Environmental Statement - Maersk Oil

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6.5. ENVIRONMENTAL RISKS ‐ FATE OF OIL IN THE MARINE ENVIRONMENT 6 ‐ 24 Balloch Field Development Environmental Statement Section 6 Accidental Spills When crude oil is spilled on the surface of the sea it is subjected to a number of processes. These include spreading, evaporation, dissolution, emulsification, natural dispersion, photo‐oxidation, sedimentation and biodegradation. The fate and effect of crude oil is dependent on the chemical and physical properties of the oil. This is taken into account in the modelling scenarios. The physico‐chemical changes to which the oil is subjected vary depending on oil type, volume spilled and the prevailing weather and sea conditions. Some of these changes can lead to its disappearance from the sea surface while others, for example emulsification, may cause it to persist. Evaporation and dispersion are the two main mechanisms that act to remove oil from the sea surface. Evaporation is the main mechanism by which the mass of oil is reduced immediately after a spill. It also causes considerable changes in the density, viscosity and volume of the spill over time. The light fractions of the oil (aromatic compounds such as benzene and toluene) evaporate quickly. Substances like diesel (the most likely type of hydrocarbon to be spilled) have a greater percentage of light hydrocarbon fractions and will therefore evaporate relatively quickly in comparison with heavier oils. A large proportion of even a very large spill of diesel will evaporate within the first 24 hours of release. Evaporation is enhanced by warm air temperatures and moderate winds. The oil remaining in the slick will have a higher viscosity and specific gravity. The processes of dissolution, dispersion and photo‐oxidation will also act to break down the oil. The aromatic compounds of diesel can be toxic to planktonic organisms in the vicinity of the spill. Figure 6‐14 Fate and behaviour of spilled oil at sea (adapted from Koops, 1985). After the light fractions have evaporated from the slick, the process slows down and natural dispersion becomes the dominant mechanism in reducing the slick volume. This process is dependent upon sea surface turbulence, which in turn is affected by wind speed. Water‐soluble components of the oil mass will dissolve in the seawater and the immiscible components will either emulsify and
Balloch Field Development Environmental Statement Section 6 Accidental Spills eventually disperse as droplets, or aggregate into a viscous mass. The rate of emulsification is dependent upon the oil type and sea state. In certain sea states, emulsions may increase in volume, containing up to 70 ‐ 80 % water depending on the oil type, and form a thick layer on the sea surface reducing slick spreading and natural dispersion. By diminishing the amount of surface area available to be weathered and degraded, these emulsions can be difficult to break up using dispersants and some mechanical recovery devices. The light oil encountered in this project is unlikely to form emulsions. Emulsions normally require the presence of long chain molecules known as asphaltenes. The impacts of oil spills on marine organisms are well documented. A synopsis of potential impacts from the proposed operations is summarised in the following sections. 6.6. SUMMARY OF ENVIRONMENTAL SENSITIVITIES AND POTENTIAL IMPACTS This section provides a summary of key relevant data on the environmental sensitivities and potential impacts of an oil spill at the Balloch location. Full details of environmental sensitivities can be found in Section 3 of this ES. Table 6‐8 provides a summary of the risk assessment of the environmental sensitivities within the Balloch location; these have been derived following the assessment criteria detailed in Table 6‐6 and Table 6‐7. Table 6‐8 Summary of risk assessment of environmental sensitivities within the vicinity of the proposed Balloch location. Feature Key Sensitivities present Impact of Small Spill (Tier 1) Impact of Medium Spill (Tier 2) Impact of Large Spill (Tier 3) Plankton Low vulnerability Insignificant Insignificant Slight Benthic communities Fish Marine Mammals Offshore Seabirds Protected Sites and Shore Birds Commercial fisheries Oil and Gas operations Shipping Tourism 6.6.1. PLANKTON Low / moderate sensitivity, species specific. Spawning and nursery for some species in winter / spring Low/moderate abundance in area Overall Moderate vulnerability 184 km from nearest shoreline (UK) Some commercially important species present Nearest are MacCulloch FPSO Northern Producer FPU and Balmoral FPSO Moderate area of activity 184 km from nearest shoreline (UK) Insignificant Slight Moderate Severe Slight Slight Moderate Insignificant Insignificant Slight (cetaceans) Moderate (seals) Slight Moderate Slight Moderate Moderate Severe Insignificant Slight Moderate Moderate Severe Insignificant Slight Moderate Insignificant Insignificant Slight Insignificant Insignificant Slight Insignificant Insignificant Moderate Although low concentrations of hydrocarbons (
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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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