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Hunt - Operational experience worldwide
70,000 tonnes of Nile Blend crude oil. The grounding damaged a number of cargo tanks and
estimates put the spill size at around 7,000 tonnes based on ullage measurements. With
almost no wind, the oil moved with the tides back and forth along the length of the Singapore
Strait, eventually contaminating shorelines in all three countries bordering the Strait: Singa
pore, Indonesia and Malaysia.
A response was mounted by the ship's managers, under the direction of the Singapore Mari
time and Port Authority (MPA), and the at-sea response initially consisted of the aerial appli
cation of dispersant, which is understood to have been judged successful by those on site at
the time. Further application was requested by MPA, but due to the oil's high wax content
and pour point (33-36 q C), the ambient sea temperature (28°C) and the effects of more than
24 hours' weathering, it was considered highly unlikely that this would be effective. Since the
sea surface temperature was 3°C below the pour point of the oil, the viscosity was estimated
to be considerably greater than 50,000 centistokes (cSt) and therefore way beyond the enve
lope of effective dispersion (i.e. typically up to around 5,000 cSt, and not greater than
10,000 cSt). The situation was further exacerbated by calm weather conditions with little
wave energy to promote effective dispersion.
No previous testing had been carried out to assess dispersant effectiveness on this little
known crude oil but visual observations showed that the oil was semi-solid as early as the
first day after the spill. A comprehensive evaluation of subsequent dispersant spraying op
erations from boats was conducted by two independent scientists from the UK on 5 th October
2000, along with ITOPF technical staff on site. This investigation included aerial surveillance
and the use of in-situ fluorometry. The results of the fluorometry verified the visual observa
tions and predictions based on the oil properties and it was concluded that the spilled oil was
no longer amenable to dispersion.
The strategy for oil pollution counter-measures at sea thus turned to containment and recov
ery. A number of obstacles still had to be overcome however: the semi-solid nature of the oil
along with the associated heavy debris presented severe problems for skimming and pump
ing, whilst the lack of suitable vessels, particularly barges to receive the recovered oil, de
layed these operations by several days. Approximately 1,000 tonnes of oily waste was re
covered at sea, which was sent to Indonesia for disposal.
Bearing in mind the issues associated with application of dispersants in this case, the
NATUNA SEA spill is considered to be a good example of an incident that demonstrates the
critical importance of prior testing of dispersant effectiveness before conducting widespread
spraying, and the value of continuous monitoring and re-evaluation of the response strate
gies.
Case Study: TASMAN SPIRIT (Pakistan 2003)
When devising an appropriate spill response strategy it is important to take account of the
potential implications of each technique under consideration, and weigh these against the
likely benefits, before coming to a decision. This decision making process is often referred to
as a Net Environmental Benefit Analysis, or NEBA. The aim of this process is to mitigate the
environmental and socioeconomic damage from the oil spill as much as can be realistically
expected. This process is particularly pertinent when considering the application of disper
sants, since if the approach is carried out effectively, oil is removed from the sea surface and
transferred into the water column, and hence the fate and trajectory of the oil, and the re
sources it may affect can be rather different than for floating oil slicks or oil stranded on the
shoreline.
One particular case that demonstrates the NEBA process with respect to the potential bene
fits and negative implications of dispersant use is the TASMAN SPIRIT in Pakistan in 2003.
