Environmental Statement - Maersk Oil

6.3.1. STOCHASTIC MODELLING
6 ‐ 6
Balloch Field Development Environmental Statement
Section 6 Accidental Spills
Stochastic modelling involves using a variety of wind and current data to simulate the most realistic
probability outcomes of a spill event. For the blowout scenario the stochastic models were run for
the time it is expected to take to drill a relief well (90 days), with an additional 30 days added in order
to model the fate of the oil after the spill has been controlled (total = 120 days).
A stochastic analysis of the uncontrolled flow rate during the two blowout scenarios and loss of diesel
inventory was undertaken by modelling 40 scenarios utilising a broad range of weather and current
data. Table 6‐5 shows the locations and main parameters for modelling undertaken at each of the
drilling locations.
Table 6‐5 Main parameters used for modelling of oil spills.
Scenario Coordinates Volume Gas:Oil Ratio API
Diesel
inventory
loss
Balloch well
blowout
58 o 22’18.035’’N
00 o 53’03.319’’E
58 o 22’18.035’’N
00 o 53’03.319’’E
6.3.2. DETERMINISTIC MODELLING
8,642 bbls
(in 1 hour)
124,000
bbl/d
Release diameter
(inches)
‐ 36.4 ‐
87.3 scm/scm 40 9 5 / 8
In addition to the stochastic analysis, a deterministic analysis was also undertaken for the worst case
scenario of oil beaching identified in the stochastic modelling, in order to gain a more detailed insight
into the fate of oil and to understand the water column and sediment distribution of the oil.
Additionally, the fate of hydrocarbons in the presence of unvarying offshore and onshore winds at
30 knots was modelled for both cases to determine the fate of each spill, according to DECC
requirements. Air and water temperatures from January were assumed in order to represent worst
case parameters.
6.3.3. IMPACT ASSESSMENT CRITERIA
The environmental resources within the vicinity of the Balloch field have been identified and assessed
for their susceptibility to oil spills. Seabirds and fisheries are not normally affected by routine
offshore oil and gas operations, but are among the environmental aspects most at risk in the unlikely
event of an oil spill. The overall impact of spilt oil on the marine environment will vary seasonally due
to variations in species abundance and behaviour. Potential effects on fish populations from spilt oil
will be greatest during periods of fish spawning. In the event of a very large oil spill, fisheries may be
closed as a result of fish ‘tainting’. Seabirds spending time on the sea surface are also vulnerable to
oiling following a spill. Cetaceans passing through the area may also be vulnerable to oil spills.
Drilling of the Balloch well is currently scheduled to begin in Q4 2012. Environmental sensitivities
noted within the block during this time are:
Fish nursery/spawning periods for Nephrops, Norway pout and blue whiting;
The Offshore Vulnerability Index (OVI) for seabirds in the area is very high in
November.
The assessment criteria used to evaluate the environmental sensitivities in this section are
outlined in Table 6‐6 and Table 6‐7.