Don Field Decommissioning Programme (pdf, 7.8MB) null - BP

DON-BP-001Don Field Decommissioning ProgrammeRisers and umbilicals run to the Thistle topsides through an opening in the 30inconcrete-filled caisson, which is fixed to the seabed.Approximately 2km from the Don manifold, the four lines cross over the Northern LegGas Pipeline (NLGP).Further details on these lines are provided in Section 10, which should be referred tofor details of pipeline construction and stabilisation, current status, decommissioningoptions and selected decommissioning methods.4 Materials on the Seabed4.1 Drill Cuttings4.1.1 IntroductionDrill cuttings are small pieces of rock that are broken up by the drill bit as it penetratesthe rock during drilling of wells. The cuttings are carried back to the surface by ‘drillingmuds’, which are special fluids used to cool and lubricate the drill bit, transport thecuttings and contain the downhole pressure in the well.Drilling mud consists of a base fluid, such as water, oil or synthetic oil, plus othercomponents, which are added to improve performance.4.1.2 Don Field Drill Cuttings HistoryDrilling of the Don Field wells, as detailed in Table 4.1, commenced in July 1989 andcontinued through to June 1996.Wells were drilled from a MODU that relocated to above each well being drilled. Thisresulted in minor drill cutting accumulations in proximity to each well.4.1.3 Total and DistributionFrom information in the reports of wells drilled between 1989 and 1991, the overallvolume of cuttings discharged was 14,000m 3 .A full quantitative and qualitative survey, performed in 1999 [4.5], estimated that theremaining volume of cuttings at the Don manifold area was 1763m 3 spread over a largearea (47,745m 2 ). This would indicate that the cuttings accumulation has diminishedthrough time owing to natural forces. There is no physical cuttings pile.Prior to the commencement of decommissioning work, a photographic survey and studyof the area will be conducted. On completion of decommissioning work, a further surveywill be carried out.4.1.4 CompositionThe seabed around the Don manifold displayed low to moderate reflectivity during theJuly 1999 survey, interpreted as representing a low relief cuttings comprising very poorlysorted coarse sands and silt.Descriptions of Items to be Decommissioned4-12 May 2011