Event Tree Analysis

Failure Tracing Methods

2 Integra Training and Consulting

TA

Event Tree Analysis starts with the initiating event (eg loss of containment), asks if controls will work, ends up on right with consequences

Failure Tracing Methods

3 Integra Training and Consulting

F A

Fault Tree Analysis starts with the top event (eg accident), asks Why, work down until root causes are revealed

Define system

Analyse Consequences

Hazardous Events

Continuing Hazards

Identify Hazards

Estimate Risks

Is Risk Tolerable?

RiskControlStrategy

Monitor

Event Tree Analysis

Inductive technique to analyse the consequences of an event, action or decision.

Based on decision trees using binary logic. Start with initiating event and identify

consequences. May be quantified.

ETA

Initiating Event

Example - Fire

Initiating event- Fire starts

Yes

No

Firedetected?

Fire Detection and Sprinkler System

Water

Smoke detector

Valve

Water tank

ETA example - Fire

Initiatingevent

YES

NO

Detector Alarm Water

Success

Minor Damage- wet people

Major fire- possiblefatalities

Fire

Valve

Personnel escapeMajor damage

Quantification of Event Trees Allocate probability to each event. Use binary logic. Probability event occurs + Probability it

does not occur =1. P Yes + P No =1 P Yes = 1 P No

ETA example - Fire

Initiatingevent

Success

Failure

Detector Alarm Water

Success

Minor Damage- wet people

Major fire- possiblefatalities

Fire

Valve

Personnel escapeMajor damage

Quantification of Event Tree Probability of each outcome is obtained by

multiplying the probability of each of the events.

E.g. If probability of each event is: 0.8 and 0.9 Probability of both events occurring is 0.8 x

0.9 = 0.72

ETA example - Fire

Initiatingevent

Success

Failure

Detector Alarm WaterSuccess

Minor Damage- wet people

Major fire- possiblefatalities

Fire

Valve

Personnel escape

P= 0.9

P = 0.95

P = 0.9P = 0.95

P = 0.1 P = 0.1

P = 0.05

P = 0.05

P = 0.1

P = 0.731

P = 0.0385

Calculation of Risk

Need to estimate the frequency of the initiating event.

Multiply this frequency by the final probability of each branch of the tree.

Can calculate Individual Risk if time exposed and vulnerability are estimated.

Example LPG Release

Event tree exampleA low pressure storage vessel is connected via pipework to a manufacturing plant which could, in the event of malfunction, generate a pressure great enough to rupture the vessel. To prevent this a pressure detector is installed in the low pressure storage vessel. If pressure starts to rise above an acceptable level the detector activates a valve control system. This in turn closes the inlet valve to the vessel isolating it from excessive pressure. It has been estimated that pressure great enough to rupture the low pressure storage vessel would be generated once every four years on average.Reliability data for the system is given below

Component ReliabilityPressure detector 0.95Valve control system 0.99Inlet valve 0.8

(a) Construct an event tree for the protective system described above AND use it to calculate the frequency of a rupture of the low pressure storage vessel. (12)

(b) It is proposed that, in addition to the protective system described above, the low pressure storage vessel is also fitted with a suitable pressure relief valve (reliability 0.9). Assuming that the vessel would only rupture if both the protective system and the pressure relief valve failed at the same time, calculate the frequency of rupture of the low pressure storage vessel in these circumstances. (4)

(c) Outline the issues that would need to be considered when deciding whether both protective systems were needed on the low pressure storage vessel. (4)

Answer (a) The frequency of rupture of the low

pressure storage vessel as 0.062/yr or once every 16.2 years by using the following Calculation = 0.047 + 0.0024 + 0.0125.

Credit was also given for alternative andcorrect methods of calculating the frequency of rupture

Answer (b)The required calculation involved multiplying the system failure probability from the first part of the question (0.062) by the failure probability of the pressure relief valve (0.1) to give a combined system failure probability of 0.0062/yr or a frequency of once every 161 years. Alternative and correct methods of calculating the frequency of rupture also received credit.

Answer (c)The issues that would need to be considered when deciding whether both protective systems were necessary include: the probability of rupture; the consequences of rupture; legal requirements and HSE guidance; industry codes of practice and guidance; risk tolerability criteria such as those contained in Reducing Risks, Protecting People and the cost of the protective systems and their reliability. While answers were generally better than those for the first two parts of the question, many still did not fully understand the relevant issues.