To leave or not to leave: a tiered assessment of the impacts of scale residue from decommissioned offshore oil and gas infrastructure in Australia

The current liability for decommissioning offshore oil and gas infrastructure in Australia is estimated to exceed US$ 31 billion over the next 50 years. This is founded on the base-case regulatory position of complete removal of all infrastructure, with over half the liability occurring in the next 10 years. Naturally occurring radioactive materials (NORM) are ubiquitous in oil and gas reservoirs worldwide and may form contamination products including scales and sludges in production infrastructure. Globally, assessment of the ecological impact of in-situ decommissioning typically fails to consider the long-term effects of exposure to potential contaminants contained within pipelines. There remain broad gaps in our understanding of NORM contaminants in the marine environment and how to effectively assess and measure the risk of NORM to marine organisms. This article discusses the application of a tiered assessment process to determine potential radioecological impacts of pipeline scale related to decommissioning practices using real-world NORM scale: (1) advanced nuclear and imaging techniques to characterise NORM-contaminated scale; (2) conducting radiological biota dose modelling using MicroShield and ERICA for different decommissioning scenarios; and (3) performing a series of marine infauna-exposure tests to understand the bioaccumulation and organ biodistribution of scale-associated constituents. This research provides a methodological approach that contributes valuable insights to inform infrastructure decommissioning planning. Several research gaps remain in our understanding of the fate of NORM contaminants in subsea oil and gas systems relative to decommissioning risk assessments including investigating radiobiological effects in marine organisms at elevated NORM levels and the establishment of a NORM inventory to better understand its characteristics and formation from different types of infrastructure.