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Occupied Building Risk Assessment

Protect site employees from major process incidents

Occupied Building Risk Assessment (OBRA) is a tool used to demonstrate that people occupying buildings on process sites are adequately protected from hazards such as explosions, fires, chemical reaction fallout and toxic releases and that they can safely escape from those buildings.

Risk assessment for all occupied buildings on process sites to protect employees.
Risk assessment for all occupied buildings on process sites to protect employees.

The Flixborough explosion in 1974, followed by more recent accidents such as Hickson & Welch’s explosion and fire in 1992, and the Texas City accident in 2005, all illustrate the dangers to people in nearby occupied buildings from dangerous substances on process plants.

As a result, both the UK Chemical Industries Association (CIA) and the American Petroleum Institute (API) have both published guidelines for the assessment, location and design of occupied buildings on sites handling dangerous substances. The latest CIA guidance was published in 2020 (Guidance for the location and design of occupied buildings on chemical manufacturing and similar major hazard sites’ 4th edition). OBRA is now a requirement of major hazard sites in the UK and elsewhere.

With the help of our extensive knowledge and experience we are the right partner to determine how OBRA can be applied to the specific needs of your site.

Your Benefits

  • Reduction of hazards and risks to building occupants (explosions, fires and toxic releases)
  • Definition of safety design and safety measures as well as identification of safe site areas
  • Evaluation of the viability of different occupied building or process storage/plant options
  • Meeting requirements of established guidance for occupied buildings

Our Approach

Occupied Building Risk Assessment (OBRA) aims at assessing the process risks that affect anyone routinely working in a building on a site where explosions, fires or toxic releases can occur. Therefore, major hazard sites, such as those governed by COMAH, are in greater need of a detailed occupied building risk assessment than ordinary users, who can opt for a limited assessment.

Five phases to perform a risk assessment for occupied buildings
Recommended approach for an occupied building risk assessment

Regarding occupied building risk assessment, the following phased approach is recommended:

A detailed site visit to review the number, type, construction and occupancy of buildings used by people and to identify all process hazards that might affect them. Hazard screening at this stage is an efficient and cost-effective way of highlighting which buildings require a more detailed assessment and which ones place occupants at negligible risk.

While explosions are momentary, and buildings should offer people sufficient protection from them, fire and smoke are a more lasting threat. By simply plotting the thermal radiation zones on a layout drawing, employees can see the safer escape routes and avoid danger zones. Marking and identifying such escape routes complements existing fire risk assessments.

Some buildings will need a more rigorous assessment, including more detailed consequence modeling in conjunction with quantified or semi-quantified risk assessments. This is to provide realistic hazard levels to ensure buildings provide sufficient protection to occupants.

The impact of the hazard levels is then assessed and appropriate remedial measures specified. Where the cost of remedial measures is very high, we undertake a cost benefit analysis ensuring ALARP principles are adhered to.

Finally, once any necessary remedial measures are in place, the results of the assessment are documented in order to demonstrate to the regulatory authority that risks to building occupants have been reduced to tolerable levels and are being managed on an ongoing basis.


  • We use our knowledge of process risks and their assessment to determine how OBRA can be most effectively applied to a site.
  • We ensure consistency in developing and documenting the OBRA assessment.
  • We draw from working within many process industries handling potentially dangerous, toxic and flammable materials.
  • We assist at any stage in the process of assessing occupied buildings and review existing submissions to identify shortfalls.

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