Investigation of a North Sea oil platform drill cuttings pile

Abstract

A comprehensive study of the drill cuttings pile at N W Hutton was undertaken in August, 1992. Fifty one welIs have been drilled in the field, mainly using mineral oil based drill fluids, with the cuttings discharged to sea. The cuttings pile was mapped using a 3D side scan sonar system and the periphery was defined by towed side scan sonar and gamma ray spectrometer surveys. The pile was cored by vibrocorer to a maximum depth of 2.35m. The cores were assessed geotechnically and subsampled for physical and chemical analyses.
Environmental impact was investigated by grab sampling at 12 stations out to 7500m, Selected on the basis of cuttings distribution. All surveys and samples were located using a differential global positioning system. The results are relevant to the corrosion and long-term environmental effects of oily cuttings piles, the remove/leave alone debate, and abandonment planning.
Beneath the platform the pile is 5m high, with a low angle of repose and extending out to about 200 m.
The cuttings in the cores were SOJ, with laminations and only minor compaction in deeper layers. The top 1-1.5m resembled freshly discharged material. The cores were sub sampled for hydrocarbon, trace metals and sulphide content and grain size analysis. Metals analyses included identification of metal species to estimate bioavailability and implications of pile disturbance.
Estimates of oil migration within the pile are made from correlation of the chemical analyses results with the drilling history, in particular the change from diesel to low toxicity base oil in 1984.
Strong gradients were found in the faunal data which correlate well with the physical and chemical results. Dense populations of opportunists species were present adjacent to the platform, including a novel molluscan opportunist.
This is the first comprehensive study of an oily cuttings pile and is a contribution to the debate on their long term impact and fate on abandonment.