Decommissioning – Relative Effects of Alternative Management Strategies (DREAMS)

Globally, there are over 7,500 oil and gas platforms in the waters of 53 countries that will become obsolete over the next several decades. Under current international legislation, at end-of-life (referred to as 'decommissioning'), most will be removed from the sea, the cost of which is estimated to be $210 billion globally. As the world transitions to greener energy, wind turbines and wave-energy devices are being added to the marine environment but will also require removal at end-of-life. The removal of oil and gas platforms is a complex engineering process, logistically difficult, costly, and accompanied by serious human health and safety risks, and which can negatively impact the marine environment. However, the current understanding around the environmental effects of different decommissioning strategies is incomplete and there is a widespread acceptance that complete removal may not always be the preferable option.

DREAMS was a highly integrated project designed to develop new understanding of how marine artificial structures, and different decommissioning strategies for them, influence the structure, functioning and dynamics of marine ecosystems and affect delivery of ecosystem services from a whole ecosystem perspective. Through novel integration and analyses of existing data from a wide range of sources, using structured, systematic and meta-analytic approaches, as well as state of the art ecosystem models, the overall aim was to provide scientific evidence and inform decision-makers and stakeholders about the relative benefits and detriments of different strategies for decommissioning structures for various stakeholders, including the effects on ecosystems, services, conservation goals and socio-economic outcomes.

A systematic map of evidence highlighted the paucity of case studies (from real-world decommissioning of structures) describing the ecological effects of different decommissioning options (Lemasson et al., 2022a; summary for policymakers: Lemasson et al., 2022b), which makes deciphering the effects of different decommissioning options a considerable challenge for evidence-informed decommissioning (Lemasson et al., 2023). This could prevent decision-makers from taking defensible and decisive action regarding structures at end-of-life, and crucially, could also hinder potential support for policy change (see our policy brief: Lemasson and Knights, 2024).

Nevertheless, the map catalogued a substantial amount of scientific evidence on the effects of offshore structures while in the sea (prior to decommissioning), particularly associated with purpose-built artificial reefs and investigating ecological or biological effects for fish and invertebrates (knowledge clusters). Clear knowledge gaps remain, for instance associated with more recent or novel structures such as marine renewable energy installations, or relating to other outcomes such as effects on hydrography, sedimentary processes, or nutrient cycling, or other taxon such as algae, plants or mammals.

This wealth of information was used to infer the potential effects of decommissioning. Using robust meta-analytical techniques based on over 530 data points from 109 independent studies, we found that offshore structures can enhance ecological function compared to natural sedimentary habitats, but not more so than natural rocky reefs.

We also found clear differences in effects between structure types (i.e. not all structures have the same ecological effects). Overall, we find that there is limited evidence that oil and gas or offshore wind structures would provide substantial ecological benefits if decommissioned as artificial reefs (by leaving them, reefing them, or repurposing them in situ) (Lemasson et al., 2024).

There are still very large gaps in our understanding about the linkages and interactions between marine artificial structures and ecosystem services (Watson et al., 2024), mostly in association with wind turbines. However, this is a field of research that is accelerating rapidly. Negative impacts on biodiversity and flows to ecosystem services from offshore wind is prevalent overall but the presence of these structures may be beneficial or have no effect on existing ecosystem services and local human community groups.

At the North Sea scale, an extended ERSEM biogeochemical-ecological model showed that the effects of offshore oil and gas structures on the primary productivity and biogeochemical fluxes is spatially limited and practically negligible at the scale of the model resolution ranging from hundred meters to few kilometres. This contrasted with that of offshore wind farms structures, for which placement density is much higher leading to detectable impacts on primary productivity, chlorophyll and nutrients within the wind farm and adjacent areas (Al Azhar et al. in preparation). Using a combination of high-resolution hydrodynamic modelling (FVCOM), biogeochemical modelling (FVCOM-ERSEM), particle tracking (PyLag) and graph network analysis (iGraph), we generated a spatial Connectivity Importance Index (CII), under a novel ‘everything is everywhere’ framework. This index provides a generic overview of maximum potential connectivity (James et al. in preparation).