Event trees to understand hazards

Introduction

In complex industrial environments, anticipating and mitigating risks is crucial to ensuring safety and preventing incidents with potentially catastrophic consequences. Event trees provide a structured approach to analyse and visualise the potential outcomes of initiating events, allowing organisations to assess and manage risks effectively. This article aims to clarify the concept of event trees, their applications, and how they contribute to risk assessment and safety management in industrial settings.

What is an event tree?

An event tree is a graphical logic model providing a clear, visual method that identifies and quantifies the possible consequences of initiating events. It serves as a visual representation of the sequence of events that may unfold following a specific incident or scenario. Each branch of the tree delineates a potential course of action or failure, leading to different possible consequences. By quantifying the probability of each outcome, teams can prioritise risk mitigation strategies and allocate resources efficiently.

An event can be defined as an undesirable deviation from the desired or expected state of the operational intention.

A top event is a serious system failure or crash that forms the top of the fault tree and event tree.

The fault tree is used to identify the root causes of top events by tracing the chain of causality through “logical gates”.

When to use an event tree?

Event trees are employed both proactively and reactively in risk management:

• Before an accident: Proactive utilisation involves assessing the effectiveness of protection systems designed to prevent the occurrence of an incident with dangerous consequences. By assessing potential failure modes and their consequences, organisations can enhance the robustness of their safety measures.

• After an accident: In the aftermath of an incident, event trees help in identifying and quantifying the dangerous consequences of the incident. This retrospective analysis aids in understanding the vulnerabilities in existing safety protocols and informs future risk mitigation strategies.

How to use an event tree?

Each loss of containment event (top event), categorised by its physical state, corresponds to a unique event tree. There are three types of event trees, each delineating distinct consequence events and necessitating specific risk management strategies:

• Liquid release event tree

• Gas release event tree

• Pressurised liquefied gas (PLG) release event tree

Each event tree event that follows the start event is affected by the course of the previous event. Utilising an event tree involves categorising events and analysing their potential ramifications:

Type of release

Events are classified based on the nature of the release, distinguishing between the following categories:

• Instantaneous: a release of the entire contents of equipment in a very short time, and is usually related to scenarios when a vessel or tank ruptures completely.

• Continuous: a gradual and ongoing release over an extended period, usually through a hole in equipment.

Ignition vs. Dilution

Events involving the accidental release of flammable substances into the atmosphere are further categorised based on ignition potential. From there, several possibilities can unfold:

• Dilution: If the gas cloud is not within the flammability limits or lacks an ignition source, it may disperse without further incident.

• Direct ignition: Direct ignition can lead to rapid fire outbreaks.

• Delayed ignition: Ignition can occur after a certain time, potentially leading to more dangerous scenarios.

Confinement vs. No Confinement

For flammable substances, events are differentiated based on whether the released material is confined within a space. This classification is used after ignition and identifies subsequent explosion events.

• Confinement: if a flammable cloud is captured in a space with a certain degree of confinement, the explosion scenario needs to be taken into account.

• No confinement: if a flammable cloud remains unconfined, the explosion scenario is usually neglected and only fire scenarios are taken into account.

Utilising X-Suite for event tree analysis

Advanced software tools such as FLACS, EFFECTS, and RISKCURVES which are all part of Gexcon’s X-Suite, can facilitate simulation and analysis of event trees. These tools enable organisations to conduct comprehensive risk assessments, evaluate various scenarios, and develop robust safety plans.

Conclusion

Event trees serve as invaluable tools in comprehensively assessing and managing industrial hazards. By systematically analysing the potential consequences of initiating events, organisations can proactively mitigate risks, enhance risk protocols, and safeguard both their workforce and surrounding communities.

In the subsequent chapters, we will delve deeper into general event trees that specifically focus on identifying the consequences of an incident based on various loss of containment scenarios discussed earlier. These event trees will outline the potential sequences of events following a loss of containment incident.

References

Lees F. P. (1996). Lee's loss prevention in the process industries : hazard identification assessment and control (3rd ed.). Boston: Elsevier Butterworth-Heinemann.