What happens after oil and gas decommissioning? A global systematic review of marine environmental effects.

The thousands of oil and gas (OG) platforms placed at sea for fossil fuel extraction have introduced new hard substrate to the marine environment. Over time, these structures can become colonized by a diversity of marine life, fostering novel ecosystems. However, an increasing number of OG platforms are reaching decommissioning age and decisions regarding their fate must be made. Some view these artificial structures as litter that ought to be removed; others view them as valuable contributors to marine biodiversity worth preserving. Evidence of the environmental effects of these structures following different decommissioning strategies is needed to identify the potential benefits of each option and make informed decisions. Here, using a systematic synthesis approach, we show that our understanding of the effects of different decommissioning options is greatly limited by a lack of empirical evidence. Only three articles addressed the effects of OG removal, preventing firm conclusions either for or against this option. Most research focused on Rigs-to-Reefs options, revealing that reefed structures can create biodiverse systems, although with clear differences between reefing methods. Decommissioned structures with higher vertical relief (e.g., standing or topped) may offer higher ecological value than those with lower relief (e.g., toppled). Risks related to the decommissioning methods (e.g., harm from explosives, non-native species introduction) are discussed. Despite the urgency, empirical research on decommissioning environmental impacts remains limited, particularly from the southern hemisphere. We call for coordinated international effort to establish standardization across decommissioning procedures, and monitoring and reporting requirements, to ensure that robust data are available to address this complex environmental challenge.