Jip 4 At-sea monitoring of surface dispersant effectiveness

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Mesoscale tests

Mesoscale dispersant tests are typically conducted in wave tanks or flumes. These have an

advantage over bench tests alone in that operational effectiveness can also be included.

Dispersant effectiveness is calculated by either assessing how much oil is remaining on the

waters’ surface, or by understanding how much has dispersed into the water column. Calculating

how much oil is dispersed into the water column can be ascertained either through the analysis of

discreet water samples or by using fluorometry. However, the tanks are of a fixed volume and

incur boundary limitations that skew the results compared to the realities of open water

dispersant use.

Determining effectiveness at sea

The results of dispersant effectiveness tests produced in the laboratory and expressed as

‘percentage effectiveness’ can lead to the expectation that similar results could be produced in

real oil spill incidents. This is not currently possible for a variety of reasons, but in many cases the

effectiveness of dispersant application in the field will exceed that observed in laboratory tests.

The ‘percentage effectiveness’ of operational dispersant use on spilled oil on the sea surface

could be quantified if it were possible to accurately measure:

1. how much oil is present on the sea surface before dispersant use;

2. the reduction in the amount of oil remaining on the sea surface at various times during

dispersant use; and/or

3. the amount of oil dispersed into the sea at various times during dispersant use.

These requirements cannot currently be met during oil spill incidents, hence it is not possible to

calculate the ‘percentage effectiveness’ of dispersant use during at-sea operations. 

Measuring the amount of floating oil

In many oil spill incidents at sea, the amount of spilled oil on the sea surface is not known and

can only be estimated from its visual appearance using the Bonn Agreement Code


or by

determining the amount of oil lost from the damaged vessel, pipeline, tank, etc. It is currently not

possible to accurately measure the amount of floating oil by visual means or by using remote

sensing techniques such as UV (ultraviolet), IR (infrared), MWR (microwave radiometry) or satellite

imagery. The amount of oil floating on the surface before the application of dispersant is therefore

difficult to quantify and, as such, the most effective dispersant-to-oil ratio (DOR) is also difficult to

quantify, but may be estimated for a larger area.

IPIECA-IOGP Oil Spill Response Joint Industry Project



The Bonn Agreement Oil Appearance Code (BAOAC) is a series of five categories or ‘Codes’ that describe the relationship

between the appearances of oil on the sea surface and the thickness of the oil layer. See:


In most cases this makes it difficult to determine the exact dispersant-to-oil treatment ratio and,

as a consequence, it is impossible to calculate the effectiveness of dispersion application in terms

of ‘percentage effectiveness’.

An accurate measure of reduction in the amount of floating oil that may remain following

successful dispersant use is also difficult to determine for the same reason, but estimation is

possible using aerial surveillance.

Measuring the amount of oil dispersed into the sea

UV fluorometry is a technique that can be used, with suitable calibration, to measure the

dispersed oil concentration at locations in the water column. However, the behaviour of dispersed

oil at sea makes it impossible to use UV fluorometry results to construct a ‘mass balance’ that

would enable a ‘percentage effectiveness’ to be calculated. 

Localized plumes of dispersed oil droplets are created as breaking waves pass through the

dispersant-treated slick. The concentration of oil (as droplets) in the upper water column rises

rapidly to a peak of between approximately 50 and 100 ppm at these scattered locations, but

then swiftly decreases as (a) the smaller, permanently dispersed oil droplets are diluted into the

surrounding water, and/or (b) as the larger, non-dispersed oil droplets float back to the sea

surface. As the oil slick drifts under the influence of the wind, wave action will cause localized

plumes of dispersed oil to be produced in the water column at locations that are some distance

from where the previous plumes were produced and subsequently diluted.  

A further complication is that the plume of dispersed oil may be moving at a different speed and

in a different direction to the surface slick, meaning that sampling under the plume may not yield

reliable data.

Currently, no techniques for measuring oil-in-water concentrations are available that can be

deployed with enough resolution in space (at least 1-metre intervals in all three axes) or in time

(multiple measurements would be required at all locations under a slick almost simultaneously) to

accurately quantify the total amount of dispersed oil at any time.

Open water experiments

It is possible to estimate, although not quantitatively determine, dispersant effectiveness in

carefully controlled sea trials by comparing the behaviour of a dispersant-treated test slick with an

untreated control slick. This enables the relative effectiveness of dispersant use to be compared

with the consequences of no dispersant use.

Open water (at sea) experiments have been conducted by various organizations, primarily in the

1980–90s. They involved controlled releases of oil onto the sea, followed by treatment with

dispersant. The nature of these experiments allowed scientific monitoring and observation of

dispersant effectiveness to be scheduled, planned and executed in a manner not usually possible


At-sea monitoring of surface dispersant effectiveness

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