Environmental Statement - Maersk Oil

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Balloch Field Development Environmental Statement
Section 6 Accidental Spills
6.4.2. SCENARIO 2: UNCONTROLLED WELL BLOWOUT FROM A SURFACE RELEASE FOR THE FIRST TWO DAYS,
FOLLOWED BY A CONTINUATION OF THE RELEASE SUBSEA
This scenario represents a blowout that occurs initially through the semi‐submersible drill rig.
Surface oil modelled outputs
The model outputs are summarised in Figure 6‐7. In general, much of the oil from the surface release
of the diesel travels in an easterly direction towards Norway. Norwegian waters are at high risk
(100 % probability) of an oil slick being present at some time during the period. A very small
percentage of the release is predicted to travel northwards outwith the modelled area. The slick also
travels in a southeasterly direction putting Danish waters at high risk of a slick (60 ‐ 70 % probability)
being present at some time. German and Dutch waters are at a low risk (10 % probability) of a
surface spill being visible. The flow of hydrocarbons from a Balloch well blowout when the entire
release is from the seabed is 19,700 m 3 /day. This is a worst case scenario as it is expected that the
flow of oil would decline over time.
Shoreline modelled outputs
There is a possibility of hydrocarbons beaching on Shetland, Orkney, mainland UK, Norway and
Denmark following a 90 day subsea release. 3,285 tonnes are predicted to reach the coastline, which
could represent 16,425 tonnes of emulsion (based on 80 % water content predicted by the
weathering analysis). The minimum time to reach shore is 11.8 days. The hydrocarbons reaching the
Norwegian coastline are more likely to be in the form of dispersed oil in the water column rather than
a surface slick. The graphs in Figure 6‐8 show the amount of oil beached and the time it takes to
beach, as well as the cumulative percentage of scenarios in which this happens. They also provide a
summary of the modelled outputs.
Water column and sediment modelled outputs
Water column concentrations are not expected to exceed 50 ppb beyond approximately 450 km from
the release, except very locally around patches of surface oil. Sediment concentrations of around
1700 mg/kg (peak) and 30 mg/kg are predicted, which again could be higher locally.
Oil is predicted to deposit into sediments where there is dispersed oil in the water column in contact
with the seabed, according to partitioning algorithms in the model. Consequently, oil is predicted to
deposit in sediments in the shallow water either side of the deep trench near Norway but not actually
within this trench, as shown in Figure 6‐9.
Fate of oil and fixed wind analysis
The fate of the hydrocarbons associated with a 2 day surface release and subsequent 89 day
subsurface release is shown in Figure 6‐10. This shows that 30 days after the release has been
controlled, approximately 45 % has decayed, 25 % has evaporated, 25 % is in the sediment and less
than 5 % is dispersed in the water column.
Fixed wind analyses predict that in the presence of 30 knot offshore winds, hydrocarbons will cross
the median line in 1.125 days and in 9 days in the presence of 30 knot onshore winds. The outputs
are summarised in Figure 6‐10.
Typical manifestation of oil during release
Figure 6‐11 presents instantaneous snapshots of the appearance of the surface slick and water
column plume. The surface example is chosen to represent a period of relatively calm winds that
allow oil to accumulate on the surface, which in this scenario occur on day 60, i.e. it is a relatively
pessimistic prediction of the extent of the surface slick. The water column example is chosen at the
end of the release period, after which concentrations would be expected to decline.
The cross section image shows the oil accumulating in the water column above the release site on the
seabed. Oil does not appear in the entire water column upon release from the well; it moves rapidly
up towards the surface where it then accumulates.