Occupied Building Risk Assessment

Protect site employees from major process incidents

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

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.

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 2010. OBRA is now a requirement of major hazard sites in the UK and elsewhere.

Customer Benefits

  • Reduce the hazards and risks to building occupants from major hazards (explosions, fires and toxic releases) to acceptable or at least tolerable levels.

  • Define design and other measures to ensure the safety of people in occupied buildings.

  • Identify safe site areas for occupied building location.

  • Evaluate the viability of different occupied building or process storage/plant options from an occupant risk perspective.

  • Ensure site occupied buildings meet the requirements of established guidance.

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

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.

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

Initial review:

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.

Clear planning of escape routes:

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.

More detailed assessment:

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.

Demonstration of ALARP:

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 up-to-date 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 share our wealth of knowledge drawn from working within many process industries handling potentially dangerous, toxic and flammable materials.
  • Our consultants can draw on our proven track record of carrying out fire and explosion risk assessments for a range of clients across broad industry sectors.
  • We are happy to assist at any stage in this process of assessing occupied buildings or review existing submissions to identify shortfalls and answer specific questions set by regulatory authorities.

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