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OCR for page 75
SHIP CONTROLLABILITY
J.P. Hooft
Introduction
This discussion gives special attention to methods for taking ship
controllability into account in designing a waterway.
In evaluating the merits of a waterway tharbor entrance or port),
economic considerations will be based on the comparison between the
costs (building and maintaining) and the benefits (amount of cargo to
be transferred in the port). Both the costs and the benefits are
influenced by (among many other factors) the navigability of the
waterway."
When determining the navigability of the waterway, the
controllability of the ships is an integral part of a complicated
system. 2/3 For this reason, attention is increasingly devoted
nowadays to the controllability of ships as traffic densities increase,
maneuvering properties change (owing to the increase in the sizes of
ships), and more ships carry hazardous materials.
The controllability of ships Is determined by the combination of
the ship's maneuverability and the actions of an appropriate
~ ~ In addition, one will find
that for a given combination of ship and control systems, the
controllability of the two-component system will change with
environmental conditions (such as harbor configuration). For this
reason, one should be more interested in the navigability of a waterway
as determined by the effects of the total "ship-control-environment
system" rather than in the maneuverability of the ships alone.
Reluctance to determine the navigability of a waterway, or even to
determine the controllability of ships in a given waterway results from
the fact that such determinations do not hold generally. For each type
of maneuver {approach, stopping, docking) in each type of waterway
(approach channel, canal, port or berthing area at sea), different
man-mach~ne control system.s/ 6/ 7
solutions will be found.
Although the quantification of ship controllability will differ for
each case to be considered, the method will always be based on
operational research involving statistical descriptions of systems.
The results of such studies provide the possibility of performing risk
analysis.
75
\
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76
Even in the face of the difficulties mentioned, it would be very
beneficial to develop systematic information on the controllability of
ships for various waterway configurations. This information would be
most useful in establishing the preliminary design of the waterway.
This preliminary design can then be evaluated and the design corrected
and refined. This second attempt--consisting of one or two
alternatives--will need a detailed study of navigational aspects, B
taking into account the ship's controllability. These detailed studies
are often performed by means of simulation techniques (in a model
basins or simulators.
General Description
Throughout this paper, the term "ship" should be understood to
denote Ship-control system. n Separate consideration of the inherent
characteristics of the ship in the dual system will be indicated by the
term ~maneuverability. n
Since no uniform definition of ship controllability is presented in
the literature, use is made here of the following description,
illustrated in Figure 1: A ship is defined to be controllable when it
can be handled in such a way that the deviation of the actual maneuver
(described by all stated variables of the system) from the desired
maneuver remains within pre-set limits.
The essence of the description lies in two items:
a. Knowledge of the discrepancy between the actual maneuver and
the intended maneuver.
b. Knowledge of the preset limits indicating the acceptability of
this discrepancy relative to Abe available space (domain
available for the maneuver).
With respect to (a), it should be remembered that the ship's
controllability will depend on the environmental conditions as they
influence the actual maneuver. With respect to item (b), the
environmental conditions influence the ship's controllability as they
affect the degree of acceptability of certain risks.
This interaction between the influence of the ship's controllability
on the requirements of the layout of a waterway, and the influence of
the waterway configuration on the ship's controllability, necessitates
complex definition and analysis of the navigability of a waterway.
The executed maneuver shown in Figure 2 brings out these points.
Of the many possibilities, two will be discussed here.
1. Assume the preset limit reads: The controllability of the
ship should be such that the ship will never hit the banks of
the approach channel. The ship in this case is taken to be a
tanker, and the banks are rocks. It now will be obvious that
the maneuver actually performed deviates so much from the
intended maneuver that the preset limit has been exceeded.
The loss of control in this situation could have been caused
by:
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77
disturbance
i=desired
manoeuvre
· i_
.
1
l ~
W:}'
.
S= ship
characteristics
O=actual
manoeuvre
S = inherent ship characteristics - ship's manoeuvrab~lity
- = behaviour of controlled
1
ship ~ ship's controllability
Figure 1. General description of the controlled ship by
means of a block diagram.
lo
!
l
!
l
l
l
1
~ i
l
l
l
sh ip touches bank
1
1
l
l
Figure 2: Manoeuvre performed with a simulated ship
entering a harbour through a dredged channel.
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78
a) Inadequate ship maneuverability,
b)
c)
Inability of the mariner to control the ship,
Malfunction of hardware elements in the steering system
or the navigational aids,
d) Poor channel conf figuration, or
e) Unacceptable environmental conditions such as wind,
waves, or currents.
2. Assume the preset limit reads: The controllability of the
ship should be such that the banks will only be hit by a ship
once in, for example, 10,000 passages through the channel. In
this case, the channel bottom is muddy and the ships are dry
cargo ships. It now will be obvious that the controllability
of the ships passing the waterway is acceptable when the
executed maneuver is a rare example of many maneuvers during
which the banks have been cleared.
Since the ship's controllability depends on so many items, it might
be of interest to ascertain a basic value of controllability for a
given ship. This value is principally sought to serve as a reference.
Such a reference value would represent controllability resulting in
minimum deviation between actual and projected maneuvers, or in other
words, the ship is optimally controllable when it performs maneuvers
that show the closest agreement with the hypothetical maneuver designed
for the waterway. In the paper by the SNAME H-10 panel, the
suggestion is offered that this reference value be def ined as the
n inherent controllability.
In addition to this suggestion, the following considerations might
also be of interest. Returning to the description of ship's
controllability, for the allowance of deviation of the actual maneuver
f rom the intended, some area is required at each stage of the passage
if many ships pass. This so-called width of lane can only be
determined with some chance that the ships will pass within the area.
According to the S NAME H-10 panel, this width of lane is determined by
the piloted controllability of the ship. The reference (optimal)
amount of piloted controllability is called "initial controllability, n
and can be defined as that amount of controllability for which the
width of lane will be minimal for the situation considered, ignoring
all types of disturbances.
To s bow the difference between this latter concept and the
definition of the H-10 panel, the following observations can be made:
.
Inherent controllability refers to the best abilities of the
ship resulting from its maneuverability characteristics.
Initial controllability refers to the best behavior of the
ship resulting from the combined effect of
"ship-controller-waterway" characteristics.
For the evaluation of the navigability of a waterway, both
considerations--~inherent controllability" and "initial
controllability" (= best piloted controllability)--have to be
considered to arrive at a most beneficial waterway design
(minimal costs and risk of accidents).
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79
A hypothetical example will be discussed in the next section to
elucidate the analysis of ship's controllability in a particular
waterway.
Considerations in Designing a Port
Starting Points of the Design
A harbor is to be designed alongside a coast for the docking of LNG
carriers only of 125,000 ma or smaller. (The schematic plan for the
design is illustrated in Figure 3.) The port is to accommodate the
arrival of 136 ships per year over a period of 20 years (about 5500
passages in the harbor). In the approach channel, the ships sail
through currents and waves, while the channel depth is designed for 15
percent keel clearance to the ship. The maximum current amounts to 3
knots while the ships sail in prevailing winds of either 5 Bft or 8 Oft.
The first decision to be made is the approach speed of the ships.
Assuming a 2750 m length for inner and outer harbor--based on experience
from earlier studies--it is stipulated that the ships will pass the
outer piers at a speed of approximately 5 knots, with a maximum
variation of 1 knot, while 4500 m in front of the outer piers their
velocity is 8 knots.
The port design will also be based on the fact that the tugs will
fasten inside the outer harbor region. Another shore-based decision
for the design stipulates that only one ship at a time will approach
and dock in the harbor.
At this stage, the question arises what the dimensions of the
approach channel (to be dredged) and the distances between the piers
should be. When design charts for the width of shipping lanes are
available, a compromise can be attained for the optimum harbor mouth.
This compromise would fall somewhere between as wide a harbor mouth as
possible for navigational purposes and as small a harbor mouth as
possible to minimize wave penetration into the harbor.
Exploring the waterway dimensions required to facilitate the entry
of ships into the harbor, the inherent controllability will lead to a
width of the approach channel dependent on the ship's drift angle
against current and wind, while the dimensions of the harbor mouth and
the area behind it will depend on the current shear in front of the
outer piers.
Further exploration will show that the initial controllability of
the (piloted) ship leads to the following design alternatives, assuming
the ship approaches a channel 500 meters wide under conditions of no
current, but some wind disturbance typical of normal operations.
Design Alternative A
Available width of outer harbor mouth 500 m
Required width of lane in the approach channel -290 m
Required width of lane between the outer piers -240 m
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outer piers inner piers
at
Cal ,rre
.
I ~
approach channe 1
( dredged )
outer .~ inner
harbour ~ harbour
L
berthing area
leading 1 ine
of 1 ight s
900 m
1 850 m
Figure 3. Schematic plan of the design.
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81
.
Required width of lane in the outer harbor ~30 m
Design Alternative B
Available width of other harbor mouth
Required width of lane in the approach channel
Required width of lane between the outer piers
Required width of lane in the outer harbor
300 m
-600 m
-220 m
-245 m
The results presented in Figures 4 and 5 have been reduced from the
average value and standard deviation of many maneuvers of ships
entering the harbor in the conditions specified. For the winds blowing
from starboard, half the required width of lane is determined by the
average and the standard deviation presented in Figure 6.
Assessment of Initial Controllability
Before comparing the two options to be developed, more attention is
devoted to the theoretical meaning of the information provided. The
question arises: are the required widths of lane in Figures 4 and 5
completely described by the initial controllability of 125,000 m 3 LNG
carriers in the harbor considered?
This question can be answered affirmatively if all boundary
conditions {ship speed, prevailing wind, etc.) mentioned in the
starting points of the design are taken into account. This means that
in the option of an outer entrance 300 meters wide, the ships'
controllability is such that an approach channel at least 600 meters
wide is required. The channel width has to be 600 meters sat least, n
because the initial controllability is considered to provide the
minimum deviation between actual and intended maneuver. During normal
operations, the ship's controllability will be less (leading to larger
channel widths) than the initial controllability, as will be shown
later.
The navigability of the waterway can only be improved when the
starting points of the waterway design are changed or by reference to
another ship system (maneuvering characteristics of the ship in the
combination-of-control method). The controllability of the ship can be
improved, for instance, by giving the pilots special training, by
providing other aids to navigation to the pilots, 12/~3 or by
increasing the water depth, by which the turning ability of the ship
increases.
Taking these additional considerations into account, it can be
assumed that from a practical point of view, the results presented in
Figures 4 and 5 represent the initial controllability of the
125,000 m3 LNG carrier in the two alternatives.
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-
L
500
m 290 m 240 m 230 m
required width of lane
Figure 4. Initial controllability of ship in design
alternative A.
~ l
_~_ ~ ~
600
m 300
.
l m
220
m
required width of lane
245
m SOO
Figure 5. Initial controllability of ship in design
alternative B.
approach channe 1
.
~ 4)
4, E
~ a. 2 5 m _
~ · - -
5 0 m _
o
SO m_
Oc 25 m~
·. -
._,
Q.
c
v
up
o
outer piers outer harbour
4 km3 km 2 km
11 1
1 km ~ centre of channe 1
__~
4 km 3 km 2 on 1 km
width of outer
harbour mouth
300 m
500 m
Figure 6. Description of the ship 's tracks in design
alternative A or B.
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Assessment of Ship's Controllability in the Design Port
In order to proceed to the design of the harbor, some decisions
have to be made from a practical point of view. For example:
At this stage, changing the starting points of the design to
improve the initial controllability of the ship is not
recommended.
It is assumed that a harbor entrance of 500 m is acceptable
from the point of view of wave penetration in the berthing
area.
Widening of the approach channel from the point of view of
initial controllability of the ship has to be rejected.
Based on these arguments, the development of the harbor design now
continues with alternative A presented in Figure 4.
It is decided that the time ships wait to enter the port at an
appropriate current velocity has to be minimal. When the ships have to
enter the port at any moment of the tide, the following values are
found:
Required width of lane in the approach channel
Required width of lane in the outer entrance
Required width of lane in the outer harbor
Required width of lane in the inner entrance
-620 m
-410 m
-525 m
-385 m
With respect to the values indicated in Figure 7, the following
comments should be made:
2.
3.
1. The widths of lanes determined are preliminary values that
hold only for the initial design stage, in which the starting
points of the design have not yet been evaluated from
economic, hydraulic, and other points of view.
The widths of lanes have been determined in a more or less
ideal environment in which, for instance, the visibility is
clear and information about the current speeds is known to the
pilots. When the hydrographical information to the pilots is
not accurate, then the waterway has to be much wider to allow
the pilot to experience the environmental conditions in which
he is sailing.
The widths of lanes have been determined using the average
track and standard deviation of many maneuvers, as shown in
Figure 8.
For the determination of the width of lane, it is assumed that
there is a chance (P) of 50 percent that never during the 5500
maneuvers in the waterway will the width of lane be exceeded. When
taking into consideration the number of extreme deviations of an outer
point of the ship (taking into account ship's length and breadth) from
the centerline of the waterway, one finds n = 6930 extremes during the
20-year lifetime of the harbor considered, leading to a chance (1-p) of
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r
84
Boo
~ m 5~
~ L
385
required w, dth of lane
Figure 7.
;25 m_
I
~ . -
SO m_
m
Required width of lane of the ship in the
first draft design.
approach channel
4 km :3 km
-
outer piers inner piers
1
, , , , ~ 1
/1\ 1
2 km 1 km
-
/ i~
·_i
._,
0
In
n
.. 0
o
4 km 3 km 2 km 1 km
Figure 8. Description of ship's track in the first
draft design (see Figure 7)
-
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O.9999 percent that the largest extreme will not exceed the boundaries
of the width of lane (P = (l-p~n). Based on the chance p of 0.0001
percent of exceeding the boundaries, these are determined by a factor
n, the relation of the maximum and the standard deviation:
n =~-2 In 0.0001 = 4.29 (average)
from which
w = 2 (n om + am)
in which
w = required width of lane
om = standard deviation of plots of extreme points of ship
am = average value of plots of extreme points of ship.
4. The values shown in Figure 7 are a consequence of the high
level of safety used in the calculations presented above.
However, in the initial stage of design, the harbor dimensions
seem acceptable relative to the controllability of the ship
considered when one neglects these required widths of lane,
instead considering the chances of exceeding the given
waterway dimensions. One then obtains the following picture:
chance of exceeding dimensions of an extreme in the 500 m
approach channel p = 0.015.
chance of exceeding dimensions of an extreme between the 500 m
outer piers p = 0.001.
chance of exceeding dimensions of an extreme in the 500 m
harbor p = 0.004.
From the preliminary values in Figure 7 (determined by the ship's
controllability), it can be decided that a first-draft design of the
harbor can be:
width of channel 500 m
width of outer entrance 500 m
width of inner entrance 500 m
This draft plan should be further evaluated from hydraulic and
economic aspects. It is advised that a detailed draft developed in
this way be tested afterwards for its navigational merits. In such a
final nautical study, a search can be made for optimum navigability by
improving the ship's controllability through a variety of measures
specific to the harbor.
In such a detailed nautical study, due attention should also be
given to real-life disturbances that exercise an adverse influence on
the ship's controllability. Suab disturbances include the breakdown of
machinery onboard the ship, failures in connecting tug boats, and
hindrance of unforeseen obstacles (e.g., maintenance dredges).
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Finally, the values presented in this section have been determined
from experiments performed at the Netherlands Ship Model Basin ship
maneuvering simulator. These figures hold only for the conditions of
this specific harbor design and cannot be copied for any other
situation without correlation to a range of experiences acquired under
other conditions. The figures have been used in this paper only to
demonstrate the recommended line of thinking for assessing the ship's
controllability in the design of port and harbor entrances.
Effect of Ship's Controllability on the Navigability
of a Waterway
It has been noted in the preceding section that many factors will
influence the controllability of a ship in a waterway. Decision makers
must consider such factors as acceptability of ship size, ship speed,
tug assistance, aids to navigation, and others In relation to the
available water depth, width of waterway, current patterns, and layout
of the port (recommended maneuver).
It will be of no interest to assess the influence of these factors
within some subsystem (as for instance, the influence of tug boats on
the turning ability of the ship, or the influence of position
information on the performance of the pilot). On the contrary, each
factor can have tremendous effects on the total system (the piloted
ship in the waterway).
The controllability of a ship has been described to this point by
the performance of the ship indicated by the deviation of the actual
maneuver from some reference maneuver (an intended maneuver or desired
maneuver). To assess the navigability of the waterway in a broader
sense, one should consider the sensitivity of this performance. In
this respect, a very important aspect of the navigability of a waterway
is the description, "ease of performing a given maneuver (sailing
through the waterway) during operational conditions." The following
example illustrates this idea. Compare approach channels, both 300
meters wide, to two different ports, A and B. For port A, the width of
lane is required to be 290 meters for ships of different types while
port B is designed for a specific type of ship for which a lane 200
meters wide is required. When the chance of an accident in port B is
large for a ship that differs slightly from the specified type, then it
will be obvious that the navigability of port A is more acceptable than
that of port B. The same illustration could be given for the influence
of the approach speed on the navigability of a port: the conditions in
some ports are such that a variation in the approach speed will not
affect the required width of the lane, while in other ports such a
variation can lead to undesirable risks.
From these examples it will be understood that the navigability
(indicated by "the ease of sailing through the waterway") depends
largely on the sensitivity of the ship's controllability to
disturbances in daily operational conditions. The general definition
of sensitivity leads to the following:
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81
Ap/p
AK/E
in which
s = sensitivity--the navigability Improves when s
becomes smaller
P = performance of the ship determined by its
controllability
AP = change of performance
E = external factor influencing the ship's
controllability
AE = change of external factor
When the external factor changes randomly, then the quantity ~
can be indicated by the standard deviation of the varying factor, while
E is the average value of The varying factor. In this case, UP is
indicated by the standard deviation of the performance index of the
ship's controllability.
As no routine exists for developing harbors from a nautical point
of view, no methods have yet been developed to analyze the navigability
of a waterway according to the paraphrase given above for the
sensitivity of the ship's controllability to external disturbances.
The most important missing aspect to develop for the analysis of the
navigability of a waterway is criteria. In the future, when experience
has been gained in using the term "navigability of a waterway, n
practical criteria can be developed that provide a common-sense
understanding of the acceptability of the waterway from a nautical
point of view.
In the absence of criteria to answer the question whether a
waterway is acceptable when the sensitivity s is known, an elaboration
of the meaning of the paraphrase for the sensitivity s will be given
here with the help of a few examples.
Example 1. It was seen in the previous section that in zero-current
conditions the required width of lane in the approach channel changed
from 290 m to 600 m when the width of the port entrance changed from
500 m to 300 m. The sensitivity of the ship's controllability to the
500-meter entrance will be:
310/290
s = --- = -2 ~7
we -200/500 ~
in which
S D sensitivity to the width of entrance.
we
The sensitivity swe of the ship's controllability at a 300-meter-
wide harbor entrance is -0.775. The conclusion now reads: the
controllability in the approach channel is acceptable with a 500-meter
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88
.
wide harbor entrance; however, the sensitivity to the width of the
harbor entrance is large. Small changes in the width of the harbor
opening will have large effects on controllability. However, in the
300-meter-wide harbor entrance, the ship's contrc~-ability is
unacceptable, while its sensitivity to the width I-- the harbor entrance
is small. Little gain in controllability can be achieved by widening
the opening'
Example 2. It was seen in the previous section that for the design
concept A, the required width of lane in the approach channel changed
from 290 meters when there was zero current to 620 meters when the ship
had to sail in a variety of crosscurrents with a maximum speed of 3
knots. Since the external factor (current velocity during each
maneuver) is randomly changing, the sensitivity to current is a little
bit more complicated than in the previous example. From the maneuvers
performed one determined:
Required width of lane: 290 m at zero current
Required width of lane: 480 m at currents with a magnitude of
either -1.5, 0, or +1.5 kn during various maneuvers
Required width of lane: 620 m at currents with a magnitude of
either -3, -1.5, 0, +1.5, or +3 kn during various maneuvers.
When it is assumed that the 290-meter width of lane is indicated by
the initial controllability of the ship in the design port without
disturbances (no current), then the 290-meter width of lane is the
initial (zero) width to be considered for the port. An additional 190
meters of width is required when the port is designed for ships to
enter during crosscurrents of 1.5 knots maximum (see Figure 9~.
However, when the port is designed for ships to enter during
crosscurrents of 3 knots maximum, then 290 meters has to be added to
the required width of lane at the initial controllability. In this
way, one finds a sensitivity to current of 1 at zero current (~P/~E
= P/E at E ~ current velocity = 01. Note: this amount of
sensitivity has no absolute meaning, as it is used only to define a
relative measure to the sensitivity at higher values of the
crosscurrent. The sensitivity to currents of the ship's
controllability in the design port is presented in Figure 9.
From the results obtained earlier, it was concluded that the ship's
controllability decreases at increasing current velocity: P (required
width of lane}, and increases at increasing E (current velocity).
However, it can be seen from Figure 9 that the port designed for ships
entering as various currents reach the maximum velocity is considered
the best navigable port when the channel width corresponds to the
required width of lane.
Note: this conclusion would have been reached much easier by
considering ~P/~E. This latter consideration, however, only
applies to the present case, and has been ignored because it seemed
more interesting to show the general meaning of the definition of
sensitivity offered previously.
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89
w
a
300 m
200 m
100 m
O m
100 m
m/see
Aw /A
a c
w = additional required width of lane when
the port is designed for ships entering at
current velocities which are maximal
as indicated on the base
`~ :
_
. _
-
-
_
O kn 1 kn 2 kn 3 kn 4 kn
, , , 1
current velocity
Aw /Ac
. _
Aw /w
a a
Ac/c
1
-__
1 , , , 1
O kn
1 kn
2 kn
current velocity
Figure 9: Schematic indication of reduction of the
sensitivity to current.
3 kn
4 kn
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go
Example 3. In reference 14, the controllability is considered of a
200,000 OWT tanker sailing through a crosscurrent of 0.5 knots average.
A peak exists in the crosscurrent of which the amplitude varies between
1, -1.25, -1.5, -1.75 and 2 knots during various maneuvers. When the
exact magnitude of this peak is known to the pilot, a required width of
lane of 350 meters is observed. However, when the information to the
pilot about the magnitude of the peak current is less exact, then the
required width of lane increases to:
410 m at an accuracy of 87%
670 m at an accuracy of 75%
One thus finds:
sad= 60/350 = 1.32 for 100% accuracy
13/100
sad= 320/410= 2.72 for 87% accuracy
25/87
sad= 260/670= 2.42 for 75% accuracy
12/75
in which:
Sal = sensitivity to accuracy of information.
From the above results it can be concluded that the navigability of
a 350-meter-wide waterway with 100 percent accuracy of information
about the current is better than the navigability of a 670-meter-wide
waterway in which the accuracy of information about the current is 75
percent, while this latter design offers better navigability than a
450-meter-wide waterway in which the accuracy of current information is
87 percent.
In other words, from the results obtained, one could recommend a
choice between two alternatives. Alternative 1 is a waterway of
restricted dimensions in which correct information about the current is
supplied to the mariner. Alternative 2 is a very wide waterway in
which the information about the current to the mariner is only a rough
estimate.
Safety of Navigation
In the preceding sections, the navigability of a waterway, as
influenced by the ship's controllability, is regarded only from the
point of view, more or less, of economical operations. Some attention
teas been given to the optimum use of a port: as many ships should
enter the port as easily as possible under most conditions.
However, when considering the controllability of the ship during
its passage through the port, the ultimate test is the mitigation of
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91
accidents. Of course, the safety of navigation depends largely on the
ship's controllability, but the execution of a risk analysis will cover
more than ship controllability alone.
Though the navigability of a waterway is closely related to the
safety of navigation in the waterway (and both are influenced by the
ship's controllability), different procedures will be employed to
specify each.
A true risk analysis necessarily implies a three-step procedure.
The first step includes the establishment of the probability of the
occurrence of hazards and their associated consequences. (This would
presumably include human errors that initiate a chain of events
creating a hazard). The second step is an evaluation process to
determine the level of risk the system is expected to be subject to,
and the third step is the procedure whereby the originally derived
level of risk is mitigated by the introduction into the system of
certain design changes, actions, operative restrictions, and other
factors.
In evaluating any marine transportation system from the point of
view of safety, one must utilize a systematic process that infers the
level of safety from the aggregate of the individual risks, rather than
the individual risks alone. This in itself suggests that a systematic
process of risk identification and analysis is necessary to measure the
safety of a system.
The problem to date has been the inability to derive a systematic
evaluation process that correctly considers all the complex interactive
elements that contribute to the occurrence and activation of hazards;
namely,
The ship's inherent hydrodynamic characteristics,
The ~skill" of the mariner in controlling the ship,
The peripheral aids (either on board or external to the ship)
that furnish data or control to the mariner, and
The effects of a particular environment (port geophysics,
wind, current, channel width and depth, other vessels, etc.)
on the vessel and the operator.
To approach this complicated problem, it is of primary importance
to acquire reliable data from actual practice. To this end, good
correlation must be available between what in fact occurred and the
reports of persons involved. It is no surprise that accurate
measurements are limited by instrumentation and the conditions under
which marine casualties occur. Moreover, human perception is highly
subjective. This has led to the generally accepted theory that there
are more accidents than are actually reported. This problem can only
be solved when the reports are scrutinized more closely and the
hypothesis that accidents are intentionally concealed is disregarded.
Another discrepancy in the reporting system derives from the
physiological and psychological characteristics of men involved at the
time of an accident. Since among other things, a clear definition of
mental load is lacking, it is difficult to establish a criterion of
Allowable stress.
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Concluding Remarks
Ship controllability exerts a large influence on the navigability
and safety of a waterway. In assessing a ship's controllability, no
absolute measures can be defined because many factors related to the
properties of the ship and of the waterway are important.
To reach an improved understanding of a ship's controllability as
it determines the nautical requirements to be imposed on the dimensions
of a waterway, more basic research must be conducted. The results of
such investigations would provide port designers the information to set
up a first-draft design of the port.
When such a draft has been evaluated from economic, hydraulic, and
other points of view, adequate means are available to ascertain the
final merits of the port from a navigational point of view.
References
7.
1. Van Dixhoorn, J. et al., "Development and Criteria for the Design
and Construction of the Port Approach and Harbour Area Entrance of
Rotterdam Europoort, n Paper presented at Symposium on Aspects of
Navigability of Constraint Waterways, I.A.H.R., Delft, 1978.
2. Keith, V. F. et al., Realtime Simulation of Tanker Operations for
the Trans Alaska Pipeline System, n Paper presented at Annual
Meeting of The Society of Naval Architects and Marine Engineers,
New York, 1977.
3. Hooft, J. P., "Handling of Large Ships, n Paper presented at West
European Conference on Marine Technology, The Hague, 1974.
4. Mandel, Ph., "Ship Maneuvering and Control, Principles of Naval
Architecture (New York: Society of Naval Architects and Marine
Engineers, 1967~.
Crane, C. L., Jr., Estate of the Art on How to Include Human
Control into the Method of Investigation, n Paper presented at
Symposium on Aspects of Navigability of Constraint Waterways,
I.A.H.R., Delft, 1978.
Paymans, P. J. "Human Factors in Shiphandling, n Paper presented
West European Conference on Marine Technology, London, 1977.
Hooft, J. P., et al., "The Influence of Human Behaviour on the
Controllability of Ships, Paper presented at Spring Meeting,
Society of Naval Architects and Marine Engineers, New London,
Connecticut, 1978.
8. Hooft, J. P., The Influence of Nautical Requirements on the
Dimensions and Lay-out of Entrance Channels and Harbours, n
International Course on Modern Dredging, Technical University,
Delft, 1977.
9. Boylston, J. W., "Is Port Study Model Testing Really Worthwhile?"
Marine Technology, 1974.
10. Hooft, J. P. and P. J. Paymans, Four Years' Operations Experience
with the Ship Control Simulator," Paper presented at S.T.A.R.
Symposium of The Society of Naval Arobitects and Marine Engineers,
Washington, D.C., 1975.
at
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13.
11. Panel H-10 of The Society of Naval Architects and Marine Engineers,
"Proposed Procedures for Determining Ship Controllability
Requirements and Capabilities, n Paper presented at S.T.A.R.
Symposium of the Society of Naval Architects and Marine Engineers,
Washington, D. C., 1975.
12. Van Dixhoorn, J., "Feasibility and Profit of Navigation Information
and Navigational Aids Offshore, n Paper presented at 23rd
International Navigation Congress of the Permanent International
Association of Navigation Congresses, Ottawa, 1973.
Atkins, D. A. and W. R. Bertsche, "Evaluation of the Safety of Ship
Navigation in Harbours, n Paper presented at Spring Meeting of the
Society of Naval Architects and Marine Engineers, Coronado,
California, 1980.
. Oldenkamp, I., and P. J. Paymans, influence of Cross Current in a
Channel on a Man-Steered Ship, n Paper presented at National Meeting
on Simulation for Service of Traffic, Bremen, 1975.
Porricelli, J. D. and V. F. Keith, "Systematic Processes for the
Marine Decisionmaker in Regards to the Safety of the Waterborne
Carriage of L.N.G. in Bulk, n Testimony before Senate Committee on
Commerce, Washington, D.C., 1974.
DISCUSSION
WEBSTER: In your Figure 7, you show a required mean width of
lane. As a result of the studies you conduct, is that the width you
recommend to be dredged? If you were trying to minimize port costs,
would you dredge the lane that way, or would you try something else?
HOOFT: For these figures, suppose you had 500 meters
available, and available information indicated that you required 620
meters. Then I would say, look at the higher requirements you would
apply to this figure; for example, "I want 50 percent safety over 2000
maneuvers of ships," etc., and when you look at the other possibilities
available for mitigating the chance of accidents at 620 meters, then I
would say which is the just concept or first draft? Then look to see
if 500 meters is acceptable from a hydraulic point of view. Do you
have acceptable wave penetration in this inner harbor? Is the wave
penetration of the docked ships excessive? Then you must take other
measurements. For designing a harbor, you want some indicative
requirements for deciding dimensions: information about
controllability, for example. Then you must look from all the other
points of view to see if the evolving design is acceptable for the
controllability assumed and for safety.
KRAY: The maneuverability of the ships you discussed, is that
for an automatically controlled ship excluding all human elements. For
example, is the delay in the transmission of orders to the engine room
considered, or the response of the ships to the actions of the
handler? It appears that you have considered rudders of the
conventional type in these studies. Have you given attention to the
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flat-belt type that is far more effective in keeping the ship on course
and in preserving its maneuverability?
HOOFT: What you are indicating is that the total system is
composed of so many aspects' The human element you mention, for
example. Your question is, was the skill of the shiphandler considered
in the performance of the maneuver? The overall system can be improved
by increasing the skills of the people involved. The ship's
maneuverability can be influenced by the rudder configuration, the
stern configuration, the dimensions of the ships. My plea in making
this presentation is that when you are not satisfied with the ship-
harbor interaction, it will not do--as was common 10 years ago--to
blame the dimensions or characteristics of the ships. In the past
three or four years, it has become common to cite human error. In
another four years, the blame for accidents may fall on the
navigational aids' Elements of the system cannot be singled out, as
you and Bill Webster indicate by your questions. The decision about
channel width and any other in the design is a compromise effected
among all the concerns the designer is trying to meet, most
importantly, the navigability of the waterway.
HARDOW: On one hand, we're talking about balancing the
capital costs and the operating and maintenance costs of a whole series
of steps one might take dealing with the ships, the harbor, the
channels, and so forth. On the other hand, we're talking about
accidents. We should be looking at the consequences of certain kinds
of accidents that will occur if proper steps are not taken, and the
costs. We have never tried to do this in a systematic way that I know
of, but if we did, we would have to look at other items, and make a
full systems analysis.
SEARLE: I want to endorse that. There is too great a
tendency when an accident happens--and all I see is accidents--to cite
human error as the cause. I've seen many accidents that were
inevitable. The unusual aspect of many accidents is that more bave not
occurred in the same place. There have been two major accidents in
Tampa Bay since the first of the year. Both were inevitable.
Seconding what Gene Harlow said, systems analysis ought to pinpoint
those hazardous locations. Your presentation highlights the
integration of ship maneuverability or controllability with harbor
design: the system also needs hazard" analysis, failure mode and
effects analysis.
CRANE: I'm fully in accord with full systems analysis. We
must accept certain constraints and givers: while it would be helpful
if all shiphandlers were fully trained, for example, their range of
ability must be accepted. Then we are in a position to work with
channel dimensions, aids to navigation, vessel traffic systems, and
other parameters to improve safety.
Representative terms from entire chapter:
approach channel
