Environmental statement - Flyndre and Cawdor - Maersk Oil

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Flyndre and Cawdor Environmental Statement Section 6 Hydrocarbon Releases automatically and shut in the well. This blow‐out scenario in the completion phase will result in a smaller oil spill than scenario 3 as the well is restricted by the internal diameter of the 5½ upper completion, 5½” tubing retrievable sub‐surface safety valve (TRSSSV) and 5” subsea tubing hanger landed in the horizontal production tree spool. In addition, most barriers in this section are fail closed making it a less likely worst‐case scenario. 5. A scenario where more than a single well blowout occurs simultaneously. This would require the loss of integrity of two or more wells simultaneously which is extremely unlikely as there is only one drilling rig planned to be operating over either drill centre at any one time. 6. A scenario where a well being drilled intersects with a completed producing well at a relatively shallow depth. This would require failure of directional drilling/surveying procedures/safeguards and result in either scenario 3 or 4 occurring. Oil spill modelling was conducted on scenario 3 which is considered the worst case scenario and extremely unlikely. As required by DECC, the modelling assumes no intervention: that is; it is assumed that there will be no response to mitigate the impacts by, for instance, the use of booms to contain the spill or dispersants to enhance degradation. In this sense, the modelling gives an unlikely outcome. When crude oil is spilled on the surface of the sea it is subjected to a number of processes including: 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 which has been taken into account in the modelling. The International Association of Oil & Gas producers has issued datasheets (OGP, 2010) on blowout for offshore operations of North Sea Standard (NSS) where the operation is performed with BOP installed including shear ram and where the two barrier principle is followed. The dataset is derived from the International SINTEF blowout database where a blowout is defined as an incident where formation fluid flows out of the well or between formation layers after all the predefined technical well barriers or the activation of the same have failed. The blowout frequencies indicated that for gas reservoirs the likelihood of a blowout is higher in comparison to oil reservoirs (Table 6‐3). Operation Category Average Gas Oil Unit Exploration Deep normal wells Blowout 3.1 x 10 ‐3 3.6 x 10 ‐3 2.5 x 10 ‐3 Per well drilled Well release 2.5 x 10 ‐3 2.9 x 10 ‐3 2.0 x 10 ‐3 Per well drilled *Denotes the blowout proportion that would happen at the surface and the platform Fraction Subsea* Table 6‐3 Blowout and well release frequencies for offshore operations of North Sea Standard (OGP, 2010). 6.3. HYDROCARBON SPILL MODELLING The extent of impacts of total diesel discharge from the drilling rig and blowouts during maximum flow rate from the Cawdor and Flyndre wells were modelled. For each scenario a stochastic analysis was carried out 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) capable of producing a visible sheen (Table 6‐4). The BAOAC states that oil films below ≈ 0.04 µm thickness are considered invisible. 6 ‐ 4 D/4114/2011 0.39 0.39
Flyndre and Cawdor Environmental Statement Section 6 Hydrocarbon Releases Code Description ‐ Appearance 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 Table 6‐4 Bonn Agreement Oil Appearance Code. 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 this 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. This is a very conservative approach since this treats all the oil as the most toxic component. The blowout model is run assuming it will take Maersk Oil no longer than 90 days to complete a relief well and get the spill under control. The model is also run for an additional 30 days in order to model the fate of the oil after the spill has been controlled. In addition to the varied wind data used in the above modelling the fate of the hydrocarbons in the presence of unvarying offshore and onshore 30 knot winds is presented, as per the DECC guidance. 6.3.1. LOSS OF DIESEL FROM THE DRILLING RIG The NTvL drilling rig has a fuel capacity of 1,143 tonnes (i.e. 8,642 bbls) of diesel. Modelling of this volume of diesel discharged onto the sea surface over 1 hour and dispersing for 30 days has been undertaken to simulate a total loss of inventory from the NtvL. A stochastic analysis was undertaken by modelling 20 scenarios utilising a broad range of weather and current data. The model output indicates the calculated probability of a surface sheen > 0.04 µm thickness being present at a specific location at any point in time over the duration of the model run per well. Figure 6‐2 shows that in the unlikely event of total loss of diesel from the NTvL drilling rig the predicted surface oiling at a thickness of > 0.04 um would be visible within a radius of ≈ 70 km. A visible surface sheen is not predicted to reach any shore. D/4114/2011 6‐5
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Environmental statement - Flyndre and Cawdor - Maersk Oil

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