Ship hydrodynamics and the design of port approach channels

National Maritime Institute

Ship Hydrodynamics and the design

of Port Approach Channels

by

I WDand

Paper presented at Portech 82

Singapore

June 22 - 26, 1982

NMI R148

October 1982

National Maritime Institute

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Middlesex TWJ4 OLQ

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PORTECH 82

Singapore

June 22-26, 1982

SHIP HYDRODYNAMICS A

THE DESIGN OF PORT APPROACH CHANNELS

By

Dr Ian. W. Dand

National Maritime Institute., UK

1982 Marintec S.E.A. (Pta) Ltd

Reproduction of this paper, either in part or whole,

is expressly forbidden without the written permission

of Marintec S.E.A. (Pta) Ltd

SHIP HYDRODYNAMICS AND THE DES ION OF PORTAPPROACH CHANNELS By Dr I W Dand, National Maritime Institute, United Kingdom

Introduction

It may be thought that the flow around a ship has little to do with the design of a port approach channel. How, for example, can the handling of a ship be affected in any way by the width, depth and alighnment of the waterway in

which

a ship finds herself?

-Consider the flow over the aft-body of a ship model in deep water illustrated in Figure 1. The flow is seen to be well-behaved with an adequate delivery to the screw propeller and rudder,

implying a ship that is efficiently propelled and responsive to the helm as one would expect from a well-designed hull-f orm.

Consider now the situation illustrated in Figure 2 which shows conditions identical to those of Figure 1 except that the depth of water has been changed to simulate that in a shallow port approach channel. The difference in flow is dramatic; no longer is there a smooth in-flow to the propeller but there is every suggestion of severe flow separation. Moreover flow over the rudder is far from ideal and, were it not for. the propeller slip-stream passing over the rudder blade, the separated flow would render the rudder almost totally ineffective. This suggests that the

propulsion and handling of the ship will change as it moves from deep to shallow water an4 so indeed is the case.

This shows that a comparatively simple change in the geometry of the. waterway - a reduction in depth - has a profound influence on the flow over the ship and, by implication, this wi.0 have a

profound effect on its behaviour.

-For several years the United Kingdom's National Maritime Institute (NMI) has been studying: the behaviour of ships in shallow and confined waters through the use of both physical and mathematical models combined with full scale measurements. It is the intention of this paper to use results of this and ocher work to present the thesis that, as. the behaviour of a ship is profound.ly affected by the geometry and alignment of any confined waterway in which it operates, due consideration shOuld

be taken of this and the waterway desigued accordingly. Other aspects related to the ship are of course important and will be mentioned as necessary. Principal among such effects is the

relationship between ship behaviour, and that of the helmsman and/or pilot and this will be illustrated by some results. also obtained at NMI.

Effects of the Waterway on the Ship

The changes in the flow over a ship induced by the bed or banks of a waterway naturally affect the hydrodyuamic forces and moments felt by the ship. Its resistance changes, it changes its equilibrium position in the fluid, it may interact with the banks or other ships and it will 'feel' different to the ship-handler. We now discuss some of these effects and their causes.

2.1

Resistance

and Speed

For displacement craft such as tankers,' container ships, ferries etc., a reduction of water depth increases resistance. This is illustrated in Figure '3, takEn from reference 1,, which shows the change in resistance to motion estimated for a large tanker in various depths of water.

The implications of these changes, induced by tht constriction of flow around and under the vessel, are that, for given propeller revolutions, the

ship

must inevitably experience a drop in epeed when entering ahallow water. Moreover some ships may have trouble maintaining a set transit speed in a

rery confined waterway, such as a canal, from this cause. This of course has implications in the programeiag and capacity of a waterway; the geometry of the waterway therefore should not be incompatible with the transit speed required of the particular ship using it.

2.2 Squat

A reduction in water depth naturally increases the likelihood of grounding but thia is further increased by the effect of shallow water, on the flow around the ship. The tendency for a ship to be sucked toward the seabed with increase of forward speed is well-known and..has recently been the subject of several studies. (See for example references 2 and 3)

The resulting 'squat' arises from a bodily sinkage of the ship together with a trim. The running trim is usually by the head for a displacement ship at sub-c.itical speeds (i.e. Froude Depth Number less than unity) if it. has a level trim at rest. 3ut at-rest departure or arrival trims may - be by the head or.by the stern; the former causes an enhanced tendency for the ship to trim by the

head while underway, and the latter may result in the vulnerable aft end of the ship, grounding at speed due to the bodily sinkage with, perhaps, an enhanced trim by the stern (ref.. 2). When two ships pass they may interact with one another (see sections 2.4 and 2.5 below) and although this is usi.ially discussed in terms of. its effect on uianoeuvring behaYiour, it can also give, rise to a transient increase in squat as the ships pass (ref.. 4).

Clearly the depth of the waterway should contain some allowance for squat and it is fortunate that it is 'a phenomenon which lends itself to reasonably accurate estimation (ref s. 2 or 3 for example). Shown in Figure 4 is a chart from ref erence 5 which allows the squat of large tankers in shallow water to be estimated. Similar charts can be produced for different ship types using computer models developed at NMI and elsewhere.

2 '3 Mano'euvring

It has been shown above that longitudinal drag increases in shallow water; so also does the resistance to lateral motion, the virtual masses of the ship, in surge and sway together with the

virtual inertia in yaw. ' , , V

All of this has an effect on the behaviour of the ship when turning in shallow water. In such a situation it behaves rather like a two-dimeesional aerofoil with a high lift/drag ratio which reau.Lt8 in a much smaller drift angle in shallow water compared to deep. Its turning circle

in

shallow water would therefore be expected to have a larger radius than that in deep water as Figure 5 (from reference 6) shows.

A ship turning in shallow water will therefore not lose speed as it would in deep water (where the drift angle is of necessity large to generate sufficient sway force - or 'lift' to

counter centripetal. effects') so that a sharp turn in an emergency in shallow water would not' be advisable because: V

the loss of speed is not as great as it would be in deep water;

there may be far less sea room to manoeuvre in shallow water as the ship may be in a narrow, dredged fairway..

It is clear therefore that a ship may have a more limited ability to manoeuvre in shallow water and.may well respond more sluggishly to the helm.

The manoeuvring characteristics therefbre depend on' the ratio of water depth h to hull draught T (h/T) as well as on hull form. This izmnediacely raises the important point that in any encounter one ship may be more eeaily able to mnoeuvre out of danger than the other partly because of her inherent manoeuvrhbility and partly because of her proximity to the seabed.

However, not only are the: hydrodynamic characteristics of the hull changed in Shallow water, so also is the effectiveness of the rudder. This has been illustrated in Figures 1 and 2 which show the effect of. water depth on the flow over the aft-body of a VLCC. model. It is further illustrated in Figure 6 (from reference 7) which shows the initial yaw moment imposed on a cargo-liner model by its rudder in different water depths. This figure demonstrates the importance of a strong flaw over the rudder in shallow water. Loss of this flOw by stopping the engines means a

correspondingly greater loss of control in shallow rather than deep water.

Other changes in manoeuvring behaviour in shallow water occur when a ship' is forced to perform a crash-stop manoeuvre. In such a case the ship may take a pronounced.sheer to port or starboard. A single screw ship tends to turn rather more often to starboard. The extent if this sheer is likely to be very much greater in. reduced water depths. The implications of this for a

_ship

when stopping or slowing in an emergency, are obvious and due consideration should be. given to such events in the planning and design of a waterway and its 'operational procedures.

2.4 Interaction with Side-banks

tnceraction between a ship and the side banks is an important consideration in the design of a waterway. The effect of .a bank on the. track of a ship can dramatic. Figure 7 shows the results of simulations of a ship moving along a narrow channel and affected by the flooded side-banks. The

rudder angle was kept at a constant value of 0.50 to starboard throughout the simulations, and the resultant tracks are typical of the bank rejection which may befall the unwary. It is seen that the initial bank rejections are comparatively mild, unlike the subsequent rejections which may ultimate-ly'result in a grounding. Even if a grounding did not result, a sheer off a bank can be disabling, especially to a twin screw ship which may severely damage a screw or rud4er.

When a ship moves parallel to a straight bank therefore, it experiences a. sway force and yaw moment caused by the presence of. the bank changing the hydrodynamic pressure distribution .around

the ship. The sway force may be directed towards or away from the bank depending on speed (see ref. 8) while the yaw moment invariably causes the bow to turn away from the bank towards the centre of the waterway. If the bank is surface-piercing (such as that of a canal) the fortes and moments experienced will be larger than if it is flooded and if it is short compared. to the length

of the ship, transient forces may be important.

It is of interest to note in passing that the ship which by virtue of its shape, design and depth/ draught ratio is more. manoeuvrable by being more responsive to forces applied by the rudder may also be. more responsive to external forces such as those arising from bank effects. This is illustrated

by Figure 8 from reference 7 which shows the trajectories Of two ship models moving over a contoured sea-bed. Model A represented a ship which, in the centre of the channel, was quite manoeuvrable. Model S an the other hand represented a tanker with a fairly sluggish response which was aggravated by its extremely small underkeel clearance at a depth/draught ratio of about 1.10. It is seen that model A developed a severe.sheer to port arising from interaction with, the talus at Q. The model then moved into the path of the on-coming tanker which was unable to take avoiding action and a collision resulted. This modelled an actual incident which resulted in heavy loss of life and gave. a very salucory lesson in the effect on ship behaviour of its local environment as defined by the

wa Cerway..

Of course while the bank can have an effect on a ship, so also can a ship have an. effect on the bank due to wave action and draw-down. Wave action arising from excessive speed is difficult to quantify although some information is given in references 3 and 10. Draw-down has a scouring effect and is caused'by the passage of the ship sucking the waterway' from the banks. This is illustrated in Figure 9 which shows the flow through an entrance to an enclosed harbour induced by the passage of a large ship along the fairway. Obtained by photographing particles on the surface of the water of a model of the harbour, induced flow velocities of up to one knot were measured in the entrance and the effect on small vessels moored near the entrance could be severe. A method of escimating the scouring effect of draw-down and surface waves is given in reference 11.

2.5 Interaction with Other Ships

If two-way traffic is contemplated or ships are to be berthed to one side of a waterway the problems which can arise from ship-ship interaction may need to be considered. This has been discussed at some length in, for example, references 4, 5 and.8 and arises, as with ship-bank interaction, from

the mutual interference of the pressure fields o.f the vessels in question.

2.5.1 Interaction at Speed

The interaction which can occur'bétween ships moving on parallel or reciprocal courses increases in severity with:

- reduction in depth/draught ratio - reduction of passing distance - increase of passing speed

ClearLy all of these are within the control E the designer of. the approach channel and an allowance can be made depending oâ the operation proposed for the port. Data in referenCe 4 for example can be used in conjuction with a ship simulation model to determine safe passing distances and speeds as well as invest-igàting whether overtaking or head-on encounters can be tolerated.

Such decisions, arising directly from the effects of ship hydrodynamics, can he important for port operations and may well have a profound effect on the capacity of the waterway. For example it may prove to be hazardous to allow overtaking encounters in a channel unless it is excessively wide.

This in turn will determine whether ships must pass throubh the waterway in line, astern or not, with consequences on port operation. Furthermore, if vessels must nave

itt

line astern, the longitudinal separation will be determined by, among other things, the stopping distance needed by each vessel t. stop safely in an emergency. This in turn is determ.tned by the hydrodynamic characteristics of the ship in question.

Interaction also arises between tugs and the ships they are handling. Ac speed. this can cause hana1in. problems for

the

tug attempting to pass a line to a ship and some discussion of this is given in

reference 12.. Figure 10, from this reference, indicates the nature of the. interactiOn forceà and moments imposed on a tug keeping station with large ship. It may be seen that for a 'conventional' tug (i.e. one with propeller(s) and rudder(s) aft) there can be situations where the control force required from the rudder can in fact add to the interaction sway force induced by the ship. These interaction forces and moments can also change rapidly as the tug changes its longitudinal position alongside the ship and clearly adequate manoeuvrability must be incorporated in the tug design to allow such effects to be handled.

Some account of tug capabilities can be allowed for in port operations, especially as regards the speeds at which tug and ship initally come into close contact.

2.5.2 Confined Water Interaction at Low Speed

A special type of tug-ship interaction occurs at very low or zero ship speed when the wash of the tug can affect the ship it is handling. This effect is magnified if it occurs in water which is confined in both width and depth and is discussed in reference 13. Figure Il, from this reference, shows the effect on bollard pull of the screw wash of a tug, stationed near the bow of a ship, which impinges on the hull.

The wash of the tug nay not only directly reduce its effective bollard pull, it may also have an in-direct effect by causing changes of pressure around the ship's hull which is then caused to move This is illustrated in Figure 12, also from reference 13, which shows that at a small enough depth! draught ratio the turning moment induced on the ship can be increased markedly above that. to be expected. from its static bollard pull. This is due to the indirect effect of the screw wash o:

the-tug 'sucking.' the ship round and aiding the the-tug in its attempts to turn the ship.

Such effects caxt be considered in the detailed design of swinging areas etc. associated with a waterway by ensuring adequate room for the tug to manoeuvre.. If the tugs operate in a 'pulling' mode, increasing the distance between tug and ship reduces the effects of screw wash interaction so that adequate allowance should be made to allOw efficient tug operation.

2.5.3 Moored Ships and Passing Ships

A particular type of interaction relevant to waterway width is that which occurs when a ship moored at the side of the waterway is passed by a ship moving along the waterway. This is discussed -in references 14 to 16 in which the ranging induced on the moored ship by the passing ship is shown to be capable of achieving sufficient magnitude to break the moorings of the stationary ship.

This effect can be. reduced by imposing a speed limit on the passing ship- and/or increasing its passing distance. By far, the most effective is the imposition, of a speed limit, but local widenizig of the waterway may be necessary near to berths or jetties to prevent such undesirable interaction effects.

2.6 Ship Motions -Induced by Wave Action

Ship motions due to wave action can have an effect on the operation of the waterway as well as directly entering the waterway depth determination.

Operations may be affected by waves which, while not large enough to hinder a large ship entering a waterway, may be too large for the smaller vessels such as tugs which must handle the vessel during the berthing and turning phase of an approach to a berth. Clearly such vessels are designed with good manoeuvring and sea-keeping properties, but it is possible that their operational limits may be lower than those of the vessels they are handling. Due account should therefore be taken of this when considering the port operation.

The depth of a waterway exposed to waves must take intO account not only the squat of the vessels which use the waterway but also the vertical motions induced. These motions are related to the design of the ship, the waterway width and depth and obviously the waves themselves The wave characteristics will depend on wind strength and fetch and hence, indirectly, on the positioning of the waterway and its environment. Yet again we see che intertwining of ship hydrodynamics and waterway design.

Unfortunately shallow water ship motions have not received the same attention as those in deep water so that there is lack of useful data in the open literature However some results for a full-form shipe are given in ref ence 17 where transfer functions in roll, pitch, yaw and heave are given for various wave encounter directions and depth/draught ratios down to 1.5.

If the wave spectrum for the area of interest is known, it is possible. to use the resule of ref e-rence 17 to obtain a prelim.inary estimate of the maximum vertical notion of some part of the ship under the action of waves. This may be done by combining the. transfer functions of appropriate linear and rotational motions to obtain that of the vertical notion z'(f) at the point on the hail of interest:

z'3(f) z'(f) +1'(f)

(1)

or z'j(f)

/ZI(f2

+ (]J,1(f))2

where f is frequency

z'(f) is a transfer function vertical linear notion (heave) 1 is an appropriate lever

is a transfer functionof a rotational motion of interest (roll or pitch) Equation (l)0is used when the motions are in-phase while equation (2) is appropriate to motions which are 90 out-of-phase. Accurate phase information is hard to come by and equations (1) and.

(2) may be used. to provide, bounds to the notiOn. Clearly, a lower bound to the motion will occur when the linear and rotational motions are anti-phase so that equation (1) becomes a subtraction rather than an addition.

From equations (1) or (2) an encounter spectrum of the vertical motion with energy density Se(fe) may be computed where:

S(f)

[z'(f)J2. _S(fe) . (3)

where S(f) is the.wave spectral energy density at a frequency f. Due account must be taken of the Oncounter frequency

e in equation (3).

The first integral of the vertical motion spectrum m0 is now calculated where:

m

fSf.

_dfe

(4)

where F is some upper frequency limit.

It is now possible to calculate the vertical motion z likely to be exceeded once in every a oscillations (i.e. with a probabIlity P of 1/n) where

-4.605 log10(P)

(5)

from reference 18. Equation (5) is derived on the assumption that the amplitude oscillations of a ship under given sea conditions obey the Rayleigh distribution law. There is some evidence chat this applies in shallow water of depth/draught ratios between 1.7 and 2.0 in reference 19 in which reference the principle of linear superposition implied by the use of equations (1) to (5) is also vindicated at such water depths.

The method outlined above can be. used to produce design charts such as that shown in Figure 13 for a given ship and given:.vave spectrum. This, when combined with a squat estimate, allows underkeel clearance requirements to be deduced. However use of such a method implies aaaumptions, which may requite further study to justify. The principal assumptions arC:

linear superposition is valid in shallow water at all ship speeds.

scale effects may be neglected when using model results. This assution may be opened to question in the case of roll motion for which there may be considerable uncertainty regarding the effects of scale on viscous damping.

Nevertheless, in the absence of more comprehensive data, the method above can be used to provide preliminary estimates of extreme vertical motions and their probability of exceedence which may be useful in the early stages of waterway design.

2.7 Ship Hand.ith

Ship hydrodynamics and humon behaviour _{are both of the greatest Importance where ship handling is} considered. _{The behaviour ol the human ship controller and the way he manages the hydrodynamics of} the ship, manifest in its manoeuvring _{characteristics, determines, among -other things}

the lane width required for the basic control of _{the ship in the. absence of wind,} waves nd current.

the operational liijts of wind, waves and current.

the radii and width- of any curved portions of the waterway. the distance-off and speeds of passing ships.

The severity of the waterway from the

point of view of ship handling is often judged by the amount of rudder activity and rudder angles used _{to navigate a given ship along it.}

While this can be a useful measure, it _{houid ne used with caution because rudder activity is as much a measure of the helmsman} as it is of the ship or the waterway.

As an example Figure 14 shows results obtained at NMI using a large, free-atlng model which was controlled by two _{operators, A andB, under identical conditions.} Their task was .o maintain, as nearly _{as possible, a straight course (defined by two leading marks)}

in the absence of- any external disturbances _{such as Chose arising from wind or} current.-it is apparent from Figure 14 that the.

two operators used quite different rudder activity and angles to achieve virtually the same end in terms of the width of swept track, w. _{Therefore design criteria} based on the rudder activity should be _{treated, with caution.}

But it is important in waterway- design _{to estimate the probable width of swept track for hand1ing} Model studies in reference 20 suggest a value of 1.6 tines th _{beam of the ship while reference 21} suggests values of l.b or 1.8.

Clearly such model results should be checked at full-scale and Figure 15 shows the track of a ship in Southampton Water obtained by NMI using theodôlite tracking -techniques. _{Such methods, described in reference 22, provide}

data which is not only useful in valid--aring model. and simulator predictions but _{a-iso can provide some information on the mathematical model} which describes the manoeuvring behaviour of a ship (ref.23)

3. Design

What then can be done to incorporate ship _{hydrodynamics into the -design of a waterway?}

It has been shown that -waterway configuration and

geometry have a major effect on the behaviour of the ship and methods have been evolved whereby waterway width,depth and alignment can be obtainedhavirg due

regard for ship behaviour. _{Also ships which operate in shallow or ccnfined waters}

can- have features incorporated in the-jr design which have due regard for the effect of the waterwa".

3.1 Waevway Desj,n

Ship behaviour may be incorporated in waterway design using: -a preliminary desigfl study.

-a fast-time simulator study.

-a real-time full-ship-bridge simulator study. -a physical model study.

3.1.1 Preliminary Design Study irelininary design studies are rapid preliminac' estimates of many such methods is the study ships alone the Panama Canal,

bank clearance., and.. 1an. width

usually done using computer predictions or design charts which enable waterway width, depth and a1ignzenL to be made. The progenitcr of

described in reference 21 and carried out for the navig.tion of large in this reference recouunenoations ara given for safe passi.g _distances, for aasic control.

Reference 24 uses data from reference 21 and oiher published work to provide resuit of rsor

relevance to niod'rn shis. _{A further up-dating is done in refereiice 25 wiicI describes}

ho the reu.lts of recent studies at NMI may be used for prelimirary design.

In all cases ship dynamics anc _th

complex interactions between che human in control of he ship ai.d the ship and its. envjroit are treated in a relatively crude nancLer. _{For example the bank} clearance distance i_s deduced from bank effect _{and ship manoeuvring data by assuming. that the ship} achieves equi1ibrum drift -and rudder angles which exactly balance bank effects. By a suitable choice of limiting equilibrium rudder angle, a safe bank clearance may be-claculated.

The problems of cour:o, risea in hoing limiting equilibrium rudder angles to provide safe shLp navigaCion in general tiire is litLie uidauce on this. It is, in effect, the same as choosing suitable safety factors which satisfy both economic and safety criteria. Usually limiting values are obtained based both on past experience and advice from practising mariners.

However preliminary deisgn methods are useful and may provide more than adequate information to determine the suitability of une particular channel compared to another. Channel width is arrived at from consideration of bank clearance, interaction, wind and current effects and basic control while depth is determined by the draught of a suitable 'design' ship, squat and wave motion.. A1.l of these features have been dealt with above and data to enable their estimation is contained in references such as 21, 24 and 25. To show the approach adopted at NMI a preliminary waterway design-spiral is shown in Figure 16 from reference 25.

3.1.2 Fast-time Simulation Study

Simulation models which adequately describe the manoeuvring behaviour of a ship have been developed at MMJ. and eLsewhere (ref. 26). Although these are generally used in ship simulators which operate

in real time, they can be used in their own right to compute ship performance very rapidly. In order to allow fo.r human control two techniques may be used:

an operator watches a simple representation of the 'bridge window' scene displayed on a visual display unit which is rapidly up-dated. Periodically the program pauses and allows the operator

to key in-, say, changes in engine revolutions or rudder angle. In this cru4e manner a ship may be 'manoeuvred' along a channel with some human intervention.

a model of the human pilot may be incorporated in the simu.lation which then runs extremely fast with no actual human intervention (ref. 27). Naturally a problem of this method is -the provision of an adequate human controller model.

Fast-time simulation using Monte-Carlo techniques has also been used (ref. 28) and the advantages of all these methods are:

They allow a rapid collection of data On ship tracks etc. to enable envelops of lane widLth to be deduced. This allows a partial check on estimates made at the preliminary design stage. They allow identification of fruitless areas of study which would be wasteful of expensive

ship-simulator time.

They enable the behaviour of the ship to be changed within limits which nay be of importance in both ship and waterway design.

If human intervention is used in these techniques there-. are disadvantages which ariàe from the contracted timescale used and the lack of visual-realian..

3.1.3 Real-time Full Ship Bridge Simulation

Disadvantages of timescale and visual realism are either eliminated or minimised by the- use of a real time simulator. In such a simulator the effects of visibility, navigation mark positioning (ref. 29)

wind and current can be studied in a simulated rea time-scale and environment. Handling- near a bank or while two ships pass can also be studied so that any problems of control can be identified. The use of simulators in waterway design and piloted control studies is well documented elsewhere

(see for example refs. 3O,3lanI 32) and will not be dealt with herein detail. It is sufficient to state that the simulator is a powerful tool in such investigations. ut they can be expensive in both time and money terms so that it is important to have a clear Idea, at the outset, of the major. waterway parameters.

3.1.4 Physical Model Study

Some waterway problems however are too complex for solution by means of simulation or desk study techniques. Mathematical modelling is only as good as our knowledge Of the physics of the problem will allow. It is fair to say that some aspects -of ship hydrodynamics in restricted waters are insufficiently understood to allow for adequate mathematical modulling.

When the proposed waterway has features which cannot be simulated easily, a physical model study is often the only method by which an investigation nay be carried out.

The methods, advantages and disadvantages of physical modelling of ship behaviours in restricted waterways are described in reference 1, while Figure 17, from this reference, shows a radio-controlled

model under test at NMI for a harbour fairway study.

The results shown in Figure 8 were obtained using physical models in which the seabed, and ships, were reproduced at model scale. The complexity of the seabed. as shown, in this Figure is such that mathe-matical simulation of its effects on the ships would be difficult if not impossible with the present state of kunwiedgd of bank effects..

The added complications of non-uniform currents and .wave action often require some physical modelling and 'set-down' due to very long period wave motion may be present in shallow water wave motions which, again, can. be studied using physical models.

As with simulation models, the physical ship models can be tracked and their track envelopes provide information on the adequacy of channel width. Rudder activity can also be measured to provide further information on ease of handling in a given.waterway.

3.2 Ship Design

While accepting that the majority of ships spend a small percentage Of their time in shallow or restricted waters it is nevertheless true that, while in such waters, they are often subjected to

the greatest navigational risk.. Restricted waters tend to be more conjested than the open sea, the manoeuvring behaviour of the ship changes as shown above, and it may be subjected to bank. effects which can be severe and unexpected. The consequences of a grounding in a busy waterway, especially if the waterway is blocked, could be far-reaching both environmentally and economically.. Many of the ship behaviour problems which arise in confined waterways can be overcome by the skill and training of the pilot in charge of the. ship but it is not without interest to consider some aspects of ship design which can ease his task. We consider here ocean-going ships and exclude those that specif i-cally designed for a lifetime in shallow or restricted waters.

3..2.. 1 Control Devices

Many control devices are available which are particularly appropriate for ships needing great

manoeuvrability in confined waters. These include various forms of, active rudder, cycloidal thrusters, vectored propeller-type thrusters and lateral thrust units. Of these the ocean-going ship is. likely

to have only a lateral thrust unit and these are by no means universal. This unit ceases to be effec-tive at forward speeds in excess of about three knots so that it is a device which is unlikely to be effective for much confined water navigation.

The control device which is universal tO all ocean-going ships is the rudder. It has been shown in section 2.3 and Figures 1,2 and 6 that the screw race is of great importance in ensuring rudder effectiveness in shallow water. Clearly therefore rudders which are placed in the screw race are likely to be of more use in shallow water than those that are not. Therefore single screw/single rudder and twin screw/twin rudder stern arrangements are likely to orovide easier confined-waters

handling than, say, a,win_screw/sg cencreline ridder configuration. . If this last type is

installed than an attempt at the design stage to ensure that some part of te rudder blade enters the screw race when put over will aid handling in confined waters. In some cases interlocks have been fitted to such ships which allow large rudder angles (up to 40 ) to be used at low speeds in confined waters (ref. 33).

It i interesting to note however that the advantages of rudder angles in excess of the usual

+ 35

mcimtmi

are not confined to twin screw/single rudder ships. The single0screw ship can also

benefit therefrom as discussed in reference 34 where rudder angles up to 4. 60 are recouimended. Clearly this has implications in the design and construction of the rudder and steering motor, but some increase in rudder angle may be possible by modifying existing designs.

A further benef it to be gained from the rudder which is immersed in the screw race is that 'kicks ahead' may be used to advantage in low speed manoeuvring to ease a ship through a particular manoeuvre. This technique is denied those ships with rudders which are not within the screw race.

3.2.2 Vulnerability

The vulnerability of various parts of the ship is naturally much increased in restricted waters In some waterways it may be the practice to moor the ships with some partof the hull on the water-way bank; examples are ships which must be 'gared up' in an emergency on the Suez Canal and pueh-tows which are often 'rested' on the banks of large inland waterwayssuch as the Mississippi F..iver in the United States. For ocean-going ships it is important that their structural strength is such

as to prevent local damage to bilge keels or lo'cal plate buckling due to such practices.

However even in normal transit, vulnerability is increased. It has been shown that squat increases 'dramatically in shallow water and a properly-designed waterway should be deep enough to allow for

this effect. If the ship is initially trimmed by the stern however, the rudder(s) and propeller(s) are in danger of grounding due to squat; in such an instanèe the aodern'open water' sterns are note vulxierable than the 'closed' type (see Fig. 18).

Damage to bow or stern may occur in a confined waterway from any sheers close to a bank when the propellers of twin-screw ships are particularly at risk. The QS*.OU. of a Uoping hank relative to a ship's propeller when the hull grounds at two points Aand B a distance '1' apart with B on the cantreline (Fig. 19) nay be obtained from:

can(tan

cos )

(6)

Y zr/tan

l 12 tan

---(7)

= tan(b/l)

(8)

where = slope of bank perpendicular to centreline plane of ship

- bank angle

-b - half-beam of ship at tangent point A

position of toe of bank to one side of ship's centre-plane -vertical distance from bed of waterway to the tangent point

12 distance from B to the propeller.

Equations (6) to (8) enable cross-sections through the bank, in the body axes of the ship, to be plotted on the body plan for any cross-section of the hull a distance 12 from B once points A and B have been chosen.

Therefore it wouid bdpddniblëta hdck, at the desfga stage, the iulnerability of, say, a screw propeller to passage along a canal should this be considered necessary.

3.2.3 Other Features

Some other features of ship design are relevant to passage along a waterway. Among thes are: Directional. Stability

Directional stability-is a function of ship design, waterway depth and waterway width. It nay change in different depths of water (ref. 7) and may pose a problem to the ship handler. For example, a -ship which is. highly directionally stable may be ideal for long voyages on the open ocean, but wi1l be difficult to manoeuvre around bends in confined waterways. Conversely a ship which is less direct-ionally stable may manoeuvre well in confined waters but require excessive use of rudder to maintain a Straight course in the open sea. Excessive use of the rudder can increase fuel bills and can excite roli notion in a tender ship. While at sea rudder-induced roll may be uncomfortable, in shallow water it. may, in conjunction with squat, cause the ship to ground at the bilge. As an example, a roll angle

of: 1 on a ship with a beam of 50 metres reduces underkeel clearance at the bilge by about 0.4 mattes.

Rudder, and Engine Response

Ship-handling in some confined waters, notably canals, demands great concentration and anticipation on the part of the ship-handler In many cases he must 'force' a ship to stay on course and to do so will require a rapid response from the control systems at his disposal. This applies both to rudder and engines and the slower the response, the greater the problem for the ship-handler. It should also be borne in mind chat propeller efficiencey may be reduced in some cases, especially with highly loaded screws which may tend to cavitate under the additional thrust loading required at a given speed In confined waters. This is due to the increased resistance experienced by a ship in shallow water, but it is a phenomenon which can be checked in the design of the ship.

Superstructure and Windage

Ships with high superstructure and shallow draught are particularly susceptible to probleme in wind. Ships such as vehicle carriers or ferries nay be able to counter this at sea by proceeding at a high

enough

speed to keep the key parameter the ratio of wind speed to ship speed - at an acceptable

level. But in confined waters this may no longer be possible so that the ship proves increasingly difficult to handle as her speed drops. This nay entail the provisioQ of additional nanoeuvring de',ices or the earlier attendenace of tugs. Reference 35 provides some information on the handling

of' h1gh-sidd slips

iü

wlnJ

ltuaua.n Factors

Although It is not the Lntention to deal with human factors in ship (and waterway) design in anything. but the

briefet

detail; it should b borne in mind that human beings vary in their performance and

training. This reflects in the way a ship is handled and therefore on waterway parameter requirements. Some account of human variability must be taken in waterway design. but how much is noc always clear. Waterway parameters are usually based on the assumption that the 'design' hip' is well-handled by

trained personnel, iully conversant with shallow water phenomena and ship behaviour, in charge of a ship which handles well and has an adequate supply of aids for navigation. While this nay not always be the case, It Is possible at the ship design stage to ensure that unnecessary problems for the ship-handler are avoided. For example, all-round visibility from the vheel-house should be as good as possible (pilots. may only have leading marks astern to guide them out-bound along a channel), the wheel-house position should be such as to allow safe navigation (perception of ship manoeuvring behaviour stay vary depending on the position of the wheel-house in relation to the pivot point of the ship), and aids to navigation should be conveniently placed. A foremast or 'steering post:' are veil-known and invaluable aids when navigating in confined waters.

It is fortunate that many of these human factors can be studied ashore using simulation which, in conjunction with mariner training, can do much to reduce the problens of confined water. navigation

3.3 Safety.Criteria

The main thesis of this paper is that ship behaviour and waterway design are inextricably linke4. Clearly therefore the safe navigation of the ship must relate In some measure to the design of the waterway and its operational procedures.. For example, the waterwaymust be sufficiently wide so that bank effects pose no problem, wind and wave limits must be considered for safe operation, convoy spacing in, say, a canal must be determined with due' regard to the abIlity of. a ship to stop safely and sufficient aids to navigation must be provided for safe navigation.

But..what'is meant, quantitatively, by 'safe'? All predictions made using the design schemes discussed above, whether they use the moat sophisticated simulation, techniques or whether they use the simplest approximations, ultimately, require some. sort of safety criterion. This is discussed in reference, 25 and several types of criterion, ranging from a gldbal statement of a socially-acceptable accident ratS to various secondary criteria relating to specific areas of waterway design are discussed.. 'For preliminary design at NM the concept of limiting equilibrium rudder angles has been used together

with a steady-state analysis of ship behaviour in sway and yaw (ref. 25). This technique works reason-ably well with regard to wind and bank. effects where the concept of a-ship attaining a 'balance'

condition of rudder and drift angles is reasodably close to the truth. It. is also convenient in that It translates forces and moments acting on a ship into entities which have more meaning to the mariner -rudder and drift angles. It has a further advantage -in that limits to rudder angle-exist (usually + 35) which immediately set limits on the external force and moment which acts on ce ship. if the.

aailable ruddcr angle is now sectored with. an allowance of', say, x for control, y for bank effects,

z for wind and so on, safety criteria have been set which, in the case of bank effects, also determine an element of channel width, as bank effects (and hence equilibrium rudder' angle) vary with

distance-off.

-This simple steady-state -analysis' is not satisfactory for dynaiic- effects such as ship-to-ship interaction and the only suitable technique for use in such -an investigation. Is a full manoeuvring simulation. 'Some work on this has been done at NMI and elsewhere, but again suitable safety criteria Should be set.

Finally consideration should be given to which body shOuld set the safety criteria. Should they be advisory or mandatory? Should they be specific or couche4 in more general terms, as guide-lines or codes-of-practice perhaps? Clearly any criteria should be determined with the active, participation of port operators and deaigners,. ship-handlers and possibly, government agencies so that a compromise, acceptable to all parties may be agreed.

4.' Concluding Remarks

The thesis of this paper has been that the behaviour of a ship is profoundly aff'ected by the-geometry and alignment of any confined waterway in wnich it operates. This has been illustrated by results obtained at NML which have' led to a compendium of data which may be used in preliminary design, physical model studies or simulator investigations.

-Throughout all of such studies however- it is essential thor the views of practising mariners, port operators, civil engLneers and stiipowners be sOught with tit ultimate goal of providing saic al,o economic wateays for the passage of the world's ships.

5. References

hAND, I W: 'Model techniques used to Study Ship Behaviour in Canals and Channels' Symposium on Hydraulic Modelling Applied to Maritime Engineering Problems, Lnsttcution of Civil Engineers, London, October1981. Paper 3.

BAND, I W & FERGUSON,. A M: 'The Squat of Full Ships in Shallow Water' Trans. Royol lnstitu1on of Naval Architects (RINA), vol. 115, 1973. p.237

'The Prediction of Squat for Vessels in Shallow Water' National Ports Cuoucil, London, February 1981, ISBN 0 86073 054 9.

DAND, I W 'Sothe Measurements of Interaction Between ShIp Models Passing on

Parallel Courses' NM1 report Rl0S, April 1979.

BAND, I W: _{'The Physical Causes of Interaction and its Effects' Nautical Lnsçitute}

Conference Un Ship-Uandling, Plymouth, 1977 (also 4I- report R38)

6.. CRANE, C L: 'Manoeuvring Trials of the 278 000 dwt Tanker 'Esso Osaka-' in Sha1low and Deep Waters' Exxon International Report Ell.4TM.79 January 1979. BAND, I W: 'Hydrodynamic Aspects of Shallow Water collisions' I Report R6,

November 19Th.

DAN!), I U; _{'On Ship-Bank Interaction' Trans. RINA, voL 124, p.24-40, 1982}

JOHNSON, 3 W: 'Ship Waves in Shoaling Water' Coastal. Engineering Conference, institution or Civil Engineers, London, Sept. 1968, paer 92

HAY, 0.: _{Ship Waves in Navigable Waterways'Coastal Engineering Qonference,}

Institution of Civil Engineers London,. Sept. 1968, paper

93.-11 BAND 1 U & WHITE, U R _{'Design of Navigation Channels' Symposium on Aspects of Navigability} ul Constraint Waterways Including Harbour Entrances, Delft, 1978, volume 2 paper 3, (also NMI Report R78,April 1980).

BAND, I W: _{'Some Aspects of Tug-Ship Interaction' Fourth International Tug Convention,}

New Orleans, 1975, Thos. Reed Publications, (also NMI Report Ri). BAND,

I U:

'Tug Wash Effects in Confined Waters' Seventh International Tug,

Convention, London, June 1982.

REMERY, C F H: 'Mooring Forces Induced by Passing Ships' Paper 2066-, Offshore Technology Conference, Houston, 1974, p.351.

15.. DAND, I W: _{'Simulation of the Behaviour of a Moored Ship when Passed by Other}

Ships ' NMI Report RIll, Aug.

1981.-16. LEAN, C 6 PRICE, W A 'The Effect of Passing Vessels on a Moored Ship' Duck and-Harbour Authority, vol. LVlll, no. 684, Nov. 1977

17.- TAKAXI, M: 'On the Ship Motions in Shallow Water (Part 3)' Trans. Society of Naval Architects

of

West Japan, no. 54, 1977 p.lO3-114.

PRICE, U C & BISHOP, R E- B: 'Probabilistic Theory of Ship Dynamics' Chapman & Hall, London, 1974 WANG, SHEN: 'Full Scale Measurements and Statistical Analysis Qf Ship Motions in a

Navigation Channel' ASNAME 'Marine Technology'vol. 17, no. '+, OctOber

1980, p.351-370.

BAND, I U:. _{'A Study of the Effect oi Human Factors On the Handling of Two Ship} - Models' NMI RepOrt k112, January 1980.

GARThUNE, R S et al: 'The Performance of Model Ships in Restricted Channels in Relation to the Design of a Ship Canal' David Taylor Model Basin Report 601,

- 1948.

-WRIDE, A T A, WILLS, A-E &

LECKENBY, R.M 'Behaviour of-Large Ships in Shallow and Confined Waters (Southampton)' NPL Report Mar. Sd., R12l., 1975

DM10, I W:

GIL, AD:

SHOOMN, M L:

SUKSELAINEN,

MCfl.ROY,. W:

23. GILL., A 0: A Ship Tracking Exercise on the 'Esso Rotterarn'' NPL Ship Division T4 327 ApriI 1974.

24.. 'Port Approach Design - A Survy of Ship Behaviout Studies,

ois. I & II' National Po±ts Council, London Sept. 19-75.

'An Approach to the Design of Navigation Channels NML Report RiOt, May 1981.

'The Analysis and Synthesis of Ship Manoeuvring' Trans RINA Supplementary Papers, 1980, (also NMI report R90).

Models of Helmsman and Pilot Behaviour for 1anoeuvring Ships' National Maritime Resea.rch Center, (NMRC) New York, report CAORF 40-7901-02 March 1980.

'On Ship Manoeuvring and Waterway Width' Helsinki University of Technology, Report no. 8, 1975.

'An Investi4ation Of the Factors Influencing Piloted Con ollabiU.ty in Restricted Waterways' NMRC, New York, Report CAORF 24-7804-01, July 1980.

3O WALD, E 0 & MCILROY, W: 'Piioted'Controllability Research at the Computer Aided Operations.Research Facility.: i979-1980' Proc. MARSIM !81 Second International Conference on Marine - SimulatiOn, NMRC, New York, 1981.

cRANE, C L: State of the. Art on HOw to Include Human CoCtrol into the Method of Investigation' Symposium oil Aspects of Navigahility of Constraint Waterways etc., Délft, April 1978.

DIXOORN, .1 V1 GEMA,- J F, KOELE, L A & ROOSE, W A: Development and Criteria fOr the Design and Construction of the Port-Approach and Harbour Area Entrance of Rotterdan - Europoort Symposium on. A.spects of Navigability of Constraint Waterways etc., Delft, April 1978.'

vossNAcK, E: Discussion on 'The Structural Design of the OCL Container Ships' by M Meek, Trans. RINA, vol. 114, 1972 p.276-278.

ENISH, 3 W: 'Propellers,'Rudders and Manoeuvring Devices' Nautical

Ship Behaviour in Ports-and Their Approaches, Part 6 - -The- Determination of. Channel Width, Reqüi±ewefltS PJ.lowiflg

for the Effects of Wind Research Transport lieadquarterS, Dept. of Transport, LOndOn, March 1982.

6 Acknowledgement

This.paper is publshêd with the permisSion of the Director of the National Maritime Institute.

7.. NomenclatUre

b half-beam of ship at forward tangent point

f frequency

ehcounter frequency F upper frequency limit h water depth

1. lever

12 distance from. aft tangent point

(on centreline) to propeller firSt integral of vertical mOtion spectrum

-n number of osciLLations

P probability- of metion a- being. exceeded once

in

n oscillations

S(f) spectra.]. energy

density-5e

e?countOE spectrum energy density

-P, T8 draught

w width of swept track

pOsition of toe of bank. tO one side of ships s centreilne plaie

a vertical motion

z1 vertidal distance from bed of waterway to tangent point

tranSfer function of vertical. tion at point of interest

a babk angle

a1 slope of. basic. perendii1.ar to centreline plane of ship rotational motioS transfer function

V

FLOW AROUND AFT-BODY AT 6 KNOTS FORWARD SPEED - DEEP WATER

0

C'-r

-r

FLOW AROUND AFT-BODY AT 6 KNOTS FORWARD

SPEED-SHALLOW WATER

-n

p

4

1 0O0O

5,000

10

TOTAL RESISTANCE

+

+

x

+

x

____+

;_ ;_

-+

-'-F

WATER DEPTH

22 fl

26

0-

-0--co

-I I I; I I I I I 11 12 13 14 15 16 17

18 VKNOTS

ESTIIMATED

RESISTANCE OF VERY LARGE CRUDE CARRIER IN VARIOUS

4U

£

LT.LUT TLI1I.I/lOO cv ITIIN

LTSCIT TI -L,..IIT sTy iILó SQUAT

ESTIMATION

CHART

FIG.4

0$

II

I 2 5 10 50 4050100 500 400 51,5(1 245 10

Ii

t1A11 505010 50C 1000 P5(5 -o S

I

S S $0 II -14 Is

IMIP s.uo - INOTI INRUCTtOSS

1 INTIL SNIP STIED II INOTI (PoiNT A)

3 OILS L1 A S TO .I?I*SICT appIopIlail

SATES -DIPTN 00NTOUS AT S

3. bn sui SC UDiCULAL TO A B o TITIASECT

CONTOUL P05 SOw 05 STEIN LPPSOPS1A?I TO. 'NI

ai..csi i..0 ó iwi sP fronn C)

4 STOP PInPINOICULAS C 0 IC .NT(NICCT APPIOPII*?I

sscp (NITN coinoi,I AT 0

S. Dm*p tint 0£ PINOICULIn TO 0 To Gist

sow/STEIN 1

* .Ints (peii.T 5)

CONVI5IOSSB

I WIT1 ITSOS PElT

EFFECT OF. WATER DEPTH ON TURNING CIRCLE OF

A LARGE TANKER (FROM REF.6)

RUDDER

350

PORT; APPROACH SPEED 7 KNOTS

RUDDER

ANGLE

14

LANE WIOT.H/ EAM

₁₃

RUDDER A N G, LE

100 5

10° p

12

I

10

1005

loop

LANE WIDTH/

BEAM

1'3

l2

11

10

20 SEC MODEL SCALE

'-4

.1

1

OPERATOR A

TIME

.

OPERATOR B

TIME

_-HANDLING PERFORMANCE OF OPERATORS A and B

TANKER MODEL; hiT= 1 04 ;

Fnh

0 18

SHIP

GROUNDED

-r

+

"START

a

N

'N

STA PT

SIMULATED TRACKS OF A SHIP IN A FLOODED FAIRWAY

SPEED:8 KNOTS, RUDDER:O5° STARBOARD: h/T=127

FIG .7

+ -r

+

±

1

WATER DEPTH

₂₇₄

+ + + +

N

N

W&TER DEPTH

_{= 12}

MODEL 'A'

(12-KNOTS

---L

210

FULL SCALE)

FAIRWAY

h=1o99 T MODEL 'A']

BUOY Ii

1312 T MODEL 'A'J

SEABED cONTOURS

I .1

LENGTH OF MODEL 'A'

I

I

LENGTH OF MODEL 'B'

FIGURES ON TRACK ARE TIMES IN SECONDS TO COLLISON

.COLLIS:ION DUE TO BANK EFFECTS

COLLISION ANGLE

-350

DEL 'B'

I KNOTS

SCALE)

SHIP TRACK

FLOW TRACERS

C)

FLOW INDUCED IN HARBOUR ENTRANCE

DUE TO PASSING SHIP

KEY

:-INTERACTION FORCE & MOMENT

f

RUD'ER:INDUCED FORCE & MOMENT

POWER REQUIRED TO MAINTAIN STATION

TUG - SHIP INTERACTION

SHIP TWO-DIMENSIONAL WAKE

CORRECTING RUDDER

KIIY

p

p

-a 7UG P p SHIP

/

APPROXIMATE STREAMLINES

i:oo

x

XE

lOPE

Ys o

YE

r

f

1-2

13

SUBSCRIPTS

S =

MEASURED VALUE ON SHIP

E =

EXPECTED VALUE ON SHIP

SURGE .FORCE

14

.

15

h/I5

+

Oo

TOW ANGLE

450

TO SHIP CL lo

900

-

x

SWAY FORCE

iO

1.1

1-2.

1-3

1-4

1-

h/T

YAW MOMENT

100

F 1G. 11

14

15 h/T5

EFFECT OF TOW ANGLE AND WATER DEPTH ON EFFECTIVE

BOLLARD

PULL (AT REFERENCE SHAFT RPM n0)-TOWS NEAR

BOW

TUG SCREW WASH INTERACTION EFFECTS

N5. O

'

'0

NE

0

0

x-.-,--.. .1.

VI

A5 o,

-0

50

ioo L

Ns

o,

Ii

0 INE

50

"0

₀

+

SWAY FORCE

-.

10

11

12

14

15

h/Ta

YAW MOM ENT

±

C

I - I -.--- I -.- I

13

SURGE FORCE

I - I

1.5

h/T5

FIG 12

EFFECT OF TOW ANGLE AND

WATER DEPTH ON EFFECTIVE

BOLLARD

PULL (AT REFERENCE SHAFT

RPM n0) TOWS NEAR MIDSHIPS

TUG SCkEW WASH

INTERACTION,E.FFEcTS.

1 I

+

450.

X

900

t.0135°

____

10

11

12

SUBSCRIPTS

S: MEASURED VALUE ON SHIP

E= EXPECTED VALUE ON SHIP

13

14

is

h/T5

o:

02

01

AXIMUM

VERTICAL

MOVEMENT

AT BILGE

(m)

O5

06

O7

FIG:13

- --

L

P

0.8

09

10

i-i

12

SIGNIFICANT WAVE HT (m)

RUDDER ANGLE

14

LANE WIDTH/ AM

₁₃

12

1 1

10

RUDDER ANGLE

100 5

100 P

id3s

10" P

1 4 LANE WIDTH/ BEAM 1'3 1.2

10

1

A

20 SEC ,MODEL SCALE

I

I

f

10

i/i

IlL

OPERATOR A

TIME

b-OPERATOR B

TIME

HANDLING PERFORMANCE OF OPERATORS A and B

TANKER MObEL; hiT: 1 04 ;

Fnh: 0 .

MEASURED FULLSCALE SHIP TRACK

BANK PROTECTION

CHECK SILTATION

CHECK HANDLING

1 N WI NO

CHECK OFF-CENTRE

HAN DL'I N G

iNITIAL DATA: BAS15SHIP,

SHIP MIX SIDE SLOPE CURRENTS

CHOOSE DEPT H DRAUGHT RATIO

SQUAT ESTIMATE,

DEDUC E DEPTH A ND

_{SILTATION ALLOWANCE}

POWER/RESISTANCE

ESTIMATE

C H EC K HAN DL I NG

CHOOSE

L 0 CK AGE RATIO

ON CEINTRELINE

AN.D TRANSIT SPEED

DE.DUC E L AE

WIDTH AT KEEL

1\

IU\L1

L)

'CLOSED' STERN ARRANGEMENT

'OPEN' STERN ARRANGEMENT

COMPARISON OF STERN ARRANGEMENTS

WATERLINE PLANE

TANGENTIAL TO BANK

BANK ANGLE.i

BANK ANGLE.

b2

p

Yo

DEFINITION SKETCHES FOR BANK CONTACT ANALYSIS