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OCR for page 85
4
EVALUATING THE ADEQUACY OF CRITERIA FOR DREDGED DEPTHS
OF NAVIGATIONAL CHANNELS
In the course of assessing the criteria used in the United States for
determining the depth of dredged navigational channels serving major
ports and harbors, the panel requested information about the criteria
used in other countries from two international organizations--the
Permanent International Association of Navigation Congresses (PIANC)
and the International Association of Ports and Harbors {IAPH)--and
from several maritime countries. Shippers were also consulted for
criteria respecting underkeel clearance.
The panel tested the adequacy of existing depths in the United
States by taking as input to a mathematical model (developed to
provide a check of channel design) the dimensions of nine
representative channels of the East, Gulf, and West coasts.
The results of the simulation are presented in the succeeding
section, followed by the criteria of international organizations,
foreign maritime countries, and shippers.
Dredged Depths of Navigational Channels
in the Uhited States
Navigation
The criteria used to design the depths of navigational channels in the
United States are described in Chapter 2, "Regulatory and
Institutional Considerations." For the major channels of the United
States, the pertinence of the design ships used for the original
4-l
OCR for page 86
4-2
determination may have long since been superseded by the
characteristics of the ships using the channels. The panel therefore
tested the dimensions of nine major channels of the United States on
three coasts (supplied by the U.S. Army Corps of Engineers) with
passages of existing ships in a computer simulation. The computer
model ~ described in Appendix D ) is a simple technique f or testing
various channel designs. For each channel, the analysis was carried
out using existing dimensions and using a hypothetical future depth of
60 ft. In addition, selected increments of depth for the Galveston
Ship Channel were considered. The results for existing depths are set
out in Table 9, and those for hypothetically deepened channels in
Table 10.
For each of the analyses performed, the model evaluated specific
parameters resulting from the passage of the specified vesselts)
through a specified section of the channel. These parameters serve as
the basis for governing vessel operations in the channel. fit is
essential to note that all calculations included the effects of a 1.5
kn current and a 30 mph wind, both acting perpendicular to the
vesselLs). Speed limits were supplied by the U.S. Coast Guard. Speed
limits are not specified for many channels, and the following
estimates were made for the purpose of squat and other computations:
Channel
1. Oakland Harbor
2. Columbia River
3. Chesapeake and Delaware Canal
4. Calcasi eu River
Speed Limit Assumed
5 kn
6 kn
6 kn
6 kn
From the panel's observations and brief ings during f held trips,
the assumptions--including the design ships--are conservative. Even
so, the underkeel clearances of the shi ps considered are well below
the minions specified by the Corps ' criteria, squat t+3 ft. or 0.9 m)
+ rolling and pitching estimate + clearance (2 ft. or 0.6 m, soft
bottom; 3 ft. or 0.9 m, rocky or hard bottom) . Although the rule of
thumb is generous in squat allowance compared to the panel ' s results,
the speeds assumed by the panel may be low. ~ Figures in Appendix D
show how squat increases with speed for these ships in these channels.)
As indicated in the sections addressing considerations of ships
in channels, the maximum vertical excursion of a ship, and the part of
the ship that experiences it, is sensitive to many factors. It is
clear from the results of the simulation, for example, that the amount
of squat changes for the same ship in different channels of the same
depth . The tanker Lenino experiences squat of 2 . 4 f t ~ O . 7 m ~ in the
Lower Columbia River Channel ~ 40 ft. or 12 . 1 m, deep ), but only 0 . 9 ft
~ O .3 m ~ in the inner Calcasieu River Channel ~ also 40 f t, or 1; . 1 m,
deep), even assuming the same ship speed, current velocity, and wind.
Changes in speed, traffic (passing or overtaking), and the physical
environment will also affect the underkeel clearance actually
available to these ships in passage through the selected channels.
OCR for page 87
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OCR for page 89
4-5
Overdredged Depths
Many local conditions affect the accuracy of dredging processes, and
the range of various kinds of dredging equipment is not precisely
known. Nevertheless, the evidence cited in this report ~ "Dredging" in
Chapter 3 ) suggests that the "plus 2 ft" rule of thumb (actually O ft
to 8 ft. or 0 m to 2.4 m; but generally 2 ft. or 0.6 m) allowed by the
Corps to achieve the design prism is within the order of dredging
accuracy but may need to be adi usted in particular channel ~:er~ti one: or
for certain types of work.
For advance maintenance overdredging, the procedures outlined by
Trawle ~1981b~ and described in Chapter 2, "Regulatory and
Institutional Considerations," offer appropriate guidance for shoaling
analysis and progressive improvement in estimation through frequent
surveys. Frequent surveys may also yield needed information to adjust
the "plus 2 f t " pay-overdepth specif ication .
~ ~ .
Criteria of International Organizations,
Other Maritime Countries, Shippers
Criteria of International Organizations
PIANC Criteria In 1974, the Permanent International Association of
Navigation Congresses ( PIANC ~ sponsored an International Commission
for the Reception of Large Ships ~ ICORELS ~ . Working Group IV was
charged with the optimal layout and dimensions f or large ships in
shallow waterways. Group IV published a final report (ICORELS, 1980)
designed to establish criteria to regulate the problem of navigation
of large ships in shallow seas and sea straits (e.g., North Sea,
Baltic Sea, Straits of Dover, Straits of Malacca). In this report,
PIANC gives advice about technical aspects of the possible works to be
undertaken, such as the dimensions of dredged channels, navigational
aids, wreck removal, and evaluation of safety. Approximately 20
countries participated as members of ICORELS.
The report of Working Group IV includes a summery of studies and
developments pertinent to various aspects of channel design,
navigation in sea straits, and dredging for construction and
maintenance, together with conclusions and recommendations in each
area. A set of recommendations is given for determining the depth of
channels:
Recommendations The conclusion Lsection 2.1.1.31 drawn by
ICORELS is that it is not possible to state a general rule for
minimum underkeel clearances and port approaches and
maneuvering areas, because of the influence of local
conditions, currents, and swell. The Commission does note
that general criteria for gross 1lnderkeel clearances ~ section
2 .2 .2 .8 ~ can be given for drawing up preliminary plans:
-
OCR for page 90
4 - 6
Open sea area-en exposed to strong and long stern or quarter
swells where speed may be high, the gross underkeel clearance
should be about 20 percent of the maximum draft of the large
ships to be received.
Waitina area-When exposed to strong or long swells, the gross
underkeel clearance should be about 15 percent of the draft.
Channel-Sections exposed to long swells, the gross ~nderkeel
clearance should be about 15 percent of the draft.
Maneuvering and berthing areas-Protected gross underkeel
clearance to be about 7 percent of the draft. ~ Figure 37]
shows the def inition of underkeel clearances used by the
Commission in their recommendations and are described as
follows Lsection 2.2.2.41:
THE GROSS UNDERKEEL CLEARANCE is by definition the margin
between the keel of a vessel and the nominal channel bed
level, considering the water ref erence level during its
pas sage and the maximum draught of the vessel, measured at
rest in calm water.
THE NET UNDERKEEL CLEARANCE is by definition the minim
margin remaining between the keel of the vessel and the
nominal channel bed level, the vessel moving at planned speed
under the influence of the most severe wind and wave
conditions it was designed to withstand (operational limit
conditions).
The net underkeel clearance, which should be at least O.5 m
(1.7 ft), has to be assessed as a safety margin against
striking the bottom. Other factors are also involved--types
and sizes of ships, commodities transported, environmental
consequences, and density of traffic.
Summary The recommendations of the Commission are for gross
,
under~eel clearances in restricted channels of approximately
15 percent, a factor that allows for the admissible draft of
the ship plus its vertical motions due to swells, squat, due
to speed, and net underkeel clearance. Further tolerances are
added to this nominal channel bed level to allow for sounding
accuracy, sedimentation deposits between two dredging
campaigns, and tolerances for dredging, to produce the final
channel dredged level.
lAPH Criteria The International Association of Ports and Harbors
(IAPH) also assembled a Committee on Large Ships (COLS), now the
Committee on Port Safety, Environment, and Construction. In the
section "Depth of Entrance Channel," COLS (1981) cross-references
OCR for page 91
4 -7
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Figure 37 Underkeel clearances as defined by PIANC
Permanent International Association of Navigation Congresse
*SOURCE: COLS (Committee on Large Ships)
Environmental Protection of Ports and Harbors (Tokyo:
Association of Ports and Harbors)
(1981), Guidelines for Safety and
International
.
OCR for page 92
4-8
the ICORELS recommendations:
S.3.1.2.1 The depths of entrance channels are determined by
allowing a minimum margin between the lowest point on the keel
of the vessel and the channel bed (underkeel clearance) in
addition to the draught of the design vessel. There are two
types of underkeel clearance, one gross and the other net:
Gross underkeel clearance is the margin remaining under the
keel when the ship is motionless in quiet waters and net
underkeel clearance is the margin which continues to exist
under the keel when a vessel moves at a scheduled speed and
when it undergoes the expected maximum inf luence of swell and
wind.
The net underkeel clearance must be at least equal to O .5 m
( 1. 7 ft ~ for a sandy seabed and 1. 0 m ~ 3 .5 f t] when it is
rocky.
5.3.1.2.2 The minimum value of the gross underkeel clearance
taken, consistent with compliance with the lowest value of the
net underkeel clearance, depends upon the following factors:
, , _ _ _
-
Factors related to channel bed;
*the level of the channel bed below chart data,
*the allowance made for the degree of accuracy
_ in
hydrographic surveys (chart data) and dredging tolerances,
*the incidence and degree of channel silting between
maintenance dredgings and dredging tolerances,
*the latest maintenance soundings and
*the eventual percentage of suspended silt.
Factors related to tide;
*tidal variations (maximum and minimum),
*tables of predicted tide levels,
*details of any tidal surges, wind and atmospheric
pressure ef fects on water level and
*the accuracy of predicted tidal heights and the
predicted times of high and low water (for particular
tides ~ .
- Factors related to the ship;
*the actual maximum draught of the design ship,
*the increase in ef f ective draught due to the rolling,
pitching and heaving of the ship under wave action within
the channe 1,
*the estimated squat and change of trim f or the design
ship calculated f or each critical depth area based on the
maximum Fermi ssible operating ship speed and the most
constricted channel section within the critical depth area,
The normal loaded condition of the design ship and
*the draught and trim changes attributed to any change
in water density.
-
OCR for page 93
4-9
5.3.1.2.3 It is not possible to establish accurate rules
concerning the minimum depth of port channels because of the
ma jor importance of local conditions .
.
In the initial planning stages, the following generalizations
may be valuable:
- sections exposed to strong and long swell - gross underkeel
clearance to be about 15 percent of the maximum draught,
- sections less exposed to swell--gross underkeel
clearance to be about 10 percent of the maximum draught.
5 .3 .1.2 .4 Detailed recommendations for the depth of channels
are given in ICORELS Report of PIANC ~ Working Group No . IV) ....
In order to define the nominal level of dredging, it would be
advisable to allow for the accuracy of the soundings, for
siltation between maintenance dredging and f or dredging
tolerances .
Criteria Used in Other Maritime Countries
Canada' s design criteria
for channel depth primarily address the need for precise and reliable
measurement of the environmental risks associated with the location
and operation of marine terminals, particularly those for large on
tankers. They can be found in the "TERMPOL" Code of Recommended
Standards (Canadian Coast Guard, 1977 ~ . The Ministry of
Transportation, through the Canadian Coast Guard, established the code
TERMPOL ~ ire coordination with other departments .
The TE1W?OL code is a set of recommended standards used in Canada
f or the prevention of pollution in me rine term nal systems . The code
outlines acceptable ship terminal standards, def ines the required ship
terminal system analysis and assessment criteria, and sets out
operating practices and procedures for ship terminals.
Although published by the Canadian Coast Guard, the TERMPOL code
is a coordination and correlation or the separate requirements of the
Canadian Department of Fisheries and the Environment, Public Works,
And -m, and Reg~ona-l and Economic Expansion.
Each participating department is individually responsible for all
contributions made and decisions taken within its area of
responsibility. ~
Provisions of the code are not themselves mandatory, but the
assessment criteria of the code are used by the Ships Safety Branch of
the Canadian Coast Guard to determine the technical needs, if any, for
making regulations or implementing special precautionary measures to
licit navigation within the ship terminal system under review.
Canadian Criteria for Underkeel Clearance
OCR for page 94
4-10
The "TERMPOL8.
representatives of the
the assessment and can
Coordinating Committee
participating __ ~
(T.C.C.), composed of
government departments, performs
require terminal planners to submit data for
environmental impact assessment.
Port authorities seeking to improve their waterways may be called
upon by the TERMPOL Assessment Committee to provide the following
surveys:
o
o
o
o
o
o
o
a
Ori gination and Destination Survey;
Transit Time and Delay Survey;
Marine Traf f ic Volume Survey;
Fishing Vessel Operation Survey;
Approach Characteristics and Navigability Surrey;
Special Underkeel Clearance Survey;
Site Plans/Technical Data;
Environmental Stud' es.
Thus, the Canadians use a systems approach in which the
specif ication of underkee 1 clearance is integrated with studies of
marine traf f ic and required navigability .
For the purposes of this report, the study of direct interest is
the Special Underkeel Clearance Surrey. The requirements are as
follows: Nominally, the design ship's minimum underkeel clearance
should be 15 percent of her maximum permissible draft. A proposal for
a minimum underkeel clearance in approach channels of less than 15
percent of the design ship's deepest draft will be considered but
should be supported by explicit details or calculations for each of
the following factors:
o Minimum chart datum measurements supplemented with tidal
heights over a specified period;
o Accuracy of predicted tidal heights and the predicted tomes of
high water and low water;
0 Details of any tidal surges and wind setup;
o Allowances for the degree of accuracy in the hydrographic
survey ~ chart datum) and for that of dredging processes;
o Incidence and degree of channel silting between maintenance
dredgings and the identif ication of all critical depth areas;
0 Increase in effective draft due to the rolling, pitching, and
heaving of the ship under wave action within the ship channel;
o Estimated squat for the design ship calculated for each
~rit~ ; Moth - treat based ~a - anal maximum' per=rse~peret~ns ship
speed and the most constricted channel section within the critical
depth area;
o
Nominal trim and changes of trim experienced by the design
ship;
o Draft and trim changes attributed to any changes in water
density;
0 Climatological and related depth anomalies; and
0 Nature of the bottom (rock, sand, mud, etc. ~ .
OCR for page 95
4-11
Underkeel Clearance and Depth Criteria in Japan The Japanese Ministry
of Transport has established its criteria for marine facilities
through j oint action of its Bureau of Ports and Harbours and its Port
and Harbour Research Institute. These are summarized in a booklet
entitled "Technical Standards for Port and Harbour Facilities in
Japan" published in English in 1980. The Japanese version, published
in March 1979, represented the first compilation of all advanced
Japanese port and harbor engineering techniques. The English version
differs from the Japanese version only in that it excludes the
official procedures for compliance with the standards.
The Japanese relate the depth of channels to their basic
specif ication for depth of harbor basins . The design criteria for
basin depth, in turn, are ~ Bureau of Ports and Harbours and Port and
Harbour Research Institute, L980 ):
DEPTH OF BASIN
1 ) The depth of basin shall be 1.1 times full load draft of the
ship below the datum level, in considering the extent of the
oscillatory motion of the ship due to the natural conditions
such as waves and tidal currents. However, this provision
shall not apply to a basin for outf it of ships and a basin
used for special anchorage or mooring of ships. In the case
of a basin f or ferryboats, the draf t difference between stern
and bow during car-to handling, should be considered to
determine the depth of the basin. Furthermore, where the sea
level of a basin may be below the datum level, because the
seasonal changes of mean sea level are larger than the tidal
level change due to astronomical tide, or where the basin may
be attacked by high waves and swells, these influences should
be considered.
2) The depth of a basin can be determined in reference to the
values given in tTable ll] when the full draft of the ship
is not known.
The depth of waterways is related to that of basins as f allows:
DEPTH OF WATERWAY
1 ) The depth of waterway shall be an appropriate value of no
less than the full load draft of the ship in consideration of
the extent of oscillatory motion of the ship due to the
natural condition such as waves, winds, and tidal currents
and the trim. In this case, "a proper depth" means a depth
obtained by an allowance added to the depths specified in
[DEPTH OF BASINI. The allowance varies with such conditions
as roll and pitching, trim and squat of the ship and the
conditions of seabed materials. This provision may not apply
to at special waterway where the draft Of the ships mires thii-S
OCR for page 96
4 -12
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4-13
waterway is always smaller than the full draft, such as an
approaching waterway to dock of shipyard or a waterway
exclusively for partially loaded ships.
WATERWAY MAT NTENANCE
1) The depth and width of a waterway shall be maintained
properly for the smooth use of harbour and the safe
navigation of ships.
When a waterway is planned on a river mouth or a beach where
a big amount of littoral drift Is expected, the degree of
maintenance dredging requ, red in the future should be
forecasted by estimating the rate of sediment transport by a
flood or the rate of littoral drift by waves and tidal
current.
Presumably, this latter calculation is used for an initial
schedule of maintenance dredging or for determining additional depth
f or advance maintenance dredging.
Channel Depth Criteria, Port of Zeebrugge, Belgium The Belgian
approach to cnanne1 Repin criteria IS outlined in a 1~8U bulletin of
the Permanent International Association of Navigation Congresses.
Waters of the Belgian coast experience dense me rine traffic. As
a result, traffic has been channeled into compulsory routes. One of
the main routes through the North Sea arrives at the f rontier of
France and Belgium approximately 16 miles of f the coast, then runs
east/northeast to the "Scheur, " which is the access channel to the
River Scheldt estuary and the Port of Zeebrugge. The channel
continues to its terminus at Antwerp.
In 19 7 0 , the Be lgian government decided to extend the port of
Zeebrugge, using a sea lock to connect the inner port with the sea
(Simoen, 19801. The seaward extension was to accommodate vessels UD
to 125,000 DWT and, specifically, to accommodate roll-on, roll-off
(RO-RO) cargo vessels and containerships. The port will also have an
LNG terminal. Since the projected traffic involved large vessels with
draf ts of 5 0 f t or more, the Adraini stration of Waterways, Ministry of
Public Works, initiated a study of needed channel dimensions .
The study plan included a preliminary phase to def ine the design
criteria and .to chaise the chancel repute., bS~;£~ d~led~; s`~di~ Are
then made for the design of the chosen channel route which included
the determination of the channel cross-section profile, various
studies of sedimentation, the removal of wrecks and mines, and the
study of required nautical equipment. -
The dredged channel depth criteria were derived in this second
phase as follows:
Channel Depth Methodology The provisional channel depth was
first determined on the basis of a combination of design draft and
tidal "windows" for the design vessels. The necessary keel clearance
,
OCR for page 98
4-14
was then calculated in accordance with the recommendations of PIANC,
in comparison with other harbors, and by using the wave and tide
records taken at Zeebrugge.
Provisional channel depths were then selected ranging between 13
m and 16 e 7 m (43 ft and 55 ft) for a variety of design ships (LNG
tankers, VLCCs, containerships).
More detailed studies were used to determine the cross-section
prof ile of the waterways . Basic data were gathered in these studies
f or the determination of the cross-section:
o The velocity and pattern of the current in different parts of
the access channel, as well as variation during the tidal cycle
inf luence on channel width );
o The wave heights induced by dif f Brent wind f orces and
tide-level records ~ inf luence on channel depth );
o Regularly taken soundings of the channel bottom. These echo
soundings provided information about the variation of the channel
bottom ~ inf luence on clearance and channel depth ~ .
Kee 1 Clearance Criteria The channel depth needed in the access
route to Zeebrugge was determined as a function of:
o The draft of design vessels;
o The keel clearance to be observed in the access to Zeebrugge;
o The water leve 1 at the time of entering or leaving the harbor ,
which is a function of the tidal windows for each type of ship.
Keel clearances for good and bad weather were calculated
separately. The criterion for good weather conditions is normal
maneuverability. The criterion for bad weather is "the chance of the
ships ' touching bottom should be acceptably small. " Fi gure 38 shows
the method used to determine keel clearance.
Starting from a reference water level, vertical ship motion due
to squat and waves is added to ship draft. This incremental value of
keel clearance is described as a net keel clearance (about 4 ft for
LNG carriers). Further allowances are added to the clearance to allow
for the accuracy of soundings and tide measurements.
Still another allowance is made for divergence from average tome
levels, and an allowance is made for sedimentation between two
sounding campaigns, plus a tolerance for dredging work.
Finally, during bad weather conditions, it is recognized that the
ship may experience increased draft owing to wave-induced motion.
Additional allowances are specified to prevent the ship from hitting
the bottom. Given the wave characteristics, it is possible to
calculate the probability of a ship's vertical movements.
Each of these individual allowances is determined by calculation
or measurement.
Summery The keel clearance criteria used by the Belgium Port
Authorities are experimental and analytic, rather than regulatory, and
are determined f or both good and bad weather.
OCR for page 99
4-15
Figure 38 Access route to Zeebrugge, Belgium: prescription of the underkeel
clearance
~ ~ - .
SO ~
t
!
i Reference Water Level
Ship's draft
Vertical ship movement (squat and heave)
Draft keel clearance
Accuracy of the soundings and tide
measurements
Divergency from average tide level
_
Sedimentation between two surveys and
inaccuracies of dredging work
\
~ 1
OCR for page 100
4-16
Calculations are made of all key factors affecting vertical ship
movement (design draft, squat, waves, maneuverability requirements,
accuracy of soundings, and tide measurements) on an analytical or
statistical basis, or both.
Additional allowance is made for maintenance dredging to provide
for sedimentation between two sounding campaigns.
The result for the channel leading to Zeebrugge is the
specification of depth, 13.4 m (44 ft), and gross keel clearance for
normal and bad conditions of 1. 4 m and 2 .7 5 m, respectively ~ 5 f t and
9 ft).
Inasmuch as the design draft of the vessel was 11 m (36 ft), the s
underkeel clearance amounts to 12 . 7 percent of ship draft for normal
conditions and to 22 percent for bad weather conditions.
One notes that the Belgian design criteria for underkeel
clearance approximate those of other countries, although the allowance
for bad weather conditions is more conservative.
"Nautical Depth" Concept, Europoort and Rotterdam, Netherlands The
.
"nautical depth " concept ( de scribed in a preceding section ~ evolved
from study of the behavior of vessels in Europoort and Rotterdam
harbors. The present operational practice in the Europoort/Rotterdam
area requires that supertankers approaching Europoort from the North
Sea to the Maas-Center Buoy (Surogeul) have a minimum underkeel
clearance of 20 percent of their draft, and from the Maas-Center Buoy
to the pierheads an underkeel clearance of 15 percent of draft.
The approach channel is maintained at 23.5 m (78 ft) depth. From
the buoy to the pierheads, the channel depth is 22 .5 m (74 ft.) . me
Caland and Beer channels allow vessels to proceed at an underkeel
clearance of 10 percent of draft. Supertankers with a maximum draft
of 20 .7 m ( 68 ft ~ are required to proceed very slowly with a mi nim fin
underkeel clearance of 10 percent ~ 2. 1 m, or 7 f t) .
The nautical depth is defined as the depth to silty layers of
specific gravity 1.2. The specific gravity of bottom sediments is
monitored weekly by survey vessels, and decisions are made each Friday
about maintenance dredging for the following week. Density charts are
prepared showing the variations throughout the channels, as shown in
Figure 39.
Underkeel Clearance Criteria, Port of Hamburg, West Germany The
approach channel to the Port of Hamburg f rom the North Sea is 6 0 miles
long and 13 . 5 m ~ 44 ft ) deep at ELM. The tidal range is 3~ m tS .
ft). The port is now conducting tests for squat in various
conditions; in the meantime, allowances for vessels have been
determined from experience, taking into account greater squat at the
higher speeds in the approach channel, insufficient depth, and narrow
curves. For arriving vessels , the depth of the River Elbe (13.5 m, or
44 ft~ and tidal range ~3 m, or 10 ft) , as well as tidal uncertainty,
are considered, with the following results:
OCR for page 101
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4-18
Maximum draf t of ves se 1 s up to 3 0 0 m ~ 9 90 f t ~ in length:
13 .72 m (45 ft ) arri ving at Hamburg
Pilot Stat' on 2 hours before high water
Maximum draft of vessels more than 300 m (990 ft) in length:
12.5 m (41 ft), arriving at Hamburg
Pi lot Station 2 hours before high water.
The largest ship allowed in Hamburg has a draft of 14 m (48 ft) and
must be brought in at high water. Conta~nerships must have an
underkeel clearance of at least 1 m (3 ft). These are, of course,
operational rather than design criteria.
The Port of Hamburg does not pay for overdredged depths .
Underkeel Clearance Criteria in the United Kingdom Ports and harbors
-
in the United Kingdom general ly apply one standard for underkeel
clearance to sheltered waters and another to open waters, but other
criteria are occasionally used. The British Ports Association offered
the following summary to the panel:
Traffic
Underkeel Clearance
Port A Almost exclusively VLCCs
Port B
Port C
Port D
Port E
Port F
Port G
VLCCs, bulk carriers,
general cargo ships
VLCCs, containerships,
general cargo ships
and passenger ships
VLCCs, bulk carriers,
general cargo ship s
VLCCs, bulk carriers,
general cargo ships
ROW RO f erries and
general cargo ships
to 4.8 m, or 16 ft. draft
Similar to Port F ~
1096 in excess of draf t f or
well -sheltered channe 1
0 .9 m ~ 3 f t ~ f or sheltered waters;
2.3 m {8 ft) for ships to
150,000 DWT in open waters;
10% in excess of draft for ships
150 ,000 DWT to 250,000 DWT in
open waters
1 m ~ 3 ft ~ sheltered waters;
1.6 m (5 ft) open waters
1 .2 m ~ 4 f t ~ in relatively sheltered
channels
For VLCCs in excess of 12 m (39 ft.)
draft only: 0.9 m (3 ft) on the
flood tide, 1.5 m (5 ft) on the
ebb tide
O.3 m (1 ft) over muddy bottoms
{but often less); 1 m (3 ft)
over sandy bar
0.6 m to 0.8 m (2 ft to 3 ft) in
sheltered harbor
OCR for page 103
4 -19
me Association points out that "open" and "sheltered"' may be
variously interpreted from port to port, that depths change more
rapidly in some ports than in others, and that traffic conditions,
channe 1 width, and the nature of the cargo may imply a need f or
dif f Brent standards . The National Ports Council and the Department of
Transport have sponsored studies to improve understanding of ship
behavior and channel characteristics ( e . g., National Ports Council,
1976 ) .
In the Port of Southampton, the design channel depth is 50 ft
(15.2 m). The maximum draft allowed is 48.5 ft (14.7 m) at high
tide. Minimum underkeel clearance required by the port is 4 .2 ft ( 1.3
m) . The overdredged depth is 0.3 m ( 1 ft) . The South Wales Ports
specify overdredging depths of O .7 m ( 2 f t ); in areas of high
shoaling, 1 m ~ 3 .3 f t ~ . The ports operator, the British Transport
Docks Board, owns and operates the dredges .
Shippers' Criteria
Ship operators must appraise ports to decide their suitability f or
ships of particular dimensions. Among their concerns is channel
depth. Crane ( 1981 ~ gives a brief summary of the methods shippers use
to make such appraisals, including extens' on of experience, hydraulic
model studies, and simulations of varying sophistication. For
bottom-clearance appraisals, Crane points out, there are several
procedures, the simplest being experiences of other ships and the
reports of pilots. Another simple technique is essentially similar to
the criteria used for designing channel depths: addition of
allowances for squat, trim, and other factors to the ship's static
draft (Figure 40). This method will produce overly conservative
results for a particular ship, Crane notes, because it assumes the
coincidence of maximum vertical excursions. "Therefore, statistical
addition of allowances for each factor should be substituted ~ Figure
411 . "
This statistical method has been elaborated by Kimon ( 1982 ~ for
comparison against a standard for underkeel clearance, namely a very
small probability of grounding. The generalized method is a
statistical combination of all the factors known to be most important
in dete,.~ining the depth of water required by ships of certain drafts
(or required underkeel clearance, or both). The statistical
combination produces an acceptably small probability of grounding in
cocoon with a semiemp~r~cal coefficient, whose value is derived
from known ports (similar to that in question) with many ship-entry
years ' experience. The method can be applied by a ship's master to a
particular port using preprinted worksheets, sample arithmetic calcu-
lations, and a series of graphs developed by Simon.
The uncertainty associated with some factors is as great as a
factor of 10, and judgment is inevitably decisive in certain cases,
but the characterization of various f actors ~ such as ship response to
waves) can be updated as data become available.
OCR for page 104
4-20
WATER
REfERENCE LEVEL
GROWS ~
UND£RKEEL CLEARANCE
t
ADMISSABLE DRAFT
e
I`JERttCAL MOTION
)(SWELL AND SQUAT)
~ _ _ _ _ _ _ _ _ _ _ _
NET UND£RKEtL CLEARANCE
SOUNDING ACCURACY
SEDI MENTATION BETWEEN DREDGINGS
TOLERANCE FOR DREDGING
Figure 4 0 Conventional net underkeel clearance calculation, def initions
from PIANC
TIDAL UNCERTAINTY
PREDICTED TIDE
1
CHA.R:IED DEPTFt
1
CHART I'ATU M
~ _ _ _ _ _ _
STATIC
DRAFT
UNCERTAINTY
UNCERTAl~JS=~7
CHARTED DEPTH
UNCERTAINTY
NOMINAL SEABED ~
SHIP
rWATER LINE
STATIC DRAFT
4_ _ _,~
_ . _ __
-
~ W, tVE RESPONSE
UNDERCUT
SlLTAtlON
Figure 41 S-atis=~c~l ~d¢~keel clearance caTc~;~ti~*<
Permanent International Association of Navigation Congresses
*SOURCE: C. Lincoln Crane, Jr. (1981), "Concerns of Ship Owners, " Problems
and Opportunities in the Design ~ _ =
~ —
(Washington, D. C.:
National Academy Press ), pp . 4 5-52 .
OCR for page 105
4-21
This statistical technique could prove useful in the validation
of channel design. While Kimon's analysis and results are specific to
tankers, the generalized method could be applied to other ships (or
the design ship). It is interesting to note that a similar method was
used to design the channels of the Port of Zeebrugge, with a some lar
aim: a very small statistical probability of grounding.
Discussion
The criteria recommended by PIANC were developed by ref erence to
recent research and approved by voluntary consensus of many nations
~ including representation f ram the United States ) and the IAPH.
Particularly for advance maintenance dredging, the criteria assume
more f requent bottom surveys than is normal practice in the United
_ . . . . _ . . .
states, wnlcn is no more often than annual or semiannual.
Taking the largest vessel in 1980 (by draft) that transited each
of the nine channels selected by the panel, underkeel clearance was
calculated by the lower value of the U. S . rule of thumb and of PIANC
criteria in Table 12 . There channels were designed long ago ~ 2 0 years
or more ), and vessels have been built that are much larger than the
design ship or ships . The margins of saf ety that were assumed in the
original design are no longer offered to the vessels using these
channels .
Actual use of the U. S. rule of thumb in the design of channel
depths today would prove cumbersome, as it relies on several estimates
f or ship behavior . Substitution of the PIANC criteria seems a
reasonable step. The statistical technique also offers attractive
features--for example, incorporation of a sub jective judgment of
safety (an "acceptably small probability of grounding" )--that could
prove useful in builds ng consensus among ship operators, pilots, and
local Coast Guard off icials about the otherwise vexing question, How
safe is safe enough?
OCR for page 106
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Representative terms from entire chapter:
channel depth
