An Estimate of the Cumulative Environmental Effects of the Disposal in the Deep Sea of Bulky Wastes from the Offshore Oil and Gas Industry

Abstract

Abstract This paper presents an assessment of the potential cumulative environmental effects of the deposition at the North Feni deep sea disposal site of 24 of the steel platforms presently located on the UKCS. It is argued that justification for deep sea disposal could only be made if it were possible to demonstrate significant benefits in terms of reduced risks to persons during the decommissioning process. If large steel jackets could be toppled and transported in the water in one piece, and then released at the deep sea site, this would, it is suggested, reduce the risk to personnel who would otherwise be engaged in underwater dismantling and lifting operations. Candidate structures were therefore identified as those that weigh more than 10,000te in air (the maximum single lift capacity of present heavy lift vessels), and stand in water more than 100m deep (thus permitting toppling of the whole structure under present IMO guidelines). If these structures were deposited at North Feni over a period of about 24 years, the main potential source of cumulative effects would be from the release of contaminants as the sacrificial aluminium anodes were depleted. It is estimated that some 10,700te of anodes would be deposited, giving rise to a peak daily wastage rate of 1,400kg. A modified advection/diffusion model was used to predict the possible dispersion of the dissolved contaminants away from the site of deposition on the seabed. It was concluded that the peak concentrations of all contaminants in the water column extending from 0m to 20m above the seabed would be very low, and would not exceed the coastal Environmental Quality Standard (EQS). Concentrations in the water column from 20m to 40m above the seabed would be an order of magnitude less than those found near the seabed.