failure, but also to statistically identify the minimal cut sets and their importance to the system failure.

• Systems failure probabilities and minimal cut sets assessed with Monte Carlo correspond closely with those obtained from the deterministic algorithms.

COMPARISON OF METHODSFault tress– Logical representation of the relationship of

primary events that lead to a specified undesirable event.

– The construction of the fault tree is top-down, in that the undesirable event is the root of the tree and the logical combination of sub-events are used to map out the tree until the basic initiating events are reached.

Event trees– Appear similar to fault trees, and may used the

same representations.– However, event trees are used to identify the

effects of an event instead of the causes.– Rather than starting from a particular system

event, and working backward to the causes, an event tree traces a primary event forward in order to determine the consequences of the event.

– Event tree analysis is inductive as opposed to the deductive fault tree analysis.

Failure Mode Effects Analysis (FMEA)– FMEA is an inductive method used to

systematically consider the effects of all failure modes.

– The system is decomposed into its component parts, each of which has known or anticipated failure modes.

– Each failure modes is analyzed, with an examination of the cause, effects, severity, probability and prevention or mitigation of each failure mode.