An Analytical Method to Combine Environmental Attributes to Generate the Matrix of Importance of Environmental Impacts for the Decommissioning of Subsea Installations in Brazil

A new method developed to evaluate the performance measure of some sub-criteria of the environmental criterion in a Comparative Assessment of various options of the decommissioning of subsea installations in Brazil is presented. The method is based on an adaptation of that proposed by IBAMA in Technical Note N°. 10/2012 (TN 10) used for assessment of environmental impacts required for the licensing of offshore activities in Brazil. By requirement of the Comparative Assessment methodology, there is a need for a numerical evaluation that allows comparing the performance of one alternative over the others. An analytical method was developed based on the combination of environmental attributes that contribute for the magnitude of the impacts (frequency, intensity, extension and duration) and sensitivity of the environmental factors affected (relevance, resilience and reversibility). The importance of the impacts is given by the combination of the environmental sensitivity with the magnitude of the impacts, in the called "Environmental Impact Importance Score Matrix". The proposed method provides a performance measure that allows comparing and prioritizing each decommissioning option according to the proposed criteria. It reduces the subjectivity in assessing the importance of the environmental impacts and provides greater transparency and traceability in the assessment of the impacts. As an example, the proposed method is applied for the decommissioning of a rigid pipeline between two platforms in Campos Basin, where the considered decommissioning options are: (a) complete removal by cut and lift and (b) leave in situ. For this particular case, the assessment of the proposed environmental sub-criteria revealed that Option (b) was the preferred option with respect to the impacts on marine and onshore environment.