Closing the loop: Tracking stocks and flows of steel from the end of useful life to re-melting
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- Institutt for elkraftteknikk 
Recycling steel scrap from different stages in the life cycle is an important input to secondary steel production and manufacturing. While flows and losses from pre-consumer processes are well known, there are considerable gaps in the knowledge surrounding end-of-life stocks, flows, and losses in various sectors. This thesis aims to investigate end-of-life processes for steel in mining infrastructure, focusing on offshore installations.Material Flow Analysis was applied to the current stock of offshore installations in the North Sea, and a case study concerning the decommissioning of the Frigg field. The system contains the following processes: removal, transport to onshore demolition yard, primary dismantling, recycling and market for old scrap. The chosen region for the large-scale model is the North East Atlantic.Various recycling metrics and rates obtained from literature were compared to the results obtained from the Material Flow Analysis. While decommissioning companies present material recycling rates in the range of 95-99%, the actual rate was shown to be lower when obsolete stocks were included.The obsolete stock of offshore installations consists of structures that are exempt from legislations regarding decommissioning. Concrete structures that contain reinforced steel, steel installations weighing over 10,000 tonnes, and disused pipelines can be left behind when an oil field is closed down, contributing to a build-up of steel stocks that are not recovered. The analysis gave an estimate of 360 000 tonnes of steel in obsolete structures in the North Sea, not including disused pipelines.Recycling metrics are often only concerned with the amount of material that is recovered from the field, and not what is left behind. When obsolete structures are included, the rates for recycling are significantly lower than what the industry reports, which is a direct result of choosing system boundaries that skew the results. Steel embedded in reinforced concrete and steel piles from jacket type platforms are examples of losses that are not included in current recycling statisticsThe removal of offshore installations is regulated strictly by both national and international legislations, and is not driven by the demand for steel scrap. This is both due to the high cost of removal, safety considerations and due to the desires of other stakeholders such as the fishing industry.