AN INVESTIGATION OF BENTHIC SEDIMENT CHARACTERISTICS INTHEVICINITY OFTHE COSSACK PIONEER FPSO, NORTHWEST SHELF

Abstract

Routine discharges from the Cossack Pioneer floating production storage and offloading facility (FPSO) on the North West Shelf include produced formation water (PFW) from the Wanaea-Cossack and Lambert-Hermes oil fields. PFW is the highest volume discharge from the FPSO and contains a variety of naturally occurring materials from the formation, including residual volatile and non-volatile hydrocarbons not removed by the separation process, trace metals, naturally occurring radioactive materials (NORMs) and process chemicals carried over from the production stream.

In May 1999, a total of 22 sediment samples were collected at the Cossack Pioneer location, using a remotely operated vehicle (ROV) deployed from a support vessel. Sampling stations were located along four transects, aligned along and perpendicular to the prevailing current direction. The objective of this survey was to investigate the physical and chemical characteristics of benthic sediments in the vicinity of the Cossack Pioneer to determine whether there have been any significant inputs of trace metals, hydrocarbons and NORMs into benthic sediments as a consequence of PFW discharge from the facility.

Trace metal concentrations were low in all the samples analysed, with levels at all sites generally below those detected at the control site, approximately 16 km from the Cossack Pioneer location. Low concentrations of TPH were detected in all of the samples, apart from the sample collected at the control site. These TPH levels were solely composed of a series of high molecular weight alkanes (C27−C34 n-alkanes) the distribution of which is not consistent with those commonly observed in crude oils, distilled petroleum fractions or synthetic oil- based drilling fluids. Concentrations of NORMs were also generally below those detected at the control site and were comparable with levels measured in uncontaminated sediments in the Gulf of Mexico.

Overall, the concentrations of trace metals, TPH and NORMs measured in sediments adjacent to the Cossack Pioneer location did not indicate any significant anthropogenic input resulting from PFW discharge, and were below levels that have been shown to cause adverse biological effects in marine sediments.