Environmental statement - Flyndre and Cawdor - Maersk Oil

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Figure 2‐5 Schematic of well design for Cawdor well. 2.5.7. DRILLING MUD, CUTTINGS AND CHEMICAL DISCHARGES During drilling, fluids are required for a number of reasons including; transportation of the cuttings to the surface cooling and lubrication of the drill bit managing hydrostatic pressure. Flyndre and Cawdor Environmental Statement Section 2 Proposed Development Table 2‐6 summarises the Phase I Cawdor well drill cuttings and mud masses. It is assumed that drilling of the Phase II Cawdor wells will produce similar mud and cuttings volumes. Section diameter 36’’ 17 1 /2” Drilling fluid Seawater and high viscous sweeps Seawater and high viscous sweeps Mud volume (m 3 ) Cuttings volume (m 3 ) Cuttings mass (Te) 225 37 94 1160 165 422 Fate of cuttings Discharge to seabed Discharge to seabed 12 1 /4” OBM 740 178 458 Rotomill 8 1 /2” OBM 345 81 209 Rotomill Table 2‐6 Cawdor mud and cuttings mass. 2‐ 8 D/4114/2011
Flyndre and Cawdor Environmental Statement Section 2 Proposed Development While drilling the Cawdor well(s) the 36” and 17 1 /2” tophole sections will be drilled riserless using seawater with viscous sweeps. Cuttings from these sections will be discharged to the seabed. The 12 1 /4” and 8 1 /2” hole sections will be drilled with OBM, and the cuttings recovered from these sections will be processed using a Rotomill system. Use of the Rotomill for treatment and disposal of oil based mud and cuttings removes the requirement for shipping large numbers of skips of OBM back to shore for processing and disposal of the solids to landfill. Base oil will be recovered from the Rotomill and stored for reuse, while the recovered cuttings powder is mixed with recovered water and seawater and pumped to sea via an existing overboard chute. It is necessary to mix the cuttings powder with the recovered water to form a slurry which helps avoid formation of surface flocs of powder due to trapped air. Recovered powder and water will be monitored to ensure they meet all discharge limits i.e. oil content of >1% dry weight. As a contingency a ship and skip system will be in place. The mud formulation and drilling chemicals will be finalised during final well design and Petroleum Operations Notice 15Bs will be submitted to DECC prior to drilling. This statutory document will identify, quantify and assess the risks associated with all the chemicals that will be used. 2.5.8. CEMENTING CHEMICALS Steel casings will be installed in the wells to provide structural strength to support the subsea valve trees as well as isolate unstable formations, different formation fluids and separate different wellbore pressure regimes. Each steel casing will be cemented into place to provide a structural bond and an effective seal between the casing and formation. During cementing excess cement may be produced. Uncontaminated cement will be treated and discharged to sea. It is anticipated that all cement will be mixed as required and as a result there should be limited operational discharge of any mixed cement or mixwater. All chemicals to be used will be selected based on their technical specifications and environmental performance. Chemicals with sub warnings will be avoided where technically possible. The cementing chemicals to be used have not yet been determined but will be detailed and included in subsequent PON 15B applications. 2.5.9. OTHER RIG DISCHARGES Water generated from rig washdown may contain trace amounts of mud, lubricants and residual chemicals resulting from small leaks or spills and rainfall from open deck areas. The volume of these discharges depends on the frequency of washdown and amount of rainfall. Liquid storage areas and areas that might be contaminated with oil are segregated from other deck areas to ensure that any contaminated drainage water can be treated prior to discharge and accidental spills contained. Drainage water from these areas and machinery spaces is collected, treated to remove hydrocarbons (less than 15 ppm hydrocarbons in water, as required under the MARPOL Convention) and the cleaned water is discharged to sea. Black (sewage) and grey water is also collected, treated to meet the requirements of the MARPOL Convention and discharged to sea. These are all relatively low volume discharges containing small residual quantities of contaminants. Maersk Oil will ensure that the rig is equipped with suitable containment, treatment and monitoring systems as part of the contract specification. Maersk Oil will also ensure good housekeeping D/4114/2011 2‐9
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Environmental statement - Flyndre and Cawdor - Maersk Oil

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