Seabed Recolonisation: N'Kossa Case

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractDuring 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.

Comparison of benthic foraminifera and macrofaunal indicators of the impact of oil-based drill mud disposal

“…For all three years, there is a conspicuous maximum at station 6, 70 m W of the disposal site. In 2000, the maximum hydrocarbon concentration at this station was about 170 g/kg dry weight, in 2002 and 2003 it was somewhat lower, with 65 and 110 g/kg, respectively (Dalmazzone et al, 2004;Durrieu & Bouzet, 2004). Away from the disposal site, hydrocarbon concentrations rapidly decrease, until only trace amounts are found at 500 m distance.…”

“…Also for Barium, the highest value is observed at 70 m W from the disposal site. In 2000 and 2002 this maximum concentration was about 150 g/kg dry weight, and in 2003 about 95 g/kg dry weight (Dalmazzone et al, 2004;Durrieu & Bouzet, 2004). The values show a rapid decrease until 500 m where they become insignificant.…”

“…Usually, benthic macrofauna and/or meiofauna is used to monitor drill mud disposal (e.g. Davies et al, 1984;Dicks et al, 1988;Gray et al, 1990;Daan et al, 1994;Olsgard and Gray, 1995;Shimmield et al 2000;Gage, 2001;Grant and Briggs, 2001;Borja et al, 2003;Breuer et al, 2004;Dalmazzone et al, 2004;Durrieu and Bouzet, 2004;Muxika et al, 2005;Gass and Roberts, 2006;Flaten et al, 2007;Olsen et al, 2007). The distributional patterns of macrofauna and meiofauna around oil drilling platforms resemble the patterns found around other point sources of pollution (Dalmazzone et al, 2004;Parr et al, 2007;Schaanning et al, 2008;Venturini et al, 2008).…”

Aged Drilled Cuttings Offshore Gabon: New Methodology for Assessing Their Impact

“…It clearly indicates a poor extent of toxicity of sediments from the discharge point after 12 years when compared to recently dumped sites where sometimes 100% of larvae are abnormal [1,[4][5]. It clearly indicates a poor extent of toxicity of sediments from the discharge point after 12 years when compared to recently dumped sites where sometimes 100% of larvae are abnormal [1,[4][5].…”

“…[1][2][3][4][5] Off Congo, the last survey was realized 4 years after the end of cuttings discharge. We focused on the impact of oily cuttings, • To assess the impact of cuttings contaminated by oil based drilling fluids and the capacity of regeneration of the environment over time.…”