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

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DON-BP-001Don Field Decommissioning ProgrammeA debris survey was carried out in 2004 [8.2] which confirmed that there is no significantcuttings pile and that cuttings do not obscure any seabed features such as smallboulders and seabed scars.2 Cuttings Composition2.1 Environmental Cuttings SurveyIn July 1999, an environmental cuttings survey [8.1] was undertaken in the vicinity of theDon manifold to: Define the size and shape of the drill cuttings Obtain seabed samples to determine the physio-chemical and biological conditions ofthe seabedTwo survey lines were run at an offset of 50m and 150m from each face of the manifoldusing sidescan sonar and swathe bathymetry. A line at an offset of 50m was also rundiagonally past each corner of the manifold. In 2004, a photographic seabed assessmentwas performed at various locations within the Don Manifold area to: Identify seabed conditions Check for presence of Lophelia and any other protected species2.2 Particle SizeThe 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. The immediate area of the Don manifold was characterisedby more highly reflective sediments, comprising a superficial cover of coarse sands withexposures of the underlying stiff clays of the Tampen Formation together with bouldersand a number of depressions. Grab samples indicated the sediment to be poorly to verypoorly sorted sand and silt. Particle sizes taken in grab samples were indicative of thedistribution of cuttings over the seabed.2.3 Sediment OrganicsThe spatial distribution of sediment organics and organic carbon was consistent with thespread of cuttings in that the cuttings pile area exhibited a greater percentage of carboncontent as organic carbon.2.4 Total OilsTotal oils were above the North Sea background level of 5µg.g -1(Gardline unpublished)at all but three of the survey sites indicating localised contamination between 100m and500m radius from drill-related activity, with some spread at distances >100m along theline of the dominant current flow to the south-east. Both total oils and n-alkanes wereelevated with a petrogenic/biogenic bias at certain sample stations.Drill Cuttings8-2 May 2011
Don Field Decommissioning ProgrammeDON-BP-001The samples showed large variation (by a factor of over 9000) across the survey sites,ranging from 1.3µg.g-1 at 500m south-west to 11,766µg.g-1 at 100m south-east. Thislatter survey location (number 1007) shows high levels of other pollutants but, as thehigh levels are only found at this single station, is not considered representative. Notehowever that this station lies downstream of the cuttings.2.5 Polycyclic Aromatic HydrocarbonsPolycyclic Aromatic Hydrocarbons (PAHs) were recorded in concentrations above theNorth Sea baseline (240ng.g -1North Sea Task Force (NSTF) 1993) with a bias towardthe lighter volatile Nitro-o-PhenyleneDiamine (NPD) fraction at certain sample stations.PAH concentration exhibited a strong positive correlation with total oil concentrationwithin the cuttings pile sediments.2.6 MetalsThe spatial pattern for metal concentrations was a reduction with distance from the Donmanifold. Most of the metals surveyed were higher in concentration at the locations100m south-east and south-west of the manifold. However, as shown in Table 8.1, at500m from the manifold, total levels of Barium (Ba), Chromium (Cr), Lead (Pb) andVanadium (V) exceeded the baseline for the North Sea, indicative of a veneer ofdrill-related material.Metal North Sea Baseline Highest Concentrationfound near Don ManifoldBarium
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