66 
Colcomb - Operational experience in the UK and sea trials 
forces caused by a breaking or cresting wave. Instead of forming oil droplets, the 
spilled oil layer is temporarily distorted and deformed, but subsequently retains its 
coherent form. This is, to some extent, sea-state dependent; rougher seas with 
more frequent and more intense breaking waves are more capable of creating oil 
droplets than calmer seas. 
These two effects are often congruent and it has not been possible to say which is more 
dominant. The practical effect is to create a limiting oil viscosity for effective dispersion. This 
is of operational significance to oil spill responders since it imposes a limitation of the use of 
dispersants as an effective oil spill response method. 
Attempts to correlate results with laboratory testing of oil spill dispersants with performance 
at sea have been difficult because of the inherent limitations of laboratory test methods; none 
of them can ever be said to be an accurate simulation of the mixing conditions at sea. In ad 
dition, the wave conditions at sea vary over an enormous range from flat calm to severe 
storms and, although a particular lab test method might simulate some aspect of some sea 
condition, it has not proved possible to correlate any lab test to any particular sea-state. 
The work described in the trials final report was a ’return to the basics’ of using dispersants; 
an attempt to use a matrix of oil viscosity, dispersant brand, dispersant treatment rate and 
prevailing sea conditions to provide information on the limiting oil viscosity of dispersion by 
using a very simple method of visual observation to determine whether dispersion was or 
was not occurring. 
The main findings - under the conditions of testing which were a sea temperature of 15 e C, 
producing oil viscosities of 2,000 cP (IFO-180 grade fuel oil) and 7,000 cP (IFO-380 grade 
fuel oil) and waves associated with wind speeds of between 7 and 14 knots - were that: 
(i) The IFO-180 fuel oil appeared to be totally and rapidly dispersed by Dispersant C 
used at a nominal DOR of 1:25 at 12 knots wind speed. Dispersant B and Disper 
sant A appeared to be somewhat less effective, but still caused moderate disper 
sion when use at a nominal DOR of 1:25. At lower wind speeds of 7 to 8 knots, 
Dispersant C at a nominal DOR of 1:25 was seen to be less effective, but still ap 
peared to cause moderately rapid dispersion of IFO-180. 
(ii) The IFO-380 fuel oil did not appear to be rapidly and totally dispersed by any of the 
three dispersants when used at any of the treatments rates, ranging from nominal 
DORs of 1:25 to 1:100 at wind speeds of 7 to 9 knots. At wind speeds of 13 - 14 
knots, the performance of both Dispersant B and Dispersant C at a DOR of 1:25 
improved to produce moderately rapid dispersion of IFO-380. The performance of 
Dispersant A was less than that of the other two dispersants, but was not tested at 
the highest wind speeds. 
Many more important findings are contained within the main sea trials report (Lewis 2004). 
Comparison of the results from the sea-trials with results obtained using the WSL test meth 
od (the efficacy test used for the approval of dispersants in the UK) showed that a high level 
of visible dispersion was only achieved at sea by those combinations of test oil, dispersant 
brand and dispersant treatment rate that produced over 80 % WSL results and that moderate 
visible performance was achieved by combinations that produced over 60 % WSL results. 
These WSL result ‘thresholds’ are applicable to wind speeds of between 7 and 14 knots. 
The report concludes that some oil spill dispersants will be an effective response to oils with 
viscosity of 2,000 cP, but will not be effective on oils with a viscosity of 7,000 cP or more, in 
waves associated with wind speeds of 7 to 14 knots. The precise limiting viscosity between 
2,000 and 7,000 cP is not known. The limiting viscosity will increase with wind speed; it is