Guidelines for the use of dispersants for combating oil pollution at sea in the Mediterranean region
Part II: Basic information on dispersants and their application
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Page 18
mammals to oil can lead to changes in the ability of animals to deal with the uptake, storage
and depuration of hydrocarbons whilst acute exposures can result in mortality in particular with
young mammals which are more susceptible to the toxicological effects of oil.
Oiling causes reduction in fur insulating capacity and dispersants have been experimentally
used for the removal of crude oil attached to fur. These experiments resulted in the removal of
natural skin oils together with crude thus destroying the fur's water-repellency. Surfactants can
increase the wettability of fur or feathers, allowing cold water penetration and subsequent
increase of the thermal conductance. This is particularly dangerous to animals that are buoyed
or insulated by their fur or feathers. Records of animal deaths due to direct ingestion of oil
during grooming also exist. Extremely limited information on the influence of dispersants or
dispersed oil on marine mammals exists, nevertheless the use of dispersants may not reduce
the physical threat of spilled oil to some fur-insulated sea mammals.
7.4
The use of dispersant on underwater oil releases (e.g. blow out)
Dispersant can be used to disperse under water releases such as blow out coming from sub
sea well head.
This technique has been used
recently during the “Deep Water Horizon” incident
, in the Gulf of
Mexico in 2010, where large quantities of oil were released at sea (high pressure / high shear
rate). Large quantities of dispersants have been injected directly at the source of the spillage at
the sea bottom (1300 m depth) into the damaged riser to disperse the oil just released (fresh),
in order to:
reduce the quantity of oil resurfacing from the damaged well;
reduce the amount of volatile in the atmosphere close to the damaged well (for
security reason);
reduce the amount of oil liable to be drifted to the sensitive Louisiana shoreline
(environmental issue).
The formation of a large plume of dispersed oil between 1100 and 1300 m, with low oil
concentrations has been observed. At the time this text has written there was still debate on
the effects (efficiency and impact) of this peculiar dispersant application.
In terms of efficiency, uncertainties remain on the real effect of dispersants (e.g. what was
really dispersed by the dispersant and what would have been naturally dispersed).
Waiting for further investigations, the preliminary feeling of the scientists community
is “that the
use of dispersants and the effects of dispersing oil into the water column has generally been
less environmentally harmful than allowing the oil to migrate on the surface into the sensitive
wetlands and near shore coastal habitats” (“Deep Water Horizon” Dispersant Use Meeting
Report
–
May 26-27, 2010 CRRC).
It should be highlighted that the usual recommendations for regular dispersant application on
surface slicks may not be suitable to the sub-sea application of dispersant on a sub-sea
blowout plume. In sub-sea application the oil is fresh with its light ends, (the most toxic
fractions) while surface slicks are usually partly weathered. However, considering ultra-deep
environment the environmental conditions are so different (temperature, ecological sensitivity
and diversity, temperature… etc.) in comparison with surface water (photic zone) that the usual
way to assess the possible impact of chemical dispersion (as described in Part II and Part III of
these Guidelines) is not applicable as they are (e.g. regular NEBA process as described in the
next chapter 8).
Guidelines for the use of dispersants for combating oil pollution at sea in the Mediterranean region
Part II: Basic information on dispersants and their application
–
Page 19
If required, this chapter on sub-sea dispersant application could be further expended in a future
edition of the Guidelines when the results of the ongoing studies on the Deep Water Horizon
incident and its consequences will be made available.
Figure 13: Under water blowout in ultradeep environment
during Deep Water Horizon incident (source SINTEF)
8.
NET ENVIRONMENTAL BENEFIT ANALYSIS (NEBA)
As the aim of the spill response is to minimise the overall environmental impacts on natural and
economic resources, the decision on the use of dispersants should be based on the following
comparison: “
What would be the impact of the pollution when treated with dispersant and
when non treated with dispersant?
”.
This comparison is named: the NEBA
2
(the Net Environmental Benefit Analysis).
The NEBA should:
consider the behaviour (drift and ageing) of the treated oil (drift according to the
stream, speed of dilution of the plume) and of the non treated oil (drift according
to the stream and the wind);
assess consequently the different resources which will be concerned either by
the treated oil or by the surface oil;
assess the sensibility of the different resources at concern towards the
dispersed oil and toward the surface oil (non dispersed);
consider also the time frame of the restoration of the items which may be
impacted.
These analyses assist decision makers when considering whether or not the use of
dispersants is appropriate, to minimize the environmental/economic damage.
Often, the NEBA can lead to compare the damage of the oil dispersed at sea with the damage
of the slick drifted on the shoreline.
In most cases, the damages at sea caused by the dispersed oil are less than those generated
by the oil weathered (often persistent) and stranded on the shoreline. However, the closer it
affects the shoreline, the more difficult is the NEBA, as depending on the response strategy,
2
The concept of NEBA can be found in the literature under different names : NEEBA (Net Environmental &
Economic Benefit Analysis, or NEDRA (Net Environmental Damage Risk Assessment), etc….