Shell U.K. Limited BRENT FIELD DRILL CUTTINGS DECOMMISSIONING TECHNICAL DOCUMENT CGI image of the Brent Field installations. A supporting document to the Brent Field Decommissioning Programme Shell Report Number BDE-F-SUB-BA-5801-00001 February 2016 Intentionally left blank BRENT FIELD DRILL CUTTINGS DECOMMISSIONING TECHNICAL DOCUMENT Contents 1 The Owners of the Brent Field .......................................................................................... 7 2 Brent Field Decommissioning Documentation ....................................................................... 8 3 Executive Summary ..................................................................................................... 10 4 Introduction ............................................................................................................... 12 5 The Brent Field Drill Cuttings Piles ................................................................................... 13 5.1 Historic Drilling Activities ............................................................................................. 13 5.2 Regulation of Drill Cuttings Piles .................................................................................... 14 5.3 Location of Drill Cuttings Discharge Points in the Brent Field ................................................ 15 5.3.1 Brent Alpha ........................................................................................................ 15 5.3.2 Brent Bravo ........................................................................................................ 15 5.3.3 Brent Charlie ...................................................................................................... 15 5.3.4 Brent Delta ......................................................................................................... 16 5.3.5 Brent South ......................................................................................................... 16 5.4 Surveys of the Brent Drill Cuttings Piles ............................................................................ 16 5.4.1 Drilling Record Estimates of Drill Cuttings Volumes ...................................................... 16 5.4.2 Survey Estimates of Drill Cuttings Volumes ................................................................. 16 5.4.3 Brent Delta Cell-top Cap Survey ............................................................................. 19 5.5 Estimates of Drill Cuttings Volumes ................................................................................. 20 5.5.1 Introduction ........................................................................................................ 20 5.5.2 Other locations of drill cuttings ............................................................................... 20 5.5.3 Conclusions ........................................................................................................ 22 5.6 Drill Cuttings under OSPAR Recommendation 2006/5 ...................................................... 23 6 Status of the Brent Drill Cuttings Piles ............................................................................... 24 6.1 Introduction ............................................................................................................... 24 6.2 Physical Changes of the Seabed Drill Cuttings Piles .......................................................... 24 6.3 Biological Thresholds and Effects................................................................................... 26 6.4 Brent Alpha Seabed Drill Cuttings Pile ............................................................................ 27 6.4.1 Physical Extent .................................................................................................... 27 6.4.2 Composition of the Brent Alpha Seabed Drill Cuttings Pile ........................................... 33 6.4.3 Biological Effects from the Brent Alpha Seabed Drill Cuttings Pile................................... 34 6.5 Brent Bravo Seabed Cuttings Pile .................................................................................. 35 6.5.1 Physical Extent .................................................................................................... 35 6.5.2 Composition of the Brent Bravo Seabed Drill Cuttings Pile ............................................ 39 6.5.3 Biological Effects from the Brent Bravo Seabed Drill Cuttings Pile ................................... 40 6.6 Brent Bravo Cell-top Drill Cuttings Pile ............................................................................ 41 6.6.1 Physical Extent .................................................................................................... 41 6.7 Brent Charlie Seabed Drill Cuttings Pile .......................................................................... 43 6.7.1 Physical Extent .................................................................................................... 43 6.7.2 Composition of the Brent Charlie Seabed Drill Cuttings Pile .......................................... 47 6.7.3 Biological Effects from the Brent Charlie Seabed Drill Cuttings Pile ................................. 48 6.8 Brent Charlie Cell-top Drill Cuttings Pile .......................................................................... 48 6.8.1 Physical Extent .................................................................................................... 48 Page | 3 BRENT FIELD DRILL CUTTINGS DECOMMISSIONING TECHNICAL DOCUMENT 6.8.2 Composition of the Brent Charlie Cell-top Drill Cuttings Pile .......................................... 53 6.9 Brent Delta Seabed Drill Cuttings Pile ............................................................................. 58 6.9.1 Physical Extent .................................................................................................... 58 6.9.2 Composition of the Brent Delta Seabed Drill Cuttings Pile............................................. 63 6.9.3 Biological Effects from the Brent Delta Seabed Drill Cuttings Pile .................................... 64 6.10 Brent Delta Cell-top Drill Cuttings Pile ............................................................................. 64 6.10.1 Physical Extent .................................................................................................... 64 6.10.2 Composition of the Brent Delta Cell-top Drill Cuttings Pile ............................................. 67 6.11 Brent South Seabed Drill Cuttings Pile ............................................................................ 68 6.11.1 Physical Extent .................................................................................................... 68 6.11.2 Composition of the Brent South Seabed Drill Cuttings Pile ............................................ 73 6.11.3 Biological Effects from the Brent South Seabed Drill Cuttings Pile ................................... 74 6.12 Summary of the Biological Effects of the Brent Seabed Drill Cuttings Piles .............................. 74 6.13 GBS Cell-top Oil and Gas Debris ................................................................................. 75 6.13.1 Introduction ........................................................................................................ 75 6.13.2 Brent Bravo ........................................................................................................ 75 6.13.3 Brent Charlie ...................................................................................................... 77 6.13.4 Brent Delta ......................................................................................................... 77 7 Summary of Technical Studies ........................................................................................ 79 7.1 Safety Risk ................................................................................................................ 79 7.1.1 Assessment of Safety Risk to Project Personnel ............................................................ 79 7.2 Drill Cuttings Modelling Studies .................................................................................... 79 7.2.1 Introduction ........................................................................................................ 79 7.2.2 Long-term Fate Modelling ...................................................................................... 80 7.2.3 Human Disturbance Modelling ............................................................................... 82 8 Assessment of the Brent Drill Cuttings Piles under OSPAR Recommendation 2006/5 ................... 87 8.1 Comparison with the OSPAR thresholds .......................................................................... 87 8.1.1 Seabed Drill Cuttings Piles ..................................................................................... 87 8.1.2 Cell-top Drill Cuttings Piles ..................................................................................... 88 8.1.3 Tri-cell Drill Cuttings .............................................................................................. 89 8.1.4 Conclusion ......................................................................................................... 90 9 Technically Feasible Options for the Management of the Brent Drill Cuttings .............................. 91 9.1 Disturbance of the Seabed Drill Cuttings ......................................................................... 91 9.2 Technically Feasible Options for the