Jip 4 At-sea monitoring of surface dispersant effectiveness

during accidental spills. The key outcomes and findings are consolidated and summarized as

follows:

l

All the experiments involved visual observation as a basic method to assess whether

dispersion is enhanced by the application of dispersant. Visual observation requires good

viewing conditions. Successful use of dispersant will cause the spilled oil to be transferred into

the water column as a light-brown (‘café au lait’) or reddish-brown coloured cloud, or plume,

which slowly fades from sight as the dispersed oil is diluted into the water. The plume of

dispersed oil may not be formed immediately, as wave action is required to disperse the

dispersant-treated oil. The absence of an immediate cloud does not therefore mean that the

dispersant is not effective. The plume of dispersed oil may drift under oil remaining on the sea

surface and be obscured from view. A milky white plume will be present if the dispersant has

missed the oil or has run off very viscous or highly emulsified oil.

l

Remote sensing, using a combination of side-looking airborne radar (SLAR), and IR and UV

sensing techniques, can provide additional information on the presence or absence of floating

oil. This can be coupled with visual observation to provide a higher degree of confidence in

the removal of floating oil after dispersant treatment.

l

With appropriate calibration, fluorometry may be used as a comparative technique to measure

the ultra-violet fluorescence (UVF) of oil in the water. The UVF signal at various water depths is

measured at locations where (i) no oil is present on the sea surface (background), (ii) oil is

present on the sea surface (natural dispersion) and (iii) where oil has recently been sprayed

with dispersant (chemically dispersed). Significantly higher UVF signals from locations under

dispersant-treated oil compared to either background measurements or signals from locations

under untreated oil indicate that oil has been dispersed into the water. Samples of the water

containing dispersed oil may be taken to calibrate the UVF signal, but UVF in these

circumstances cannot be quantitative because measurements are only made in a small

fraction of the water that could contain dispersed oil. It is therefore not possible to calculate a

‘percentage effectiveness’ value for the entire oil volume sprayed with dispersant.

l

All the sensing tools provide qualitative information concerning the presence or absence of

floating oil or oil droplets in the water column. The mechanism of dispersion by wave energy

and associated turbulence and currents invariably leads to a varying and uneven distribution of

droplets as an oil slick disperses. It is not possible to comprehensively and simultaneously

sample all areas under a dispersing slick in three dimensions, even during experimental

circumstances. Furthermore, it is not possible to accurately quantify either the volume of

floating oil prior to dispersant application or that which remains afterwards. Accurate mass

balance calculations are, therefore, not possible for dispersant operations; at best, only

estimations and approximations can be attempted.

l

The primary purpose and value of monitoring dispersant operations is to verify that dispersion

is being enhanced by the application of the dispersant. This supports the operational decision

concerning continued application or its cessation.

IPIECA-IOGP Oil Spill Response Joint Industry Project

8

Preparation of a

fluorometer to monitor

dispersant effectiveness.

Source: USCG 

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If contingency plans include the surface application of dispersant as a response technique, and

the circumstances of an incident lead to a decision to apply dispersant, it is important to establish

a monitoring programme to verify that the operation is effective. If the application of dispersant is

not capable of achieving the intended dispersion of floating oil, the exercise will be a waste of

resources and there will be no environmental benefit.

Mobilization of a response is typically a race against time. Operations will be most effective prior

to the oil spreading and fragmenting over a wide area of sea. Furthermore, oil weathering will lead

to the oil becoming more viscous and rendering dispersant either less effective or potentially

ineffective over time. However, there may be some scenarios where more time is available to plan

a dispersant operation and related monitoring, e.g. a vessel presenting the threat of a spill or an

ongoing release.

It is good practice to undertake a ‘test spray’ and/or field effectiveness test (see below), to help

determine and confirm dispersant effectiveness prior to full-scale deployment of the dispersant

spraying operation. In many spill scenarios, the initial monitoring will be limited to visual

observation. Only in larger-scale operations will additional monitoring techniques be available and

deployed. This is reflected in the so-called SMART Protocol described below, which was

developed in the USA but forms a reasonable basis for an operational monitoring programme at

any location.

It is the nature of oil spill response that flexibility and adaptability are essential for success. A

monitoring plan is dictated by factors such as the availability of equipment and personnel, the

on-scene conditions, and the window of opportunity for dispersant application. The need for

flexibility in monitoring design, effort and rapid deployment (possibly using a vessel of opportunity),

may dictate the nature and extent of the monitoring. The following should therefore be taken as

guidance to be adapted to an incident’s circumstances.

Shipboard field effectiveness tests

Relatively simple and portable kits have been developed that enable the testing of dispersant

effectiveness during an incident on board a vessel. While these tests do not replicate the open

sea conditions, the results are designed to provide operational decision makers with additional

supporting information concerning the viability of dispersant and its use in the emergency context.

The tests may also provide guidance on whether a particular dispersant product may perform

better than other products under prevailing conditions.

In essence, the field tests involve a simple but standardized procedure for the on-site collection of

small volumes of seawater and oil samples, and the addition of dispersant. This is performed in

glass tubes or jars (note that these are not the same procedures used in the laboratory for

efficacy testing). After the addition of shaking energy, the appearance of the samples with

dispersant added is compared with the appearance of the samples containing no dispersant. This

can provide an indication of possible effectiveness prior to, or in conjunction with, a test spray. An

9

At-sea monitoring of surface dispersant effectiveness

Operational monitoring

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