International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 1

Number1

March 2007

Assessment of Navigational Safety in Vessel

Traffic in an Open Area

Z. Pietrzykowski

Maritime University of Szczecin, Poland

ABSTRACT: The assessment of a navigational situation is of major importance for safe vessel conduct as

well as countermeasures and reduction of threats resulting from undesired events. These events include,

among others, technical failures and human errors. To describe cause and effect relationships fault tree and

event tree analyses are used. An algorithm has been presented for the determination of vessel collision

probability in an open area, based on the methods mentioned.

1 INTRODUCTION

The level of navigational safety depends on a variety

of factors: the present navigational situation, correct

functioning of technical equipment and systems, and

navigators’ knowledge, experience and skills. The

probability of ships’ collision is one of the indicators

of navigational safety level. The determination of

this probability makes up a basis for collision risk

determination and risk management.

The fault tree method is one of the most

frequently used methods of system analysis. This

analysis has a deductive character and focuses on a

specified undesired event (usually a failure or

inoperability of a system), and its aim is to indicate

possible ways for a desired situation to take place.

The fault tree is a logical scheme showing the

manner in which a fault or system inoperability may

be caused by other events. The development and

analysis of a fault tree for the process of vessel

traffic are based on information on the process,

equipment and human factor. The main stages of the

FTA are:

− describing the system for FTA purposes and

setting the system limits,

− identification of threats (risks),

− error tree construction,

− qualitative and quantitative analysis of the error

tree.

The above method was used for the assessment of

a navigational situation in an open sea area – the

determination of ships’ collision probability.

2 ASSESSMENT OF NAVIGATIONAL SAFETY

- ASSUMPTIONS

While constructing an algorithm for the deter-

mination of ship collision probability the following

assumptions were made:

1 The examined area for which the ship collision

probability is to be determined is an open sea

area.

2 An encounter situation involving several ships is

considered and analyzed by decomposing it into

encounters of two ships.

3 Ship encounter situations are described by the

parameters of ship status vectors (position,

course, speed, size, manoeuvring characteristics).

4 Ship’s operational states are considered, such as

rudder or main engine failures, and a blackout.

5 Hydrological and meteorological conditions are

taken into account in the algorithm.

6 The algorithm accounts for the human factor

involved into the decision making process of ship

conduct as well as navigators’ errors that may

result in a ship collision.

7 Cause-and-effect relations of ship collisions are

described by methods of fault tree and event tree.

8 Cause-and-effect relations of ship collisions

implemented in the algorithm enable indicating

possible areas of risk reduction.

9 Events that may bring about ships’ collision are

described with the probability of their occurrence.

10 The effect of algorithm operation is a determined

value of collision probability of two encountering

ships.

3 GENERAL CHARACTERISTIC

OF THE ALGORITHM

The algorithm of determining the probability of ship

collision in an open sea area is based on probabilistic

methods of risk analysis and assessment. Fault trees

and event trees of ship collisions in an open sea area

have been developed.

While constructing a fault tree model, the cause-

and-effect relationships for ship collision events

were analyzed. On this basis a model of ship

collision event tree was developed, presenting the

influence of basic failure components on the effect,

i.e. ships’ collision.

Both external and internal events were taken into

account, as well as operating states that might lead to

a collision, such as equipment failures, human errors

(human factor), the environment. The considerations

herein concern the technical and procedural

solutions in use, aimed at an enhancement of the

reliability of system elements and the whole human-

ship-environment system.

A model of ships’ encounter was built (Fig. 1),

taking into account navigators’ decision processes

(Fig. 2), aimed at a solution of collision situations.

Much attention was paid to the human factor and

errors done by humans, errors that might lead to

ships collision.

hydro-

meteorological

conditions

own ship

area

other ships

area type:

open, restricted

size (length,

breadth, draft),

properties

Navigational situation

regulations in

force

size (length,

breadth, draft),

properties

wind, current,

visibility, waves,

tides, icing

COLREGs

local

regulations

parameters

parameters, state

vecotor values

parameters, state

vecotor values

parameters

detailed

regulations

Fig. 1. Model of ship encounter situations

Human errors done at each stage of the decision.

information

acquisition

sit. analysis

and assessment

making

a decision

taking

action

Fig. 2. Stages of the decision making process

On this basis detailed fault trees for normal and

failure states were drawn.

Making use of the available literature and

gathered experimental data, density distributions or

probability values for elementary events in the

modeled event trees were determined.

Basic operators of probability sum and probability

product were used for the determination of ship

collision probability based on the models designed.

The algorithms was verified using accident

statistical data (collisions of ships) in the examined

areas.

4 COLLISION CAUSES

Marine accident statistics show that human errors

and failures of shipboard equipment and systems are

main reasons of ship collisions.

In this analysis of ship collisions and the

construction of a relevant fault tree human errors

were classified into two groups:

− errors and mistakes: absent-mindedness, lack of

attention;

− offences (violation of regulations or procedures):

absence on the bridge, falling asleep, accident,

illness, alcohol).

In the case of errors and mistakes, there is a

possibility of detecting the error based on the three-

minute cycle of ship conduct, while in the case of

close quarters (collision) situation, due to the

obligation of constant observation (Fig. 3).

ship conduct

Process

error

detection

Start

Stop

Fig. 3. Ship conduct process

Equipment and system failures were divided into:

− propulsion failure,

− rudder failure,

− blackout.

These events may occur at various stages of the

ship conduct process:

− observation (distance between ships 8-12 Nm),

− decision making (distance between ships 4-8 Nm),

− taking action (distance between ships 2-4 Nm) -

collision situation.

Taking action by performing a collision

preventing manoeuvre follows mostly when the

minimum values of the closest point of approach and

the time to closest point of approach (CPA

and

TCPA

) are exceeded.. Normally one nautical mile

distance is assumed as the closest point of approach

and 10 minutes as the time to CPA.

5 FAULT TREE METHOD IN THE

ASSESSMENT OF SHIP COLLISION

PROBABILITY

The fault tree method consists in writing down

relations between events being causes and other

events being their effects.

When two ships encounter, possible causes of

collision may be both internal to either of the ships,

and external to the ships, resulting from traffic

conditions in a given area. The causes of undesired

events may be divided into two categories:

1 Ship conduct: the ship is capable of navigating

safely. Collision risk is only due to navigator’s

errors;

2 Operational states and the environment: the ship

is not capable of navigating safely. The causes are

failures to shipboard equipment and systems. A

risk of collision is due to: a) restricted or lacking

information necessary in the process of analyzing

and assessment of a navigational situation, e.g.

due to radar failure in poor visibility; b) restricted

manoeuvrability or difficult navigational

conditions prevailing in the area.

A collision may be a consequence of such an

error as deviation from the collision course. The

ships will collide if the error is done on both ships

(Fig. 4).

ship collision

Error of deviating from

collision course by ship 1

Error of deviating from

collision course by ship 2

Fig. 4. Fault tree of ship collisions

6 FAULT TREE OF DEVIATING FROM A

COLLISION COURSE

Causes of collision course deviation errors are these:

− error of collision situation identification,

− error of preventive manoeuvre performance.

The fault tree of the collision course deviation is

presented in Fig. 5.

error of collision

course deviation

error of collision

situation

identification

no error

detected

error of preventive

manoeuvre

performance

no error

detected

Fig. 5. Fault tree of collision course deviation

7 FAULT TREE OF COLLISION SITUATION

IDENTIFICATION ERROR

The errors consisting in wrong identification of a

collision situation are as follows:

− error in the determination of the closest point of

approach (radar or gyrocompass error),

− human errors,

− radar failure in poor visibility.

It is possible to detect and correct an error in

determining the closest point of approach or an error

due to absent-mindedness in subsequent cycles of

ship conduct. The fault tree of the collision situation

identification error is presented in Fig. 6.

8 FAULT TREE OF PREVENTIVE

MANOEUVRE PERFORMANCE ERROR

The causes of errors in performing a collision

preventing manoeuvre may be these:

− failing to perform a manoeuvre due to human

errors,

− failing to perform a manoeuvre due to rudder or

propulsion failure or blackout,

− wrong manoeuvring parameters (insufficient

course or speed alteration).

− wrong ship command (error in giving a

command, error in performing a command).

Due to the fact that in open sea navigation the

most frequent collision avoiding manoeuvre is an

alteration of the ship’s course, the fault tree does not

take include speed alteration (Fig. 7).

poor

visibility

radar

failure

collision situation

identification error

CPA determination error

no error

detected

offences

Absence

falling

asleep

accident

illness

Fig. 6. Fault tree of a collision situation identification error

control error

no error

detected

preventive manoeuvre

performance error

wrong parameters of

manoeuvre

offences

no error

detected

failures

rudder

failure

propul-

sion

failure

blackout

Fig. 7. Fault tree of preventive manoeuvre performance error

The presented fault trees provide a basis for

building corresponding event trees, enabling the

determination of ship collision probability in open

sea navigation.

9 SUMMARY

A method of navigational safety assessment based

on probabilistic methods has been presented. This

method enables the determination of ship collision

probability in a situation where ships encounter in an

open sea area. The collision probability has been

determined by using methods of fault tree and event

tree analyses. On this basis we can estimate the

navigational risk and examine ways of counteracting

and reducing risks in vessel traffic – risk

management in sea transport, which is of major

importance for the assurance of safe navigation, thus

for the safety of people, cargo, ship and the marine

environment.

REFERENCES

Guidelines for Formal Safety Assessment (FSA) for Use in the

Imo Rule-Making Process, IMO MSC/Circ.1023, April

2002.

IALA Guidelines on Risk Management, December 2000.

Jaźwiński J., Ważyńska-Fiok K., 1999, Systems safety, PWN

Warszawa (in Polish).