Characterising subsea gas pipeline contaminants to inform decommissioning practices, an ecosystem perspective.

Operation of offshore oil and gas infrastructure may result in contaminant accumulation on the internal surfaces of infrastructure such as subsea gas pipelines. Contamination varies due to factors including the type of substance being transported, internal surface coating, and the type of maintenance cleaning. Successful decommissioning of subsea pipelines requires an ecological risk assessment of contaminant residues, especially when in situ decommissioning is considered. This study characterised the inorganic and radiological constituents of pigging residues collected from a subsea gas pipeline (in operation). Residues contained high concentrations of total mercury (1.7% dry mass), with metacinnabar (βHgS) the dominant species. Radionuclides from the 238U series (210Pb and 210Po) were detected above exemption limits of 1 Bq/g. Nickel, manganese and cobalt from pigging dust demonstrated solubility in seawater above Australian marine water quality guidelines after 30 d leaching at 0.2% solids:solution ratio, in a closed system. Metals, including mercury, and radionuclides also demonstrated solubility in dilute acid, suggesting there may be potential for bioaccumulation of such contaminants via ingestion of residues. Radiological dose modelling suggested that benchmarks for potential adverse effects to organisms from 210Po were exceeded following pipeline corrosive breakthrough and leaching. However, this risk is time dependent.