National Maritime Institute
An Approach to the Design of
Navigation Channels
by
IWD.and
NMI R104
May1981
National Maritime Institute
Feitharn
Middlesex TW14 OLQ
Tel: 01-977 0933 Telex:263118
Technische Hogeschool
Deift
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PROJECT 252001
NATIONAL. MARITIME INSTITUTE
AN APPROACH TO THE DESIGN OF NAVIGATION CHANNELS
BY
IWDAND
Summary
This. report discusses some aspects of water-way design from the point of view of safe ship navigation A method for designing the cross-section and alignment of canals or flooded channels is suggested in which use is made of experimental results obtained recently at NNI. The method (which applies essentially to the conceptual stage of design) is illustrated by means of a worked example.
Bibijotheek
van de
AtdeIin
en Sceauje
Introduction 1
Waterway Design 1
Assumptions and Definitions 3
3.1 Hydrodynamic Effects 3
3.1.1 Squat 3
3.1.2 Interaction 3
3.1.3 Resistance and Blockage 4
3.2 Speed and Depth 4
3.3 Ship Handling 5
3.4 Curved Waterways 5
Cross Section Design - Str ght Waterways 6
4.1 Fairways 6 4.1.1 General Remarks 14 4.2 Canals 4.3 Final Considerations 6.1 General 6.2 Secondary Criteria 7.1 The Problem
7.2 Design Procedure - One Way Traffic
14 18
4.3.1 Waterway with Moored Ships 18
4.3.2 Waterway Bifurcation 19 4.3.3 Depth Determination 20 Curved Waterways 21 5.1 General 21 5.2 Curved Fairways 5.3 Curved Canals 22 5.3.1 Effect of Current 25
5.3.2 Increase of Width in a Canal Bend 25
Safety Criteria 26
26 27
6.2.1 Moving Ship Passing a Stationary Moored Ship 27
6.2.2 Two Ships Passing on Reciprocal Courses 27
6.2.3 Two Ships Passing on Parallel Courses 28
6.2.4 One Ship Passing Close to the Edge of a Fairway 28
6.2.5 Basic Controllability 28
6.2.6 Wind and Current Effects 28
6.2.7 Controllability in a Curved Fairway 29
6.3 Examples of Secondary Criteria 29
6.4 Other Considerations 31
7. Worked Example 32
32 34
7.2.2 Ship Speed and Power Requirements 35
7.2.3 Choice of Minimum Distance From Bank 36
7.2.4 Lane Width for Basic Controllability 36
7.2.5 Handling in a Cross Wind 37
7.2.6 Width & Depth of Fairway: Summary 38
7.2.7 Moored Ships 38
7.2.8 Curved Portion of the Fairway 40
7.3 Two-Way Traffic 41 7.4 Overall Summary 43 8.' Concluding Remarks ' 44 References 45 Acknowledgement 50 Nomenclature 51
Appendix A - Resistance Estimation in Fairways & Canals 53
Appendix B - Typical Manoeuvring Coefficients 57
Appendix C - Estimation of Sway Force & Yaw Moment Induced by Bank
Proximity. '
59
An Approach to the Design of Navigation Channels: by I W Dand
1. Introduction
In a study of the behaviour of ships in port approaches (ref. 1) carried out by the National Ports Council and using information available in the open literature up to 1973, it was clear that there was a paucity of knowledge of ship behaviour in confined waters. Accordingly the National Maritime Institute was commissioned by the UK Department of Transport with the technical guidance of the National
Ports Council, to investigate some of these less-explored areas of interest.
In the event, the investigation covered four main areas which were:
ship-ship interaction
ship manoeuvring characteristics, especially when in the proximity of banks
squat
the virtual mass of a ship while berthing
One of the aims of the study was to provide a compendium of information which could be used by port designers, operators and users alike. The purpose of this report is to indicate one way in which the results obtained can be used in a waterway design scheme. Throughout, emphasis is laid on design of the waterway as
it relates to the behaviour of the ship; no attempt is made to discuss the effect of the resultant design on such features as tidal flow, wave climates,
sedimentation etc which were outside the scope of the study. Use is made of
results publishedin references 2, 3, 4 and 5 while other more recent results are incorporated in the body ofthis report where appropriate.
2. Waterway Design
The design of a waterway is a complex process involving several disciplines. The
economics of a design are of paramount mportance to the successful operation of
the waterway whether itbe a port approach channel or a canal allowing the transit
of seagoing ships (see refs 6, 7 and 8).
Indeed, so important are the economic aspects, that the component parts
diagranunatically in Figure 1. It is clear from this figure that while results from the engineering and operational studies are needed for the economic analysis, so also are results from that analysis required to provide guidance on the
suitability or otherwise of the other studies.
The overall design process is therefore seen as one in which the various disciplines involved must work in parallel. If necessary, a continual review must be made
of the overall design as the work progresses.
It is seen from Figure 1 that one aspect of the overall scheme relates to the design of the waterway cross-section and alignment; this report deals with such aspects only and does so by laying emphasis on the safe navigation of a ship, making only passing reference to other engineering problems such as bank protection and siltation. Therefore, results obtained from the method outlined below should be used in conjunction with civil engineering expertise on the construction and maintenance of the waterway.
Several methods can be adopted for the design of the waterway cross-section and alignment, but they may all be classified into groups which are either probabilistic or deterministic. The probabilistic methods assume that sufficient data are
available to allow a simulation of many ship transits thereby allowing optimum waterway width and depth to be determined by means of Monte Carlo techniques.
Such methods are discussed in references 9 and 10 where the conclusion is reached
that insufficient dataareat present available to allow such a simulation to be constructed.
A form of probabilistic approach makes use of a ship-handling simulator in which many runs along a proposed waterway can be made with suitable modifications being made to channel dimensions and alignment in the light of the results obtained (refs
11, 12 for example). For example, if a curve proves to be particularly difficult for experienced ship handlers to negotiate in a simulator, the radius of the bend, the bend angle or the waterway width may all be adjusted until an optimum solution is found.
But such a method requires some prior knowledge of a width, depth and alignment of the
proposed channel, which may then be modified as necessary. To obtain such a
preliminary design, deterministic methods may be employed, based on the results of model experiments. It is with such a preliminary design scheme that this report is concerned. By using it, the engineer should be able to progress from
some prescribed initial conditions to an estimated waterway width and depth
together with an assessment of the severity of any curves. Once such a preliminary design is obtained it may then be subjected to further analysis by engineers
and experienced mariners in conjunction with the use of a ship-handling simulator, if required.
The method, which extends that described originally in reference 13 and further in reference 1, is straightforward in its use and does not necessarily require
substantial computational support. However, some of the estimation methods outlined below are contained in computer programs held at the National Maritime Institute.
3. Assumptions and Definitions
We assume that the waterway comprises a combination of basic elements that are either straight or curved. The major part of the design method is concerned with
the elementary straight sections, the assumption being made that if the width and depth of the straight sections allow safe navigation, then these should be used
(and increased if necessary) in the curved sections.
We alsoassume that the effect of-wave action on the motion and handling of the
ship is negligible although some reference will be made to the action of the
pressure field generated by the ship (and manifest as surface waves) on other ships and banks.
3.1 Hydrodynamic Effects
It is known that when a ship moves in a waterway it is subjected to hydrodynamic phenomena which affect its behaviour. These are:
3.1.1 Squat
This is the tendency of a ship to settle bodily in the water when under way,
less than that expected.
3.1.2 Interaction
When two ships or a ship and a bank are in close proximity, the pressure field over one ship is changed due to the presence of the other ship or the bank. This
causes lateral forces and moments to act on the ship under consideration, and, as this effect increases in shallow water, it can give rise to serious handling problems.
3.1.3 Resistance and Blockage
Resistance to motion is greater for a ship in shallow water than in deep
It increases still further if a width as well as a depth restriction is placed on the body of water in which the ship moves. In such a case - a ship in a canal in fact - the resistance (and hence speed) is clearly related to the blockage
ratio in the canal, this being defined as the ratio of the immersed cross-sectional area of the ship to the cross-sectional area of the canal. If the blockage is high, very large resistance increases can occur with a corresponding effect on
speed.
In a flooded fairway the blockage is theoretically zero so that the ship experiences a moderate increase in resistance due to the
restricted water depth. If however the banks were to dry out at low tide, the channel would become in effect a canal, blockage would become important and large increases in resistance (and interaction forces) would occur.
3.2 Speed and Depth
In all shallow water work an important parameter is the Froude Depth Number,
Fh, relating the speed of the ship U to the water depth h as follows:
Fh =
U//gh (1)where g is gravitational acceleration. In general mostrnerchant ships will have insufficiei power to exceed Froude Depth Numbers of about 0.6, due to rapid increases in resistance
3.3 Ship Handling
In attempting to assess ship handling in a given fairway, several parameters are used. These are:
Lane width, w
This is defined as the maximum width of the track swept out by the ship.
Course or cross-track error, y
This is defined as the difference between the mean course achieved and the 'target' course. It is a measure of the accuracy with which the ship can be positioned in a fairway for a given set of conditions.
Basic controllability
This is a measure of both the handling characteristics of the ship and of the skill of its pilot. It is defined by means of the basic lane width, course error and rudder activity of the ship as its pilot attempts to maintain a course along the centreline of a fairway in the absence of wind or current.
Equilibrium rudderangle,
This is the rudder angle required in a steady-state situation to balance any external forces and moments imposed on the ship. Port rudder is counted positive.
Equilibrium drift angle, m
This is the drift angle required in a steady-state situation to balance any external forces and moments imposed on the ship. It is usual for non-zero
equilibrium rudder and drift angles to occur simultaneously. It is counted positive
when the ship is moving to starboard.
3.4 Curved Waterways
Bend radius, R
This is the radius of the centreline of the waterway in the bend.
Angle of bend, y
This is the angle between the centrelines of the straight portions of the waterway at entry and exit to and from the bend, extended until they cross.
Parallel bend
This is a bend in a waterway whose width remains constant throughout and equal to that of the straight posiitons of the canal at entry and exit.
4. Cross Section Design - Straight Waterway
4.1 Fairways
We consider for the purpose of this report that a fairway is a waterway whose sidebanks are flooded and of effectively unlimited lateral extent. We now
introduce the concept of a design spiral for a fairway as shown in Figure 2. This shows diagrammatically the steps in the proposed design procedure, the diagram indicating the logical flow of the process as well as the cross checks required. The process is essentially iterative with the possibility of several passes around the spiral before an acceptable design is obtained.
It is assumed at the outset that the following are known before the design begins:
a) The dimensions (length, beam and draught) of the 'design' ship. This will be determined generally from technical and economic considerations, but it may be necessary in some cases to consider more than one 'design' ship. For. example a port may handle both VLCC's and vehicle carriers, both of which have their own problems; those of the VLCC associated with its size (in particular its draught) and those of the vehicle carrier associated with handling in strong winds due to its high windage. Therefore some parts of the fairway may have their depth determined by the VLCC while their width should be checked for ease of handling of both a VLCC and a vehicle carrier.
Environmental data, in particular that relating to prevailing wind and currents.
Bank slope which will be determined by the angle of repose of the material in which the fairway is constructed.
It is appropriate to discuss some of the steps in the design spiral and this is done below, but it should be remembered that other aspects of design, not
considered in this study, may have to be inserted at appropriate points in the
cycle.
Steps 2 - 4: Choice of Depth/Draught Ratio, h/T, and Squat
The choice of depth is governed by:
ship draught
at-rest trim of the ship
maximum squat corresponding to a) and b)
variations in draught due to variations in water density siltation
safety margin
Of these items it is assumed that a) and b) are known and e) may be determined by methods similar those outlined in reference 14, but which lie beyond the scope of this study. Item f) relates directly to safety criteria which are discussed in section 6 below, but is is perhaps of interest to note that a safety margin on
depth of 0.6 metres is suggested inmference 15.
Squat may be estimated using the method of reference 16. This requires a knowledge of ship dimensions and trim as well as the depth/draught ratio, h/T, to produce estimates of squat for a range of speeds in wide open shallow water. In the first instance the final h/T value will not be known and must be guessed. However squat is not very sensitive to small variations in h/T so that a reasonable initial estimate may be made.
The naximum squat value thus obtained may then. be corrected -fr batik ptödmity using the method of ref 17 This method, however, requires some knowledge of
distance. off the bank and in th 'first pass through.the deign cycle this is nOt
known at' this stp. The correction is however fairly 'small unless the ship
is very near the bank and could be negleted at this point in the first pass.
A final correctiOn to the stimatéd maximum squat should be 'made if ships ae to pass in the- fairway for, as' shown in ref erené 2, increases iti mean sinkage and changes in trim occur during the passing phase. Tables for estimating the,i-ncrease over the squat for wide shallow water are given in ref etençe 18 but requite once again a knowledge of the distance frqm tIie bank to the ship As mentioned above, this is not known at this stage .of the design cycle on the first pass and it is
pro'ab'1y adequate to make an initial' allowance by doubling -t-he maximprr squat of the
larger of the two ships. This can be refined On subsequent passes through the design spiral. when an approximate fairway width has been obtained.. ' -Steps 5 and 6: Power/Resistance/Speed Estimate
1t is clearly of ipOance that a realistic transit speed for the waterway should
be -chosen. The capacity 'and ttaff;ic prograirg of the port are. intimately connected
with the transit speed of the ship and it is therefore essential that the major fairway pararneters do not result jn an increase in resistancewhjch would make the
requjred transit speed unobtainable.
'In' wide shallow ate, caith water tesistance'can be estimated reasonably accurately using Schlichting's met-hod (ref. '19,Appendix'A) which r'equires information relating
'to' the' hi.p and water depth that are available at this stage of 'the design.
Model results described in referefice 3 indicate that, provided the Froude Dept-1
Number is below about 0.3, the effect of the banks on resistance estimated in this way is small so that it could be safety neglected in the first pass around
the désigrtiral.
' '' ' ' ' -
-AthethOd of estimating the change. in resistance of the ship when close to flopded 'banks is giver in Appendix A which would allow a more accurate resistance esftimate
Once a curve of .resistance, RT, against speed has been obtained.,, the effective power of the hull
'F is then-obtained from.
.
. . s .= RT.0
The .deli ered -power -PD can be calculated by use of the quasi-propulsive coefficient
thus . . .
"D. (2)
The shaft power is then derived from after dne allowance has been made for shafting
losses. ' . - .
A' simpler. method is to use a propulsive coefficient lip for the ship type in question and obtain directly from . . . ,
- .-.. '(3)
Values, of n' based on estimated P values for single-screw ships in open shallow water, and various h/T have been measured at NMI and results are shown in Figure 3. ..Values of fl/flD(where flD- is the-quasi-propulsive efficiency-in deep water for the
ship in question) are shown both for various blockage ratios m (where
Ax/Ac . .. (4)
A being the cross-sectional area of the canal and A the midship section of the ship) and also'for wide shallow water. The latter, appropriate to ships in
fairways, show. some scatter but have a mean value of lip/liDo. of 0.75 with a standard error of 0.045.
It is clear from Figure 3 that in canals there is a marked increase in lip both with reducing Fflh and with increasing m. Thus although the siip suffers
a-resistance increase,this is partially offset by the favourable propulsive coeff:icients
'at lower Fflh values. This assumes that the estimated values used to prepare
The service rating of the thain engines of a ship is usually taken as 90% of the
installed power so it is possible to determine from the poer/seed data thus
obtained whether the ship is likely to be able to advance at the. required transit
speed.
Further checks can be made on the l.ikelihood of propeller cavitation at the high thrust loadings imposed on the screw in shallow water by means of simple
cavitation criteria such as that of Burrill (ref. 20), fo example. Iii:oder to do this values of thrust deduction and wake fraction in shallow water are required; some
information on this is contained in reference 21.
Step 7: Choice. of Minimum Distance from Bank
The assessment of fairway width begins with an estimate of the minimum distance from ship to bank that can be accepted before interaction with the bank becomes severe enough to hinder ship handling. The apprppriate distance off the bank may be calculated fOr a given equilibrium rudder angle on the assumption that the ship is able to proceed parallel to the bank in a state of equi1ibrium its
rudder and drift angles inducing forces and moments on the ship which exactly balance those induced by bank interaction. In other words we assume that, inthe balance situation, the following simultaneous equations apply
+ Y'.8m + 'm = 0
... (5)+ N'.m
=0
.... (6)Solution of these equations, in which linearity has been tacitly assumed, yields
= (N' Y. - Y' N) / (Y'N' - N' ) (7)
m = '' ' / . .
.... (8)
where . .
N' are non-dimensional external force and moment coefficents, arising here from bank effects.
N (Y0/B) = N' (Y0/B) (12)
Y', N'
are partial derivatives of sway force and yaw moment induced on the shipby a rudder angle .
and
Y', N'
are the partial derivatives of sway force and yaw moment induced on theship by a drift angle .
Values of Y'6, N'd, Y', and N' may be obtained from manoeuvring tests on ship
models (see ref. 22 and Appendix B for examples suitable for the flooded bank case).
In equations (5) to (8) Y' and N' are the non-dimensionalised sway force and yaw moment induced on the ship by bank interaction. Values may be estimated for flooded banks directly from reference 3 or by use of the method outlined in Appendix C. Reference 23 discusses transient effects associated with flooded banks of
various finite lengths rather than the infinitely long banks assumed above.
Values of and may be computed for various water depth h to bank height DB
(h/DB) ratios with due account being taken of the bank on the other side of the fairway The effect of the far bank has been discussed by Norrbin (ref. 24) who has shown
that Y' and N' for the 'near' bank can be estimated from
Y'(Y0/B) = Y'(Y0/B) - Y'(w0/B - Y0/B) (9)
N'(Y0/B) = N'(Y/B) - N'(w0/B - Y /B) ... (10)
0
where Y0 is the shortest distance between the aft perpendicular of the ship and the toe of the near bank, w0is the bottom width of the waterway and B is the breadth of the ship.
In the first pass through the design cycle a value of w0 will not be known and it would be necessary to assume that the effects of the near bank dominate so that
Values of ôm and should be. calculated for several Y0/B and the results plotted. This then allows a determination of the minimum Y0/B-' Y0/B mm- compatible with safe handling provided some form of crjterion that relates and to safe and acceptable handling is known. The question of such safety àriteria is discussed in section 6 below.
Step 8 - Determination of Lane Width fQr Navigation
Step 7 determines in effect two strips att'he edge 'of the propbsed fairway into which: the ship should no trespass 'if handling problems 'ate to be kept to an
'acceptable level. The width of the fairway at the keel, WI, therefore consists of a 'navigation lane' of width wN in which it is safe to navigate together with the
two, less-safe strips mentioned above. That is
wi/B = wN/B + 2 (y*/B
mm)
(13)where is the distance off at the keel given by
y*
lB =c,/B +n (h/T-l)/(B/T) (14)
where the bank slope is 1 inn.
Values of are determined by the follwing:
The lane width w required by a ship to navigate along a straight fairway. This requires a study of the ability of the ship-han4ler to navigate a ship, firstly in
'perfect' conditions with no wind, waves or currnt (basic controllability) 'and
secondly in the presence of such disturbances. The effct of wave action o'ri handling will not be discussed here, it being assumed that the fairway is sufficiently
sheltered for wave action to be insignificant. That this is not always so is' mentioned however in reference 25.
The additional lane width andclear separation required when operations involve traffic':overtaking or passing'head-on.
Results given in reference 5 suggest that an appropriate lane width for
basic controllability is 1.6 times the beam of the ship (compared to figures of
Therefore for single lane traffic, the lane width for basic controllability is now determined. For two-way traffic, the determination of an adequate separation between the passing ships remains.
This can in principle be tackled by the use of equations similar to (7) and (8) to determine instantaneous and m values as the ships pass. Use of the
interaction data of reference 2 provides values of Y' and N'. Clearly the
and values thus obtained would not necessarily be the same as those used in
an actual passing situation forwhich the manoeuvring dynamics of the passing ship should be considered. But, if used in conjunction with safety criteria based on experience, such instantaneous values can be used to determine a safe separation distance between passing ships. Certainly at the preliminary design stage of the waterway, the use of instantaneous equilibrium rudder and drift angles as a concept
to aid design seems justified, especially if the safety criteria are also based on such instantaneous values.
Step 9 - Check on Handling in Wind and Current
At this stage in the design cycle estimates of the width and depth of the waterway in 'perfect' conditions have been obtained. A check should now be made on the
lane widths and rudder angles required to maintain a course within the fairway in the most probable winds and currents in the area. If the width that has been estimated proves inadequate a decision should be made as to the advisability of
increasing the design width
assessing the wind strength and direction at.which operations within the fairway become unsafe from a handling point of view and of using this as an operational
limit.
Clearly, if the design width should prove inadequate to allow safe navigation in the prevailing currents of the area then the width would need to be increased However this may also have an effect on the currents themselves.
It has been shown in reference 5 that, whereas values in a beam wind actually attained by human operators on models are well-predicted by equation (8), the resultant lane widths are not if they are taken as the 'projected width' of the
It is seen therefore that the design procedure outlined above addresses itself t.o the various components that can be sunmied to give the appropriate width and depth of the fairway in question. Figure 5 shows this approach diagraatjca1ly. This is essentially the approach outlined in reference 13 and for it to be successful
requires an informed! and sensible selection of. afety criteria based on
equilibrium rudder angles and acceptable lane widths.. This is discussed further be lOw.
In step 9 operating limits due to ind and curreht werementioned. In this context it shoUld be remembered that an operating limit due to wind may be an indirec.t one in that the wind may induce waves pf a size in whch tugs. may not be able to
operate. Clearly if the wave height is toO great for the safe operation of tugs, an operational limit will have been met.
4.2 Canals
When the tide causes the water level in a fairway to fall below the level of the
flooded banks (i.e.. ..h/DB = 1.0) the fairway becomes a canal. Other canals are of
cOurse purpose-built and may be of the sea-level type with no locks (such as the
Suez Canal) or locked (uch as the Panama Canal). While the presence of locks will prescribe the size of the largest basi. or design ship, the basic esign procedure
outlined. here is unaffected by whether the canal is a sea-level or locked type. ship at . In the same reference owevet tesults are given that allow lane
widths to be deduced for beam winds and various values of the important ratio Vu/U where V is wind speed and U is ship speed. Diagrams such as that in Figure 4 (from ref. 5) provide a means of estimating lane width due to a cross-wind acting on a large tanker. The same diagram could be used to estimate the
lane width for a cross current rather than a wind, provided current direction is properly interpreted.
Clearly, the determination of the ability of the ship to operate in wind of a given strength depetds On some criterion of safety being set as in steps 7 and 8
above. In this report we again cOnsidei Uch a criterion to be related to values, calculated by the use of equation 8.
The design spiral for a canal is similar to that for a fairway and is illustrated in Figure 6. There are however some differences and these are best indicated by a reexamination of the main steps of the design cycle in sequence.
Steps 2 - 4 Choice of Depth/Draught Ratio, hIT, and Squat
The various components that make up the depth of the canal are the same as those
considered for the fairway in section 4.1.
Squat may be estimatedbvmeanSfthemethOd
of references 26 and 27 for example, but it should be remembered that squat in acanal depends not only on speed through the water and depth/draught ratio h/T but
also upon the blockage ratio m defined in equation 4. These parameters are not known for the first pass around the design cycle, and so estimates should be made for a
range of these two important parameters.
Steps 5 and 5 Power Estimate, Choice of Blockage Ratio and Transit Speed
Estimates of the power required to propel the basis ship at various speeds and various blockage ratios may be made by use Of the method outlined in section 41.
Appendix A gives a means of estimating hull resistance that is suitable for canals.
The appropriate blockage ratio and transit speed may then be chosen from due consideration of the installed power of the design ship, the likelihood of severe and damaging propeller cavitation, the magnitude of the power from the ship that the canal structure is able to absorb without severe erosion or damage, and acceptable squat.
It is useful, however, to bear in mind that in canals there is a limiting speed beyond which it is unlikely that a displacement vessel can travel. A
curve of limiting speed against blockage ratio is given in reference 28 and reproduced in Figure 7. It is usual to assume that a speed equal to 70% of that derived from Figure 7 should be used as a practical working limit.
Step 7 - Choice of Available Lane Width
Once h/T and m have been chosen, the lane width w available to the ship (namely
of the geometry Of the cross-sectiqn. ltis.convenient.to approximate the
cross-sectional shape of the canal by a rectangle, trapezium or parabola The first
choice poses few problems in determination of lne width frob/T and m (see
equation (16) below) while the choice of a parabolic cross-section usually stems from a desire to represent an originally trapezoidal shape whose upper banks have eroded, the sediment being deposited near the toes of the banks. ,For the
trapezoidal cross-section the following relations apply:..
A= (w0 + w2)h/ 2
l/(m.h/T) - ' n(2-h/T) B - B B/T w0 .2ri(l-h/T) Bwhere w2 is the width of the water.surface..
These relations are shown graphically in Figures 8 to 12 where n refers to the bank slope as before.
These Figures allOw the effect of variations of h/T and kanic slope on lane width
to be assessed and may be used for the range o blockage ratio 0 25 m 0 10 Linear interpolation is appropriate within this range as equations (15) and (1) show that a linear
relationship holds between w1/Bandm mat constant h/',. BIT and flo Also shown on
these Figur.e are values of w0/B in the range l.0(0.5)2..5 which indicate whether the chosen combinations of design parameters will combine to form a physically realistic canal cross-section. It may be assumed that where w0/B curves are not shown, the. vaiue;of w0/B will be in éxcess of 2.5.
Steps 8, 9 and 10, ChcJcs on Handling
Checks on handling. may follow the procedure of section 4.. 1 with canal bank effect data taketi from. eferences 3, 13 and 29 or obtained by the method of Appendix C.
While it is possible to obtain currents flowing across a fair-way, .thj.'. js unlikely in a canal. In a sea-level canal, currents niay be induced by
differences in sea. leyels t th e4s of the. ..can.al and thes.e wi]4 of...
course result in currents with or oppbsingthedirect.ion-of the.ship. For a
.staight canal the effect of such heading or following cuIrents on the lane widths needed for controllability appear: to:be small..: Figure 13 illutrates this byshowing
some- results obtained by NMI using a 1-arge: manned single-Screw VLCC. -model in, two
straight canals. Widthsof lane swept. dut by the model: are--plotted against Fh based on speed through the water and,it.is.:c,lear that the effect of a longitudinal current on handling is small.
This result is of some interest for it seems intuitively obvious
that afoliowin'g current would mke.andlingmoe difficu1.t.,
but,as sugges.ted in ref erene. 13, t-he problems are morepsyho.l-ogical than
-..hydrodynamic:in origin. Many of the.helmsmans visual 'dues' are-based on land
when transidnga canal; .so that a foliowing'current makes rthem pass by more rapidly than: in a heading curent r initially still 'water. . Te-ituation,theef.Qre appears
more dangeroas than .it actually i.s .. . . ..
:...
Although the dominant current direction in a canal would be expected ;be t-hat of
the canal itself, it should of course be borne in mind that local features such as river tributaries, outfalls and inlets at an angle to, the canal axis can have a
local effect on handling and should be taken into consideration. - Some inforirietion
on the effect on handling of such features is given in reference 3O
Step 12 * BnkProt-ection
A step in.the design cycle, for canals which does not occur 'in that for, fairways results f-rdm the consideration that. be give.n to the protetion.of the canal
àhksfrOm. the .effèt of-. the passingsh±ps. .-D'amae t-o:bai1cs ari'ses..mainly-from
the'draw-down and waves steing from.the local..- andfree-wavé systems;..oftheship.
While the design'ofsuit'ab1ebankprotection'i'beYOTd the scope of this report,
some information on ship wave action on a' sloping bank is .given.in refernc3l
4.3 Final Considerations
The above procedures should provide across-section-design suitable for safe
navigation along a straight waterway. The cross-sction thus obtained may be
regarded as the 'basic' cross-section which may be locally modified as necessary. Such local modifications would arise in curved sections of the waterway (dealt with in section 5 below), in regions where ships are moored alongside the fairway and where the waterway bifürcates1 as might happen for example at each end of a bypass in a canal. These are now discussed briefly.
4.3.1 Wateray with Moored Ships
If ships are moored to one side of a wateway, they will interaät with any ships
that pass. along the waterway itself. This may give rise to excessive movements of the moored ship which
could
impose Unacceptably large loads in the mooring lines, possibly causing them to break. Moreover the moored ship will sink and trimwhile passing is taking place, inducing yet greater loads on the mooring lines and possibly causing the ship temporarily to ground at its thoorings.
These interaction-induced effects can be lessened by
Increasing the passing distance Reducing the passing .speed
Changing the mooring .and their geometry
Changing the mooring line pre-tenson
Clearly, of these options, a) and b) are related to the local waterway cross-section,and the waterway design in the egion of moorings must be undertaken with due consideration of the moored ship. The effect On mooring line loads and
movements of the moored ship can be studied by means of a simulation model sich as that described in reference 4 so that the appropriate passing distance and speed
may: be determined. Other effects on the passing ship, such as bank interactiàn
and handling in wind, are then incorporated in the.local cross-section design as described in sections 4.1 and 4.2 abOve... . .
4.3.2 Waterway Bifurcation
Some handling problems may arise when a waterway divides and, in so doing widens asymmetrically over a short region. This is shown diagrammatically in Figure 14 where the entrance to a canal by-pass is illustrated. To demonstrate the handling problems which might arise, a ship moving in the main canal in direction 1 is
initially considered. The following points arise:
Assuming the passing ship is moving along the centre of the main canal, at position A it will be using small rudder angles to maintain its course and will
feel no effects from either bank due to symmetry.
As the ship moves to point B, maintaining its original course, it is no longer in the centre of the canal section, but is closer to the bank on the starboard
side. It is therefore subjected to bank rejection which will tend to turn it to
port requiring the application of correcting starboard rudder.
As the ship progresses down an ever-widening canal to point C.the 'restoring' rejection from the bank to port diminishes and more starboard rudder must be applied to prevent the ship sheering toward the by-pass or heading for the headland (point D)
at the junction of canal and by-pass. (Course P in Figure 14).
As the ship passes the mouth of the by-pass and approaches point E, it is once more symmetrically placed between the banks of the main canal and should require little or no helm to counter bank effects which will once again cancel. However, if the response of helmsman and steering motor is too slow so that the
large amount of starboard rudder required at point C is not taken off in time, the
ship may steer into the starboard bank at point E as shown in Figure 14, course Q.
It has been shown that the power required to maintain a given speed depends on the shape of the ship and the shape of the canal cross-section; in general, the smaller the blockage, the less power is required to maintain a given speed. Thus it is clear that if the ship maintains the same propeller revolutions from point A to point C it would be expected to speed up from A to C as the blockage reduces and then to slow down again at point E after the rapid increase in blockage ratio
between C and E. The speeding up between A and C coupled with the handling problems in this area could give rise to a loss of control which might increase chances of grounding at point D.
A ship moving in direction 2 will be subject to similar if less severe problems. On moving from E to C it would experience a resistance decrease and so would increase speed if constant revolutions were maintained. The position of the ship in the canal at point C would immediately cause a sheer to starboard which would require port correcting rudder. If such a sheer developed however, the shape of the bank on the starboard side, as it angles toward point A would tend
to reject the ship toward its target course.
Clearly some care is required in the design of the waterway where it divides if severe handling problems are to be avoided. A gradual widening of the canal as
the by-pass is approached would cause any incipient sheers to develop more gradually and therefore be more easily controlled (see scheme 1 in Figure 15). Likewise the
local width and YO/B could be checked so that excessive equilibrium rudder angles are avoided.
A symmetrical widening with one bank 'bulging' as shown in scheme 2 in Figure 15 might be appropriate. This could provide a mooring basin for small vessels
although this might be subjected to large changes in water level by passing
ships.
It is clear however that while some general guidelines can be given for this particular waterway feature, insufficient data exist to allow accurate assessment
of all its effects on ship-handling. This is a case therefore where it would be prudent to check handling using physical model studies possibly in conjunction with
shiphandling studies on a simulator.
4.3.3 Depth Determination
A problem arises in an accurate assessment of the depth of a channel dredged
through a mud or silt area. The bottom of such a channel may be poorly-defined due to dense layers of suspended sediment that may occur. Here it is usual
to define a 'nautical depth' from the water surface to sediment with a density equal
to or greater than 1200 kg/zn3. Research has shown that suspensions of a lower
5. Curved Waterways
5.1 General
It is the aim of this section to summarise results obtained at NMI and elsewhere and to indicate how these may be used to assess whether the 'straight' waterway cross-section obtained by the above procedures is suitable for use in curved waterways.
It is in the navigation of bends in waterways that interaction may in fact be used to aid rather than hinder the ship-handler. This is a feature of navigation in canals rather than fairways and therefore will be a major consideration in the design of
curved canals. However, little fñll-scale or model experiment data are generally available for curved waterways, so that the information given here is of necessity less complete than that given for the straight sections of the waterway. But it is assumed that, once the cross-section relevant to the straight sections of the waterway has been obtained, this should provide the 'basic' cross-section for the
curved portion, being modified if necessary to allow for bebaviour of the ship in the bend in question.
It should be noted that the discussion concentrates upon ships chat negotiate
curved waterways unaided and at manoeuvring speeds. Clearly with tug assistance,,arid at
very low speeds, ships can navigate sharp bends successfully provided there is a minimum lane width available that physically allows them to advance.
Geometrical considerations indicate that this is given by:
w1(min) / B = ( Ri + 1)2 + (Lpp)2 - Ri
-
-r
where R1 is the inner radius of the available lane width at the keel (see ref. 33), and a parallel bend in the form of a circular arc has been assumed.
An extension of equation (19) to take into account the drift angle of tow-barges is given in reference 33 while in reference 34 a similar equation is presented which allows the inclusion of a safety criterion. This equation may be generalised to give
= (A1 + A2)2
L2
B1O.25
wj(min)(mm...! B
(19)
where R is the radius of the centreline of the bend, assumed to be parallel and an arc of a circle.
A1 is the linear distance from the bow of the ship to the bank as a fraction of L (see fig. 16)
A2 is the distance of the pivot point from the bow as a fraction of
(see fig. 16), where the pivot point (which is a function of h) is the point about which the vessel turns.
As an example, assuming that A1 = 1.0, A2 = 0.5, L/B 7.8 and R/L = 10,
equation(20) gives a value of w1 (min)/B of about 1.6.
But these geometrical considerations ignore the path taken by a manoeuvring ship. The turning radius and width of swept track are determined by the manoeuvring characteristics of the ship and the ability of the ship-handler. Manouevring
characteritics of several ships have been measured at NMI and elsewhere and allow manoeuvring simulation models to be constructed. With such a model it is
possible to determine the turning circle radius and width of swept track of a given ship once the rudder is set to a known, constant angle. Results obtained from the NMI manoeuvring simulation model are given in Appendix D and such
information gives guidance on the minimum radius and width of the curved waterway being particularly relevant to curved fairways in which bank effects are seldom
important in handling.
This leads naturally to consideration of the way ships are handled in curved fairways and canals.
5.2 Curved Fairways
Two main features distinguish handling techniques in a curved fairway from those in a curved canal:
- it is not possible for the ship-handler to see the banks and therefore much reliance must be placed on navigational aids, skill and local knowledge for the correct
- bank effects are unlikely to be used to aid the turn so that the ship is likely to attempt to follow the centreline of the waterway.
The first feature is important in determining lane width requirements in a bend, for without accurate visual position-fixing, as given by leading marks for example, the positioning error of a ship in a curve may be greater than that in the straight sections of the waterway. This is discussed at some length by Sukselainen in
reference 10 who used a dead-band in a controller in a simulation model to determine the additional lane width necessary in a curved fairway. Results from his
simulations are shown in Figure 17 for a bend angle of 300 and provide some guidance for lane width requirements. It is of interest to note the reduced additional width requirement as h/T reduces; this is in fact a demonstration of the fact that, while shallow water reduces turning ability, this is more than compensated by the reduction of drift angle, which accompanies reducing depth (see Appendix D).
5.3 Curved Canals
In a curved canal, the ship-handler has the advantage of being able to see the
limits of the waterway to a greater or lesser degree depending on bank slope. He has the added advantage that rejection from the outer bank can be used to aid the turn.
Bank rejection is an extension of the bank effects considered for a straight bank
which occur as a ship approaches a bank at an angle to its course. A pressure 'cushion' builds up between bow and bank, giving rise to surge and sway forces which increase as the bank is approached and may ultimately cause the ship to be pushed bodily away from the bank. As the pressure cushion occurs near the bow of the ship, a powerful yaw moment simultaneously turns the ship away from the bank in its path. An example of the wave system generated around a ship model near to a curved, surface-piercing bank is shown in Plate 1.
This phenomenon can be used by canal pilots to aid in navigating a canal bend, by skilfully positioning the ship so that it obtains some rejection from the outer bank, using, if necessary, check rudder to counter the sheer which may develop. Sometimes sheers will develop when the 'balance' position in the bend is not found and
the ship will move round the bend on an oscillatory course. An example of this is shown in Figure 18 in which the track of the model of Plate 1 has been plotted as it
negotiated a 620 curved bank. It is clear from this that a sheer may develop as the ship moves from the straight entry section into the curve and
the resulting oscillatory course, with sheers opposing the applied rudder, is significant. It means in fact that the lane width in a curved canal must be sufficient to allow for the occurrence of, and recovery from,bank-induced sheers.
Measurements of the width of the track envelopes of free models navigating curved canals of various widths have been made at NMI and elsewhere (notably ref. 13). These results are summarised in Figure 19 which includes NMI data and results for
the 26° parallel and widened bends of reference 13. It is seen that the results cover two hIT values and indicate the width of the swept track (wIB) for a given lane width at the keel (w1/B). Also shown are rays that represent maximum values of the criterion (see sections 6.2.6; and 6.3 below):
w/B .< x (w1/B) ... (21)
The bend angle y varied from 26° to 62° although the 62° results apply to a single bank rather than to a canal of finite width.
An interesting feature of this plotting is that it indicates that the wider the available lane width, the greater the width of the swept track. This suggests that
there is a trade-off in the narrower waterways between the problems of restricted width and the benefits of rejection from both banks preventing the development of
serious sheers.
Figure 19 can be used as an aid to design in that it indicates the width of swept track that should be obtained in curved canals similar to those tested and moreover relates this to the safety criterion of equation (21). For example, if y = 260, h/T = 1.18, and the canal has a parallel bend, the minimum value of w1/B for x = 0.70 in equation (21) is about 2.7; it would appear from the results obtained
in the y 450 bend that, at an hIT of 1.18, the criterion of equation (21) with x = 0.70 is not likely to be satisfied in general.
Some indication of the rudder angles used in the NNI model experiments is given in Figure 20 where values of the mean absolute rudder angle Sm1 are given, averaged over all runs. It is clear that when no current flowed, the 450 bend required greater
5.3.1 Effect of Current
The effect of a current flowing in the canal is well illustrated in Figure 20. In the 450 bend while values of w/B were not found to differ, significantly in the presence or absence of a current, the rudder angles used to achieve them depended markedly on the current. It is seen in fact that a following current improved
matters so that ImI were consistently lower than those measured with no current
which in turn were lower than those obtained in a heading current.
This echoes results mentioned above for straight canals, and for curved canals in reference 13 and is at variance with what would appear to be intuitively obvious -that a following current would constitute the greater hazard and give more handling problems. In fact, as stated in ref. 13 'The higher ground speed due to a following current creates a mental hazard, but not an actual risk'.
5.3.2 Increase of Width in a Canal Bend
It will be noted that the discussion on curved canals has dealt exclusively with parallel bends. Little mention has been made of the need for additional widening in the bend analagous to that required for a curved fairway. This question is discussed in reference 13, in which it is concluded that if the straight entry and exit sections of the canal are wide enough, parallel bends should be adequate. This
view was confirmed by the NMI model experiments which showed in fact that most of the handling problems occurred at the point where the curved section of the canal started or finished. This is illustrated in Figure 18 where a major sheer at the entry to the curved section is shown. Any widening contemplated in this transition region would have to be carried out with care and ideally should be done on the inner rather than the outer bank so that the effectiveness of the rejection of the outer bank is not lessened. Any width increase that is necessary can be deduced from Figure 19.
There seems little reason to depart from a circular arc for the curved section of the canal with suitable blending in to the straight entry and exit sections; any
sudden changes of curvature, especially on the outer bank, should be avoided for they can cause correspondingly sudden changes in bank rejection, increasing the probability of a sheer.
6. Safety Criteria
An essential ingredient in the deterministic design scheme outlined above arises from detailed probabilistic considerations which are outside the scope of the study described herein. However it is necessary to consider their results briefly at this
point because they relate directly to the ability of the waterway design to allow safe navigation.
6.1 General
At several points in the design process described above, safety criteria have been tacitly assumed without any firm recommendations being given. The concept of an overall design goal or criterion for the safe operation of a transport system is common in the fields of aviation and, to a lesser extent, road transport. Such a goal might be given in terms such as 'The probability of x accidents occurring in y years must be less than z', and this could be used to guide the design of airports,
landing approaches, aircraft etc.
Springing from such a global criterion would arise a host of secondary criteria
each relevant to some particular part of the system. To eontinue the aviation analogy, secondary criteria would be of the type that state 'runway roughness must be less than x', or 'runway lengths must be greater than y' or 'the aircraft under consideration must be able to take off in a distance no greater than z'. Furthermore traffic
separation schemes on road, sea or in the air impose constraints on design and operation and must tacitly assume some sort of global safety criteria, related to accident rate, similar to the one mentioned above.
In the field of port approach design no well-defined global criterion exists at present. Accident data in sufficiently comprehensive form are not readily available
and the possibility of control of ships in port approaches, analogous to air traffic control, while the subject of discussion, is not always accepted.
In the absence of a widely-accepted global safety criterion, secondary criteria must be used that have been formed by an iterative process involving ship-handlers, ship operators, waterway designers and port operators so that a compromise
acceptable
to all parties may be
agreed. These secondary criteria may take the form 'The rudder angle must not exceed x degrees for more than y% of the transit time', (see for example reference 34). Such a criterion can be used in a designscheme such as that described in sections '4 and 5 abàve. Other, less sóphi'ticated, criteria simply state that the fairway'width must not be les than a cértàin
nuthbëtof'bèth'tidths of the:design s'hi and pay iio:rèard' to-the ossibie"trde
off a between-safety and benefits in an edonomIc sense.
The philosophy adopted in this report is that secondary safety criteria should be
used to produce a first design for which: a. full risk,. anAlysis ,couldbe carried: out
once the global criterion was established.
6.2 Secondary Criteria
Secondary criteria used in this report consfstof e4uilibriuth rudder angles in some of the situations listed below. It should be emphasised that the rudder'angle is used here as a design tool and the values given below should not be interpreted as
recommendations to ship handlers for use in given situations. Rather they should be considered 'as a convenient and easiLy-understood.way of describing the resultant of the complex forces and moments acting on a ship in a given situation. Moreover, by äsuit'able cihdicé of-equlibrium rudder angle, adequ'ate safetystandards may be, set.
6.2.1 Moving Ship Passing a Stationary Moored Ship
The secondary criterion used here 'would be the limiting movement of the moored
ship. It is Often conveniently described in terms of an acceptable movement
envelope relating to a given point on a ship. For' aiioil tanker this may . -be the' position dn t'he'ship at which the loading arm is connected while for a
terry with a stern-loading ramp it might rélãte to a pOi.tion on :the ramp. Clearly the. site and shape of the envelope indirectly: determines the speed and distance-off of the passing ship and hence the local waterway width. '
. ...'
6.2.2 TwO.Ships Passing' on Reciprocal Cduses
Here the secondary criterion is probably most conveniently expressed, in -:
terms of rudder angles, chosen so that adequate control is maina.tined before, 'after afld during passing. EÜati'ors (7) and '(8)'- allOw: 'instan,t"aneous' .. an m
values t.o"be calculated during the passing encountet based.. on Y and N: values induced by the other ship. It' is-suggested 'thatasuitableecondary crierion1 which-takes
root mean square value of
R' together with the maximUm - calculated. The criterior
would then state that.SR and max. should not exceed certain prescribed.values. This would then, determine, safe passing speeds and distances and therefore waterway. width; it would also determine whether such a !head-on' passing encounter could be tolerated.
6.2.3
Two Ships Passiflg on Parlel ço,rs
In an overtaking encounter, the ships will be close to each other for a longer period of time than in a reciprocal encounter. Because of this the ships will have mote
time to respond to the transient interaction forces and ioments imposed on them,
and there will be. an. increased risk of collision,. . The values of and max. should .therefo'e be lower than those for the reciprocal encounter.
6.2.4.,'One. Ship Passing Close to the Edge of a Waterway
Use of an equilibrium rudder angle to limit speed and distance-off from the bank at the boundary of the waterway has been discussed above. Thi is .a straightforward application of the equilibrium rudder angle concept and. requires no fur;her discussion.
6.2.5 Basic 'Controllabi].i.ty
The definition of basic controllability in section .3.3 relates it to the basic lane width, course error and rudder activity to uiaintain a course along a fairway in the absence of. wind or current. Values of lane width .(incorpoatthg course ertor) are given in reference 5 It would seem appropriate to use in addition a criterion based on rudder activity althQug1, as stated in reference 5, this should b done with caution because rudder activity is as much a' measure of the ability of the helmsman as it is of the 'handiness' on the ship or the suitability of the waterway.
6.2.6 Wind and Current Effects
It is shown in reference 5 that in a simulated cross.-dnd estimated equilibriu rudder angles agreed well with. those actually obtained, by human helmsmen using
large models.' It therefore seems justifie to. use an equilibrium helm angle
The global safety criterion ii: considered in reference 35'which gives as, an example for a 10km long fairway that "the probability that.no acCident'. will occur during 10 criterion which could state that "the 'esültant lane width when Oun'teting wind and
current should not exceed x% of the available lane width'. "A criterion of this type, al-ready introduced in section 5.3 is thèntionedin ref erènce 34-in connection with overall handling where a value for x of 70 was suggested. ;
6.2.7 Controllability in a 'Curved Waterway'
'It woUld seem appropriate in curved channeis to 'adapt crieriä similar 'iñ nature to those used for wind and current effects namely, criteria involving' allowable: rudder angles and lane width. It is worth remembering however that the effect of
'a wind will vary ma curved waterway, the ship beingmore severely affected in
some parts of the curve than others.
6.3 Examples of Secondary Criteria
It is appropriate'at this stage toconsid'er examples of secondary'criteriathat have been used elsewhere. In reference 10 the author lists the following criteria
used in Finnih waterways:
Straight Channels: minimum width 'lOT or 4B'
narrOw lOT or 4B
normal 30T or l2B)
open waters 50T or 20B) natural fairways
Basic cOnt'rollabili'ty lane width " 1.4 to 2.0B
Bank Clearance 0.6 to l.5B '
Clearance Lane 30 metres
Curved Waterways: Bend radius 1OL minimum
-' Bend angle 30° maximum
where L,and 'T are length, beam and draught of the basis ship.
In reference 30 cumulative frequency lane widths measured during model experiments are given. Limiting prObabilities of exceederice Of lane"width and assirg;'
-years of operation is equal. to 0.6". On the basis of the statistics of extremes obtainei
from runs on ahiphandling simulator, it is shown how waterway width may be
determined Such a risk analysis however implies the existence of a preliminary waterway design.
In reference 36 it is suggested that for large tankers in straight-channels, th r.m.s. value of rudder angle should not exceed 100 to give suffjcient reserve fot emergencies, while in reference 34 the criterion for a straight canal is that "f or safe navigation
the rudder angle i5t not exceed 15° for more than 10% of the navigation time.
the maximum rudder angle for a ship with good manoeuvring qualities must not
0
exceed 120
I.
the maximum lane width used by the ship (w) must be less than 70% of the
available lane width at the keel (Wi)".
An additiOnal criterion for curved canals is given in the same reference when it is stated that a distance of one ship length, measure4 frdtn bow to bank
in a straight line along the ship's centrelin; must be available, (i.e A1 = 1.0 in equation (20)).
In relation to a canal, reference 13 gives the following criteria, which may be compared with those of teference 10:
Basic Controllability Lane Width: 1.7 to l.8B - one way ttaffic
1.6 to l.7B (manoeuvrable ship) - Two way traffic 1.7 to l.8B (less manoeuvrable ship)
Bank Clearance: 0.6 to 2.OB using 5° and7 equilibium rudder angle
Clearance Lane l.OB
Navigatipn: in wind is considered in reference 37 where an equilibrium rudder angle of 25° is allowed as a maximUm.
Finally, in relation to moored ships. and their allowable movements, refeetice 8 gives the following. criteri4 for moored VLCC': . .
- allowable loads in any one mooring line should not exceed 55Z of its Minimum Breaking Load (MBL)
- allowable vessel movement is ± 3m in surge and 3m in sway.
For container ships it is probable that much smaller movements would be needed due to the more precise positioning requirements posed by the container guidance systems. Although the ideal relative movement between ship and container crane should be nil, mooring elasticity prevents its achievement, so that a value of ± O.25m i.n surge is
suggested as a preliminary figure.
6.4 Other Considerations
The secondary safety criteria discussed above are sufficient to allow waterway width and depth to be determined. But other factors, (some of which are appropriate
to the mote detailed design phase, beyond the scope of this report) should be borne
in mind.
This comment refers particularly to the assumption implied throughout this report the shiphandling personnel are well-trained, fully conversant with the aspects of shallow
water phenomena and ship behaviour with which they will have to deal, and able to respond
to a given situation in a manner similar to that of the 'ideal' human subjects used in the experiments described above. This assumption may not always be valid so that the waterway designer and operator are faced with the choice of changing the waterway to allow for below-average ship-handling, or to ensure that no ships and
ship-handlers that are known to have less-than-satisfactory reliablity use the waterway.
The factors of safety in some waterways can of course be significantly improved by the use of tugs to escort and handle ships. The design process outlined in this report considers only the ship that is navigating unaided under its own power. Limitations of operation are set by weather and handling criteria as discussed
above, but these limitations may be considerably alleviated by the use of tug assistance. Decisions as to whether to employ tugs or not at a certain part of the waterway or
for a given size of ship could be made at the preliminary design stage and further investigated when detailed design investigations are made.
Similarly, the effect of visibility on lane
width
etc can be cOnsidered at the detailed design stage hen the views of local pilots can be sought, augmente4ifpossible by experiments on a ship handling simulator. The preliminary design method, discussed above, assumes good visibility.
Finally ond additional criterion is sometimes employed. This states
that the width of the waterway should be at least equal to the length of thedesign
ship. This would allow the ship to be tUrned if, in an emergency it was ñeCêssry
to return the ship to sea.. Often the waterwaywidth already complies with thisL
criterion, but in canals and other places, where such a criterion would be economically unacceptable, other arrangemefits mUst be tiiade to cater for an emergency. These
include operational procedures such as convoy systems, adequate separation beteen ships to allow for emergency stops, tug escorts and provision for mooring alongside in the waterway.
Worked Example.
The design method described above is now illustrated by means of a worked example involving a fiëtitious waterway. It should be emphasised that the example has been chosen simply to illustrate the procedures inyolved and should not necessarily
be taken as representative of any particular waterway.
7.1 The Prdblem
A deep-water channel,shown schematically in Figuè 21,is to be dredged to
accept 75,000 dwt bulk carriers, which are to transit at high tide. Transit speeds are to lie in the range 6 U 12 knots and winds of up to 24 knots are liable
to blow across the waterway.
The channel is to be dredged through a shallow water area 7 metres deep at high tide which ef:fectively extends to infinity on both sides of the proposed channel.
How deep should the chaPnel be dredged and what is its minimum width for.safe orce-way navigation Of the basis ship? TidalheghtVari?ti0nS may be ignored.
Tankers are to be moored at the side Of the deep-water channel. What is the appropriate distance off and passing speed for the basis ship?
At one point the dredged channel must hange' direction by 26°. Canthe design
ship manage this within the speed range quoted an4 what is the minimum wjdth and radius of the bend to allow this?
Can two-way traffic be allowed in the hafiriel afld if so, that ize and' speed
restrictions should be imposed? (Consider head-on passing only).
The following initial data may be assumed:
Basis Ship Particulars:
Length between perpendiculars, Lp: 255.Om Breadth Transit Draught Initial trim Block coefficient Installed power No of screts No. of rudders Deadweight Type Moored Ship:
Particulars as for design ship.
Mooring scheme as indicated in Figure;22
Safety Criteria for Straight Sectionof Channel:
Equilibriumriidder'ang1es: Basic Controllability 5°
Wind 25°
- Bank Effects 5.9
Mirimüm underkeel alearañce on channel C.L. .O.6m
Maximum lane width to beatmost 70% of available lane-width i.e. 0.70 w.1..
B: 36.Om T: l3.7rn level CB: 0.80 P1: 15,000kw
'1
1. 75,000 'tontiesSafety Criteria for Curved Channel Section:
- width of swept track tobe no greater than 70% of available lane width at keel.
- rudder angles no greater than 350 to be used while navigating the bend.
(It is assumed that the ship navigates the curved section unaided).
For two-way traffic OR must not exceed 200 and max. must not exceed 350 during passing.
Bank slopes to lie in the range 1:3 to 1:4.
7.2 Design Procedure - One Way Traffic
7.2.1 Depth oF.Channel (Steps 2 to 4 of the 'Fairway' Design Spiral')
In the specification of the channel, no guidance is given that would allow an
initial depth/draught ratio to be ähosen. In order to start the design, therefore, squat and resistance estimates must be made for several h/T values.
This has been done here for hIT values from 1.05 to 1.2 in steps of 0.05. The maximum sinkage was invariably found to occur at the bow, and plots of predicted variations of underkeel clearance at the FP with forward speed are shown in Fig. 23.
It is clear from this Figure that the ship would ground at a speed less than the upper design speed of 12 knots if the h/T were 1.05. Indeed the underkeel clearance becomes dangerously small at 12 knots for h/T = 1.10. It would seem appropriate
therefore to ignore these two h/T values.
It is stated in section 4.1 that a safety margin and an allowance for siltation should be made, once squat has been allowed for. We assume here that a safety margin of 0.6 metres should be allowed as well as a further 0.4 metres for siltation, a figure which can be checked at a later stage. We seek therefore a minimum underkeel
clearance at 12 knots of 1.0 metre and this is obtained at an h/T of 1.15 as seen from Figure 23.
This gives the following parameters for the channel:
Water depth, h: 15.76m Bank height, DB: 8.76m
h/DB: 1.799
Froude Depth Number Range: 0.248 - 0.497
7.2.2 Ship Speed and Power Requirements (Steps 5 and 6 of the Design Spiral)
Before a final decision can be made regarding the initial depth/draught ratio, a
checkon the power required by the ship to proceedatspeedsintherange6 to 12 knots
should be made. At this stage of the design we assume that an estimate for wide shallow water at the correct hIT is adequate. Using a deep-water resistance curve, estimated using reference A4, we use the method outlined in Appendix A to determine the. shallow water resistance curve, both shown in Figure 24.
For this ship type a value of
1D of 0.77 is not unreasonable so that, from Figure
3 we have liD = 0.75x0.77 = 0.58. Using this value of together with values deduced from the resistance curve, we can derive the curve of P5 against speed shown
in Figure 24.
The installed power P1 is 15,000kW and the service rating would be 90% of this or
13500kw. It is clear from Fig. 24 that the upper required speed of 12 knots is
attainable with this ship, This confirms, for the moment, the choice of an h/T of 1.15.
It is also of interest to note that the limiting speed in this water depth (assuming that open shallow water may be represented by a channel whose breadth is ten times that of the ship) is, from Figure 7, 16 knots. It is stated in section 4.2 that a practical working value of limiting speed is about 70% of that derived from Figure 7 which in this case is 11.2 knots, close to the upper speed required. While not at this stage a cause for alarm (because the powering estimate suggests that the ship could attain the 12 knots required), this result should nevertheless be borne in mind should later stages in the design cycle indicate that depth may have to be
7.2.3
Choice of Minmu Distance from Bank (Ste2)
We now use equations (7) and (8) in conjunct-ion wih bank effect data from Appefldix C
to caIu1ate values of
and
for various
a1ue
of YQ/B.
Thee results' are
plotted in Figure 25 and were obtained from the following coefficients appropriate
for a bulk carrier at an hIT of l..5:
:
4.4 L03
1.7
52.O
ici3
N :
-47.l 1O'
Shown in the Figure are results for the maximum and minimum speedsquoted of 12 and
6
