A methodological approach for an objective environmental impact assessment to support the decision-making in the decommissioning of oil and gas subsea installations in Brazil

The lifecycle of offshore oil and gas installations has reached an age at which many need to be decommissioned. The challenges are many, as both the types of structures and the marine environments in which they occur are diverse. We present the framework of a method for assessing the environmental impacts of the decommissioning activities of oil and gas structures, focusing on the characteristics of the Brazilian installations, its marine ecosystems, and legislation particularities. The magnitude of the environmental pressures is derived from the technical features of the decommissioning process and uses, as much as possible, quantitative indicators with continuous or discrete scores. Environmental sensitivity, which is also expressed in numerical scores, is based on the component sensitivity and the presence of species threatened with extinction within the area of influence of the decommissioning activity. The objectivity sought in our approach makes the decommissioning process more transparent and traceable from the planning to its execution. This is likely to minimize conflicts among stakeholders and to smooth the licensing process by the regulatory environmental authorities. Moreover, the approach described here for subsea structures may be transferred to decommissioning other structures, advancing towards a unified method for the whole oil and gas sector.