International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 6
Number 1
March 2012
11
1 INTRODUCTION
Traffic conflict refers to the event of vessel interfer-
ence, which occurs in port waters due to the special
characteristics of port traffic in limited sea space,
high traffic density, and complex operational regula-
tions. As undesirable incidents, conflicts have a di-
rect effect on the safety of navigation. A conflict
without proper resolution may lead to a collision re-
sulting in a loss of life and property, and even
threaten the ocean environment.
In recent years marine traffic has been increasing
greatly due to the sustained growth of seaborne
trade. As a result, the port traffic network becomes
finely meshed and intensively used. The demand for
the use of sea space sometimes exceeds the available
capacity, such that even a small interaction (i.e. a
conflict) between vessels may have a large impact
on the entire network. The most common product of
a conflict is time delay, which results from the eva-
sive maneuvers of vessels to avoid a collision. With-
in a saturated network, these delays can slow the
speed of traffic stream, increase vessel-waiting time
and the length of waiting queue. Traffic congestion
would arise accordingly.
The world’s busiest ports are faced with potential
risk of traffic conflicts. However, maritime control
centers often can only play an advisory role, which
cannot satisfy the demand on traffic management
arising within port waters. There is no positive con-
trol as to conflict avoidance.
If conflict risk could be predicted in advance, we
could take appropriate measures to resolve or elimi-
nate conflicts so as to avoid incidents/accidents and
reduce the impact of conflict on network efficiency.
However, to the best of this researcher’s knowledge,
no systematic method has been developed for detect-
ing marine traffic conflicts. A review of past studies
related to marine traffic safety revealed that almost
all were focused on collision avoidance. Neverthe-
less, a conflict can be considered as a collision risk
with a low degree of danger. Hence, works in colli-
sion avoidance are worth reviewing, which could
provide valuable reference to this research.
A Simulation Model for Detecting Vessel
Conflicts Within a Seaport
Q. Li & H. S. L. Fan
School of Civil and Environmental Engineering, Nanyang Technological University, Singapore
ABSTRACT: Conflicts represent near misses between two moving vessels, and often occur in port waters due
to limited sea space, high traffic movements, and complicated traffic regulations. Conflicts frequently result
in congestion and safety concerns. If conflict risk can be predicted, one could take appropriate measures to re-
solve conflicts so as to avoid incidents/accidents and reduce potential delays. To the best of this researcher’s
knowledge, no systematic study has been carried out on the issue of detecting marine traffic conflicts. In this
paper, we present an algorithm designed to determine a conflict using the criterion of vessel domain. The al-
gorithm aims to evaluate the relative positions of vessel domains to detect potential conflicts. To implement
the algorithm, a simulation model has been developed in Visual C++. The model at present provides a single
function for conflict detection but can be expanded to a multi-functional system for resolving conflicts in fu-
ture work.
12
Two criteria are used in past studies for determin-
ing a collision risk: the closest point of approach
(CPA) and ship domain.
The criterion of CPA is applied with two parame-
ters: distance of closest point of approach (D
CPA
) and
time of closest point of approach (T
CPA
).The value of
CPA parameters indicates the relative position be-
tween two vessels. For example, a smaller CPA in-
dicates a higher risk of collision. The CPA parame-
ters are applicable in a collision avoidance system,
which can guide vessel to execute proper anti-
collision maneuvers. An example is Lenart’s studies
(Lenart 1999, Lenart 2000) on what speed and/or
course maneuver should be undertaken to achieve
the required CPA time and distance.
The criterion of CPA is difficult to use in restrict-
ed waters such as narrow fairways. In view of this,
the concept of ship domain has been proposed as a
more comprehensive and accurate criterion. It can be
explained as “a water area around a vessel which is
needed to ensure the safety of navigation and to
avoid collision” (Zhao et al. 1993). Vessel domain
was first presented by Fujii et al. (1971). Based on
field observations, Fujii’s study established a do-
main model for a narrow channel. Later, Goodwin
(1975) developed a domain model in open sea. Be-
sides presenting a model, the study also analyzed
how traffic density and length of vessel affect the
size of vessel domain.
The shape and size of a vessel domain are affect-
ed by a number of factors (vessel’s speed and length,
sea area, traffic density etc.). As different factors are
taken into account, ship domains proposed by vari-
ous studies differ from one another. Many studies
have focused on improving the vessel domain model
(Davis et al. 1980, Coldwell 1983, Zhu et al. 2001,
Pietrzykowski 2008).
In a port traffic system, vessels traveling along
fairways are required to keep various safety clear-
ances in accordance with the ports regulation. The
domain of a vessel can thereby be referred to as the
clearance area around it. This paper would attempt
to design an algorithm to detect conflicts using the
criterion of ship domain. That is, the relative posi-
tions of the domains of two vessels will be evaluated
before they actually encounter each other. If the do-
main of a vessel will interfere with the domain of the
other, a potential conflict is indicated.
A simulation model is developed to implement
the algorithm, using Visual C++ 6.0. In the simula-
tion model, conflicts can be detected for a given de-
mand schedule of marine traffic within a seaport.
The first and most important goal of conflict detec-
tion is to enable safe navigation and avoid collision
between vessels. For system optimization, attention
should also be paid to reduce the impact of conflicts
on network efficiency so as to improve traffic condi-
tions within the seaport.
This paper is structured as follows: Section 1 in-
troduces the issues addressed; Section 2 presents an
overview of the simulation model; Section 3 de-
scribes the algorithm for conflict detection; Section
4 focuses on simulation model implementation; and
Section 5 summarizes findings and proposes future
work.
2 OVERVIEW OF SIMULATION MODEL
2.1 The seaport traffic system
A seaport traffic system can be treated as a network
of nodes and links. Within the network each link in-
dicates a section of a fairway, and a node can be a
berthing/anchorage area, a boarding point for port
pilots, an intersection area of fairways, or a separa-
tion point dividing a fairway into two sections due to
differences in widths and/or traffic regulations. The
route of a vessel can be represented by a path in the
network consisting of a series of nodes and links.
Figure 1 shows a seaport traffic system we use in
the simulation model, where black dots represent the
nodes and a rectangle between two nodes indicates a
link. The width of a rectangle indicates the width of
the link. A vessel is only allowed to travel within the
link.
Figure 1. A seaport traffic system for Singapore.
2.2 Flowchart for conflict prediction
A seaport traffic system usually involves a large
number of vessels. We need to detect a potential
conflict between any pair of vessels. For any pair of
vessels, the system will check whether they will con-
flict or not in a time interval (t
0
, t
3
).
There are two situations in conflict detection:
Node conflict prediction: two vessels traveling
toward the same node are on different links.
Link conflict prediction: two vessels traveling
toward the same node are on the same link.
13
In the first situation, the two vessels may have a
conflict when they are passing the node. Thus, be-
fore the two vessels reach the node, the system
needs to predict whether the two vessels will have a
conflict.
In the second situation, the two vessels may en-
counter a conflict on the link. However, if the fair-
way is sufficiently wide so that a vessel can overtake
the other safely, the conflict will not occur. Thus, the
factor of the link width should be considered into
conflict detection on a link. These are described in
the next section.
Note that, the relative position between two ves-
sels varies as vessels are moving. The conflict situa-
tion would change accordingly. Suppose that the
vessels have a risk of conflict during a certain time
period. According to the changes in vessel trajecto-
ries, this time period is divided into several time in-
tervals. The system needs to separately evaluate the
conflict situation during different time interval.
Figure 2 shows the flowchart for conflict detec-
tion (the notations t
0
, t
1
, t
2
, t
3
are defined in Section
3).
Figure 2. Flowchart for conflict prediction.
3 DETERMINE A CONFLICT BETWEEN TWO
VESSELS
3.1 Preliminaries and assumptions
Denote a vessel as V (O, d, Φ, Ψ, Φ
ˉ
, Ψ
ˉ
1
, Ψ
ˉ
2
) as
shown in Figure 3. For the purpose of simplifying
analysis, a vessel is regarded as a rectangle V, whose
dimensions are Φ (width) and Ψ (length). Suppose
O(x, y) is the center of the vessel. At present, it is
travelling along the direction d.
The clearance area of a vessel is defined as a zone
within which the vessel can keep enough distance to
avoid conflict with each other. The clearance area
varies according to differences in a vessel’s outline,
dimension, technical parameters and fairway charac-
teristics. In this research, the shape of a vessel’s
clearance area is assumed as a rectangle R. The pa-
rameter Φ
ˉ
refers to the vessel’s lateral clearance.
Vessel’s longitudinal clearance is represented by pa-
rameter Ψ
ˉ
1
in the direction of the bow and Ψ
ˉ
2
in the
direction of the stern. The values of these parameters
(Φ
ˉ
, Ψ
ˉ
1
, Ψ
ˉ
2
) are specified by regulation. These pa-
rameters can be set up in a simulation system as in-
put data.
Figure 3. A vessel and its domain.
3.2 Node conflict prediction
Two vessels, V
1
and V
2
, on different links travel to-
ward the same node. Table 1 lists the navigation in-
formation, where t
1
< t
2
, i.e. V
1
will reach the node
before V
2
.
Table 1. Two vessels on different links
______________________________________________
V
1
V
2
______________________________________________
Position A E
Velocity before the node v
1
v
2
Velocity after the node vˉ
1
vˉ
2
Time to the node t
1
t
2
Time to the next node
ˉ
t
1
ˉ
t
2
______________________________________________
Suppose t
0
= 0, t
3
= min (
ˉ
t
1
,
ˉ
t
2
). We aim to check
whether there is any conflict during the time interval
(0, t
3
). The movements of V
1
with respect to V
2
are
different in three different time intervals
In the time interval (t
0
, t
1
), the velocity of V
1
with
respect to V
2
is w
1
= v
1
- v
2
.
In the time interval [t
1
, t
2
], the velocity of V
1
with
respect to V
2
is w
2
= vˉ
1
- v
2
.
In the time interval (t
2
, t
3
), the velocity of V
1
with
respect to V
2
is w
3
= vˉ
1
- vˉ
2
.
Figure 4 shows the movement of the center of V
1
with respect to the center of V
2
. With respect to V
2
,
starting at A, V
1
passes B at t
1
, moves from B to C
during [t
1
, t
2
], and reaches D at t
3
. Thus,
( )
( )
( )( )
( )
12 1 2 2
212 1 212
33 1 2 3
,
,
.
AB t t
BC tt tt
CD t t
= =-
= -=- -
= =-
w vv
w vv
w vv
14
At location A, the domain of V
1
follows its mov-
ing direction v
1
(Fig. 5(a)). Similarly, the domains of
the vessels at different locations can be obtained
(Table 2). Suppose q
5
ij
= q
1
ij
, i = 0, 1, j = 0, 1, 2, k = 1,
2, 3, 4. Table 2 tells that
Q
ij
is a domain of the vessel V
i
at t = t
i
,
q
k
ij
is the k-th corner of the domain Q
ij
,
q
k
ij
q
ij
k+1
is the k-th edge of the domain Q
ij
.
The movement of the domain of V
1
with respect
to the domain of V
2
is denoted as the relative move-
ment of V
1
to V
2
. For example, referring to Figure 4,
Figure 5 shows the relative movements of V
1
to V
2
,
in the three different time intervals.
Figure 4. The movements of the center of V
1
with respect to the
center of V
2
: (a) In time interval (0, t
1
); (b) In time interval [t
1
,
t
2
]; (c) In time interval (t
2
, t
3
).
Table 2. Domain of vessels at different locations
______________________________________________
Location Domain of V
1
Domain of V
2
______________________________________________
t =t
0
Q
10
(q
1
10
,q
2
10
,q
3
10
,q
4
10
) Q
20
(q
1
20
,q
2
20
,q
3
20
,q
4
20
)
t =t
1
Q
11
(q
1
11
,q
2
11
,q
3
11
,q
4
11
) Q
21
(q
1
21
,q
2
21
,q
3
21
,q
4
21
)
t =t
2
Q
12
(q
1
12
,q
2
12
,q
3
12
,q
4
12
) Q
22
(q
1
22
,q
2
22
,q
3
22
,q
4
22
)
______________________________________________
For any j=0, 1, 2, in the time interval (t
j
, t
j+1
), the
velocity of V
1
with respect to V
2
is w
j+1
. The move-
ment of the corner q
k
1j
with respect to V
2
is a line
segment q
k
1
p
k
1j
where
p
k
1j
= q
k
1j
+ (t
j+1
- t
j
) w
j + 1.
Thus, the movement of the edge q
i
k
q
i
k+1
with re-
spect to V
2
is P
k
j
= q
k
1
q
k+1
1j
p
k+1
1j
p
k
1j
(Fig. 6). If V
1
and V
2
conflict with each other, the movement of at least
one edge of V
1
will intersect with the domain of V
2
,
i.e.
P
k
j
Q
2j
Ø.
Figure 6 shows an example when there is no con-
flict between V
1
and V
2
. Figure 7 is another example
when there is a conflict between V
1
and V
2
.
In summary, V
1
and V
2
will conflict in the time
interval (t
j
, t
j+1
) if and only if
(P
k
j
Q
2j
)
Ø.
In this way, the conflict detection turns to check-
ing whether two parallelograms intersect with each
other or not.
3.3 Link conflict prediction
Suppose a vessel V
1
follows another vessel V
2
along
a link (see Fig. 8(a)). Table 3 lists the navigation in-
formation of these vessels. The velocity of V
1
with
respect to V
2
is
w
1
= v
1
- v
2
.
If v
1
is not larger than v
2
, V
1
and V
2
will conflict if
and only if
12
2
LL
AE
+
<
.
Suppose t
3
= min(t
1
, t
2
). We need to check wheth-
er the two vessels will conflict with each other dur-
ing (0, t
3
). After that, the two vessels will not be
conflicting on the link, because one vessel leaves
this link. If v
1
is larger than v
2
, during (0, t
3
), the
relative movement of V
1
with respect to V
2
is shown
in Figure 8(b), where
Figure 5. The relative movement of V
1
to V
2
: (a) In time inter-
val (0, t
1
); (b) In time interval [t
1
, t
2
]; (c) In time interval (t
2
, t
3
).