Taxpayers and operators worldwide have significant current liabilities associated with decommissioning of offshore Oil & Gas (O&G) assets. Consequently, decommissioning is at the forefront of industrial, governmental, and non-governmental agendas. Decommissioning is a highly complex activity with health, safety, environmental, social, economic, and technical implications. Increasing scientific evidence supports that manmade subsea structures create hard, artificial reef habitats that provide ecological and social benefits to society. Given the significant uncertainty regarding how subsea structures should be retired at the end of their operational lifetimes, it is necessary for governments, taxpayers, and operators to understand the risks and benefits associated with potential decommissioning options. Currently, the North Sea decommissioning process is based on the policies and direction of the Oslo and Paris Convention’s (OSPAR) Decision 98/3 and follow comparative assessment (CA) multiple-criteria decision analysis (MCDA) guidelines to determine the best overall strategy for decommissioning subsea structures; however, CA MCDA processes can be biased, ambiguous, difficult to use, interpret, and replicate, and limited in their consideration of multigenerational benefits. Consequently, to assist decision-makers in understanding and evaluating options and associated benefits for decommissioning subsea structures, this study adapted the net environmental benefit analysis (NEBA) framework to supplement and strengthen the CA process for evaluating decommissioning options for offshore O&G facilities. The net environmental benefit analysis based comparative assessment (NEBA-CA) framework is presented that addresses the growing need for a practical, quantitative, scientifically robust, defendable, and transparent MCDA approach to determine optimized decommissioning strategies for subsea assets. Increased transparency in CAs will provide an additional layer of credibility with regulators and society. The approach is data driven and a desktop analysis mainly relying on existing data. Using a North Sea case study, this work demonstrates the ability of NEBA-CA to resolve inherent complexity in comparing decommissioning options, thereby supporting operators in working with regulators to decommission assets in a way that maximizes ecosystem service benefits to society while managing site-related risks and costs. The NEBA-CA framework supplements and strengthens the standard CA process by 1) incorporating quantified metrics including multigenerational ecosystem service benefits and risks, 2) excluding front ranking (scoring) or weighting of metrics, and 3) providing consistent graphical displays to support visual differentiation of options and metrics.

Presented herein is a newly developed quantitative approach for assessing potential ecological risk resulting from long-term degradation of deep-sea plastic-containing infrastructure. The risk characterisation involves four iterations of modelled ‘risk’ through forward or backward calculation of a deterministic hazard quotient, mathematically defined as the ratio of estimated exposure to a reference dose (or concentration) for a similar exposure period. The assessment focuses on direct effects of microplastics exposure, wherein exposure concentrations are based on modelled estimates of microplastic mass formation resulting from structure deterioration over time. Predicted no effect concentrations (PNECs) protective of slightly-to-moderately disturbed ecosystems and ecosystems of high conservation value were determined based on a species sensitivity distribution (SSD), in accordance with the current Australian and New Zealand Guidelines for Fresh and Marine Water Quality. Each iteration of risk characterisation is performed irrespective of burial, with varying exposure unit dimensions (i.e. geographically localised and broader regions of microplastic dispersal) and degrees of plastic degradation, designed to conservatively bound the risk characterisation. Additionally, two SSDs derived from different ecotoxicological data sets prioritising either particle shape or marine species are also provided for a sensitivity analysis of the PNEC. Thus, the bounding exercise encompasses all possible outcomes. The risk characterisation approach is reviewed for a case study of two larger plastic-containing flowline assets in an oil production field offshore of Australia. The outcome of the risk assessment is the same for all model iterations: degradation of the subsea plastic-containing flowlines does not pose a risk to the local marine community.

Australia’s oil and gas sector expects approximately 56 billion (AUD) to be expended for decommissioning costs over the next 50 years. The view of the National Offshore Petroleum Safety and Environmental Management Authority is that the designated decommissioning approach must provide equal or better environmental outcomes when compared to full removal of the infrastructure (considered the ‘best case’ expectation under current legislation) and meets as low as reasonably practicable (ALARP) levels of risk. Decommissioning alternatives are generally evaluated through a comparative multicriteria assessment approach, for example, a net environmental benefit analysis-based comparative assessment (NEBA-CA). Decommissioning options cover a range of possibilities, from full removal to leaving subsea structure in situ (in part or in whole). NEBA-CAs we have conducted suggest that some subsea structures are projected to generate significant ecological and social value over multiple generations to come. Thus, in these cases, management in situ can provide greater benefits to the public when compared to full removal. During operation, oil and gas assets typically incorporate a safety zone. If subsea structure is selected to be managed in situ, a question that arises is, should the safety zone be maintained or removed? Our experience indicates that if the safety zone is removed after decommissioning an asset that is left in situ, there may likely be a greater adverse impact on ecological and social values, depending upon the asset location. This abstract showcases the value of the use of safety zones to maximise environmental value while managing risk.

Presented on Thursday 19 May: Session 23 Australia’s oil and gas sector expects approximately 56 billion (AUD) to be expended for decommissioning costs over the next 50 years. The view of the National Offshore Petroleum Safety and Environmental Management Authority is that the designated decommissioning approach must provide equal or better environmental outcomes when compared to full removal of the infrastructure (considered the ‘best case’ expectation under current legislation) and meets as low as reasonably practicable (ALARP) levels of risk. Decommissioning alternatives are generally evaluated through a comparative multicriteria assessment approach, for example, a net environmental benefit analysis-based comparative assessment (NEBA-CA). Decommissioning options cover a range of possibilities, from full removal to leaving subsea structure in situ (in part or in whole). NEBA-CAs we have conducted suggest that some subsea structures are projected to generate significant ecological and social value over multiple generations to come. Thus, in these cases, management in situ can provide greater benefits to the public when compared to full removal. During operation, oil and gas assets typically incorporate a safety zone. If subsea structure is selected to be managed in situ, a question that arises is, should the safety zone be maintained or removed? Our experience indicates that if the safety zone is removed after decommissioning an asset that is left in situ, there may likely be a greater adverse impact on ecological and social values, depending upon the asset location. This abstract showcases the value of the use of safety zones to maximise environmental value while managing risk. To access the presentation click the link on the right. To read the full paper click here