Full Title: A Holistic Approach to Fire Risk Assessment: How to Combine Bow-Ties of Standard Fire Scenarios to Develop a Creative yet Robust, Barrier-Based Fire Safety Strategy

By Luca Fiorentini

Introduction

Nowadays, a performance-based approach to fire safety can be considered a specific discipline in fire engineering activities. A performance-based approach verifies the fire safety strategy by considering specific fire scenarios and the negative effects (for occupants, environment and assets or business continuity) in a specific fire dynamic.

Fire scenarios should become the basis of the assessment for the evaluation of the fire strategy: Given predefined thresholds, performance has to be verified as consistent with the fire risk associated with the scenario. Due to the amount of time needed to perform a quantitative assessment of the outcomes of credible and relevant scenarios, it is fundamental to start from a precise fire risk assessment. Performance-based approaches to fire engineering have shown that risk-based decisions and fire scenarios are essential elements of fire safety strategy assurance, but fire risk assessment has always been a challenging issue.

Robust fire strategies are not those based on complex studies, but are those that represent reality and are can be maintained over time and in all the possible modifications. The performance also should be able to be monitored over time (audited) to guarantee a proper level of residual fire risk (acceptable) or mitigate an increase of risk with alternative measures (technical and organizational). This is an invaluable advantage when you have to identify alternative strategies for new or existing buildings.

Alternative solutions can be evaluated by taking the risk reduction of different strategies into account and by assessing different elements of fire strategies themselves, as well as costs, with a modern “as low as reasonably practicable” (ALARP) approach. Different strategies are based on safety critical elements (SCE) and barriers. In the end, in the initial performance-based approach, the focus should be on fire safety barriers management over time.

Application of a Barrier-Based Management System to Fire Safety

Barriers are the key elements of a fire safety strategy. They should be maintained over time to assure a proper level of fire safety and monitored to manage any deviations that are likely to cause a fire risk level that is not compliant with acceptable defined thresholds (in terms of frequency or consequences). A fire safety barrier-based management system (BBMS) can be an invaluable tool for focusing on SCE (identification, associated risk reduction factor and current status). This provides an overview of the current fire risk level throughout the facility and makes it possible to compare against different assets (sites) of the same owner, especially for those characterized by a high complexity.

Railway stations, ports and airports are nowadays among several specific locations characterized by the presence of masses of people, often in fast transit — a clear example of infrastructures open to the public that are constantly subject to modifications. Changes take place to install temporary stores, new stores or facilities (even schools, healthcare facilities and hotels), advanced security measures due to recent episodes, new utility systems, and networks.

Occupants and users of these facilities/infrastructures are particularly heterogeneous and much more different from the general population. These spaces have become centers open to the public where people can find commercial stores, services, restaurants, etc. The modification process in several cases is even more “dangerous” because these infrastructures are sometimes located in heritage buildings that pose both physical and permitting-related constraints. New expansion is often designed to meet green building criteria that pose new treats to fire safety and particular architectures can be characterized by complex geometries and distributed across many interconnected buildings.

As an example, railway stations are built around train tracks and often composed of several buildings, with some underground floors (usually connections to subway stations or utility spaces, or warehouses for stores located on upper floors).

Barrier-Based Methods Starting from Bow-Tie

The Bow-Tie method (from the oil and gas quantitative risk assessment) is the proper tool to:

• Represent the main cause and consequences in a logical and coherent description;
• Identify both prevention and protection barriers that can prevent the occurrence or mitigate the consequences (or reduce both) of the identified outcomes;

Consider both technical and management barriers.
Bow-Tie is characterized by a clear and simple methodology; it is appropriate to use to:

• Explain fire design considerations to both upper management and field operators;
• Identify the need for new/different barriers;
• Make comparisons among areas, assets and stations at a global level;
• Describe the current and intended level of fire safety;
• Conduct technical audits and use the results to visualize the current status of barriers against the minimum established level (e.g., using inspections result data on active fire protection systems to update relevant barriers and update the related key performance indicator/s).

A top event can be developed with engineering tools, as well as used as the basis of an emergency and evacuation plan. Bow-Tie can also be used to demonstrate/verify the impact of modifications (in organization, systems, assets and compartments), especially with the support of a HAZard IDentification (HAZID) method to verify the impact of temporary modifications on the overall fire safety level (for instance, closure of emergency exit due to temporary construction sites inside the station and impact on evacuation strategy) to meet the requirements of a specific management of change process.

The Bow-Tie tool incorporates different methods of analysis, both qualitative (for a preliminary assessment) and quantitative (for the development of a detailed analysis). It is characterized by the possibility of analysing complex events that, in the face of a single cause, can determine several consequences or similar effects for different source terms. Bow-Tie diagrams are used extensively in risk analysis, especially for assessments of the degree of safety of complex structures.

Bow-Ties are composed of a fault tree connected to an event tree. The junction point — the center of the bow tie — represents the critical event under examination. With the use of one or more fault trees, one identifies each cause that contributes to the occurrence of the critical event; through the event tree, the incidental sequences that may originate are identified.

A Bow-Tie diagram can easily be created by defining a series of aspects, specifically:

• the event to be prevented;
• the causes that can determine the onset of the problem;
• the consequences of the event in case it manifests itself;
• prevention measures;
• protection measures.

The Bow-Tie represents causes and possible consequences of fires while measuring (in/expected) prevention and subsequent mitigation (limitation of expected consequences). Identifying the measures that actually exist helps pinpoint the missing or inadequate links in a chain and determine the most urgent improvement actions to guarantee the absolute minimum of fire safety.

A simple Bow-Tie can be used to identify the main safety functions and the main barriers associated with those functions.

The initial event is a small fire in a compartment. To avoid a large fire, the ignition function is in place (work permits, safety of electrical systems, fire quality and reliability of the building, compliance with the smoking ban and control of other heat sources, inspections, maintenance). After a large fire, several barriers are available to avoid the severe negative effects of a large fire: alarm, evacuation, smoke control and fire control.

A quantitative assessment can be made by means of a specific worksheet to simplify data input where, as in a layer of protection analysis (LOPA), specific measures have been applied to all the elements in the diagram.

The Bow-Tie can become the central tool to manage the fire risk cycle (identification, assessment, mitigation and control).

This provides the possibility of creating pictures of the current status of the barriers of each control function and grouping them to form a dashboard of data for the building owner.

Conclusions

A barrier-based approach to locations of great complexity (railway stations, airports, subway stations, etc.) shows a lot of benefits and demonstrates how to manage all the requirements of applying several regulations in complex realities effectively. The entire process could even be supported by a cloud-based information technology platform. Application of the BBMS also resulted, in the working cases conducted, in the ability to collect information, data, documents and performance indicators to support a fire certificate request to the authorities, make better informed decisions, and demonstrate to all the stakeholders the activities in place in real time.

Employing an information technology systems would allow building owners and fire protection engineers to measure maturity levels of barriers and underline (site by site, in the case of complex realities distributed in the territory) their quality.

Luca Fiorentini is a Executive Director with Tecsa s.r.l.