Decommissioning of offshore oil and gas structures – Environmental opportunities and challenges

Science of the Total Environment 658 (2019) 973–981

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Review

Decommissioning of offshore oil and gas structures – Environmental opportunities and challenges Brigitte Sommer a,b,⁎, Ashley M. Fowler a,c, Peter I. Macreadie c, David A. Palandro d, Azivy C. Aziz d, David J. Booth a a

School of Life Sciences, University of Technology Sydney, Sydney, NSW 2007, Australia School of Life and Environmental Sciences, The University of Sydney, Sydney, NSW 2006, Australia School of Life and Environmental Sciences, Centre for Integrative Ecology, Deakin University, Burwood, VIC 3125, Australia d ExxonMobil Upstream Research Company, Spring, Texas, 77389, United States b c

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• After decades at sea, oil and gas structures support diverse marine life. • Decommissioning decisions need to reflect this changed ecological context. • Approaches to weigh risks, benefits and trade-offs of decommissioning options vary. • Assessing a wide range of decommissioning options and environmental aspects is key. • This will help better understand the environmental effects of decommissioning.

a r t i c l e

i n f o

Article history: Received 26 October 2018 Received in revised form 12 December 2018 Accepted 12 December 2018 Available online 13 December 2018 Editor: Damia Barcelo Keywords: Marine Decommissioning Oil Gas Environmental impacts Comparative assessment

a b s t r a c t Thousands of offshore oil and gas structures are approaching the end of their operating life globally, yet our understanding of the environmental effects of different decommissioning strategies is incomplete. Past focus on a narrow set of criteria has limited evaluation of decommissioning effects, restricting decommissioning options in most regions. We broadly review the environmental effects of decommissioning, analyse case studies, and outline analytical approaches that can advance our understanding of ecological dynamics on oil and gas structures. We find that ecosystem functions and services increase with the age of the structure and vary with geographical setting, such that decommissioning decisions need to take an ecosystem approach that considers their broader habitat and biodiversity values. Alignment of decommissioning assessment priorities among regulators and how they are evaluated, will reduce the likelihood of variable and sub-optimal decommissioning decisions. Ultimately, the range of allowable decommissioning options must be expanded to optimise the environmental outcomes of decommissioning across the broad range of ecosystems in which platforms are located. © 2018 Elsevier B.V. All rights reserved.

⁎ Corresponding author at: School of Life and Environmental Sciences, The University of Sydney, Sydney, NSW 2006, Australia E-mail address: [email protected] (B. Sommer).

https://doi.org/10.1016/j.scitotenv.2018.12.193 0048-9697/© 2018 Elsevier B.V. All rights reserved.

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Contents 1. 2.

Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental and ecological considerations for decommissioning . . . . . . . . . . . . . . . . 2.1. Biodiversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Biomass production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Conservation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Connectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Energy consumption and carbon footprint . . . . . . . . . . . . . . . . . . . . . . . 2.6. Direct physical disturbance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7. Dispersal of contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Evaluation of decommissioning options – current practice. . . . . . . . . . . . . . . . . . . . 4. Information needs to gain an ecosystem understanding of offshore O&G decommissioning . . . . . 5. Looking ahead: expanding the toolkit to advance the understanding of the ecology of O&G structures 6. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Background As offshore oil and gas (O&G) structures (structure refers to the platform segment in the water column) reach their end of production life, understanding the environmental effects of decommissioning and of potential alternative use options is a priority. Over 600 structures are projected to cease operations in the next five years and a further 2000 by the year 2040, with annual costs of decommissioning estimated to increase from $2.4 billion in 2015 to $13 billion by 2040 (Hem et al., 2016). Apart from the financial burden, decommissioning also presents safety, technical, environmental and socio-economic challenges for stakeholders and regulators (Fowler et al., 2014). The United Nations Convention for the Law of the Sea (Article 60) and the International Maritime Organization (Resolution A.672) provide guidelines for offshore O&G decommissioning used to direct policy. Nations and regions have taken different approaches to decommissioning, with policies ranging from complete removal of structures in the North Sea to ‘Rigs-to-Reefs’ and alternative use options (see Box 1) in the USA and Southeast Asia, and other regulators currently reviewing permissible alternatives to complete removal (e.g., in Australia; Chandler et al., 2017). In jurisdictions where complete removal of structures is the default option (e.g., the North Sea), environmental assessments tend to focus exclusively on limiting the impacts that arise during removal activities (e.g., potential dispersal of contaminants; Fowler et al., 2018). In contrast, the scope of environmental assessments in jurisdictions that permit alternative use options (e.g., US Gulf of Mexico) is broader, because it also considers the potential benefits derived from leaving structures in place, or from using them as artificial reefs elsewhere as habitat for biological communities (i.e., the reef effect; Macreadie et al., 2011). Removal of structures may therefore come at an ecological cost through the loss of flora and fauna and associated ecosystem functions and services. From a management policy perspective, the North Sea approach reflects the view that removal of structures minimises negative effects on the marine environment and/or that ecosystems will return to their pre-existing state once structures have been removed. In contrast, approaches that allow part or entire structures to be left in place or relocated, acknowledge that ecosystems have changed since structures have been installed and consider these altered ecosystem values in decommissioning decisions. Indeed, offshore O&G structures tend to co-locate in certain regions (Fig. 1) where habitats and ecological communities are now modified from their pre-existing configurations (e.g., the North Sea; Halpern et al., 2008), to the extent that a return to the historical state may no longer be possible nor preferred (Lusseau et al., 2016). This is consistent with the science on ecological regime shifts (e.g., Folke et al., 2004) and novel ecosystems (Hobbs et al., 2014), which indicates that once ecosystems have moved beyond reversible thresholds, they defy conventional restoration.

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Current ecosystem values (e.g., ecosystem functions, services) and options beyond a return to the ecosystem's pre-existing state therefore need to be considered if important ecosystem functions and services

Box 1 Beyond rigs-to-reefs - alternative use options of offshore O&G structures. There is increasing interest in the repurposing of offshore O&G structures after their production lives have ended, although developing a robust business case and the aspects of future liability will likely continue to make implementation challenging. With the exception of reefing, repurposing can prolong the life of an offshore infrastructure, although in the eventual future, the offshore infrastructure will need to be decommissioned. Apart from reefing, O&G structures could be adapted for a range of alternative uses including tourism, recreation, mariculture, alternative energy generation (e.g., wind turbines, wave energy), carbon capture storage, ocean instrumentation, and research facilities (Fig. 3). Alternative use potential is governed by a range of factors such as location (e.g., distance from shore), water depth, the type and condition of the structure, local environmental conditions, oceanography, technical and financial feasibility. Some projects are already in operation. For example, a platform in Malaysia was converted to a hotel and dive resort (Fig. 3b), and in the northern Gulf of Mexico many reefed platforms attract recreational fishermen and divers as part of the Rigs-to-Reefs program (Fig. 3c). Platforms have also been used as CO2 capture and injection sites for carbon capture and storage schemes. The Sleipner facility in offshore Norway has been injecting over 17 million tonnes of CO2 into an offshore sandstone reservoir since 1996 (Fig. 3a; Leung et al., 2014). While CO2 capture and injection were part of the Sleipner facility's original design, technical challenges remain in repurposing decommissioned offshore infrastructure for CO2 capture and injection. Other alternative uses are still in development. For example, Azimov and Birkett (2017) recently proposed a design for the integration of a wave power station with a decommissioned platform (Fig. 3d). Offshore water depth and clarity are considered favourable for offshore mariculture, with platforms investigated to anchor cages and ocean-pens, store feed, house solar panels that provide energy for operations, and provide accommodation for personnel (Buck et al., 2017).

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Fig. 1. Global map of the distribution of major offshore oil and gas structures. Numbers of structures based on Parente et al. (2006). Numbers exclude pipelines and smaller subsea structures.

are to be maintained (Hobbs et al., 2014). Ecological communities from artificial and natural habitats interact in complex ways that affect ecosystem functions and services (Hobbs et al., 2014), and need to be considered when comparing decommissioning options. A pragmatic approach to offshore decommissioning that sets realistic goals and incorporates a broader range of options including partial removal, is therefore likely to improve the environmental outcomes of decommissioning (Fowler et al., 2018; Macreadie et al., 2012). Nevertheless, history and public sentiment can strongly influence policy directions and much work may be needed to integrate science, public sentiment and policy in some regions (e.g., following the Brent Spar controversy in the North Sea; Jørgensen, 2012; Macreadie et al., 2012). Relative environmental outcomes of decommissioning options (i.e., complete removal, topping, horizontally reefing in place, reefing elsewhere, alternative use options; Figs. 2 and 3) and how they are influenced by local and regional factors (e.g., oceanography, biogeography, surrounding habitat) are not fully understood (Fowler et al., 2014). O&G structures are located in a wide range of ecosystems, from

shallow tropical coral reefs (e.g., Southeast Asia) to cold-water deepsea environments (e.g., North Sea), and the marine communities inhabiting these structures vary widely due to different local and regional factors. While most environmental work in relation to artificial reefs and offshore O&G structures has focused on understanding the effects of installation and continued presence in marine ecosystems (Bishop et al., 2017; review by Cordes et al., 2016; Heery et al., 2017), comparatively less is known about the effects of en masse removal and alternative use options. Here, we (1) conduct a global review of the state of knowledge on the effects of different decommissioning options on marine ecosystems, (2) present case studies and lessons learned from decommissioning practice, (3) identify information needs and future research directions, and (4) outline analytical approaches that can be incorporated into existing decision frameworks to close knowledge gaps and gain a holistic understanding of the environmental costs and benefits of various decommissioning options, as well as expanding our knowledge of marine ecosystems.

Fig. 2. Conceptual figure of decommissioning options, including ‘complete removal’, ‘reefing in situ’ (including leave in place, topping, and horizontal reefing), ‘reefing elsewhere’ and ‘alternative uses’.

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Fig. 3. Alternative use options of offshore O&G structures. (a) Carbon capture and storage at the Sleipner platform in Norway, photo Kjetil Alsvik/Statoil; (b) the Seaventures Dive Rig located off-shore Mabul Island, Semporna, Sabah, photo Jesse Schoff; (c) diverse marine life on a reefed structure, photo Shutterstock; (d) design for platform-based wave energy by Azimov and Birkett (2017).

2. Environmental and ecological considerations for decommissioning Although hundreds of structures have already been decommissioned globally, few published studies have monitored ecological communities and environmental conditions beyond the removal or reefing of the structures. Here, we review the environmental effects of decommissioning options, drawing on knowledge of the ecology of the marine communities on the structure, and on general biology and ecology in other systems. Generally, due to their location (e.g., most are on the continental shelf or slope) and unique physical and habitat characteristics (e.g., high vertical relief throughout the water column with internal cavities), the biological communities of the O&G structures differ in community parameters from those of the surrounding habitat (often low-relief sedimentary) and from natural reefs nearby (Bishop et al., 2017; Cordes et al., 2016; Dafforn et al., 2015; Heery et al., 2017; reviewed by Macreadie et al., 2011).

bottom habitat. The relative effects of the various partial removal options are more nuanced and depend on the structure and composition of associated communities (Ajemian et al., 2015; Simonsen, 2013). Studies comparing biological communities between natural reefs, operating and reefed platforms highlight the influence of depth, vertical relief and physical characteristics of structures on associated fish assemblages, and suggest that reefing will likely alter assemblage composition (Ajemian et al., 2015; Claisse et al., 2015; Simonsen, 2013). For example, in the Gulf of Mexico, fish assemblages significantly differed in composition and community structure among operating and horizontally reefed platforms, but did not differ among operating and topped structures, suggesting that the latter would impact fish assemblages less (Ajemian et al., 2015). Research on fouling and benthic communities has been limited, with no discernible differences among operating and horizontally reefed structures for coral density (Sammarco et al., 2014), or for benthic communities both b0.25 and N1.5 km from platforms in the Gulf of Mexico (Daigle, 2011).

2.1. Biodiversity 2.2. Biomass production Decommissioning options likely differ considerably in their effects on biodiversity, particularly between partial and complete removal options, the latter of which will result in almost complete loss of associated reef biota (Claisse et al., 2015; Pondella et al., 2015). Structures can enhance taxonomic and functional diversity by adding hard substrate in areas characterised by soft-bottom communities. For example, structures in the North Sea facilitate the presence of the mollusc Mytilus edulis that further modifies the habitat to facilitate other organisms (van der Stap et al., 2016). Complete removal is therefore likely associated with localised biodiversity reductions in areas dominated by soft-

O&G structures support significant biomass of sessile invertebrates and fish assemblages (Macreadie et al., 2011), with fouling communities increasing the total weight of structures in the North Sea by up to 30% (Pors et al., 2011). Although these biological communities may, to some extent, represent biomass redistribution, rather than production (Bohnsack, 1989), the fish production potential of the structures was recently demonstrated off California, with structures in this region having the highest secondary fish production per unit area of seafloor of any marine habitats investigated (Claisse et al., 2014). While complete

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removal of the structures will eliminate most of the existing biomass (Claisse et al., 2015; Pondella et al., 2015), partial removal typically leads to decline in fish biomass and production for species associated with the shallow portions of the structure that may be removed, such as schooling planktivores and large pelagic predators (Claisse et al., 2015; Simonsen, 2013). Deeper-dwelling demersal species are generally less affected. This suggests that the types of biological communities associated with the structure and their depth stratification influence the site-specific outcomes of decommissioning, and it is plausible that in systems where demersal fishes are dominant (e.g., in California; Claisse et al., 2015), the loss of production caused by partial decommissioning is reduced. For example, in California partial removal would likely retain on average 80% of fish biomass and 86% of secondary fish production (Claisse et al., 2015). Generally, habitat value appears to vary greatly among structures, even when they are in similar ecological settings (Schroeder and Love, 2004) and productivity data from one platform should be used with caution when considering productivity of other platforms (Fowler et al., 2015).

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structures to distant recycling locations), dismantling and onshore processing activities, and ‘indirect energy consumption’: ‘recycling energy’ (mostly steel) and ‘replacement energy’, a theoretical amount accounting for the production of new materials that replace materials left at sea. Complete and partial removal options generally involve higher direct energy consumption and emissions s than leave in place option (Pors et al., 2011). For example, the energy consumption to remove and recycle the steel jackets of the Ekofisk platforms in Norway was estimated at 40% of the annual electricity consumption of a city with 100,000 residents (Phillips Petroleum Company Norway, 1999). Nevertheless, energy use and atmospheric emissions need to be evaluated on a case-by-case basis to account for variation in the types and duration of marine operations for different structures (Pors et al., 2011). Moreover, the relative energy performance of decommissioning options might change when replacement energy is considered (e.g., Phillips Petroleum Company Norway, 1999). Emissions calculations add further complexity, with direct energy use generally incurring higher emission factors (a combination of fuel efficiency and engine design) than recycling and material production (Pors et al., 2011).

2.3. Conservation 2.6. Direct physical disturbance O&G structures are considered de facto marine protected areas (Inger et al., 2009; Schroeder and Love, 2004), because they protect biota and habitats from fishing within the safe-working exclusion zones that surround platforms. Decommissioning options that leave entire or partial structure in place have the potential to act as exclusion zones for specific gear types, particularly trawling, through the risk of entanglement and collision. In addition to conservation benefits, the exclusion of trawling has the potential to enhance fisheries, due to the export of fish and larvae to surrounding areas (‘the spillover effect’; Russ and Alcala, 2011; Williamson et al., 2016) and the protection of sensitive benthic habitats that act as nursery areas for fish. For example, in Norway, between 30 and 50% of Lophelia pertusa reefs have been damaged or destroyed by bottom trawling, with reports of reduced catch rates of line fishermen in damaged L. pertusa reefs (Fosså et al., 2002). The complete removal of structures will enable trawling in former safety zones, which could disturb drill cuttings and resuspend contaminated sediments (Pors et al., 2011). In the North Sea alone, complete removal of all structures would open up ~400 km2 of seabed to trawling.

All decommissioning options involve some physical disturbance to biota and habitats. Leaving structures in place involves the least disturbance, because only the wellbore needs to be plugged. Dismantling and severance of structures from the seafloor typically involves diamond wire cutting, abrasive water jetting, hydraulic shears or explosives (NOAA, 2017), which have the potential to damage organisms and habitats. O&G structures made primarily of steel will slowly disintegrate and collapse if left in place, with full corrosion of the structures expected to take over 500 years (Picken et al., 1997). Decommissioning options that involve handling and transport of structures pose safety risks (e.g., collisions, accidents, spills), increase the risk of damage to existing infrastructure and natural habitats, particularly in shallow coastal areas with high prevalence of sensitive habitats (e.g., coral reefs), but also in the deep sea, where cold-water corals are particularly slow to recover (Roberts and Cairns, 2014). 2.7. Dispersal of contaminants

The spatial distribution of structures in the marine environment affects connectivity and consequently biodiversity and ecosystem function at a range of spatial scales (reviewed in Bishop et al., 2017). Ecological communities from artificial and natural habitats interact in complex ways and the removal of structures that have been in place for extended periods may disrupt ecological processes. In regions with sparse natural reef habitat (e.g., North Sea, Gulf of Mexico), structures may facilitate connectivity over large distances for taxa with widely-dispersive life stages such as corals (Atchison et al., 2008; Sammarco et al., 2012), and may provide stepping stones for species (Bishop et al., 2017; reviewed in Cordes et al., 2016). A potential risk of decommissioning options that involve transporting structures is the unintended spread of non-native species. For example, a towed rig that became unintentionally stranded on a remote island in Brazil transported 62 nonnative sessile species to the new location (Wanless et al., 2010).

Decommissioning can disperse contaminants through the disturbance of drill cuttings located on the seafloor beneath the O&G structures (Breuer et al., 2004; Cordes et al., 2016). Estimates indicate that up to 12 million m3 of drill cuttings lie on the seafloor in the North Sea alone (Breuer et al., 2004). Associated contaminants are likely to remain within the cuttings pile unless they are disturbed (Breuer et al., 2004), which is typically the regulatory direction given to O&G operators. The severance of the structures from the seabed has the potential to resuspend contaminants, with potential disturbance of drill cuttings where topped sections of platforms are placed beside the base of the structure. Decommissioning options that leave components in place contribute to localised contamination by slow degradation of materials, such as anode material commonly used in marine environments to protect structures against corrosion (Picken et al., 1997). To address this, O&G operators are typically required to characterise substances associated with the structures, the extent to which they may be dispersed during and after decommissioning, and the potential exposure of biota to contaminants (e.g., OSPAR Decision 98/3).

2.5. Energy consumption and carbon footprint

3. Evaluation of decommissioning options – current practice

All decommissioning options involve the consumption of fuel and the production of greenhouse gases, with energy use and atmospheric emissions part of the prescribed assessment criteria in certain jurisdictions (e.g., UK). Energy use is typically divided into ‘direct energy consumption’ for diesel-powered vessel movement (e.g., towing

Operators and regulators currently use a variety of approaches to weigh the risks, benefits and trade-offs of decommissioning options. Evaluation approaches vary widely, but typically involve multi-criteria decision analysis (MCDA) and quantitative, semi-quantitative or qualitative assessment methods to integrate a range of environmental,

2.4. Connectivity

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Table 1 Glossary of terms that are commonly used in relation to decommissioning, often interchangeably. Term

Definition

Alternative use options

Repurposing of structure following the end of O&G production, including reefing, tourism, recreation, mariculture, alternative energy generation (e.g., wind turbines, wave energy), carbon capture and storage, or ocean instrumentation. A systematic approach in which the practicality of all reasonable decommissioning options is examined, including technical feasibility, environmental, risk and safety, costs and public acceptance. An approach used to evaluate decommissioning options based on a range of criteria typically including safety, technical, environmental, socio-economic and financial aspects (Oil and Gas UK, 2015). The removal or reuse of structure following the end of oil and gas production. A reefing option where the structure is severed from the seabed and placed on its side (sometimes referred to as toppling and reefing in situ). An approach that evaluates the net environmental performance of decommissioning options by weighing the potential environmental gains against the adverse environmental effects caused by the activities. A decommissioning option, whereby a platform is deployed as an artificial reef following the end of oil and gas production, either in situ or at a different location. A reefing option where the upper section of the rig is removed for navigational safety. The topped section may be deployed in the marine environment or removed (sometimes referred to as reefing in situ).

Best Practicable Environmental Option (BPEO) Comparative Assessment (CA) Decommissioning Horizontal reefing in place Net Environmental Benefit Analysis (NEBA) Reefing Topping

safety, technical, socioeconomic and cost criteria (e.g., Fowler et al., 2014, Henrion et al., 2015). The three most commonly used MCDA tools are the Comparative Assessment (CA; Oil and Gas UK, 2015), Net Environmental Benefit Analysis (NEBA) and Best Practicable Environmental Option (BPEO; Table 1). While these tools vary widely in scope, methods and application, all three include the assessment of the environment and are based on the decision analysis principle that no single decommissioning option performs best across all considerations, and that the option that maximises benefit is likely to differ depending on the decommissioning context (Fowler et al., 2014). The scope of environmental criteria examined varies and is generally broader in jurisdictions where a range of decommissioning options are permitted, relative to jurisdictions where complete removal is the default option (Table 2). In the UK, for example, operators conduct an Environmental Impact Assessment (EIA) that facilitates environmental comparison of decommissioning options as part of the larger CA. Decommissioning EIAs in the UK address the impacts arising from the decommissioning operations and generally do not consider the potential ecosystem values of structures that have developed during the operating phase such as biodiversity and biomass production. For example, the environmental impacts identification workshop for the decommissioning EIA for the Murchison platform (Table 2), one of the

biggest steel jacket platforms in the North Sea, highlighted the presence of the coldwater coral L. pertusa and large volumes of marine growth and fish biomass (CNR International, 2013). However, based on the rationale that these biological communities would not be present in the absence of the structure and due to the restrictive permissible decommissioning options, these aspects were not considered a significant issue and were not investigated further. In jurisdictions where all or part of the structure may be left in place, operators need to demonstrate that leaving the structure in place results in a net benefit to the marine environment compared to full removal. Some operators have therefore used the NEBA approach for decommissioning, a framework initially used in oil spill management that incorporates ecosystem values and weighs the environmental benefits and costs to determine the best performing decommissioning option. For example, the NEBA of the Bongkot structure in Thailand (Table 2) placed strong emphasis on the biodiversity values of marine communities associated with the structure, with high weightings for marine fish ecology, marine benthic ecology and marine fish production in the aggregate scoring assessment (Kanmkamnerd et al., 2016). These examples highlight the variable scope of environmental assessment criteria used in the evaluation of decommissioning options (Table 2) and the potential challenges this poses in selecting the decommissioning option with the best environmental outcome.

Table 2 Comparison of two multi-criteria decision analysis (MCDA) approaches used and environmental criteria assessed to evaluate the environmental impacts of decommissioning options for the Murchison platform in the North Sea and the Bongkot platform in Thailand, using Comparative Assessment (CA) and Net Environmental Benefit Analysis (NEBA), respectively. Murchison platform CA – United Kingdom

Bongkot platform NEBA - Thailand

Total energy consumption and CO2 emissions: Total energy consumption and CO2 emissions including direct, recycling and replacement energy components.

Air quality and emissions: Direct energy use from vessel use, land vehicles, equipment, fuel. Recycling and waste disposal not considered.

Impacts of operations: Operational impacts on aspects of the marine and terrestrial environment, including noise, seabed disturbance, contamination, vessel presence, accidental events (spillage, dropped items).

Marine water quality: Sediment disturbance, release of contaminants into the water column. Spills and leaks from vessels.

Impact of end-points: Impacts of the final condition of the material or components on the marine and terrestrial environment, e.g., waste.

Sediment quality and physical disturbance: Seafloor disturbance, such as removal of platforms below the mud line. Marine benthic ecology: Disturbance of benthic fauna and infauna. Marine fish ecology: Disturbance or removal of fish habitat. Marine mammals and reptiles: Collision, disturbance through underwater sound, behavioural effects, feeding and breeding patterns. Marine birds: Effect on migratory birds. Marine fish production: Effect of altered habitat on marine fish production. Terrestrial habitat value: For options involving onshore activities (e.g., waste, transport). Coastal habitat value: Risks of collision, groundings, propeller scarring, spills, leaks near coral reefs, seagrass meadows and mangroves.

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The broad use of MCDA approaches indicates the need to optimise the decision problem by weighing potential benefits and detriments, yet the choice of assessment criteria (Table 2) and different allowable decommissioning options among regions can produce vastly different ‘best’ outcomes. In particular, the consideration of a broader range of environmental effects is necessary to select the decommissioning option with the best environmental outcome. By initially restricting options, the scope of environmental assessment criteria is reduced, potentially resulting in suboptimal decisions as some environmental and ecological criteria are omitted from evaluation. For example, excluding the evaluation of the established biological community on the Murchison structure (Table 2) due to the restrictive decommissioning options in the North Sea barred the opportunity to identify the potential environmental benefits from leave in place options and to reduce specific risks associated with complete removal. Decommissioning planning, decision-making and implementation can take many years (e.g., an estimated 9 years for the Murchison platform, 2009–2017) and assessments that include the full range of ecosystem considerations, both detrimental and beneficial, are more likely to identify the best-performing option and reduce the risk of oversights. An ecosystem approach to decommissioning also assumes a full range of decommissioning options and an understanding of the ecological values of platform ecosystems, and how they interact with surrounding natural and artificial habitats. By initially restricting options, the scope of the assessment process is reduced, potentially resulting in suboptimal decisions. 4. Information needs to gain an ecosystem understanding of offshore O&G decommissioning An ecosystem approach to decommissioning entails understanding of a broader range of environmental aspects than is often currently considered, concerning the types of aspects, as well as the temporal and spatial scales at which they operate. With new analytical approaches and improved data availability, a more comprehensive environmental perspective on the impacts of decommissioning options has become more achievable in recent years. Ecosystem approach - In general, research into the ecology of structures has focused on particular regions, such as California, the Gulf of Mexico and the North Sea, with less known about the importance of habitat provided by structures in other regions (e.g., Southeast Asia, West Africa). In less-researched regions, research focus on only a few aspects such as fish production, currently limits an ecosystem understanding of the effects of decommissioning. A redirection of research to environmental decommissioning aspects that are typically less studied (e.g., taxa other than fishes, trophic ecology, temporal dynamics) will enable a move toward more comprehensive and quantitative environmental assessment of decommissioning options. Increased emphasis on a broader range of species, and on whole-community and functional approaches grounded in general ecological theory will build our understanding of the broader biodiversity values, and ecological functions and services provided by offshore O&G structures. Trait-based approaches have dramatically improved our ecological understanding of natural systems (e.g., coral reefs; Sommer et al., 2017) and will likely facilitate a more mechanistic understanding of the ecosystems associated with O&G structures and generalisation of processes operating at regional and global scales (e.g., in relation to latitude, depth, surrounding habitat). Regional and longer-term focus - Research directed toward understanding the regional ecological context of structures, including the connectivity between structures and natural habitats and the temporal dynamics of structural communities, will inform our understanding of the cumulative effects of decommissioning decisions. This can be achieved through cooperation between industry, regulators and researchers, as exemplified by the international INSITE project (www.insitenorthsea.org), which aims to fill data gaps and increase scientific knowledge of the influence of man-made structures on

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Table 3 Environmental considerations and potential indicators to guide the identification of relevant environmental criteria for decommissioning assessment. Primary consideration

Attributes/potential indicators

Habitat and biodiversity values

Abundance Biomass Species richness Diversity Ecosystem engineers Functional groups (e.g., predators, herbivores) Trophic interactions/pathways Threatened/endangered species

Biomass production

Recruitment rates Growth rates Mortality Site fidelity

Regional ecology

Spatial configuration of habitats (natural and modified) and biodiversity Oceanographic regime Life-history information (e.g., reproductive mode, larval duration) Adult movement patterns

Conservation

Area protected from trawling Historical fishing patterns/practices

Energy, emissions, recycling

Direct energy use (e.g., vehicle movements) Indirect energy use - for recycling and replacement of material Emission factors - combination of fuel efficiency and engine design Landfill

Contamination (including biocontamination)

Resuspension of seabed contaminants Transport of seabed/structure contaminants Exotic/invasive species Disease Heavy metals/bioaccumulants

Temporal dynamics

Temporal changes in biological communities Integrity of structure over time

North Sea ecosystems. Long-term monitoring of decommissioned and reefed sites will facilitate our understanding of the long-term effects of removing offshore structures on the marine environment, including how long it takes for marine habitats to re-establish following decommissioning. The ecosystem approach to decommissioning therefore hinges on the availability of data at broader taxonomic, spatial and temporal scales than are often currently available. In well-researched areas, studies may be limited to specific knowledge gaps that still hinder the resolution of decommissioning options within the decision process, whereas in lessresearched areas, programs that deliver a broad knowledge base across many environmental considerations will best aid decommissioning decisions. To this end, Table 3 represents environmental aspects for consideration during the scoping phase of environmental assessments, particularly for areas where few investigations have been conducted. Information is scale-dependent, with regional and longer-term variables likely spanning numerous structures, such that information does not necessarily need to be collected for each structure. For example, information on ocean current trajectories may only need to be obtained once for a region encompassing many structures. This provides for the leveraging of shared datasets and can guide a fit for purpose approach to acquire new information to be incorporated into MCDA frameworks. 5. Looking ahead: expanding the toolkit to advance the understanding of the ecology of O&G structures Improved analytical techniques, computing power and the availability of habitat and environmental data provide opportunities to identify and fill gaps in our understanding of the ecological dynamics of O&G structures and decommissioning decisions at multiple scales. These

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studies typically use statistical modelling to integrate biodiversity, lifehistory (e.g., pelagic larval duration), molecular, habitat or biophysical (e.g., oceanography) data over a variety of spatial and temporal scales. While these approaches have a long tradition in land-use and conservation planning in different systems (e.g., Margules and Pressey, 2000), they have rarely been applied in the context of offshore O&G. For example, species distribution and larval dispersal models can be used to model the potential loss of fish production and habitat under different decommissioning scenarios. Pondella et al. (2015) found that structures export fish larvae to other areas and that complete removal of structures would eliminate almost all fish biomass, while topping would retain over 90% of biomass at the deeper structures in southern California. Ecological and oceanographic data have been combined to investigate the ‘stepping stone’ connectivity effect of structures (Coolen, 2017; Simons et al., 2016; Thorpe, 2012). For example, Thorpe (2012) used a Lagrangian approach to show that approximately 60% and 28% of structures in the southern and northern North Sea, respectively, are connected via larval transport, whereas the northern and southern North Sea regions are isolated from each other. In California, larval dispersal via currents explained the spread of the bryozoan Watersipora subtorquata from one to four structures and that further spread were unlikely (Simons et al., 2016). These studies exemplify how existing ecological data (e.g., species distribution patterns, pelagic larval durations) can be used in novel ways to understand the regional ecological context of O&G structures and the effects of decommissioning decisions at regional scales. 6. Conclusions O&G structures represent a considerable proportion of artificial marine habitat and, after decades at sea, they support substantial biological communities. How they are decommissioned therefore has regional ecological implications. Assessment of the environmental effects of decommissioning would benefit from a regional and multi-disciplinary approach that considers the broader ecosystem values of offshore structures across the complete range of decommissioning options. The ecological setting of decommissioning is substantially different from that of the exploration and production phases, because after decades at sea, structures support biological communities that provide valuable ecosystem functions and services. Decommissioning decisions therefore need to consider a broader suite of environmental aspects than is typically evaluated during exploration and production, including the ecosystem benefits that structures provide and the negative effects associated with the loss of this reef habitat and associated biological communities, along with those impacts more typically considered during other phases of the platform life-cycle. Moreover, an ecosystems approach that evaluates the full range of environmental risks and benefits for a broad range of decommissioning options will likely improve environmental outcomes, in contrast to some current approaches that eliminate decommissioning options before they are fully considered. Expanding the decommissioning options available and the environmental criteria assessed provides a more robust and comprehensive assessment of the environmental impacts from decommissioning that is more likely to optimise outcomes across the broad range of ecosystems in which structures are located. Acknowledgements This research was supported by an environmental research grant from the Exxon Mobil Upstream Research Company. References Ajemian, M.J., Wetz, J.J., Shipley-Lozano, B., Shively, J.D., Stunz, G.W., 2015. An analysis of artificial reef fish community structure along the northwestern Gulf of Mexico shelf: potential impacts of “rigs-to-reefs” programs. PLoS One 10, e0126354. Atchison, A.D., Sammarco, P.W., Brazeau, D.A., 2008. Genetic connectivity in corals on the flower garden banks and surrounding oil/gas platforms, Gulf of Mexico. J. Exp. Mar. Biol. Ecol. 365, 1–12.

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