Environmental Statement - Maersk Oil

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6 ‐ 14 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.
Balloch Field Development Environmental Statement Section 6 Accidental Spills Figure 6‐7 Surface slick summary outputs for a well blowout for 2 days at the surface and the remainder at subsea. 62°00'N 60°00'N 58°00'N 56°00'N Blowout ‐ 2 day surface release + 89 day subsea release Probability of visible surface oil at any time Oil type SNORRE B Release position Surface (2d) + Seabed (89d) 1,773,000 19,700 m 3 Volume released Release duration Rate of release / day Median lines crossed % likelihood Mass balance (tonnes) 120000 100000 80000 60000 40000 20000 0 5°00'W 200 km 5°00'W 0°00'E 0°00'E m 3 5°00'E 5°00'E Norway Faeroes Denmark Germany Neth's 100% 0% 60 ‐ 70 % 1 ‐ 10 % 1 ‐ 10 % Typical mass of oil on surface over time 10°00'E Statistical Map: Surface: Probability of contamination above threshold (0.000 mm) [%] 10°00'E 62°00'N 60°00'N 58°00'N 56°00'N 90 days 0 20 40 60 80 100 120 140 Time (days) Surface 6 ‐ 15
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B ‐ 2 Halocarbon release during e
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B ‐ 4 Lube and hydraulic oil spil
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Discharges to Sea Discharge from pi
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Production Phase Environmental Aspe
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B ‐ 10 Drainage water Discharge o
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Physical presence Oil and gas infra
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Major accidental events Other envir
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Environmental Statement - Maersk Oil

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