Investigation of sediments influenced by drill cuttings around the Visund offshore installation

Abstract

Two mounds (H1 and H2) which were increasing in size on the seabed southeast of the Visund offshore installation in the Norwegian North Sea were identified in the spring 2007. This made StatoilHydro ASA actuate measures in order to identify the source of these accumulations. Identification was accomplished by analyses of sediments sampled nearby the platform and comparison of various tracers for drill cuttings reference samples of mud and base oil used in drilling operations at Visund in July 2007. Sediment core samples were collected using ROV. The muds used at Visund have exclusively been based on barite. Therefore, barium was a good indicator on the amount of cuttings present in the samples. The concentration of barium in the samples as well as the composition of the extracted organic material (EOM) from the samples showed clearly that one of the two accumulations (H1) had a high content of drill cuttings down to a depth of at least 35 cm. Additionally, the source and biodegradation of hydrocarbon remnants were assessed partly by assessment of the hydrocarbon chromatogram patterns and partly by determination of the carbon isotopic composition of n-alkanes with between eleven and seventeen carbon atoms. The carbon isotopic ratios in the top layer (0-3 cm) of this pile, had patterns very similar to those of the mud used in Visund drilling operations in July 2007 and it was concluded that the surface of the pile had a high portion of cuttings or mud recently discharged from the Visund platform. The other mound (H2) showed only trace amounts of drill cuttings and it was therefore concluded to be composed of mostly non-contaminated seabed sediments.