Successful decommissioning of subsea oil and gas pipelines requires an effective and safe approach of assessing and managing chemical and radiological residues, especially when in situ decommissioning options are considered as part of the closure planning. This study characterised the inorganic and radiological constituents of pipeline scale collected from a subsea welded-steel lean hydrocarbon (dry) gas pipeline.
Leach tests in seawater were conducted alongside leaching in dilute acid. Results of inorganic contaminants in bulk material and leachates were compared against the ANZ Guidelines for Fresh & Marine Water Quality. Radiological concentrations of bulk material and in seawater leachates were used to conduct simplified radiological dose modelling to marine biota using the ERICA tool.
While pigging dust contained high concentrations of total mercury (approximately 1.7% dry mass), metacinnabar (β-HgS) was present as the major species. Organic mercury accounted for <0.01% of the total mercury. Radionuclides from the 238U series were detected (210Pb and 210Po) above international exemption limits of 1 Bq/g. Mercury, nickel, manganese, cobalt and 210Po from pigging dust demonstrated solubility in seawater above water quality guidelines. Metals including mercury and radionuclides also demonstrated solubility in dilute acid, suggesting there may be potential for bioaccumulation in marine organisms via ingestion of pigging dust. Radiological dose modelling for scenarios where pipelines may be left in-situ suggested that benchmarks for potential adverse effects to organisms were exceeded following pipeline corrosive breakthrough and leaching of specifically 210Po. However, twenty-eight-day bioaccumulation tests using Australian benthic fauna suggested limited bioavailability of all contaminants, signifying a deviation from model predictions, and highlighting the importance of conducting bioaccumulation tests with site-specific organisms. Based on these results, we proposed there would be minimal short-term ecotoxicological risk from pigging dust. Further studies are focussed on potential medium- and long-term changes to contaminant bioavailability from site-specific geochemical parameters.