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EVALUATION OF ME SAFETY OF SHI P
NAVIGATION IN HARBORS
Donald A. Atkins
William R. Bertsabe*
Abstract
Concern for safety of navigation in harbor waterways teas increased
due to the huge economic and environmental consequences of potential
accidents to those ships of rapidly escalating size operated or
proposed for operation in harbors today. The authors describe a
methodology for the determination of ship and waterway navigational
safety, including the definition of measurement indices of safe
navigation and the means for determining their values. This
methodology is the result of extensive research sponsored by the
Maritime Administration and the U.S. Coast Guard involving the study of
navigation of ships in harbor waterways through real-time simulators.
Ship operators, port authorities, and regulatory agencies can apply
the methodology to establish port and waterway designs or to evaluate
the safety of accommodating potential traffic. The methodology is
applicable to evaluation of limiting environmental conditions (i.e.,
visibility, wind, current) beyond which piloting of certain ship types
can be considered unsafe, and examination of the effects of alternative
aids to navigation, redesign of channels and turns, new traffic
policies, or less-maneuverable ships.
Specific applications of the methodology and measures of safety to
changes in ship controllability, turn design, and aids to navigation
are included. An analysis of the channel characteristics of 32 U.S.
harbors {i.e., channel widths, depths, turn angles, turn types) is
included to serve as reference material for future U.S. ship designs.
Introduction
The advent of large ships carrying cargo harmful to the environment
and the economic advantage of accommodating oversized vessels in
existing ports has focused the attention of the public, port
_. ~ ~ _ ~ agencies on the need for
improvements in the safety of navigation in U.S. port waterways. To
date, navigational safety in U.S. port waterways has been maintained at
au~norleles, sole operators . and government
*Presenter.
53
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a relatively high level by virtue of an evolutionary process. Ship
size increased at a slow enough pace that channel requirements, aid to
navigation requirements, and ship maneuverability requirements could be
determined by trial and error. Given a number of near misses and an
occasional accident, port and ship designs were improved to acceptable
levels of safety. As one port showed its capability to accommodate
particular vessels, another port sought the same type of traffic by
improving its own design to be equivalent to the first. Out of this
experience and limited research, rules of thumb and empirically derived
design criteria evolved for channel dimensions, aids to navigation, and
ship design.
Our difficulty today arises from the rapid escalation in ship size
and the potential outdating of the available design criteria. An
analysis of shipping traffic in U.S. port waterways would show that by
many existing design policies and standards, present waterways cannot
safely accommodate many of the large ships using the waterway today,
much less the larger vessels of the future. Are present operations of
oversized vessels safe or are we in a time-bomb situation? What is the
present margin of safety for navigating large ships in existing
channels? What economical improvements can be made to increase the
margin of safety?
Clearly, analytical techniques need to be developed for
quantitative evaluation of the navigational safety of narrow waterways
for large ships. The evolutionary process is too slow to provide the
criteria in a timely fashion and the environmental, economic, and
social consequences of a major marine accident are too high to risk.
Statement of the Problem
Research conducted in the area of navigation of ships in narrow
waterways was for many years focused on hydraulic channel testing and
simulation of ships' hydrodynamic response in analog or digital
computer models. These methods were used to evaluate a single transit
of a channel by a ship. Typically, autopilot rudder and propulsion
control algorithms were used to control the model or the simulation.
The advantages of such research methods were repeatability, and the
ability to isolate and study unique hydrodynamic responses. These
research methods provided valuable data about the vessel's physical
response in the waterway. The extent to which these vessels could
safely transit the waterway, however, could not be ascertained, since
theme methods failed to acount for the variability the pilot and
helmsman introduce.
Recognizing this deficiency during the past decade, several
research institutions around the world have integrated the human
element into research through the use of ship simulators. By
considering the variability man's performance adds to the piloting
process, we are truly considering the ultimate safety of the vessel in
the waterway, for a waterway can be said to be safe to the extent that
variability of ship tracks in the waterway can be contained within the
boundaries of the waterway under stated environmental conditions.
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The variability under study is that normally resulting from
differences in perceptual and cognitive behavior between different
pilots and helmsmen, and differences in behavior over time or for
unique ships or channels for an individual pilot or helmsman. Research
in this area must therefore be conducted to assure that a
representative sample of subjects has been analyzed, in order to
achieve a level of statistical significance transferable to the real
world. The research methodology and examples presented in this paper
appear to achieve these goals.
Methodology
The process for determining the requirements for safe navigation in
restricted waterways was developed to address the following critical
design and operational questions facing ship operators, port
authorities, and regulatory agencies.
.
.
.
Which environmental conditions preclude safe navigation in the
waterways?
Which operational procedures for specific ship types enhance
their safe navigation in the waterway?
What level of safe navigation is provided by the
aid-to-navigation system in the waterway, or what is the
effect of alternative aids to navigation?
What maneuvering characteristics are required for proposed
ships to navigate the waterway safely?
Is the level of safe navigation acceptable for a proposed ship
type in a given waterway, or what changes in the waterway
dimensions are required to ensure acceptable safety levels?
All these questions must be addressed using methods that recognize
it is performance of a human pilot exercising his capabilities in
navigation that must be analyzed. Safely navigating a ship which is
large for the channel is relatively routine for an experienced pilot -
the ship is maneuverable and directionally stable, and there is no
wind, current, or other perturbing influences, suab as banks or
traffic. Determination of safety, given an adverse environment with
allowance for the variability in response by the pilot, is the
objective.
The basic methodology consists of the following steps:
1. Define the characteristics of the harbor and its environment.
2. Define the operational characteristics of the ship.
3. Explore the interaction of the ship and the harbor in the
presence of environmental conditions that limit a human
operator's control of the ship during simulated harbor
transits.
Analyze the results of that interaction through appropriate
measures of safe navigation performance.
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~6
The elements of these four steps are discussed in subsequent
sections of this report.
Step 1. Define Characteristics of the Harbor and its Environment
Many categories of information are required to describe a port
sufficiently for a comprehensive study of safe navigation. The sources
of the required data, however, are few, consisting of 1) navigation
charts, light lists, and current direction and velocity data for the
harbor published by the National Ocean Survey, and 2) weather
information and statistics for the area published by the National
Weather Service. Information collected from these sources should be
compared with and enhanced by interviews with mariners and weather
observers who have extensive local knowledge. The categories of data
required for a port study include the following:
Waterway configuration
- Channel widths and depths
- Turn types and angles
- Bank and shoal locations
- Type and location of hazards
Environmental statistics
- Wind direction and velocity
- Current direction and velocity
- Visibility range
- Unique current conditions
Aids-to-navigation system
- Types of aids
- Characteristics and patterns (day and night)
- Location of aids
Operational policies and conditions
Traffic rules and congestion
Tug availability and size
Limits on operations
Types of vessels accommodated
.
The foremost limiting condition to large ships has generally been
channel width and depth. To assess the general limitations of U.S.
ports and waterways, the authors have developed a data base, resident
in a computer file, which contains data on the physical channel
characteristics of 32 major ports of the United States. Each straight
channel leg and each turn in these harbors has been examined, and data
on depth, width, aids to navigation, turn angle, etc., recorded. To
assist naval arabitects contemplating design of future vessels, summary
tables that characterize ports of the United States have been assembled
from this data. These tables and the list of ports used are provided
in Appendix A of this paper.
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57
Step 2. Define Operational 'Characteristics of the Ship
Each question of safe navigation in a waterway implies a potential
ship or family of ships. Normally, the problem involves the extension
of the operating limits of the channel to allow passage of a larger
ship or a ship of specific characteristics. It may involve guiding
ships through a channel that has been restricted by, say, channel-side
construction, or extension of port operational environmental limits to
increase possible port use. In each case, a required component in the
study is a mathematical hydrodynamic model of ship motion with the
proper set of response coefficients for the ship's propulsion and
control forces. Mathematical models of ship's motion have progressed
to a stage at which there are a number of ship types available as
models. Additionally, hydraulic model tests can produce good estimates
for models' coefficients, given the ship's physical characteristics.
Today's mathematical models include factors such as bank influence,
shallow-water effects, bow thruster and tug boat forces, passing ship
effects, and wind and current effects.
Step 3. Simulation of Waterway Transits Under Operator Control
The objective of the simulation is to determine how consistently,
given the environment (ship characteristics, channel design, aids to
navigation and possibly external help from tugsl, a pilot operating
with a helmsman can navigate the ship through the channel safely. An
appropriate simulator facility which can address this problem is the
full-scale ship simulator. The ship simulator normally consists of a
full-scale ship's bridge with all normal equipment. Typically, there
is a method for representing the visual outside world, the radar image
of the world, and the progress of the ship through that world. The
motion of the ship through the world is driven by the computer, using
the hydrodynamic model, which is in turn driven by signals from the
steering stand and throttle on the bridge. The technology of ship
simulators has been most advanced in the Computer Aided Operations
Research Facility (CAORF) which is located at the Kings Point Merchant
Marine Academy and is sponsored by the National Maritime Research
Center of the Maritime Administration. At CAORF, a 125-foot
cylindrical screen extending for 120 degrees to each side of the bridge
portrays a computer-generated visual scene containing ships,
shorelines, navigational aids, bridges, and buildings, realistically
shown and moving in real-time response to the ship's movement. The
visual scene can realistically simulate any level of visibility (fog)
under night or day conditions. The visual scene is projected on the
screen by special television projectors. The radar image is generated
by a computerized radar signal synthesizer and is programmed to
coincide with the visual scene. Pilots and masters navigating the ship
experience the equivalent sensations and use the same information from
the visual scene, the radar, and from the instruments as when
navigating in the real world. CAORF has proved to be a valid, valuable
tool for studying navigation performance with Oman in the loop. n CAORF
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58
has been used to study many port design problems, including those of
Valdez, Alaska;t Puget Sound; 2 Point Conception, California; 3
Galveston, Texas; 4 Pascagoula, Mississippi;s and the Santa Barbara
Channel. 6 A multiyear research program teas been maintained to
systematically address a study of safe navigation in restricted
waterways.
Step 4. Analysis of Simulation Results and Measures of Safety
To obtain the benefits sought in the methodology, performance
measures must be defined that relate simulation results to safety. The
objective of navigation in restricted waterways is primarily to
maintain the position of the ship in the proper location relative to
the channel boundaries or the channel centerline (i.e., establishing a
proper crosstrack position). In the absence of traffic, the normal
crosstrack position in straight channel legs is near the centerline.
When meeting other ships, this position will shift toward the starboard
boundary of the channel. The performance to be measured Is the
consistency with which pilots passing through the channel can determine
and control their crosstrack position, recognizing tbe necessity for
tighter consistency near the channel boundaries than near the
centerline. As will be discussed, measures of safety are principally
descriptive of crosstrack variation.
Along-track position in restricted waterways Is of minor
importance, except in two instances. The first instance is the
determination of the position to begin a turn, after which negotiation
of the turn again becomes primarily a crosstrack and turn-rate control
problem. The second instance is bringing the ship to a stop at some
location.
Measures of navigation performance in restricted waterways are
therefore directed to measuring consistency of crosstrack position for
repeated transits of the channel by many pilots under the same
conditions. Changes in safety of navigation are defined by determining
differences in the measures for changed conditions. Three principal
measures have been derived and effectively applied across various
experimental conditions.
1. The mean track location across the channel of the:
· Ship's center of gravity (CG),
· Port and starboard extreme points of the ship's hull.
Statistical limits descriptive of the variability about the
mean track:
· Standard deviation of crosstrack locations at points
along the track,
· Location of the 95 percent limit of the track envelope of
the CG.
3. Combined index.
Measures 1 and 2 may be easily understood by considering the plot
of these data along a sample channel. Figure 1 shows these data
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59
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60
plotted at 600-foot increments along a channel. The dashed lines
indicate the channel edges, and solid lines indicate the mean tracks of
the ship's CG and the port and starboard extreme points. The square
symbols s bow the CG standard deviation doubled on either side of the CG
mean point. If the distributions of crosstrack variance are assumed to
be normal, this envelope would contain 95 percent of all transits.
Measure 3 is called the combined index because it combines the mean
ship position in the channel and the variation of the transits about
that mean position. This combination has desirable features for
predicting navigational safety in restricted channels. Neither the
mean ship position across channel nor transit path variability alone
give a complete description of safety. When combined in one index,
however, the index can discriminate between tolerance for higher path
variability when the mean track is far from the channel edge and the
requirement for low track variability when the mean track is near the
channel edge or when passing another ship, and assign both conditions a
favorable value.
The index computation is shown graphically in Figure 2. A normal
distribution based on the standard deviation of the center of gravity
point is centered on the mean crosstrack position of the CG point. The
index value is the integrated area under the distribution curve which
lies beyond the channel edge. The values of the combined index are
plotted on the right side of Figure 1.
The two curves are for the values relative to the port boundary (P)
and starboard boundary (S). Insufficient data are available to test if
the assumption of normality is correct. In fact, it is suspected that a
truncated distribution may be more characteristic of the crosstrack
variance as the edge is approached. The assumption of normality,
however, is conservative, and sensitive to changes in pilots'
performance. Since it is not necessarily the proper distribution, the
index should not be interpreted as a probability of grounding.
The values for the combined index included in this paper have been
calculated relative to the mean ship center-of-gravity location. The
process can easily be applied to calculate values relative to the mean
port and starboard extreme point locations at along-channel locations.
The index values for the starboard extreme point would be relative to
the starboard channel boundary only, and the values for the port
extreme point index would be relative to the port channel boundary
only. The resulting index would reflect mean channel position, track
variance, and heading error.
Application of the Methodology to Port
Design Problems: An Overview of Findings
The Maritime Administration has conducted a series of experiments
with their CAORF facility to evaluate the performance of navigation in
restricted waterways. These experiments have investigated those areas
of performance in which the master's, the pilot's, or the docking
master's variability is likely to cause the ship to exceed safe
operating conditions. The experiments have provided an initial
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understanding of the complex and interdependent relationships of harbor
design parameters. They have uncovered a number of mitigating measures
that can be applied in specific problem areas to achieve satisfactory
performance in heretofore marginal situations.
Six harbor design issues have been addressed at CAORF. These are:
channel dimensions, environmental limits, operating procedures, tug
requirements, aid-to-navigation requirements, and ship maneuvering
requirements. The influence of each of these issues on the variability
of masters, pilots, or docking masters is described briefly in
subsequent sections. Specific data are not quoted in these sections,
due to the large number of experiments from which conclusions were
derived and the difficulty of comparing findings from specific
experiments. Examples of performance measurement in several of the
areas are presented separately in a succeeding section Examples of
Analysis...".
Channel Dimensions
The adequacy of channel dimensions has been addressed in several
harbor design experiments. Most recently, studies have been concluded
on the Galveston ship channel, the Restricted Waterway Experiment" IIIA
(8), and IIIB (9), and the Pascagoula ship channel. Experiments in
channel dimensions generally addressed channel width, turn
configuration, or both. Typically, worst-case wind and current
combinations were selected. Experimental conditions tested whether
subject pilots could safely maneuver in the proposed channel under the
selected conditions.
As a result of the wind and current variability and the requirement
for the pilot to maintain a high drift angle against the wind and
current, a ship's tracks displayed a high level of variability in
crosstrack position both within runs and between runs. Although this
variability does not show a large dependence on channel width, the
channel width must contain it and allow for an additional margin of
safety.
Depending on the turn design (effective maneuvering radius
allowed), the crosstrack variance in some cases was significantly
affected when exiting the turn: the smaller the required turning
radius, the higher the crosstrack variance during and exiting the
turn. Analysis of performance in turns has indicated pilot control
actions are initiated in anticipation of the turn. For small-radius
and narrow turns, the pilot's anticipated actions must be accurate in
magnitude and precisely timed. For large-radius turns, there is more
room for error in the anticipatory actions, and for making corrective
actions during the turn. Proper turn design has been shown to reduce
crosstrack variation in a narrow waterway.
-
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Environmental Limits
The selection of appropriate wind and current conditions, and even
the limits of visibility, is an important issue in any harbor design
study. Typically, a ship operator or port authority specifies limits
below which he requires 100 percent operation. Limits are defined by
the frequency and distribution of local weather conditions and the
economic consequences of occasional delays in delivery or shipment. In
studies where the port is to be open to many operators (e.g.,
Galveston), environmental conditions are selected to provide, for
example, 90 to 95 percent harbor availability based on weather and
current statistics.
The effect of environmental conditions on the ship and pilot are
twofold. First, the ship must "crab" along the channel with a specific
drift angle to maintain a ship's course equivalent to the channel
course. Second, due to the presence of high drift angles, the pilot's
perception of his position, and therefore the accuracy of corrective
orders is degraded. Drift angle increases the "swept width" of the
ship's path, thus occupying a wider portion of the channel. The effect
of the degradation of the pilot's control process is to increase the
crosstrack variability. The net effect of environmental conditions is
thus seen to be a reduction in safety, placing the extreme points of
the ship closer to the channel edges and increasing the crosstrack
variability of those points.
Current and wind combinations may also degrade performance in
turns. Typically, the most severe effect evolves from a following
current when the ship's ground speed appears high while the water speed
is low, impairing maneuverability. Excessive windage can contribute to
difficulties in turning depending on the topsides and superstructure
configuration. In cases where environmental conditions degraded turn
performance, crosstrack variation exiting the turn was high, and the
only solution appeared to be widening the channel following the turn.
Operating Procedures
Many design studies involve handling ships in new harbors or
modified waterways. Until recently, there was little experience in the
United State" with oversized vessels (e.g., 150,000 DWT tankers and
above). Most harbor design studies of today, however, involve
accommodating such vessels in U.S. ports.
With increased environmental pressures, authorities must consider
establishing operational limits, be they environmental (wind strength,
current cycle, etc.) or procedural (specified routes, speed, traffic
conditions, etc.~. Procedures also need to be established that could
act as mitigating measures to ship system failures. Several port
studies at CAORF have addressed these issues: the Valdez tanker study,
Puget Sound speed limit study, and Point Conception LNG* study.
-
*Liquefied natural gas e
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The issue of many procedural studies is to determine the safest
approach and departure routes to a harbor across the environmental
conditions. In Valdez, the departure route proved to be the design
issue. By reducing a turn angle along the route, cro"strack variance
passing by a middle rock was reduced. For the Point Conception
operations, the evaluation of the approach route concluded that
crossing an oncoming traffic lane would present little hazard.
The findings of several port-related studier have indicated that
safety may be inversely dependent on ship's speed over a limited range.
The first impression is that slow ship speeds will be inherently
safer. Data indicate, however, that with reduced speed comes a
reduction of maneuverability and an increase in crosstrack
variability. Increased speed not only increase" maneuverability, but
also significantly reduces the required drift angle for adverse wind
and current conditions.
Tug Assistance
Harbors planned for accommodation of oversized vessels often assume
the use of larger shiphandling tug. than are generally available in
U.S. ports today. Several port design experiments at CAORF have
addressed the use and size of tugs for oversized vessel operations.
Notable are the Point Conception Study, the Galveston Channel Study and
the Pascagoula Channel Study.
The use of tugs as rudders, and for slowing vessels by means of
long lines astern, i" frequently practiced in Europe and Japan for
oversized vessels, but has not yet received much attention in the
United States. The interdependence of tug power and ship type and size
with environmental conditions is important, but is yet largely
unknown. A high-fidelity simulation of tug forces has been recently
added to CAORF and will be applied in a number of experiments in the
near future.
Aid" to Navigation
Visual aid" to navigation appear to serve as a mitigating factor to
some of the perturbing environmental and channel design variables.
Providing extra aid. in a channel has resulted in lower crosetrack
variance and improved performance in difficult turns. Experimental
conditions with fewer aids resulted in higher variance and unacceptable
performance in channels of equivalent design and environmental
conditions. Deficiencies in some harbor waterways might thus be
overcome with additional aids to navigation.
Evaluation of precise radio aid navigating systems teas been
undertaken to evaluate potential performance gains achievable through a
highly accurate positioning system. Data gathered so far indicate
excellent trackkeeping performance. Just as visual aids to navigation,
advanced radio aid systems may be employed to overcome marginal
operating conditions in ports in place of port modifications, such as
widening channels.
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- 64
Ship Performance
The ef'fects-o'f ship cof~tol~a~ility on'variabi'1ity of 'trackkeeping
has suggested that newly construdted''ships'might tee 'custom-designed for
a specific pert or t'ype.of'wate`way. ING operations are particularly
suitable for't. 8-i8 'typetof investigation'6ue 'to the ships' commitments
to certain'termina-1-^s. 'I'' "''I'""' '''''' '" ' ' " ; " ''''
An experiment'conducted'at ChORF"'indfcated that back'' variability
increased~w1'th a-red~o'tia'n'i'n'th6' tbr'~ingiresponse of a Charge tanker.
Tm~'rovem'ents'~n ma*-euvera'bili~ty Off' fa'rge'vess~el's using'-a'dvanced'desig'n-
concepts~mat,'p'rove-'6ighly; behe'fi'cial 'to safe nav1'gat~ion'in restr'zcted'
waters. ~-Of interest-is' rudder side, number 'of rudaers','~umber;-of '
propellers an8-perhaps`;bul1 'form.'; If higher turning moments could be
pr~d'u;cdd at~ld'w`-spee'd''.(e.g.~,'.tWin.'-9ct'ews),'pe'rhaps'safe;;opera~elons
could^8e~~con~ducted'~'at' very low;speeds.'~"Thirs are;a of' performance is
still at the basic research level, but the gains to'be'ac'hi'eved are' ''
· ~
prOmlSlng e
_.
_ , . .
. .
..... .. _. . ~. . .
Examples of Analysis of Relative .
~ Navigational Safety in..Nar;row Waterways
Specific compar'~sons'''.$'f navigatzon^performance'evaluationifor
alternative ship character~stics,'channel design, and aids'to '
. . .
navigation have been drawn' from two recent experiments at CAORF.
During these experiments, Restricted Waterways Experiment Phase IIIA
and IIIB, trained pilots navigated an 80,000 DWT tanker along a
500-foot-wide channel containing three turns connected by straight
channel segments. This channel configuration is shown in Figure 3.
Five pilots made transits through"the channel for each variation In a
specific condition, providing a statistical basis for 'evaluating the
- .. . .. .. . . . .
relative effectts).of tbe.goodition on safe navigation. Results for
these experiments have been reported in references'8 and 9. For this
paper, several experimental conditions have been selected to illustrate
the value of analysis of navigation safety using the measures
previously described.
Ship Maneuverability
The amount of control force required to enable ships to negotiate
waterways is one factor to be considered in the design of a new ship.
There teas been a feeling among mariners that given enough training and
experience, man is sufficiently adaptable ' to overcome difficulties with
slow-re'sponding ships. The purpose of this comparison was to
determine, in relatively severe environmental' conditions, what actual
effect.a reduction of maneuverability would' have on safe navigation of
a ship in restricted waterways. Would the pilots compensate for the
slow response or would overall safety be reduced? ' '
For this experiment,'th'e'ship'was modeled with two alternative''
rudders. One rudder was the"standard rudder us'ed for an 8'0,"0'00 DWT
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65
tanker. The alternative rudder had only one-balf the effective area of
a standard rudder. The results should be of interest to naval
architects as well as port authorities and ship operators.
The channel transits through the first leg, first turn, and second
leg of the channel shown in Figure 3 were compared. The first leg
required compensation for a crosscurrent, while the second leg had a
following current. Graphic presentation of the results is shown in
Figures 4, 5, and 6.
The results show that the pilot was not able to compensate fully
for the reduced maneuverability. Transits with the less-maneuverable
ship resulted in greater variability in track position in the straight
legs and turns, as illustrated by the crosstrack standard deviations.
The mean track line is more sinuous on both straight legs for the
less-maneuverable ship. The mean extreme point violates the channel
boundaries in the first leg, as illustrated in Figures 4 and 6. The
combined index values averaged along each segment are given in Table
1. In all instances, the more highly maneuverable ship allowed smaller
combined index numbers. There is clear indication that with
less-maneuverable ships, pilots require more channel width for safe
navigation.
Turn Configuration
Turns in channels of the United States are generally of two types,
non-cutoff turns and cutoff turns. The basic difference in the two
types is that the vertex of the channel boundaries on the inside of the
turn has been cut back on the cutoff turn, while it teas been left
intact on the non-cutoff turn, Figure 7. The two types of turns are
about equally common.
Navigation through 30-degree cutoff and non-cutoff turns were
investigated during the CAORF experiments. Graphic display of the
results for turns is shown in Figure 8. Experienced pilots navigated
cutoff turns more smoothly and safely than the non-cutoff type. Their
mean cross-channel position through cutoff turns was close to ideal,
while the combined index values are uniformly negligible. On
non-cutoff turns, the pilots entered the turns and exited the turns
wider and with greater variance in track line position. There is a
focal point on non-cutoff turns at the turn apex at which the track
variance is very low. The pilots apparently must pass through this
point on the turn regardless of their position entering the turn and
without regard for the effect on turn recovery. This effect is not
apparent on cutoff turns where the pilots can establish a smooth curve
through the turn and continue the line through recovery entering the
next channel with a low crosstrack standard deviation. A rather
dramatic reduction in the average combined index for the cutoff turn
may be noted in Table 2.
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66
` ~WIND
; ~ RENT
2 NM y~\NM
VESSEL
INBOUND
FROM SEA
l
Figure 3 . Characteri sties of experi-
mental channel.
\
\\\'
· \
\ ', \
* ~ . ~
'
1
hi. 1
6;.
1 ; 1
. ~
| · d .
-/ IQ
turn.
~llj
350 ~ ~ no at. 1
O - race cam IT' o,
Effect of rudder size in 0
I.
TABLE 1. AVERAGE C - BINED INDEX
CC~PARI SON FOR NO - AL AD SMALL
RUDDY SHIP
Average Combined Index
Leg One
Cross Current Turn
Port ST8D Port STBD
Normal Rudder . 151 .014 .085 .009
Emu 1 1 lludder .23 3 .260 .1 14 .226
Leg Two
Followin ~ Current
Port STBD
I .029 L
.os'
.
.00
.0
on _ _~ _1
° 00 250 500 00 250
DISTANCE CROSSTRACK (FT)
1. ~
Normal Rudder Leg I Small Rudder Leg 1
.
500
Figure 4. Effect of rudder
size in Leg ~
1
1
.
4 _.
-
o
-
.
l
! . 1 ~
1
·~
AWL RUOOl. LlD 2 MILL RU~OER tEC 2
.`
Figure 6. Effect of rudder
size in Leg 2.
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67
W :\
\
500
-~ ~
~"~"~:
VN \
\ '\ \ \
V\$
~\
N~\~\
~\ \
\ \\ ~
:\ \
\ \ \
\ \
CU1.OFF 7UIt~
HO CUTOFF YU~
Figure 7 . Cutoff and non-cutoff
turns .
\^
CUTOFF TUIIBI \ ~
V, .-\\
V- ·~\~-
\ "ka _~.
30 KTS Figure 8 ~Ef f ects of type s of
turns .
~ ~0w, 5.
Figure 9. Experunental channels with
and without midleg buoys.
TABLE 2. EFF=T OF ~N
CONFIGURATION ON COMBINED INDEX
Turn Condition
Non~cutoff Turn with Corner Buoys
(Small Radius)
_ _.
Cutoff Twn with Geted Buoys
(Larger Redius)
_
Average Combined Index
_ Port Starboard
_
0.056 O.032
TABLE 3. EFFECT OF ADD ING
MIDL" BUOY
Without treffic
~,
n.~3 n~0ol
~I ~
_-i i ~
_ l,
`,j, 1 .
~|
`-1 1 !
~,
~,
~i
ss1 1 111: ~
. AVZ GE CO INZD NDEX _
| WITHOUT GATE | WITH ADOlTIONAL iATE 31 | | | | O
PORt STBD PORT ST8D 3.| ~1 1 ~ ~°
1 .
_ O.O0O O. 276 0.000 O.Og4
-
1
1
1. 1
_ _
o-~ c~-clt Im
fUI~ F01~? ~ - TURN ~ -
H0 ~IOLl. "n - ~e "n ~o
Figure 10. Effect of adding
midleg buoys on straight legs.
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68
Additional Channel Markings
Many channels consist of a series of relatively short (1.5 to 1.7
nm) straight legs separated by turns. The turns must be marked so
their position is known. It has not been clear, however, that the
addition of buoys along the straight legs away from the turns is
cost-effective with regard to increased safety. During the CAORF
experiments, the second leg of the channel provided an excellent
comparison of the effect of turn markings only versus the addition of
gated pair of buoys midway along the leg. The two configurations are
shown in Figure 9.
The results are presented graphically in Figure 10. The average
combined index values are shown in Table 3.
Conclusions from this comparison are that the additional buoys
clearly caused the mean track line to shift toward the center of the
channel away from the edges and reduced the variance between transits.
The combination of improved mean track line position and lower track
line variance reduced the combined index values to essentially zero.
As shown in Figure 10, these results clearly illustrate the potential
use of aids to navigation to reduce crosstrack variance in certain
channels and to increase the relative safety margin by holding the mean
track near the channel centerline.
Conclusions
Performance data gathered from experiments with the ship simulator
at CAORF have shown that a number of port design parameters directly
affect piloting variability and navigation safety in narrow channels.
The safe operational configuration of any port can be seen to be an
appropriate combination of channel dimensions, operating procedures,
limiting environmental conditions, ship maneuvering characteristics,
and aids to navigation. Such combinations must yield a variability in
trackkeeping performance that will fall safely within the defined
channel for multiple ship transits. In this context, the design of any
particular port is seen to be unique, each of the factors listed above
providing specific limitations on the design parameters. The evolving
experimental data base in port design from CAORF is increasing our
understanding of the complex relation of piloting variability to safety
and port design parameters. Using the methodology and experimental
analysis developed at CAORF, we are now able to find mitigating
solutions to many cases of identified problems that are cost-effective
and that may have minimal environmental effects.
The effectiveness of the present methodology is demonstrated by its
ability to sense changes in all critical port design parameters. The
formulated performance measures are effective in addressing the
following requirements:
Summarizing along-track performance;
Identifying specific problem locations and reflecting changes
required to solve them;
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69
.
Providing numerical indices for comparison of relative safety.
The final requirements of these measures will be to provide
absolute indication of safety relative to actual behavior at sea.
Measures indicative of the actual probability of grounding per transit
will be sought over the next several years through extended
experimentation at CAORF and at-sea data collection.
References
McIlroy, W., MA Review of Valdez Experiment,. Paper presented at
First CAORF Symposium, National Maritime Research Center, June 1977.
2. Riek, J., S. Tenenbaum, and W. McIlroy, Can Investigation into
Safety of Passage of Large Tankers in the Puget Sound Area, Report
to the U.S. Coast Guard, October 1978.
3. Reese, W. Phillip, "Maritime Risk Assessment Applied to California
LNG Import Terminals, n Proceedings, Second CAORF Symposium,
National Maritime Research Center, 1978.
4. Tenenbaum, S., investigation of Navigation into the Port of
Galveston, n Proceedings, Third CAORF Symposium, National Maritime
Research Center, 1979.
5. Cook, R., "Investigation of Limiting Channel Conditions for LNG
Transit into the Port of Pascagoula, Mississippi, n National
Maritime Research Center Report, October 1979.
6. Mara, T., P.R. Keyes, and J. Puglisi, Impact of an All-Weather
Precision Navigation System for Channel Navigation Performance and
Ship Control," Vol. 3, Proceedings, Fifth Ship Control Systems
Symposium, Vol. 3., David W. Taylor Naval Ship Researob and
Development Center, Annapolis, Maryland. November 1973.
7. Bertsabe, W.R., A.J. Pesab, J.L. Maskasky, J.G. Clark, and D.A.
Atkins, Study of the Performance of Aids to Navigation Systems -
Phase I, An Empirical Model Approach, Report to the U.S. Coast
Guard No. CG-D-36-78, July 1978.
8. Atkins, D.A. and W.R. Berteche,
9.
IIIA, Data Analysis and Findings, National Maritime Research Center
Report No. CA0RF-24-7802-01, October 1978.
Atkins, D.A., W.R. Bertsabe, and R.A. Cooper, Restricted Waterways
Experiment ITTB, Results and Findings, National Maritime Research
Center Report, May 1978.
Appendix: The Physical Characteristics
of Waterways in 32 Major Ports
Information covering physical characteristics and present aids to
navigation of 32 major U.S. ports has been collected and entered into a
computer data file. The ports selected and their regions are listed in
Table 4.
Using the most recent U.S. Coast Guard navigational charts, data
descriptive of the physical dimensions of channel segments in each port
were documented for each of the following four categories:
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t
70
TABLE 4. COASTAL REGIONS AND
PORTS EVALUATION IN THE DATA B=E
East Coast
Portland (ME)
Boston
Providence
New London
New Haven
New York
Albany
Philadelphia
Baltimore
Chesapeake Bay
Norfolk
Wilmington (NC)
Charleston (SC)
Savannah
Jacksonville
Miami
Great Lakes
_
Duluth
West Coast
.
Long Beach
Los Angeles
San Francisco
Portland (ORE)
Seattle
Juneau
Valdez
Honolulu
Coos Bay
Gulf Coast
Tampa
Mobile
New Orleans
Port Arthur
Houston/Galveston
.
.
Straight channel: the space between turns or larger areas of
water that is delineated by dashed lines on navigation charts.
Turn: a change in direction coming out of one straight
channel and going into another.
Bay: an open area of water with no dredged area or
delineation of channels. Boundaries are land masses.
River: as given on a chart. Boundaries are the river banks.
The physical data compiled were channel width, depth, length, turn
angle, and turn type (dredged configuration). The remaining data were
code numbers and chart numbers that allowed retrieval of data from the
computer data base and cross-reference to charts.
When necessary, averaged widths of the rivers and bays were
entered, and generally where there were different depths, the
shallowest was chosen. Dashed lines delineating the channels on the
charts were used as a basis for measurement. Depth is taken from the
chart tabulation table or measured directly. Only channels with depths
of 29 feet or deeper were considered for this analysis.
There were entries for 835 channel segments, of which 47 percent
were straight channels, and 46 percent were turns. The remaining 7
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71
percent were rivers and bends. Only the two larger groups by
occurrence (straight channels and turns) have been tabulated.
Straight Channels
Straight channel depth and width for each port is given in Table 5.
Figure 11 in a histogram that summarizes the number of channels by
categories of width for all ports. It is apparent from the figure that
the greatest number of straight channel segments are less than 600 feet
in width and that the majority are either between 350 and 400 feet or
between 550 and 600 feet.
The distribution of straight channel depths is shown in Table 5.
Turns
Distribution of depths and widths of turns parallel the findings
for straight channels. Physical data common only to turns are the
types of turn configurations and the angles of turns.
The determining factor of turn type (cutoff, non-cutoff, or bend)
we" delineation on the navigational charts. A series of cutoff turns
with extremely short (less than 1/4 nm) straight channels connecting
them was counted as one bend, regardless of delineation. Bends
amounted to approximately 50 nm, mostly in the ports of Houston/Corpus
Christi.
Figure 13 shows that of all turns sampled, more than 75 percent are
40 degrees or under, 34 percent are between 20 to 40 degrees, and
another 43 percent are turns of 20 degrees or less. Of the 23 percent
that are greater than 41 degrees, many represent turns on to a
secondary channel.
DISCUSSION
JOHNSON: Does this apply to large bays and relatively shallow
or dredged channels only, or does it also apply to approach channels,
deep water, jettied entrances?
BERTSCHE: It can apply to either problem. There are data
bases that give the entire bottom, and the effect of shallow water
comes in automatically.
KNIER~M: The radius of the turn at several place" in New York
Harbor where there are several marks for different circles in the same
turn, necessitate" a different wheel. You have to increase the wheel
to "et the "hip turning, then when you get in the middle of the turn,
the arc flattens out and the radius becomes longer, and you have to
ease your wheel, at times reverse wheel. At the end of the turn, you
must increase the wheel sharply and get the ship "winging to stay on
the course. Whenever possible, any turn, regardless of the degree
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72
TABLE 5. SUMMARY OF STRAIGHT CRANNEL DEPTE AND WIDTE
FOR EACH MAJOR U.S. PORT (DEPTE IN FEET)
-
RARBORS
WIDTH
400-500 500-600 600-800 .
35 40
35 35
35 35 35 35
40 40
32 32 31
35,33 35 35 35 35
42,40,45 45
40
40,38 40
38,39 34 42,38
38,35
36 36
40 42,40 40
36,33 40,30,38
45 47,47
42,40
45,30,35 35,30
40 40 40
40
l
350-400
800-1000
35
35
Portland
Boston
Providence
New London
New Haven
New York
Phildelphia
Albany
Chesapeake
Baltimore
Charleston
Norfolk
Wilmington
Savannah
Jacksonville
Miami
Tampa
Mobile
New Orleans
Port Arthur
Corpus Christ
Houston
Los Angeles
Long Beach
San Francisco
Portland
Coos Bay
Seattle
Juneau
Honolulu
Duluth
38
38
30
34,32
40
40
40,35
30
30
35
40
41,40
35
40
55
30
.
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73
of the turn, should have the same mark in the water and the same radius
BERTSCEE: Yes, I would like to comment on that. In all my
years of schooling, I really learned one thing in this area, and it
became apparent as we looked at all the charts of all the ports to
build a large statistical data base: all straightaways in the United
States are connected by turn". m e turn is such a perturbative factor
that attention to turning phenomena and their accommodation, if it can
be facilitated, enables consideration of more narrow dimensions.
.
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Representative terms from entire chapter:
safe navigation
