Ask an Expert
Tel: +1-281-673-2800
Fax: +1-281-673-2931
Find an Office
Email Us

COMAH Regulation: What is Occupied Building Risk Assessment?

EMEA Regional Focus: Process Safety Management 

What is Occupied Building Risk Assessment?

Occupied Building Risk Assessment (OBRA) is an assessment of buildings housing occupants given potential exposure to explosion, fire and toxic hazards.

As stated in the Control of Major Accident Hazards Regulations 2015 (COMAH), Regulation 5 - General duties of operators clause, "Every operator must take all measures necessary to prevent major accidents and to limit their consequences for human health and the environment." The regulator expects that all risks can be demonstrated to be As Low As Reasonably Practicable (ALARP). Therefore, it is difficult to demonstrate the risk to occupants within buildings are ALARP without undertaking an appropriate OBRA – and is typically expected to form part of the COMAH safety report by the regulator.

Sites are always changing, and as a part of the management of change (MOC) process, an OBRA should be maintained in-line with these changes. The OBRA is advised to be updated and revalidated every 5 years in line with the revision of the site COMAH safety report.

UK Guidance from the Chemical Industry Association

The Chemical Industry Association (CIA) document Guidance for the location and design of occupied buildings on chemical manufacturing and similar major hazard sites is widely considered as the industry best practice on undertaking OBRAs in the UK/Europe. Depending on the magnitude and complexity of the onsite hazards or potential risk at the facility, the following approaches are mandated:

  • Hazard (Consequence) Approach
  • Risk-Based Approach (Simple or Exceedance Methodology)

Compliance with the CIA guidance requires a detailed evaluation of fire, toxic and explosion hazards, which includes evaluation of building response, personnel protection and evacuation/shelter-in-place potential. This can be a complicated process that requires a multidisciplinary team and support from outsourced technical resources, such as ABS Consulting.

In addition to OBRAs, we also offer comprehensive services to mitigate hazards, including facility siting for new or relocated structures; building strengthening and new building design, window and door retrofits; emergency shelter evaluations; and evaluation of process changes to reduce risks. ABS Group's Extreme Loads and Structural Risk division is a leading provider of these services and uses cutting-edge technology to evaluate issues to whatever detail is needed.

6 Steps of Occupied Building Risk Assessment

OBRA is a continuous cycle. It is intended to be revised and updated as processes and facilities change, though it is not uncommon for the initial OBRA to be considered "the end" or the "goal." This ongoing cycle can be broken down into six (6) steps:

  1. Corporate Practice
  2. Pre-Study/Initial OBRA Study
  3. Detailed Assessments
  4. Remediation Planning
  5. Implement Remediation
  6. Management of Change (MOC)

There are a variety of technical approaches to performing OBRA. These approaches vary in their levels of applicability, conservatism and technical difficulty. Both consequence-based and risk-based methods can benefit from these approaches.

For example, vapor cloud explosions (VCE) hazards can be evaluated with widely used screening/traditional methods (Gaussian dispersion and VCE blast curves) or can be modelled using computational fluid dynamics (CFD) methods for both dispersion and explosion.

Fire hazards can be assessed (in order of complexity) using empirical modelling, geometric analysis (using view factor and shading from obstacles) and/or CFD fire modelling. Fire evaluation criteria can use heat flux values for certain consequences, such as thermal radiation intensity tolerable for evacuating personnel. A more detailed approach would be to perform a thermal dose calculation that integrates thermal radiation intensity exposure over time to predict personnel vulnerability.

Toxic release hazards can be assessed using traditional dispersion models or with CFD. Evaluation criteria can be based on endpoints of concentration levels, such as Immediately Dangerous to Life or Health (IDLH), or on a dose calculation that integrates exposure level over time to predict personnel vulnerability.

Apart from the simplest hazard assessment methods, most OBRAs are performed using a wide variety of software packages (commercial and in-house) to evaluate hazards with varying levels of complexity. As with any software package, technology is upgraded and additional capabilities are implemented. As more advanced software packages are utilized for this type of work, they become more prevalent in industry. This along with hardware improvements can lower the cost of analysis, improving the cost-benefit for screening level studies and detailed assessments such as CFD.

Hazards Analysis

Evaluating fire, toxic and explosion hazard consequences is required by the regulator to demonstrate worker protection and public exposure. ABS Consulting quantifies consequences to help our clients clearly understand the risks. Our knowledge and experience with a range of hazard analysis tools allows us to select and apply the most appropriate techniques for each situation. Primary services offered for hazards analysis include:

  • Accident Scenario Development
  • Explosion, Toxic and Fire Hazard Prediction
  • Consequence Based and Risk Based Evaluations
  • Portable Building Siting
  • OBRA Updates/Revalidation
  • OBRA for new process or new building construction
  • Detailed Structural Assessments of buildings for blast loads
  • Emergency Shelter Evaluations during fires or toxic releases
  • CFD modelling for dispersion, explosion or fire hazards
  • Finite Element Analysis (FEA) modelling for building response

Inherent Conservatism?

One of the key issues with OBRA is that there can be a danger of an assessment becoming a "black box" that may lead to outputs being overly conservative. Separate to the disparity or appropriateness of the assumptions made in an OBRA, a key conservatism for the calculation of risk to occupants within buildings is the building vulnerability assessment. This is the relationship of the occupant vulnerability based on the building damage due to a blast wave.

In many software packages, vulnerability functions use generic building types, based on empirical data sets. This approach is adequate in many cases, however, the question should be asked, "How do your onsite buildings compare to the empirical building?"

In our experience, structural engineers can add value by making the building vulnerability assessment directly applicable to a facility's buildings.

By understanding how the primary structure (columns, load bearing walls etc), secondary structure (infill walls, cladding) and the non-structural elements (glazing, doors etc) interact and respond when subjected to a blast wave, the following can be achieved:

  • Improved occupant vulnerability assessment
  • Targeted mitigation strategies at specific building components
  • Identify cost-effective solutions

Involving a structural engineer in the OBRA can add a great deal of value by removing potential conservatisms, creating a building specific vulnerability model and developing specific, cost-effective mitigation plans where required.

To learn more contact one of our experts in our UK office:

Back to top