Can offshore energy structures function as artificial reefs after end-of -life?Ocean Sprawl

Anaëlle Lemasson¹, Paul Somerfield², Michaela Schratzberger³, Louise Firth⁴

¹The University of Plymouth, UK - ²Plymouth Marine Laboratory, UK - ³Centre for Environment, Fisheries and Aquaculture Science, UK - ⁴University College Cork, Ireland

Thousands of energy structures are present in the marine environment, contributing to global ocean sprawl. Many of those with a finite operational period, such as oil and gas installations (O&G) and offshore wind farms (OWFs), are at or approaching end-of-life and requiring urgent decisions regarding their decommissioning. Should they stay or should they go? In certain parts of the world such as the North-East Atlantic, complete removal is the only legal options (OSPAR Decision 98/3). In others, alternative management options involving leaving whole or part of a structure in the marine environment is allowed, for instance by “reefing” it (US “Rigs-to-Reefs” programme). However, selecting the decommissioning options that will generate the most ecological, social, and economic benefits remains a challenge, in part because their effects on the marine environment, both during the production phase and during decommissioning, are still largely undetermined. A question of particular interest is whether these offshore energy structures “act as artificial reefs” (ARs), and thus provide desired ecological benefits that might be worth retaining after end-of-life. We use a meta-analysis approach of 531 effect sizes relating to the abundance, biomass, and diversity of marine fish and invertebrates obtained from 109 articles to assess the effects of offshore artificial structure around the world. We compare O&G and OWFs to accidental shipwrecks and purpose-built ARs, and to natural habitats, to gauge whether these structures “act as artificial reefs”. Our synthesis demonstrates that whilst they can have positive ecological effects akin to that of natural reefs, important idiosyncrasies exist, which should be considered when deciding on decommissioning approaches at end-of-life. These findings can also support potential reconsideration of local and regional restrictions to alternative decommissioning options, such as OSPAR Decision 98/3.