Applications of Cause-Consequence Diagrams in Operational Risk Assessment

Global Congress on Process Safety
April 3, 2012 - 12:00am

Learning Outcomes:
  • Understand the importance of hazard evaluation and risk assessment in the context of and safety and environmental management systems
  • Gain knowledge of how cause-consequence diagrams (e.g. Bow-ties) summarize and depict the hazard identification and risk assessment process of a project or facility
  • View and obtain insight from real cause-consequence diagrams examples

1. Introduction

The decades of the 70s and 80s witnessed an unprecedented series of major chemical disasters worldwide that fueled the development of comprehensive hazards guidelines and standards for processes involving flammable liquids or gases. Process risk management has been in the agenda since the first known reference to process safety was published in 1987 [1], through the 1992 Process Safety Management (PSM) key regulatory milestone for onshore facilities along with industry implementation efforts, and the latest development involving offshore exploration and production regulation by the Bureau of Ocean Energy Management, Regulation and Enforcement (BOEMRE), where operators are to submit mandatory Safety and Environmental Management System (SEMS) Plans by 15 November 2011.

2. The Pillar Element: Hazard Analysis and Risk Assessment (HIRA)

HIRA or “Risk” is pillar to every safety and risk management system and nearly all system elements either feed of are built upon process risk outputs. A key provision of regulatory based safety management systems (e.g. OSHA PSM and BOEMRE SEMSs) is “hazard analysis”, including identification, evaluation, and risk reduction. This is performed thorough an orderly systematic examination of causes leading to major accidents and what safeguards must be implemented to prevent releases of hazardous substances. The risk assessment process comprises four parts: risk identification, risk analysis, risk evaluation, and risk treatment [2]. The first and foremost step is risk identification, quoting Lloyd Nelson “figures on the most important areas of management are unknown or unknowable, and successful managers must nevertheless manage those areas” [3]. By thoroughly identifying hazards, potential sources of accidents are made known, and thus, successfully managed.

However, with more than 25 years of PSM history, and after innumerable hazard and operability study (HAZOP) worksheets have been produced, findings followed upon, and study binders filed in archives, catastrophic events still occur. Only six months earlier, one the worst Gulf of Mexico accident took place: the Deepwater Horizon Blowout of 20 April 2010. Then, what are we missing? Is it possible that safety critical elements -SCE (i.e. equipment, procedures and activities) are not being adequately extracted from the risk assessments, clearly communicated, and thoroughly performance-verified? And if so, is there a tool that can bridge the gap between qualitative and numerical hazard evaluation, and depict in one diagram what both CEO and junior operator need to know and address to prevent and mitigate accident events?

3. Cause-Consequence Diagramming and the HIRA Process

Cause-Consequence analysis (CCA) is proving to be a very useful tool to depict and maintain an up-to date, real-time working risk management system enthralled in daily operations (e.g. operational). That explains why bow-tie diagrams are becoming increasingly popular. These diagrams combine the inductive and deductive reasoning of logical diagrams (e.g. ETA, FTA) to identify the basic causes and consequences of potential accidents. Bow-tie is a less formal CCA that provides a pictorial representation of the risk assessment process. Bow-ties are extensively used in European drilling and production safety cases to demonstrate major hazards are identified and controlled to as low as reasonably practicable (ALARP).

4. Application of Cause-Effect Diagrams in HIRA

Bow-ties are beginning to be used worldwide drilling, oil and gas processing, healthcare, and nuclear, transport, and organizational applications. Some of the benefits are:

  • Illustration of the relationship between major accident and the causes or threats leading to its materialization
  • Linking accountability, performance standards, and any relevant information associated with preventive and mitigation barriers (SCE)
  • Links to the HSE Management System
  • Represents the risk management process structure to link information that demonstrates ALARP
  • Promotes workforce participation and ownership
  • Promotes “live” or evergreen documentation throughout the business lifecycle

Specific worked examples to be discussed in the presentation include:

  • Major Hazard Event ALARP Demonstration
  • Layer of Protection Analysis
  • Incident Investigation

[1] Center for Chemical Process Safety (CCPS) “A Challenge for Commitment”. 1987

[2] ISO 31000:2009 Risk Management Principles and Guidelines

[3] W. Edwards Deming “Out of the Crisis”, Cambridge, Mass.: IT 1982

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