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Legacy subsea plastics from the oil and gas industry: Environmental risks of decommissioning in situ.
Summary
When old oil and gas pipelines are left on the seafloor instead of removed, they slowly break down and could release up to 500 tonnes of microplastics into the ocean every year, in some cases more than what used to come from banned cosmetic microbeads. Since these tiny plastic particles can harm marine life and eventually work their way up the food chain (including into seafood we eat), researchers say this risk should be factored into decisions about retiring old offshore equipment and planning future infrastructure like wind farms.
Accumulation of end-of-life plastic is a global environmental problem. Plastics are widely used in oil and gas infrastructure and bring considerable benefits while in service. However, the environmental consequences of end-of-life decommissioning scenarios are largely unknown. Using the UK Continental Shelf (UKCS) as a case study we estimate the mass of plastics deployed in subsea pipelines, quantify degradation and estimate the rate of microplastic generation. Recognising that the relatively slow rate of any subsequent mineralisation leads to an accumulation of plastic particles in the marine environment, we then apply a novel approach using Predicted No-Effect Concentration (PNEC) analysis to estimate potential environmental risks of decommissioning in situ. We demonstrate physical and chemical degradation of oil and gas plastics after 22 years in the UKCS. Available data indicated 121,000 tonnes of plastics may be decommissioned in situ, and we estimate that depending on environmental conditions their degradation could generate 4.5-500 tonnes of microplastics annually. Our lowest release scenario is a similar order of magnitude to the annual releases of microplastics to the North Sea from UK rivers and our highest is considerably more than annual microbead releases from cosmetics prior to the UK ban. Moreover, our PNEC analysis indicates microplastics generated from legacy oil and gas plastics have the potential to harm marine life. Despite uncertainties in absolute levels of microplastics generated from oil and gas legacy plastics we recommend they are formally integrated into decommissioning decision-making and into planning and consenting of new installations, including offshore renewable energy infrastructure.