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
2
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
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2
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:
www.bonnagreement.org/site/assets/files/3952/current-status-report-final-19jan07.pdf
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
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At-sea monitoring of surface dispersant effectiveness