T YPES OF A CCIDENTAL E VENT

7.1.1

Hydrocarbon Spills and Leaks

The characteristics of liquid hydrocarbons used and produced during the project phases are summarised below. These include diesel, lubricating and hydraulic oils, crude oil and aviation fuel. Spills and leaks of gas and condensate have not been considered below because both liquids are non-toxic and, as they vaporise quickly, they will have a minimal impact on the marine environment in the unlikely event of a spill.

Crude Oil

There is a small chance that a crude oil spill could occur in association with the  Alma development due to:

 A collision causing loss of inventory from the FPSO and/or the export tanker

Loss of inventory from the export flowlines due to a rupture or incident such as fishing damage

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 Accidental releases of produced hydrocarbons from identified potential sources have been assessed and modelling of the relevant scenarios carried out to characterise the extent of the impact (see Section 7.3). The scenario modelled was if the FPSO and export tanker collided resulting in the total loss of inventory from both vessels. Neither vessel would be full to capacity at the same time in the same location, so as the export tanker has the largest capacity (100,000m3, 87,000 tonnes) this was the total amount modelled.

The Alma crude has an API of about 38° and modelling shows that it is likely to emulsify, with an expected water content of 80%.

The most recent UK guidance on oil pollution emergency response requires Operators to model a loss of well control (blow out), as this although an extremely rare occurrence in the UK is considered to be the worst case volume of crude oil that could be spilt from a development. After consultation with the DECC Offshore Inspectorate this modelling has not be run for the Alma development due to the low reservoir pressure. From the very start of field life, reservoir pressure is such that ESPs will be required to pump crude oil out of the reservoir. In the event that well control is lost the wells will effectively self- kill. Instead, the worst case crude spill was considered to be if the FPSO and export tanker collided, with a total loss of containment. As neither vessel is likely to be full at the same time, the larger inventory of the two (export tanker) has been modelled. The modelling results are presented in Section 7.3 and  Appendix B.

Diesel

Marine diesel used in mobile drilling rigs and support vessels is a low viscosity distillate fuel. Diesel contains a high proportion of lighter hydrocarbons, such that evaporation is an important process contributing to the removal of spilt diesel from the sea surface. Evaporation will be enhanced by higher wind speeds and warmer sea and air temperatures. The general behaviour of diesel at sea can be summarised as follows:

 A slick of diesel will elongate rapidly in the direction of the prevailing wind and waves.

Very rapid spreading of the low viscosity diesel will take place.

Some diesel fuel oils may form an unstable emulsion at the thicker, leading edges of the slick.

Speed of physical dispersion of the surface slick increases with wind speed. Up to 95 % of a slick may disperse within about 4 hours of the spill in 15 knot winds and sea conditions.

Only the worst case spill scenarios for the greatest inventory of diesel offshore for the development have been modelled. This would be from the FPSO and export tanker colliding into each other during offloading. The combined inventory of the two vessels has been modelled and the results are shown in Section 7.3.

Lubric ating and Hydraulic Oil

Lubricating oils behave in a manner similar to marine diesel but are more viscous, slowing down the spread of the slick marginally. As lubricating oils are considerably refined, they do not contain the same quantity or ratio of light-end

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They have a rapid spreading rate and generally dissipate quickly, particularly in higher sea states. Lubricating and hydraulic oils are used in a variety of equipment on both drilling rigs and support vessels and are stored in containers ranging from 20 to 1,000 litres.

 Av iat ion Fu el

 Aviation fuel is volatile and evaporates and spreads quickly. Since the fuel will mostly evaporate, leaving little or no visible mass left on the surface within 24 hours, it is unlikely there would be sufficient time for clean-up operations in the event of a spill. Aviation fuel is used for refuelling helicopters that will transport equipment and personnel to and from the drilling rig and other offshore vessels to shore.

7.1.2

Chemical Spills and Leaks

During the life of the project there is the potential for chemical spills and leaks to occur. Spills may result in localised impacts on water quality and toxicity effects on marine fauna and flora. Chemical spills include accidental leakage of hydraulic fluid or chemical inhibitors used in the wells or accidental release of chemicals during transfer between vessels.

 All chemicals used during construction and production will be permitted under the Offshore Chemical (Amendment) Regulations 2011 (OCR). Chemicals that are to be discharged undergo a risk assessment. A comprehensive list of chemicals will be developed during the detailed engineering phase of the project. Bulk chemicals stored during the commissioning and operational phase of the project that are considered in this section are:

Chemical used during the drilling of the wells e.g. oil and water based drilling muds, cement chemicals, completion chemicals, rig wash, hydraulics fluids etc

Chemicals used during production e.g., wax inhibitor, asphaltene / demulsifier, scale inhibitor, methanol or MEG, corrosion inhibitor etc

Specific effects on individual receptors would depend upon the type and volume of chemical released but is broadly similar to the receptors discussed in relation to hydrocarbon spills.

7.1.3

Dropped Objects

Dropped objects have the potential to increase the project footprint on the seabed if the objects are not recovered. They also pose a risk to other sea users as snagging hazards, can get caught in propellers or are a collision risk. Occurrences of dropped objects are most likely during drilling activities and during vessel transfer operations.

Subsea equipment associated with the Alma development will be subject to dropped object studies to ensure that the potential risks posed by dropped objects are reduced. Dropped objects that cannot be retrieved, will be reported to the DECC on a PON2 form so that other sea users can be notified of their presence.

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