Application of Bowtie Analysis to Sub-Surface CO2 Disposal

Bowtie analysis is an established barrier-based risk assessment technique^[1] that involves building a pictorial representation of plausible accident scenarios – a bowtie diagram – usually in a multi-disciplinary workshop environment. It provides a means of analysing, qualitatively and in depth, the possible causes of an unwanted event and the potential consequences should such an event occur and/or escalate. It also enables an evaluation of the individual prevention and mitigation measures – the barriers.

In the context of CO₂ disposal, bowtie analysis has become a standard technique for identifying and analysing the risks of leakage from geological CO₂ storage sites.

Bowtie diagrams can be understood at all levels, including by non-risk specialists. It is, therefore, a convenient tool for communicating various aspects of a particular hazard and the means by which it is managed in a clear, logical, and substantiated manner. This is especially beneficial in an emerging field such as sub-surface CO₂ storage, where the level of risk perceived by the public can be significant, and the nature of the risks and how they are managed can be highly technical. Bowties are very effective in enabling diverse stakeholders to visualise how risks are being managed.

The process of developing a bowtie diagram also brings significant benefits, as the bowtie workshop provides an opportunity for all parties involved in the CO₂ storage project to raise and discuss potential problems, allowing each discipline to appreciate how their decisions and plans can impact on other areas of the project. By aiding a clearer, shared understanding of all aspects of how a particular risk scenario can occur and how it is managed, the bowtie exercise contributes directly to more informed, and better, decisions being made.

Application of bowtie analysis within the oil and gas industry globally is mature; it has been in widespread use since the mid-1990s and tends to be used to illustrate how Major Accident Hazards (MAHs) are managed. In this context, MAHs include hydrocarbons being extracted, processed, stored and distributed, and the unwanted events usually relate to loss of containment, fires, explosions, etc. with potentially catastrophic consequences. Prevention and mitigation measures are typically engineered systems and operating limits.

By drawing on real-life practical applications^[2,3,4,5] and the findings from multi-year research projects^[6,7], this paper presents the challenges, and some solutions, to applying the bowtie analysis technique to sub-surface storage of CO₂, where the unwanted event is similarly loss of containment but, unlike the analysis of process plant hazards, the prevention and mitigation measures include natural geological formations which act as impermeable containment barriers.

While bowties are always subjective, the absence of conventional process industry-type reliability data for geological barriers means the analysis must adopt different ways of evaluating the barriers and accounting for uncertainty. The range of causes which can lead to CO₂ loss of containment from sub-surface storage, and the way in which these causes (or pathways) are interconnected, means similarly interconnected bowtie diagrams may be necessary. Finally, proving that the risk of unwanted CO₂ release from storage is adequately managed can be challenging, in the absence of numerical risk values and acceptability criteria.

This paper presents an introduction to bowtie analysis and references relevant case studies to illustrate how the analysis can be adapted for application to sub-surface storage of CO₂. It concludes that bowties are well suited for detailed, structured qualitative understanding of the risks of loss of CO₂ containment, with potential for some (limited) quantification.

References

1. Center for Chemical Process Safety of the American Institute of Chemical Engineers, in association with the Energy Institute. Bowties in Risk Management, a Concept Book for Process Safety
   Wiley, ISBN 978-1-119-49039-5, 2018.
2. Northern Lights - Receiving and permanent storage of CO₂ Plan for development, installation and operation, Part II - Impact Assessment,
   EL001 October 2019. https://northernlightsccs.com/en/facts-and-reports
3. ENI UK Ltd. Liverpool Bay Carbon Capture and Storage Project
   https://www.eni.com/en-IT/operations/united-kingdom-liverpool-bay.html
4. Northern Endurance Partnership, Endurance Carbon Capture and Storage Project
   https://www.netzeroteesside.co.uk/
5. https://www.portofrotterdam.com/en/news-and-press-releases/ccs-project-porthos-a-step-closer
6. Risktec Solutions BV. Bowtie Analysis Report: DETECT: Integrated CO₂ Leakage Risk Assessment,
   Report No. SGSI-12-R-07, Issue 1, 2020.
7. SECURe project (Subsurface Evaluation of CCS and Unconventional Risks). https://www.securegeoenergy.eu/work-packages/wp2-risk-assessment