The decommissioning of offshore assets has increased in recent years as platforms are reaching the end of their operating life. This trend exists for decommissioning in regulatory environments that are well established, as well as those that are stewarding decommissioning for the first time. Where regulatorily permissible, multiple decommissioning options currently exist for offshore structures. These options can be broadly bucketed into removal to shore, reefing and alternative uses. A multi-criteria decision analysis (MCDA) is often used to compare the options to determine the best possible outcome. The goal of this work was to assess the different MCDAs in order to promote a single, global methodology.

The first step was to list all existing MCDAs that have been used or promoted to be used for assessing decommissioning options globally. The MCDA models reviewed were: Comparative Assessment (CA), net environmental benefit analysis and best practical environmental option. After identification, each of the MCDA methods were analyzed to understand their process and their real-world applicability.

The results of this review process revealed that the UK Oil & Gas CA is a holistic, clearly defined process that is both rigid in scope and flexible in deployment manner. The CA includes five evaluation criteria: safety, environment, technical, societal and economic. These criteria are further categorized to sub-criteria that are relevant to the asset to be decommissioned, e.g., impacts to other users of the sea. The process is clearly defined and provides a transparent mechanism to engage stakeholders. Further, these criteria are well aligned with the International Maritime Organization's guidance and standards for the non-removal of offshore installations and structures.

This work includes the review of a case study that employed CA in the Gulf of Mexico where the outcome was the successful support for the decommissioning option of horizontal reefing in place.

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