To help achieve global aims to reach Net Zero greenhouse gas (GHG) emissions, a clearly structured method for calculating emissions from decommissioning oil and gas structures is required. In order to understand the GHG consequences of recycling secondary steel from decommissioned structures, this paper presents a new methodology in decommissioning, based on the UN’s International Resource Panel (IRP)’s Value Retention method that combines life cycle assessment principles, the waste hierarchy and the circular economy to holistically calculate GHG emissions produced as a consequence of manufacturing primary and secondary steel, manufacturing a product from this steel and the associated transport emissions. The Value Retention Model presented here combines the concept of material value and product value to obtain realistic GHG emission calculations based on end-of-use and end-of-life scenarios, including recycling and reuse options. The results show that reusing a steel jacket structure in situ will retain 55,040 tCO2(eq) in GHG emissions, not including removal operations or transport emissions. New regulation is urgently required to update the current outdated emissions calculation guidelines, enable the provision of both realistic baseline emissions figures and to provide a mechanism for reporting ‘after operations’ figures.