A synthesis of experiments on the discharge of neutral density fluids from ships

Report Nr. 392

T

LABORATORIUM VOOR

SCH EEPSBOUWKUNDE

TECHNISCHE HOGESCHOOL DELFT

A SYNTHESIS OF EXPERIMENTS ON THE

DISCHARGE OF NEUTRAL DENSITY FLUIDS

FROM SHIPS

by

Ir. M.C. Meijer.

Prepared on the occasion of the meeting of

I.C.S., Chemical Carriers Panel,

much importance to be allowed to dispose of residues

at sea. For the organic life in the sea, such a dispoal may be detrimental, if the pertinent fluid is noxious in

the concentration existing at the instant when the organisms meet the contaminated water.

Three major questions have been posed:

Which concentration of noxious fluids (c ),, can be peak

allowed in a part of the water for long periods of

time?

During which period of time (t) may larger.concentrations be allowed locally with acceptable danger ?

Which is the rate of discharge allowed, so that after

time t the concentration 0peak is not surpassed ?

Sub 1.: Depending upon the chemicals involved,

concentration values of 10 ppm or 1 ppm have

been mentioned by biologists and abcepted by

rule-makers.

Sub 2.: The time duration t has so far not been

determined.

Sub

3.:

The allowable rate of discharge has been the subject of several experimental investigations

2.

with the main object to determine the rate of dilation in a ship's wake.

2) The dilutioh experiments.

The following experiments on the dilution of discharged effluent are known to the author.

2.1 Full scale experiments with

Kthe-H", aoatál shi

discharging effluent over the stern into

thTI-stream of the single screw. The concentration has been measured by a ship following in the wake.

(refer3nce 1)

2.2 Full scale experiments with the sma].l tanker"Esso Bergen'; crossing a line (fixed relative to the water)

at full speed. The concentration has been measured very

carefuilly in the vertical plane through the marked line,.

The maximum concentration values have been found long after passage of the ship; thus the t values are lage.

(reference 2)

2.3 A simple model experiment with a coastal model in the Deift University towing tank. The concentration

measurëmeitook place at a constant distance behind the model. The conditions at measurement are stationary this way. The time after passage (t) depends on the

model speed (T):

fx) fl

t = , if(x= distance of point of measurement behind

the model.

The effluent was discharged through 2 openings in the

forward cofferdam, as opposed to all the. other

experiments, where the discharge holes were single and further aft. Moreover in this test, no screw has been

fitted.

(reference ₃₎

2.4 Extensive model tests by the Davidson Laboratory of Levens' Institute of Technology, USA, both in a recirculating water channel and ma towing tank. The model has been chosen to represent a relatively large

chemical tahker.Tgaiñ the concentration has been measured at a constant distance behind the model. The

influence of many geometrical variations has been

established. (reference ₄₎

2.5 Full scale tests with the larger tanker "EssoSlagen" by the ]nstitutt for Atomenergi in KjeIier, Norway.

The experiment was exactly like their former: test with the 'Esso Bergen"., except for the goal to keep the time

t as small as possible and for the fact that the ship

speed has been restricted. (reference _.5)

A note on the tests 2.2 through 2.5. has been published by the Ship Research Institute of Norway. (reference 6)

3)? Boundary layer and wake flow.

In accordance with reference ₄ and, others, the author,

believes that adequate mixing of fluid occs only in

turbulent rhotion. In the case of a ship, moving through

the water, this turbulence Is generated in the so called "boundary layer", which is the region immediately adjacent

to the wetted surface of the hull, where- the water particles are accelerated by friction.. With the large size of ships

and sufficient speed (even less than 1 knot) the accelerated water layers have turbulent characteristics. One characteristic

is, that water particles move perpendicular to the tirné

-average direction of flow, which is the main condition lrnecessary for a mixing process.

The thickness of the boundary layer grows with distance

behind the ship' s stem. With this- thickness the capacity of

the surrounding water to contain foreign matter grows.

Behind the ship, there is a mass of water following the ship as a result of the former acceleration. The turbulent motion persists for a long time because of the high inertia (density) and the low viscosity (friction) of water. This wake flow carries On accelerating surrounding water which was at rest before, so the wake goes on widening. However, because the

PRINCIPLES OF O ERY LAYER A1D OF WAKE

Boundery layer

J

r Wake

Boundery layer theories - - Jet theories

=- x2____

wake has no engine to replace theenergy which has been yielded to the neighbouring fluid, the original part of the

wake decelerates.

Both for the boundary layer and for the wake flows it has been

found that the width of the turbulent region may be desribedj

by power laws or logarithmic formula'ç _k1eference

_4).

The slipstream of an efficient propeller is not turbulent by

nature. It consists of' vortices of which the axes are oriented inV? _{the local stream direction. For this reason mixing}

characteristics may not be attributed to the propeller action. his is in accord:nce with certain findings in reference

_4.

Of course the propeller blades generate their own turbulent wake, but this is the smaller part of the total wake of the ship.

4) Analysis of test results.

From all the available sources everaltest results have been selected to represent truly maximum concentration values.

Contrary to what was the starting point in reference _3, in the

present analysis the concentration in the boundary layer region

is considered uruimportant, but its maximum value is supposed to

be prescribed at sach a finite time (t) after discharge, that most of the mixing may be assumed to occur Ln the true wake

behind the ship. In this case it is thought to be correct to neglect differences of ship ge6metry and to represent the size and all dimensions o the ship by the value of the length (,L).

This simplification has been proposed in reference ₄ and is

--This represents the non dimensional width of.the "container". Assuming that this nomipal width of the wake indeed is

related to some power of the distance to its source, the logarithm of.equation 2 must be compared with the

(2)

5.

In an attempt to catch the results in one single and

simple. formula, the following basic thoughts have been worked out

The volume flow rate of the chemical at discharge is

CD _D' (time : t = 0) with

= concentration at discharge of the wash water = volume flow rate Of the wash water at discharge.

This volume of chemicals which flows in the sea during every unit of time is also the volume which is redeived

by the. wakO in the unit of time. During this time the

length of the receptacle (the wake) is increased with V units Qf length (v = speed) at the time t, the width. and height of the wake has reached a certain size and

considering the fact that the concertration is not equal

in every po(int of its cross section, we may define some

cross sectional area containing the amount of chemicals which is present along the length V in the unit time

(Aw)t =

V

Because the size of the ship is important, the size of the wake is made non dimensional by the length L, thus,

logarithm of this distance

or, made non dimensional =

The logarithms have been plotted in figure 1 for the

selected test results (the question marks should be

delete.d). It must be noted that the results taken from the model experiments are real model-results which have not been extrapolated to some full scale value. In fact the knowledge to perform an extrapolation in the wake region is only now being developed and should not be

influenced by experience only valid for the boundary layer region (see also reference

6).

5) Results.

Neglecting two extreme points in figure 1, the drawn line shows the lower boundary of all points and thus represents the extreme as regards the dilution capability of all tested vessels. The line can be described by the following formula

Vt

= 0.4 log 2.7 CD _D log Cpeak V L2

(5)

(6)

in the the influence of speed, which may be of interest

middle for the.ship's officers which is equivalent to

0.002 0peak 0.4

T.16

The formula may bç used for discrete values and put in

other diagrams, for instance as straight line.s with log-log scales as is shown in fdtgure 2. Thi figure hows three different diagrams :

at left: a plot which shows the influence of the time t, which may be decided by biologists

7.

at right: the influence of the ship's size which is of interest mainly for the shipowner.

The diagams have been designed to fit the final concentration allowed in the wake : Cpeak = 1o_6

Note: CD = 1 should be acceptable, giving low discharge

speed (refereiice ₄₎

Note: The formula permits the use of any consistent

L Cpeak

V

x

= nominal width of the wake at time t

defined as in equation t..

= peak concentration a time t after ship' s

pas sage

concentration at discharge

(to)

= ship's length

= volume flow rate of discharging fluid

= _{time after ship's passage, at which Opeak} is measured

= ship' s speed

= distance of point of measurement o.f 0peak

REFERENCES

Abraham et al; "Full scale experiments on disposal of wash

fluids into propeller stream, of ship"

F.A.O. tech.conf. on marine pollution and its effects

on living resources and. fishing. Rome, Dec.

1970

Final Report on Pollution caused by the Discharge of Noxious Liquid Substances other than Oil through normal Operational Procedure of Ships engaged in Bulk Transport"

IMCO, 1973

M.C. Meijer and A. Goeman; "Model test on Discharge of

Fluid with N:eutral Density in the Boundary Layer of a

Light Ship'!

Shipbuilding Laboratory Rep. Nr. ₃₈₄ Delft University Of Tech. Sept. _1:973

Merôier, Hires and Wu; "Model Study of the Dilution of Soluble Liquids Discharged from Tankers"

Stevens Inst. of Tech.,

₁₉₇₃

"Dilution in the Ship's Wake of Tank Washings Released. from NIT "Esso Slagen",

Institute of Atomenergy, 1973

6. "Note on the Dilution of Liquid Chemicals Discharged

from Tankers";

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Motivation of future tests on mixing of effluents behind ships.

INTRODUCTION

The Chemical Carriers Panel of the Royal Netherlands Shipowners' Association

has asked the author to produce a motivation for future tests on mixing of

effluents discharged from ships. A reason for this request is the opinion of

the author that an improvement of the dilution rate with a factor of 2 may

be possible if the effluent is discharged from more than one hole simultaneously.

Such an improvement may put the owners of small ships in a less unfavourable

position.

On the other hand it is felt, that the environment is insufficiently protected

if certain aspects remain unguarded, which otherwise might prove to have an important influence on the subject.

The following explanation may be considered as an appendix to the lecture given by the author at the ICS meeting of February 18, 19114 in the Hague (report no.392

Shipbuilding Lab. Deift). Numbers between brackets refer to the

li5t

given at the end of that report.

EFFICIENCY OF CONCENTRATION DISTRIBUTION

Consider the concentration distribution along the width of the wake (direction y)

at a certain distance behind the ship x and depth below the water surface z

(C11)=f(y)

as a result of discharge from a single position, say at starboard, off the ship.

It may then be expected that, disregarding the asymmetry of flow in the screw

race,if the discharge opening had been situated at the other side (say port),

the concentration distribution may be described by the symmetrical function

(c ) =f(-y)

local x,z

if the origin of y is taken in the plane of symmetry.

If the effluent would have been discharged off both sides at the same time, the

distribution in the wake should be determinable by addition of the former

functions

(c ) f(y)+f(-y)

local x,z. (See fig. 1).

Now we may define some efficiency

(ti)

of distribution across the real wake-cross sectional area (A,) (as opposed to A, used in previous repor'ts) as

follows :

r I

JJ

(c1)dydz

nx= _Cpeak(AR)x

If it is recollected that concentration is defined as

c

_bc

local volume of chemicals per volume of water and c =peak volume of peak

chemicals per volume of water, than it is seen that the efficiency conception

replaces the ratio of the total volume of chemicals which is found to be

distributed in the wake, to the maximum amount which is supposed to be possible

with the same peak concentration value.

3. EXAMPLE

From fig. ₅ off the Esso Slagen report,

₍₅₎

the efficiency has been calculated, assuming that the real wake area A, _{equals the total area covered by the figure}

(AR=900 m2). Also the efficiency has been calculated under the assumption of discharge from two symmetrically arranged ports. The results are as follows:

single discharge, as reported : n.10.20

double discharge assumed symmetrical

Thus from one single full scale test the probability has been shown that by

symmetrically doubling of the discharge location, the efficiency of the

concentration distribution is increased by a factor of n2/n1=1.T5.

The magnitude of the efficiency obtained is rather small (0.35), which is

reason to believe that more gain may be expected by distributing several

dis-charge ports along the girth of the ship's hull.

4. DESIRABILITY OF FUETHER TESTS

The tests which have been performed so far, concern the rate of dilution of

effluents behind a ship, mostly discharged from a single port. In one case

(Deift model test) two discharge ports have been utilized, but very few

variables have been considered (3).

The Stevens Institute of Technology tests

(4)

have shown that the position of the peak concentration in the wake depends quite much on the locality of the

discharge port, but as far as known to the author no discharge from more than

one hole at the same time has been tried.

It may be considered sufficient information for the present cause to determine

the influence of simultaneous discharge, by simple superposition of the various

results obtained at "Stevens, but consideration should be given to the

following aspect, which is also true for the Deift test:

The size of the tested models has been such (L<3m) that the test Reynolds Number

has been of the order

R3x106,

whereas the ship's comparable condition is

characterized by

RRfl.3x10'

if a ship of T5 m length is considered.

A consequence of this is, that if a fully developed turbulent boundary layer

has been obtained, it's thickness along smooth parts of the hull will be 30%

too large along the model. The model size however is such, that most model

testing facilities will probably insist on using some turbulence stimulating

device, which adds even more to the boundary layer and therefore to the wake.

If a model of atout 6 m length is used, a calculation teaches that the bourdar:r layer aft, along smooth stretches is only about 8 too large and at the same time it may be agreed that turbulence stimulators may be left off for the pertinent

purpose (if the speed is not chosen too low).

In the case of very full oil tankers most ship model basins will advice to use

models of 12 m or more, or better even, to carry out full scale tests, In this

respect it must be remembered that full scale tests are the most reliable from

the point of view of hydrodynamics, but certainly not from that of measuring

techniques.

There is a third aspect to the problem of waste disposal at sea, which has had

little attention so far. This concerns the fact that the density (p) of the

effluent may differ from that of the receiving water. This aspect has up to

date only been investigated by Abraham et al, in a different set-up (ref. 1).

If much interest is taken in such high- or low density wastes, it seems

in-evitable that model tests are planned because the necessary number of test runs

is highly increased. Also it is much to be recommended that such tests are

accompanied by a hydrodynamicicist with experience in such flows, in which

Another aspect which may be important, especially in combination with high

density effluents, is density stratification in the sea (fresh water on top of

sea water, layers of different temperature).

CONCLUSIONS

There is good reason to expect that improvement of dilution characteristics can

be obtained by using multiple discharge ports, distributited along the ships

girth on both sides. The improvement is expected to be in the order of a factor

of 2.

There is reason to distrust the quantitative results of the model tests performed so far, due to the small size of the models. Models of at least 6 m leneth are recommended.

It may be necessary to perform new tests, in order to be able to judge the

influence of high- and low density effluents.

It is advisbie to perform new tests in order to bearned for the possible adverse influence of stratification of the receiving water.

Measured distribution at 3m depth C peak C peak / / / / / / / / / / / -.. -% f(y)+f(-y) C = local concentration bc. V V V V Possible distribution I I -50 -25 o +25

+50 _y

__..sideways position in wake

0 25 50 7.5 100

( scale of "Esso Slagen' report)

Fig.1. Principle of distribution improvement by discharge through both sides of the ship. (taken from fig.5 ref.5)

V

f(y)

I