Jip 4 At-sea monitoring of surface dispersant effectiveness

ability to penetrate through the oil to the oil-water interface where the dispersant, works to break

the oil into very small droplets. This is often known as the ‘window of opportunity’ for dispersant

use. There is no universally-applicable viscosity value for defining the limits of effective dispersant

use because the successful dispersion of oil will depend on many factors, such as the dispersant

used, the nature of the oil and the prevailing conditions. General guidelines on the probable

effectiveness of dispersant and oil viscosity are shown in Table 1.

IPIECA-IOGP Oil Spill Response Joint Industry Project

4

Table 1  Generally accepted ranges of the effect of oil viscosity on dispersant effectiveness 

Light distillate fuels 

(petrol, kerosene, diesel oil)

Oils with viscosity up to 5,000 cSt

a

Oils with viscosity between 5,000 and 10,000 cSt

Oils with viscosity above 10,000 cSt

Dispersant use is not advised.

These oils will evaporate and naturally

disperse or spread rapidly to very thin sheens

in most conditions.

Dispersant use is likely to be effective

Dispersant use might be effective

Dispersant use is likely to be ineffective

(though success is reported on oils with

viscosity greater than 20,000 cP)

Oil that is at a temperature that is significantly (10–15°C) below its pour point will be semi-solid

and will not flow, meaning that dispersants cannot penetrate the body of the oil and are therefore

unlikely to be effective.

To assist with contingency planning and the selection of appropriate response techniques, specific

physical properties of the oils that might be spilled should be available along with the results of

weathering and dispersibility studies, if previously conducted. Facilities for the computer modelling

of the fate of oil are often available and can make predictions concerning changes in oil

properties, including viscosity, over time under different environmental conditions. These modelling

efforts can provide supporting information about the likely effectiveness of dispersants.

a

cSt = centistoke, a unit of kinematic viscosity: 1 cSt = 10

-6

m

2

/s

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Laboratory tests

Laboratory tests have been developed to measure dispersant effectiveness,

primarily for product approval purposes. These tests are able to identify those

products which have, as a minimum, appropriate effectiveness at dispersing oil,

thereby avoiding the approval of poorly performing products. The principles behind

the majority of effectiveness tests are very similar:

i.

A known quantity of test oil is added to a known quantity of seawater in a flask

or tank.

ii.

A specified quantity of dispersant is added and allowed to soak into the oil.

iii. Mixing energy is applied using a choice of method (e.g. flask rotation, an

oscillating hoop, a shaker table) to mix the dispersant-treated oil into the seawater.

iv. After a specified period of mixing, a sub-sample of the dispersed oil in water

mixture is withdrawn and the oil content measured.

None of the laboratory test methods can simulate all the complex mixing scenarios

and energies encountered in the marine environment. Prevailing wave conditions at

sea can vary over a wide range from flat calm to very rough. While one laboratory

test method may superficially resemble a particular sea state more than another, an

accurate simulation of oceanic conditions will never be possible due to low

turbulence and lack of dilution. While this disadvantages apparent dispersant

effectiveness, this ‘stress-testing’ makes it easier to discriminate between the

effectiveness of various dispersants.

The results of laboratory testing, typically expressed as a ‘percentage effectiveness’,

should only be used to compare the relative effectiveness of different dispersants

under the test conditions. The main difference between the various tests is the

amount of mixing energy applied; for example, some tests simulate relatively calm

sea states whereas others are more representative of moderate/more common sea

states. An evaluation therefore needs to be made to determine whether the mixing

energy used in the effectiveness test is representative of the sea state in the subject

area. In any case, most tests will be able to discriminate between poor products

and more effective ones under the test conditions. It is important to note that many

accepted tests utilize considerably lower levels of mixing energy than those found in

the majority of sea areas where oil-related operations are carried out, and therefore

may not be representative of dispersant effectiveness at a given location.

This arbitrary pass mark in some tests should not be interpreted as being an

indicator of dispersant performance in the field. For example, the UK pass mark of

60% in the WSL (Warren Spring Laboratory) test method using a medium fuel oil

does not indicate that only 60% of the oil would be dispersed and 40% would

remain. The proportion of oil dispersed at sea could be 100% or less, depending on

prevailing conditions.

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At-sea monitoring of surface dispersant effectiveness

Testing the effectiveness of dispersant

Above: these sequential images

show the aerial view of a single wave

passing through dispersant-treated

oil; note the formation of the light

brown cloud, indicating successful

use of the dispersant.

Sour

ce: OHMSETT test performed by the USA gover

n

ment.

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