Study of Application of Fault Tree Analysis Method in Safety Risk

Assessment of Large-Scale Activities

Quanmin Bu

and Yuehong Xu

Department of Security Studies, Jiangsu Police Institute, Nanjing, 210031, China

Institute of Higher Education, Nanjing University of Information Science and Technology, Nanjing, 210048, China

Keywords: Large-scale Activities, Safety Accident, Fault Tree Analysis Method, Risk Assessment.

Abstract: With China's rapid economic development, the number of all kinds of large-scale activities has continued to

increase, and the scale has also expanded constantly, the population have become more and more large, and

the structure has become more and more complex. Therefore, safety accidents take place frequently in large

activities, and cause serious casualties. In such a situation, security risk assessment of large-scale activities

draws more and more attention. In this paper, the author introduced fault tree analysis method into the

security risk assessment system of large-scale activities, and elaborated on the application of fault tree

analysis method in every aspect of security risk assessment of large-scale activities.

1 INTRODUCTION

In the “Kummel La Festival” which was held in

Nasik area of Maharashtra state of India in 2003, the

situation was out of control, at least 30 persons died,

and 50 persons got injured. On February 5, 2004, a

crowd accident took place in Minong Park of Mayan

County, Beijing, causing 37 deaths. On January 22,

2005, a serious congestion accident occurred in the

Holy Land Meghan M Nile, causing about 500

injuries. The casualties and loss caused by accidents

in large-scale activities were shocking. With the

rapid economic development in recent years, various

cultural and sports activities, exhibitions were held

frequently, and the scale expanded constantly, the

population and the structure also became more and

more complex, such as the 2008 Beijing Olympic

Games, 2010 Shanghai world expo. These events

were effective propulsion of China's economic

development, and brought enormous challenge to

domestic security work. (Komal 2020) Good

prevention work has become an urgent and arduous

task for preventing accidents in large scale activities

and ensuring the development of Chinese economy.

Practice in risk assessment of large-scale activities

in foreign countries offer us some new ideas.

Compared with former experience in security job, in

the safety risk assessment of large-scale activities,

various factors affecting the occurrence of risk are

analyzed, the probability of accidents and accident

consequences are predicted through the use of

historical data, and risks of large-scale activities are

quantified. In this way, the security work has

minimum risk.

The fault tree analysis method, also known as

the method of accident tree analysis, abbreviated as

FTA, is one of the important analysis methods of

safety system engineering, it can carry out

identification and risk evaluation of various systems,

analyze the direct cause of the accident, and reveal

the potential causes. Fault tree method is clear and

logic in the description of the causal relationship in

the accident. It can be used for qualitative analysis,

and quantitative analysis, and is often used to solve

reliability problem in complex system, and can also

be used for risk assessment. (Yazdi, Korhan,

Daneshvar 2020) Because the cause of the accident

in large-scale activities is a complex system, the

introduction of the fault tree analysis method into

the security risk assessment of large-scale activities

has practical significance.

2 FAULT TREE ANALYSIS

METHOD

Fault tree analysis was firstly proposed by

A · B · Mines from American Baer telephone

laboratory in 1962.At that time, it was mainly used

778

Bu, Q. and Xu, Y.

Study of Application of Fault Tree Analysis Method in Safety Risk Assessment of Large-Scale Activities.

DOI: 10.5220/0011767600003607

In Proceedings of the 1st International Conference on Public Management, Digital Economy and Internet Technology (ICPDI 2022), pages 778-783

ISBN: 978-989-758-620-0

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

in research on the control system of missile launcher.

In 1974, American Atomic Energy Commission

carried out a risk assessment on a nuclear power

plant accident with the use of fault tree analysis

method, published the famous "Lampson report".

The report involved extensive application of fault

tree analysis for the first time, and was a great

success. Since then, the method of fault tree analysis

caused great repercussions and received wide

attention, thus was applied and promoted in many

countries and enterprises. In China, the introduction

and study the method of fault tree analysis began in

1976, at present, it has been applied and promoted in

many sectors and enterprises, and achieved great

progress. (Onur, Elifcan 2021) At the end of the 80's,

the fault tree analysis method was applied in safe

production and labour protection of the railway

transportation system, and good results was

achieved. And now, the fault tree analysis method is

also widely applied in nuclear industry, aerospace,

machinery, electronics, shipbuilding, chemical

industry and other fields.

The method of fault tree analysis adopts inverse

method of causal relationship, and is the deductive

reasoning that adapt to human thinking habits. The

method of fault tree analysis starts from results (top

events) to causes (lower level events), until

decomposition ends. In the process of analysis, a

particular accident (top event) is connected with

causes at various levels (lower level events) with

logic gate symbols, thus getting a tree graph

(namely fault tree) that describes the logical

relations in a vivid and brief way. Then the analysis

and assessment are conducted through simplification

of the fault tree and calculation.

3 THE APPLICATION OF FTA IN

PREPARATION STAGE OF

SECURITY RISK ASSESSMENT

IN LARGE-SCALE ACTIVITIES

To carry out an accurate security risk assessment of

large-scale activities, it is necessary to do a

preparatory job before the evaluation. Careful

analysis of reasons for safety accidents of large-

scale activities and the accumulation of evaluation

materials need to be done. At this stage, the fault

tree analysis method can be introduced for the fault

tree modelling of various safety accidents in large-

scale activities. The establishment of the fault tree

has the following steps:

3.1 Determine the Top Event

Top event is the accident to be analyzed. In the risk

assessment of large activities, top events are various

accidents that may occur in large-scale activities,

including fire accident, crowd accident, etc. It

should be noted that, when selecting top events, it is

necessary to concentrate on certain kind of accident,

because large events accident is a enough broad

concept, and is not conductive to the establishment

of a fault tree model. (Zhang, Yin 2020) When

selecting top events, data on the possibility of the

occurrence of accidents and severity of accidents

need to be prepared.

3.2 Investigation on Various Reason

Incidents

Reason events are all direct reasons and factors

related to the accident. After determining top events,

in order to prepare the fault tree, it is of great

necessity to find out all direct causes for the top

events. As mentioned above, various safety

accidents in large scale activities have complex

factors, including human factors, material factors,

environmental factors, management factors, etc. In

conducting the investigation, investigation and data

statistics methods can be applied. Investigation

results must be truthful, objective and

comprehensive.

3.3 Drawing Fault Tree

After finding various reasons for top events,

appropriate event symbols and logic gates can be

used to connect and analyze them until the most

basic reason event is found, in this way, a fault tree

is formed.

When connecting reason events of different

levels, if the top events occur only when all the

lower level events take place, the "and" symbol is

used. If the top events occur when one underlying

event occur, then "or" symbol is used. Connection

of logic gate is very important in fault tree, it relates

to the logical relationship between various events,

directly affects qualitative analysis and quantitative

analysis of the fault tree. (Shahabuddin, Mat

Bistaman, Subri 2020) The logical relationship

between various event should be strict, reasonable,

repeated deliberation, modification, and even restart

are needed in the process of drawing the fault tree,

until it is in line with the actual situation.

Once the fault tree model of all kinds of

accidents is established, the fault tree analysis can

Study of Application of Fault Tree Analysis Method in Safety Risk Assessment of Large-Scale Activities

779

be formed. The main purpose of the fault tree

analysis is to find out the relationship between basic

events and top events, and between different basic

events, thus reach corresponding conclusions and

offer the basis for security risk assessment of large

events and the development of countermeasures.

The main content of analysis is simplification of the

structure of the fault tree, obtain minimum cut sets

and the minimal path sets, determine ordering of the

structure importance of each basic event, then find

out the probability of each basic event through

statistical data or expert data, and calculate the

probability, importance and critical importance

degree of the top events.

3.4 Minimum Cut Set

In the fault tree, the collections of basic events that

can lead to the occurrence of top events are called

cut set. Minimum cut sets is a set of basic events

that cause the occurrence of top events, namely, top

event will not take place if any of the basic events in

the minimum cut set does not occur. One minimum

cut set represents an accident model, and all the

minimal cut sets represent all accident models.

In the fault tree analysis, first of all, find out all

the minimal cut sets of the fault tree. Figure out all

possibilities of top events. The treatment of the fault

tree is generally carried out with the use of the

Boolean algebra law, starting from the basic events,

and determine the logic relationship between all the

accidents according logic gates of the fault tree.

(Komal 2020) The logic gate "and" means that

output event occurs only when connected events

both happen, the symbol is "∩". The logic gate "or"

means that the output event occur when any of

connected events happen, the symbol is “∪”.

We judge from the number of minimal cut sets,

generally, the more minimum cut sets, the more

occurrence models for top events, and more risks for

accident system. From the combination of minimum

cut sets, the combination of basic events in the

minimal cut sets represent the ways for occurrence

of accident, and the key factors causing the

occurrence of accident can be found based on this.

3.5 Minimum Path Sets

In the fault tree, the collection of basic events that

cannot cause the minimum possibility of occurrence

for top events is called minimum path set. In the

minimum path set, the removal of any basic event

cannot guarantee any accident, therefore the

minimal path collection represents the safety and

reliability of the system. In order to obtain a

minimal path set, the fault tree is transformed into a

duality success tree, and various accidents cannot

occur, at the same time, "or" gate in the fault tree is

replaced with “and” gate, and “and” gate is replaced

with “or” gate, thus obtaining a success tree. The

minimum cut set of the success tree is the minimum

path set of the fault tree.

If the minimum path contains many basic events,

it means that accident prevention need to be done

from many aspects. If the minimum path contains

less basic events, it means the system is highly safe,

and accident prevention is easy.

3.6 Structure Importance Degree

Structure importance is the importance of each basic

event based on the analysis of the fault tree. Namely,

the influence degree of the occurrence of each basic

event on top events is analyzed, assuming each basic

event has equal probability of occurrence. Structure

importance is the influence degree of each basic

event on top event, based on the analysis of the fault

tree structure, without considering the probability of

basic events, therefore, structure importance is not

related to the probability of bottom events, and it is

a qualitative importance analysis.

Structure importance of each basic event can be

judged based on minimum cut sets and minimum

path sets. In general, basic events in minimal cut set

that contains a single event has the largest structure

importance. (Gachlou, Roozbahani, Banihabib 2019)

And structure importance coefficient of other events

is determined by the times of appearance in the

minimal cut set. More occurrences mean a larger

structure importance coefficient, and less

occurrences means a smaller structure importance

coefficient. If a basic event has a large structure

importance, its impact on top event is bigger, and it

should be the focus of prevention.

3.7 The Probability of Top Event

In order to obtain the probability of top event, it is

necessary to obtain the probability of each basic

event first. There are two ways: one is expert

inference method, namely professional personnel

who have a lot of experience in the field give the

probability of each basic event. (Abbasi,

Allahviranloo 2021) Second, collect some cases of

accidents that occur during a period of time, do

classified statistics of causes, thus obtain the

probability of each basic event. Then calculate the

probability of intermediate events according the

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fault tree and the logic relationship, thus obtain the

probability of top event.

Probability of top event is basic data for the

quantitative analysis of the fault tree, and important

evidence for the quantitative risk assessment of the

system.It should have a good accuracy. The gap

between the probability obtained and actual

situation should be within the reasonable error range,

so as to the guarantee of the accuracy of the data.

3.8 Probability Importance

It refers to change degree of probability for the

occurrence of top event caused by probability

change of basic event. A partial derivative of

probability function of each basic event of each

variable can produce probability importance

coefficient of the basic event.

In case of independent bottom events,

probability importance of a bottom event means

change rate of occurrence probability of top event

caused by minor change of occurrence probability of

the bottom event. Larger probability importance

means the larger impact on top event.

3.9 Critical Importance Degree

In general, the control of event with larger

occurrence probability is easier than event with

smaller occurrence probability. And the contribution

of cut set of probability importance and basic events

cannot reflect the problem. Therefore, it is necessary

to carry out a critical importance analysis.

In principle, the analysis of the fault tree

involves the 6 basic contents. Analysis about

minimum cut sets, minimum path sets and structure

importance degree is qualitative analysis, and

analysis of top event probability, probability

importance degree, and critical importance is

quantitative analysis. In specific analysis, different

steps, qualitative analysis, semi quantitative analysis

and quantitative analysis can be chosen according to

various analysis purposes, manpower and material

input, analyzing ability, as well as basic

dataavailable.

4 THE APPLICATION OF FTA IN

IMPLEMENTATION STAGE OF

SAFETY RISK ASSESSMENT

OF LARGE-SCALE EVENTS

In fact, the security risk assessment of large-scale

activities is already a very mature business projects

in foreign countries, especially the developed

countries, but in our country, the introduction of

security risk assessment of large-scale events has a

short history. Over the years, public security organs

mainly adopt experience-type management in large

public events. But in recent years, safety risk

evaluation of large activities draws more and more

attention in China. For example, on September 9,

2005, the Beijing Municipal People's Congress

passed the "Regulations on safety management of

large-scale social activities in Beijing City”, which

required organizers of large-scale activities to carry

out the risk assessment, submit risk assessment

reports, develop safety work plans, and emergency

disposal pre-plan for the first time. In 2008 Beijing

Olympic Games, in the application of a security risk

assessment, some progress was made.

The introduction of fault tree analysis method

which is widely used in system safety engineering

into security risk assessment of large-scale events is

an innovation. (Jeba, Johnraja, Jebaveerasingh 2021)

The fault tree analysis method can identify all kinds

of accident risks of large-scale activities, find out

key cause of accident, and help obtain accurate risk

evaluation results, and at the same time, put forward

risk countermeasures.

(1) The application in safety risk identification

stage of large-scale activities

Risk identification is the prerequisite and basis

for risk assessment. Timely, comprehensive,

accurate access to information, identification of

accident types, influence factors, accident

mechanism of large-scale activities are necessary for

accurate risk evaluation results, taking right

decisions and implementing targeted safety

measures. Safety accidents of large activities have

many kinds and have complex causes. They are

huge systems with man, machine, and environment

interacting with each other.

In identification of safety risk of large events, if

there are no clear classification and objectives, the

final conclusion will be useless. In the application of

the fault tree analysis method, the focus of

assessment objective is accidents of higher risks.

For example, in temple fair, performances and other

large activities, the focus is on the assessment of fire

accident, in contrast in some comprehensive and

large exhibitions, many types of accidents are

evaluated to get clear objectives and focus in risk

recognition.

After forming the fault tree for various types of

accidents in large-scale activities, all relevant factors

have been clear, the logic relationship between

Study of Application of Fault Tree Analysis Method in Safety Risk Assessment of Large-Scale Activities

781

various factors and the accident has been determined,

therefore, the identification of various risks should

be based on the fault tree of various accidents, with

each bottom event as the object. At the same time, a

great attention is paid to factors with large structure

importance, probability importance degree, critical

importance degree based on the conclusion of the

fault tree analysis.

(2) The application in security risk evaluation

stage of large-scale activities.

Risk evaluation is the selection of scientific,

reasonable, applicable risk assessment methods and

assessment classification according to

characteristics of accident, probability of accident,

and influence of dangerous factors. Risk assessment

mainly has three ways: qualitative evaluation

method, quantitative evaluation method and semi

quantitative method.

The qualitative evaluation method refers to the

qualitative analysis of the characteristics, activities,

organization and management, and participants of

large-scale events. This is based on experience and

intuition. The evaluation results include some

qualitative indicators, such as compliance with

safety indicators, accidents, and accidents that

caused the accident. In the past practice, the

qualitative evaluation method is widely used in the

safety risk evaluation of large-scale activities.

Although the method is simple, easy to understand,

and easy to learn, its conclusions have certain

limitations. Sometimes, evaluators may differ from

each other for different reasons and different

evaluation results.

In quantitative evaluation method, a lot of

experimental results and accident data are analyzed.

Personnel capacity, security management situation,

facilities and other aspects of large-scale activities

are calculated quantitatively. The assessment results

are some quantitative indexes, such as occurrence

probability of accident, accident injury (or damage)

range, quantitative risks, accident relevance or

importance of leading factors, Obviously, the

conclusion from the fault tree analysis belongs to

quantitative results, therefore, quantitative

evaluation method applies to the risk assessment of

fault tree analysis. Due to limited conditions, fault

tree analysis can only reach the qualitative analysis

stage, and its conclusions about the minimum cut set,

minimum path set, and structural importance are

also applicable to semi-quantitative evaluation

methods. The results from quantitative or semi

quantitative evaluation are more scientific than

qualitative evaluation results.

The main content of quantitative risk assessment

is risk calculation. The basic idea of risk calculation

is based on mathematical relationship of the theory

of risk: risk degree = accident probability* accident

severity. If we can calculate accurate risk degree, we

can carry out more precise risk classification with

the accurate risk degree, and set an accurate warning

threshold.

(3) The application in stage of risk

countermeasures development of large-scale

activities.

Development of risk countermeasure refers to

scientific deployment of resources and the

development of security implementation scheme and

emergency plan according to risk assessment results.

Risk countermeasures include reducing the

possibility of accidents and reducing accident

severity. The possibility of accident refers to the

probability of accident. To reduce the possibility of

accidents is to reduce the probability of accident. To

reduce the severity of accident, it is necessary to

take surrounding environment into account. If a

building collapses, and there is no people around, it

will not cause any casualty. If the personnel density

is high, it will lead to a large number of casualties.

Two different environments mean different severity

accidents.

Using the fault tree to develop risk

countermeasures, all possible solutions of reducing

accident probability can be found on the basis of

minimum cut sets, and then choose the best solution.

The best solution for eradicating accidents can be

found based on minimal path sets to reduce the

possibility of accident. The important points and

ranking of countermeasures can be determined

according to importance (importance coefficient)

analysis, so as to enhance the prevention efficiency.

The comparison of early-warning threshold value is

conductive to the establishment of early warning

mechanism and the improvement of preventing

capacity.

5 CONCLUSION

Fault tree analysis method has been used for

analysis and risk assessment of industrial production

accidents for many years, obvious effects have been

achieved. Since the 60s of the twentieth century, the

development of fault tree has been more and more

mature, and its application has extended to different

fields such as aerospace and nuclear industry. But

the introduction of the fault tree analysis method

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into the safety management of large-scale activities

is only made in recent years, it is a new field.

(1) After the above discussion, we can see that

the use of the fault tree analysis method in safety

risk of large-scale activities has certain feasibility,

since compared to other methods, it has specific

advantages.

(2) The use of fault tree analysis was to analyze

all kinds of safety accidents in large scale activities

can help understand the accident system, find the

key links of accident, and key points of high risks,

enhance the accuracy of safety risk assessment.

Targeted risk countermeasures can be proposed

through the fault tree analysis, making assessment

activities more efficient.

(3) In this paper, taking crowd accidents as an

example, the author tried to construct the fault tree,

and carry out qualitative and quantitative analysis.

In the practice process, it was found that the fault

tree method involves a large amount of calculation.

In the future, with the aid of computer, the

calculation can be easier.

(4) The security risk assessment of major events

will become an inevitable trend, but at present, the

safety risk assessment of large-scale activities has

many problems, especially in some basic work. The

author has consulted a large number of literatures,

trying to find out the statistical data of various

accidents, but the data is incomplete and the

statistical methods are not standardized. In order to

promote the security risk assessment in large-scale

activities, a good job in data statistics is needed.

ACKNOWLEDGMENTS

This work was financially supported by the Key

research and development plan projects of Jiangsu

Province (BE2018716).

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