Environmental Statement - Maersk Oil

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6 ‐ 4 Balloch Field Development Environmental Statement Section 6 Accidental Spills 1. The first time the reservoir is penetrated is while drilling the 12¼” hole. If the well were to flow at this stage, for instance because a large hydrocarbon influx has entered the well, it is possible that hydrocarbons may reach the surface. However, as the wellbore pressures in the 12¼” open hole will be significantly below what is required to keep the shales back, hole collapse and plugging is expected to occur within a couple of days/weeks. 2. Similarly, during the drilling of the 8½” reservoir hole section it is possible that hydrocarbons may reach the surface. Hole collapse and plugging would be expected within a couple of weeks if a blowout were to occur while the reservoir was being drilled. 3. Once the sand screens have been installed and before the completion is run, there is a full steel‐lined conduit in place from reservoir to surface with an 8½” internal diameter from the top of the sandscreens. If the well is live and the drilling rig would somehow lose station with the BOP not holding pressure and all hydraulic control lost, the well will have an unrestricted flow through the 9 5 /8” production casing with an internal diameter of 8½”. This will present a lower pressure loss conduit to the environment than once the well is completed. This scenario constitutes the worst case blow out event for the spill modelling. 4. During the completion phase, hydrocarbons are purposely introduced into the wellbore during the production clean‐up. Although this would appear to introduce more blowout risk than in any drilling scenario, there are several barriers in place: the sub‐surface safety valve (SSSV), the subsurface test tree, lubricator valves above the subsurface test tree and the surface test tree. Moreover, the SSSV, the subsurface test tree and the surface test tree have valves which fail closed. If the well is live and the rig would somehow lose station with the BOP not holding pressure and all hydraulic control lost then the subsurface test tree and SSSV will close automatically and shut in the well. This blowout 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 subsurface 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. A scenario where a well being drilled intersects with a completed producing well at a relatively shallow depth 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 (well blow out) which is considered the worst case scenario and extremely unlikely. As required by DECC, the modelling assumes no intervention, i.e. 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. In this sense, the modelling gives a pessimistic outcome. 6.2.2. LOSS OF FUEL INVENTORY FROM NTVL. The appraisal/production well will be drilled from the Noble Ton van Langeveld (NTvL) semi‐ submersible drilling rig. Any additional wells are likely to be drilled by the Sedco 704 semi‐ submersible drilling rig. The inventory for the diesel stored on the NTvL is 8,642 bbls and on the Sedco 704 is 6,610 bbls. Spill modelling was carried out on a total loss of fuel inventory from the NTvL to represent a worst case. 6.3. HYDROCARBON SPILL MODELLING This section summarises the input data and assessment methods used to model the loss of inventory from the drilling rig and the blowout scenarios. For each spill scenario, both stochastic and deterministic analyses were carried out as per the DECC guidance. The “SNORRE B” oil type was chosen from the model database to represent the closest oil type to the Balloch oil. The parameters used to make this assessment were the API and the pour point of the
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
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Environmental Statement - Maersk Oil

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