Research focus: How bow-tie analysis tightens up risk assessment

By Matt Beeson, Risktec Solutions

A bow-tie is a graphical risk analysis technique, which illustrates what happens when control of a hazard is lost, what the cause of that loss of control is and what the impacts of an unwanted event would be. As part of the SECURe project, we are using bow-tie analysis as the foundation of a risk assessment framework for carbon storage and unconventional gas operations.

Risktec Solutions Ltd is leading the process of developing risk assessment framework bow-ties as part of Work Package 2 (WP2). The diagram here shows a basic bow-tie structure. The loss of control of the hazard is known as the “top event”. The causes of this top event are known as “threats” and the impacts resulting from the top event are known as “consequences”. The chronology of causation passes from left to right on the diagram.

The bow-tie technique allows for a structured approach to identifying what causes may result in an unwanted event and how this, in turn, could ultimately lead to adverse impacts. This flow of causation is interrupted by “barriers”, which are systems, structures, operations or measures intended to prevent a threat from leading to a top event, or even mitigate a potential consequence.

Barrier analysis is a process by which risk assessors seek to understand the effectiveness of barriers and the extent to which they contribute to the overall risk. The primary purpose of bow-tie analysis is to contextualise these barriers by representing them graphically. This has two clear benefits:

  • It facilitates the scrutiny of barriers during the risk analysis process by depicting them as discrete, tangible elements in an overall context; and
  • It communicates graphically the culmination of the risk management process in a way that is intelligible and useful to non-experts in risk management and / or the hazard subject material.

It is generally accepted that bow-ties were initially developed by Imperial Chemical Industries (ICI) in the 1970s, however, the exact origin of the technique is unclear. Shell is an example of an organisation that has integrated bow-tie analysis into the analysis of Major Accident Hazards (MAHs) during the development of safety cases for both upstream and downstream facilities, and is widely acknowledged as being the first major company to do so [see Reference].

The Centre for Chemical Process Safety (CCPS) has developed a Concept Book for bow-ties and process safety [see Reference]. The barrier concept is becoming more common in regulatory regimes, and standards and guidance provided by regulators and standards organisations such as the American Petroleum Institute (API), the UK Health and Safety Executive (HSE), the International Association of Drilling Contractors (IADC) and the Petroleum Safety Authority (PSA) in Norway [see Reference]. Bow-ties have been used extensively within Carbon Capture and Storage (CCS) projects to aid in understanding and demonstration of how project risks are being managed.

As part of the SECURe project, the bow-ties were initially drafted following an extensive literature review then subjected to scrutiny by experts in the scientific and engineering fields surrounding geological storage of CO2 and hydraulic fracturing. These initial bow-ties were then reviewed in a two-day WP2 workshop in May 2019 to gain agreement of their structure, logic and completeness.

The following month, Risktec presented the bow-ties at the SECURe General Assembly. During this meeting, large prints of the bow-ties were displayed on the walls of one of the conference rooms and delegates were invited to record their comments. Two further workshops were then held in July, one with WP2 at Heriot Watt University and the second with the British Geological Survey (BGS) at Keyworth. In total, nine bow-ties have been developed.

This development process is the first major step in developing a generic risk assessment framework that can be applied to any site or project proposing to exploit unconventional gas (shale gas) or develop geological CO2 storage.

Reference: Bow Ties in Risk Management: A Concept Book for Process Safety, Centre for Chemical Process Safety of the American Institute of Chemical Engineers (in association with the Energy Institute), September 2018