Environmental statement - Flyndre and Cawdor - Maersk Oil

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5‐ 6 Flyndre and Cawdor Environmental Statement Section 5 Assessment of Potential Impacts and Control Measures arise from the support vessels and anchor handling vessels. Anchored semi‐submersibles will generate considerably lower underwater source levels than dynamically positioned semi‐ submersibles. The anchor handling vessels will use dynamic positioning and be equipped with powerful engines and thrusters. Sound arising from vessels is assessed in Section 5.2.5. Noise associated with drilling from the semi‐submersible will propagate from any rotating machinery such as generators, pumps and the drilling unit and risers. Studies have shown that during drilling, underwater sound levels increase when compared to periods of non‐drilling, this has been related to the use of additional machinery and power demands (McCauley, 1998). The noise from drilling has been found to be predominantly low frequency, less than 1,000 Hz, with relatively low source levels
Flyndre and Cawdor Environmental Statement Section 5 Assessment of Potential Impacts and Control Measures Vessel type Total fuel used (te) Emissions CO2 NOx N2O SO2 CO CH4 VOC Survey vessel 440 1408 26.1 0.10 1.8 6.9 0.8 0.88 DP reel‐lay vessel 575 1840 34.2 0.13 2.3 9.0 0.10 1.15 Trenching support vessel 680 2176 40.4 0.15 2.7 10.7 0.12 1.36 Diving support vessel 1350 4320 80.2 0.30 5.4 21.16 0.24 2.7 Rock placement vessel 150 480 9.0 0.03 0.6 2.4 0.03 0.3 Guard vessel 200 640 11.9 0.04 0.8 3.1 0.04 0.4 Supply vessel 200 640 11.9 0.04 0.8 3.1 0.04 0.4 Total from installation 3,595 11,504 213.7 0.79 14.4 56.36 1.37 7.19 Total from drilling vessels (Table 5‐3) 6,950 22,246 412.6 1.53 27.83 109.1 1.25 13.9 Total from all vessels 10,545 33,750 626.3 2.32 42.23 165.46 2.62 21.09 2008 UK domestic shipping emissions 1 ‐ 5,400,000 ‐ ‐ ‐ ‐ ‐ ‐ % of UK total ‐ 0.62 ‐ ‐ ‐ ‐ ‐ ‐ 1 Source; Department for Transport, 2010. Note: Atmospheric emissions have been calculated using emission factors from the EEMS Atmospheric Calculations Issue 1.810a (Austin, 2008). Table 5‐5 Vessel emissions associated with installation infrastructure. From Table 5‐5 it can be seen that in the worst case scenario the vessel emissions associated with drilling of the three wells and installation of the infrastructure amounts to 0.62% of CO2 generated by UK domestic shipping emissions in 2008. 5.2.2. PHYSICAL PRESENCE VESSEL ANCHORS A Dynamically Positioned (DP) reel‐lay vessel will be used to lay the pipelines and hence no anchor damage to the seabed will be associated with this vessel. However other vessels associated with laying the infrastructure may hold their position with the use of anchors and a wire mooring spread. Anchors and anchor spreads can interact with other infrastructure in the marine environment for example pipelines and cables. However, the risk of interaction between anchor spreads and subsea infrastructure occurring can be minimised by careful laying of the anchor spread to avoid existing infrastructure. As each anchor is laid the depth of anchor penetration will be dependent on the shear strength and load bearing capacity of the seabed soils; a firm seabed will result in less depth of penetration than a soft seabed. A small area of the seabed where each anchor is placed will be compressed as the anchors sink into the soft seabed, and further disturbance and re‐suspension will occur again as the anchors are retrieved. Anchor placement can cause mortality or displacement of benthic species in the immediate area surrounding the disturbed sediment, and a direct loss of habitat and mortality of sessile seabed organisms that cannot move away from the contact area. Additionally, when the anchors are removed there is the potential for scars/mounds to be left on the seabed. The seabed is soft, and damage caused from any anchors are expected to be visible for a number of years. Anchor scars are formed from the deployment and recovery of anchors in sedimentary areas, particularly where clay is present immediately beneath the seabed. Although the predominant seabed sediments D/4114/2011 5 ‐ 7
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Environmental statement - Flyndre and Cawdor - Maersk Oil

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