The effect of passing vessels on a moored ship

NOVEMBER, 1977 . ₂₇₅

The effect of passing

vessels

on a moored ship

By G H Lean BSc ARCS DIC

WA Price BSc (Eng) CEng MICE

, Hydraulics Research Station

The passage of a ship close to another moored at a berth can cause excessive ship movements and can lead to ships breaking

their moorings. As ship sizes have increased

harbour masters have become more aware of this problem and, in order to make

sen-sible decisions about speed restrictions, safe

mooring arrangements and the like, need to know what forces are produced by passing ships. To find out how the magnitude of these forces depend on the speed and size of the passing vessel, its distance off, the under keel clearance, and other factors, the Hydraulics Research Station carried out a

series of model tests for Esso

Trans-portation Department in connection with their oil jetty at Milford Haven. All

the results cannot be reported in such a short

paper but some typical examples have been

chosen to demonstrate general principles and

to show the effect of the more important variables.

The model

The model was constructed to a scale of 1:100 (Plate 1) with jetty and dolphins

arranged centrally in a tank measuring 24 in

x 30 m. The moored ship, which was

arranged to have the correct displacement

nd dynaniic behaviour, represented a proto-itype vessel of 250 000 dv:rt. It was Moored In_ a conventional way by twelve lines having

.,fhe correct elastic characteristics. The two

gements used are shown in Fig I.

fle passing ships were of 17 000 dwt.,

00 dwt. and 210 000 dwt. They were self

pelled and fitted with motors running a current-controlled supply from wet atieries Speeds could be preset, but

pre-'Nome velocities were established by timing

Jai _{ship over a measured distance. A}

Alkaight course for each ship was assured Attaching it with the aid of nylon guide

ts to a taut wire rope stretched under 1

oring loads were measured by strain

connected through amplifier units to

al channels on high

speed u/v

The movements of the ship in natal plane, and the water level Side of the moored ship, were also

Movernent recording

° probes

0 Water level recording probes Scale 30 SO (nil laisce 160 200(11) 1===) (a)Rope roods Rivaw impart

(b) Movement and water Iamb Mooring layout 1

Mooring layout 2

I Mooring ropes

(Steel ropes with 30' nylon tail) tio.1 end 2- double Nos.3-W single

(I) Above. mooring layouts.

ion, of p000sng vosoal rolotkot tornof mond ohips-7601n - 4130ro

200

Plate 1, a general vieiv of the model.

.1.2.V.174

The 80 000 dwt ship was used in the laden conditions at speeds of 3, 4 and 5

knots, arid at distances of 46 m and 69 m (150 and 225 ft).

The 17 000 dWt ship was uted in theladen condition at speeds of 4, 6, 8 and 10 knots at distances of 46 in (150 ft).

Most of the tests were made with the ropes in pretension (5 twines) or slack. Pretension means that an initial load was applied to the

Mooring ropes Moorings when the ship was at rest.

(stow ropesitth surrylantwo _{The passing ship was set in motion. and}

Hos.1-2-4-15

Nos.7-13-9-10 (drupe the recorders switched on. The vessel was dined at 3.05 ra, 6.10 m, 15.25m and 183 m (10. 20, 50 and 60 ft) distance.s. from the

moored ship so that its speed could be

checked. A typical example of the traces obtained is given in Fig 2. For each test

there were two records, one giving the loads

in the lines, and the second the water levels

Tim Bon focal.

Swayauniawst at Surge inovemera alai Sway movement al wars

at the outside and nearside probes (Fig 1) and also movements of the ship at the bow, centre and stern.

Behaviour of the moored vessel

Qualitatively, the movement of the moored vessel relative to the passing vessel was

simi-lar in all tests. The movement could most

readily be described and explained by

refer-ence to the pressure field associated with the flow round the passing vessel. This is sketched in Fig 3 in which the pressure head, h, can be taken as the local rise and fall in water leveL It will be clear from this that a stationary ship placed at A,B, will experience a force towards Ai, and there will be a moment tending to turn it towards

the passing vessel. At A,.13, a ship will tend to move bodily towards the passing ship. At A,13, the ship will experience a force towards the receding vessel, and also a moment

tend-ing to turn it in that direction. For the

moored ship, these forces are resisted by

Wales timber. Ct2fn

- Mb,

Dube

Test conditions

The test conditions may be summarised

as follows':

1. The 210 000 dvit ship was used both laden and 'in ballast', and at distances of 84m, 69m and 46m (275, 225 and 150 ft) from the moored ship: The speed range was 2, 3, 4 and 5 knots.

Colit Unkersity of Technology

Ship Hydromeohonies Laboratory

Library

Mekelweg 2 - 2628 CD Delft

The Netherlands

Phone: 31 15788873 - Fe= 31 15781838

275

C29

(1) pressure heed full alp

V. ship velocity

. Az Bak! H2

-Salim Pressure

Pressure distribution around mooring ship.

the mooring lines, but the observed motion is similar to that which would be expected from the above description.

Results

-For each test the maximum loads in the mooring lines and the position of the vessel were determined.

The most

significant

variables affecting the moVemenf. 812(1 line

forces of the moored ship were the speed, size, and clearance of the. passing vessel. A

summary of the main findings follows:

Effect on passing ship speed (pre-loaded and slack lines). .90 80 70 60 50 20 10 Suction F.P. - Bt

I-Pressure mann --w...

_40/1_,i

Pre load

i I I

1 2 3 4 5

Passing ship speed (knots)

-(1) The effect of speed. For a 210 000 -dwt

tanker passing at 45 m (150 ft) at different

speeds, the maximum mooring loads in one

of the ropes are given in Fig. 4. At other

-distances there is a similar steep rise in load

with speed. &railer variations were found with the other two ships.

To illustrate the effect of speed an empiri-cal relationship between rope load and speed

has been established. Note that this must be taken as a general relationship.

If R = Rope kad in tonnes S = Passing ship speed in knots

Then ,R =-- 2A5 S':" . ... for slack

moorings

R = 1.17 S2-6 for

moor-ings with 5 tonne pre-tension

Even with 5t pre-tension in the lines the Mooring loads are reduced by nearly a

half of what they would be with slack

lines. Also, because Of the power relation-ship small reductions in speed can produce significant reductions in mooring loads. It is interestins to note that in nearly all cases with pretensioned lines the moored ship suffered a small permanent displacement along the jetty after the passage of the passing ship. This was probably due to fender friction which was, of course, in-creased with larger pre-tensions in the fore

and aft breasting lines.

(2) The effect of clearance between the passing 210 000 dvit tanker and moored

vessel is shown in Fig 5. As might be

expec-ted the mooning loads diminish with increased clearance, the decrease being more marked at higher speeds.

(3) The effect of underkeel clearance on the rope loads, with the 80 000 dwt tanker

passing at approximately 3 knots at a

Clear-ance of 45 m (150 ft), is shown in Fig 6. There is 5 t pre-tension in each mooring.

As the underkeel clearance becomes smaller there is a significant increase in the mooring

forces.

(4) The comparison in the rope loads in

the spring lines for the tanker passing loaded and unloaded is shown in Fig 7. There VMS

a significant reduction in all rope loads for

the ballasted tanker.

(5) In one set of tests the tanker was allowed to pass at different speeds and measurements were made of the loads in

the lines necessary to keep the moored ship

still. The results are summarised in Fig 8.

They show that the necessary mooring loads

were approximately proportional

to the

(speed) of the passing ship. Theoretically

the lateral force on the ship is given by :

V' PD

Y = 0.29

w

--2g 2

and the rope load in this case is

T = Y/728

Pretension in each rope (tonnes)

90

BO

2 20

THE DOCK & HARBOUR AUTHORITY

eo

70

` 40

20

Clearance between ships 150 ft

4 knots

Pre load 4!"tee4to..0.013

0 __

20 90 60 BO 100

Mama between moored and passing wised (m) (5) Above, effect on clearance between moored and passing vessel.

3.0-3.3 knots

105 1.10 125 1.20 1.25 1.30 t35 1.40 Path/theft

(6) Above, effect' of =clerked clehrance.

Pre load

'1

1 - -_I , - .I 4

1 2 3 41 1 -5

Pinsky speed (knots) (7) Above, effect of passing ship in ballast. 2 40

a

NOVEMBER, 1977 ₂₇₇

Speed of Passing Ship in ICstots (?)

w = Weight per unit volume of

water

p =- Pulse length (m)

D Draught of ship

The effect of currents were established

in a separate series of tests. The effect is threefold. Firstly. it changes the magnitude

of

the flow disturbance caused by the

passing vessel for a given speed relative to the banks. Assuming the velocity distribu-tion in depth is uniform, the wave and

current pattern round the passing vessel will

be the same for the same relative velocity through the water, irrespective of its speed over the ground. Thus a vessel moving at 3 knots relative to the bank in an opposing current of 1.3 knots will create the same disturbance field as a vessel moving at 4.3 knots in still-water. In each _{case the ship} will be subjected to a water velocity of 4.3 knots and the pressure distribution round the ship's hull will be the same. Thus the results of the still-water experiments a/so apply to those in a current provided the

tanker is moving at the same relative velocity through the water.

Secondly, a current imposes an additional load on the moored vessel. The tests showed

that with the additional current load the

extreme fore and aft movements and

spring-line toads were much the same, but that the sway at bow and stern was increased

Thirdly, the current changes the speed with

which the flow pattern associated with the

g vessel moves past the moored vessel.

the moored ship is regarded as a simple

linear oscillator with certain natural periods

in surge sway and yaw, its response to the disturbance of a passing ship will depend

Letter to

the Editor

November, 1977 Dear Sir,

In the article of May 1977 covering the recent Isle of Wight Symposium, your cor-respondent makes the statement that

'Do-losse have a higherstability factor than any other artificial armour block.'

I beg to differ.

The statement may well be true for random placed units in two layers (I have no figures

for `Le dinosaure) but correctly designed patterned place units can easily outstrip

so eo

I 40 2

E

210,000 -dart loaded passing

tanker 150' (413ra)clearance

v-0.29 w-TA 1-117.28 Depth7S. (23m1 0 I 0 1 2 3 4 5 Wm speed (Iwes)

(8) Passing ship speed to pull moored ship dear.

on the ratio of these natural periods to the

time taken by the ship to pass. This enables the mooring loads due to a passing ship in a

current to be estimated from the toads

obtained in still-water experiments. It should perhaps be noted that the natural periods depend on the slackness or pretension in the mooring lines. For the largest passing

vessel in still water the maximum mooring

loads for a given clearance and passing speed were smaller, the greater the pretension: At

places where there is a large tidal range it is dearly easier to ensure in loading and unloading operations that the moorings are kept tight by ensuring there is a modest

amount of elasticity in the lines by inserting nylon or polypropylene tails.

Conclusions

A principal conclusion to be drawn from the tests was that the forces appear to be

due:to the pressure gradients associated with

random Placed units. The cob, I think, has an apparent stability factor of between 60

and 70.

Here in New South Wales, a new unit,

called the Seabee is currently under

develop-ment. A large number of model tests have

been run using simple alumininin test units,

which after density correction show a

stability factor in excess of 400. Using model

units to a scale of 1:20, we can only get up to apparent stability factors of 200 or so

with our apparatus, and the units are totally

inert. Experimental removal of units shows

that over 20 have to be removed in a lumped

group between two consecutive waves to induce collapse. Artificial holes of 15% -spread evenly over the surface have zero effect on stability. Due to the form of the unit, a linear relationship applied between the incident wave and unit stability, so that errors in wave data need not engender such panic.

According to our supporting theories, which are simply derived from existing

knowledge, such as Hudsons equation, units with an apparent stability factor in excess of

1 000 (one thousand) could be made if

required.

(Note. I use the word 'apparent' as it is

the .pauern of flow that accompanies the passing ship rather than with the wave system. This is because the length of the wave is short in comparison with the size of the moored ship and at low speeds the

wave heights are small.

The tests demonstrate-that slack lines are

to be avoided and that some relief in

maxi-mum line loads can be achieved by increasing the pretension.

Only a fraction of the results have been reported, 135 tests were done in alt, and where mooring loads are concerned only those for one rope are used. Consequently it would be inadvisable to apply them to

other sites. Netreithe eless the effect of speed of passing vessels, ship clearance and draught will vary in the way described.

Acknowledgements

This article is published with the per-mission of the Director of the Hydraulics Research Station. The authors would hlre to thank Mr L H Miller for his helpful advice in preparing this article for publica-tion

References

I FIRS Report EX566. Effect of passing vessels on a moored ship.

2 _{TAYLOR D W. 1909. Some model} experi-ments on suction of vessels. Trans. Soc.

Naval Architects and Mar. Engna, pp 3-21.

3 _{ROBS A M. 1949. Interaction between ships.}

TRANS RINA.

4 _{NEWTON 1CM; 1960. Some notes on}

inter-action effects between ships close aboard lc

deep water. First Symposium on Ship

ManoenvrabilitY. David Taylor Model'

No. 1461.

5 _{HIM Report 1968. Model investigation of}

wave disturbance and draw off effects on

container vesseb EX417.

preferable to use a coefficient ,denoting the

linear relationship between unit height and wave height rather than the cubic

relation-ship between unit mass and wave

height)Papers

on this unit have been submitted.

for consideration at both the Antwerp and Hamburg Conferences in 1978. A brief description of the device was given at the third Australian Coastal Engineering Con-ference in Melbourne last April (during a paper on a related topic) and was

sum-marised in the Journal, Engineers Australia, in June 1977.

Naturally, the unit is not tmpatented, and the writer, in conjunction with Uniscarch Pty Ltd and another are currently pursuing the full patents on a worldwide basis.

Of course there is a long way to go from the model to the prototype, but we hope

than another useful unit will shortly be

avail-able to all those beset by the rigours of the sea.

Yours faithfully,

C T. Brown.

Tillotson Brown & Partners,

avil & Structural & Coastal Engineers, St Ives,