Environmental Statement - Maersk Oil

Balloch Field Development Environmental Statement
Section 6 Accidental Spills
Balloch fluid. The API and pour point of the SNORRE B oil type is 39.8 and ‐6 o C while those of the
Balloch oil are 40 and ‐6.1 o C respectively.
Bathymetry data is based on the Sea Topo 8.2 and IBCAO databases (Jakobosson et. al., 2008) which
are built into the OSCAR model.
Representative water column current data from 1990 and 1991, supplied by SINTEF for modelling in
the North Sea and northeast Atlantic areas, was used. This data covers currents varying over 16
layers in the water column up to 400 m water depth. Representative 2‐dimensional wind data from
1990 and 1991 supplied by SINTEF was also used.
Given the relatively shallow waters of the North Sea, the travel time between seabed and surface is
short. Because of this, salinity and temperature variation with depth have not been included as they
are not considered to have a significant effect on the predictions.
Model runs were undertaken to determine the probability of a surface sheen >0.04 µm, the
probability of a water concentration of >50 ppb and the probability of oil reaching any coastline. The
>0.04 µm surface thickness threshold was chosen as this is the minimum surface thickness identified
by the Bonn Agreement Oil Appearance Code (BAOAC) that is capable of producing a visible sheen
(Table 6‐4). BAOAC states that oil films below ≈ 0.04 µm thickness are considered invisible. Recent
research by O’Hara and Morandin (2010) suggests that a thickness between 0.04 µm and 0.1 µm is
also the point at which there is noticeable uptake of oil into bird plumage.
Code Description ‐ Appearance
Table 6‐4 Bonn Agreement Oil Appearance Code.
Layer thickness Interval
(µm)
Litres per km 2
1 Sheen (silver/grey) 0.04 ‐ 0.30 40 ‐ 300
2 Rainbow 0.3 ‐ 5.0 300 ‐ 5,000
3 Metallic 5.0 ‐ 50 5,000 ‐ 50,000
4 Discontinuous true oil colour 50 ‐ 200 50,000 ‐ 200,000
5 Continuous true oil colour ≥200 ≥ 200,000
The water column distribution has been curtailed at a concentration of 50 ppb. Below this threshold
there is no expectation of significant acute toxic effects, as 50 ppb is the lowest acute concentration
for any oil component that is deemed to present a 5 % risk to marine life using standard ‘no‐effect’
risk assessment methodologies (e.g. EU, 2003 and ECHA, 2008). This is a very conservative approach
since this treats all the oil as the most toxic component. Following OSPAR recommendations
sediment concentrations of 50 mg/kg are used as the threshold
For the blowout from the Balloch well, the model was run to incorporate an area of ≈ 900,000 km 2
that included UK, Danish and Norwegian coastlines. A small fraction of oil (< 5 %) was found to have
moved outside these areas, at extremely low concentrations.
A release diameter of 9 5 /8” was assumed. The diameter of production tubing is 8 1 /2”; at the surface
this opens out to 9 5 /8”. The modelling parameters used are summarised in Table 6‐5. A surface spill
blowout was modelled for 2 days and then followed by a release for the remainder of the blowout
period from the seabed. It was not considered that an ongoing blowout at the surface was a realistic
scenario to model for a semi‐submersible, as there are a number of mechanisms by which the rig
should quickly detach from the well location. It was assumed a period of 90 days would be required
to arrest the blowout by drilling a relief well and the model was run for an additional 30 days to track
the fate of the hydrocarbons following cessation of the spill.
For the diesel inventory loss, the model was run to incorporate an area of ≈ 360,000 km 2 and included
UK and Norwegian coastlines. No diesel was found to have moved outside these areas. A total loss of
inventory from the rig was assumed to occur in one hour, with the fate of the diesel being tracked
over the next 30 days.
6 ‐ 5