Seabed Recolonisation: N'Kossa Case

Abstract

Abstract During offshore activities directed at the exploration and production of hydrocarbons, the living conditions of the flora and fauna on the seabed may be disrupted. Drilling operations in particular, during which cuttings are discharged to sea, induce an impact on the benthic populations living in the sediment. The study described in this paper set out to assess directly on site the real impact of the cuttings discharged, the regenerating capacity of the ecosystem and the medium- and long-term recolonisation processes. The physical and chemical parameters used as tracers of the drilling muds were: the barium and hydrocarbon concentrations, and particle size. The paper presents the main results obtained from the monitoring surveys. It discusses the methodology adopted (study of the physico-chemical and biological quality of the sediment, plus evolution of toxicity and bacterial population trends). The study has shown that the relevant parameters for describing benthic macro-fauna are: species richness, abundance and diversity (Shannon index). Where the environment is concerned, the results obtained are a basis for evaluating the kinetics of restoration of the drilling site in time and in space, and provide evidence of the limited impact of the cuttings. Introduction About 2 million tons of cuttings are generated each year by the offshore industry. Although the impact associated with the discharge of drilling cuttings impregnated with fluid has already given rise to numerous study works [1], mainly in respect of the North Sea, further information is needed in the shape of the monitoring data on the cuttings discharged today and on the fate of old cuttings. The environmental impact of these cuttings essentially hinges on the toxicity and biodegradability of the drilling fluids that impregnate the drill cuttings. It also includes the burying effect, which can destroy the indigenous fauna even in absence of any toxicity, and the inputs of organic matter (hydrocarbons), which govern the whole re-colonisation process after the first phase of acute toxicity. We therefore propose to apply the principle of measuring physico-chemical, biological and toxicological parameters to monitor recolonisation of the contaminated sediments. In order to study the way the benthic fauna regenerates and recolonises the seabed after the discharge of cuttings, a project was conducted in 2 phases between 2000 and 2003, targeting different objectives:Phase 1: To draw up an experimental protocol for monitoring recolonisation. Ecotoxicological measurements and tests of biodegradation and lixiviation were carried out to validate and select the most suitable methods for achieving this. We focused on the impact of oily cuttings [2].Phase 2: To assess the real impact of cuttings and the capacity for regeneration of the environment on a real discharge site. This particular study was conducted offshore Africa, under warm sea conditions. The results are later compared to previous studies in temperate and cold seas [3,4,5]. Description The site is located in the Congo on the N'Kossa field, some 60 km off Pointe Noire, under a water depth of 180 m. Discharge, essentially of LTMBF (Low Toxicity Mineral Oil Base Fluid) cuttings, began in November 1993 and ended in April 1999 and generated around 10 000 m3 of cuttings. A baseline study was undertaken after the first step of drilling in 1995, and was followed by 3 monitoring surveys in November 2000, March 2002 and April 2003 (Fig 1). Results for 2003 are not yet available, and consequently only the results for 2000 and 2002 will be discussed.