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Occupied Building Risk Assessment (OBRA)


Occupied Building Risk Assessment

Occupied Building Risk Assessment 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.

Periculum Global’ Services:

  • To 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 identify other measures to ensure the safety of people in occupied buildings.
  • Identify safe site areas for the occupied building(s) location(s).
  • Evaluate the viability of all the different occupied building(s) or process storage and plant options from the occupant(s) risk perspectives.


Occupied Building Risk Assessment (OBRA) aims to assess the process risks that affect anyone routinely working in a building(s) on any site where explosions, fires or toxic releases can occur. Therefore, the major hazard sites, such as those governed by COMAH, are advised to have an Occupied Building Risk Assessment. Those who don’t fall into this high-risk category tend to opt for a limited assessment.

Periculum Global recommend that a phased approach is required for your Occupied Building Risk Assessment. Our structuring is as follows:

Initial review:

One of our experienced consultants will perform a detailed site visit to review the number, type, construction and occupancy of all the buildings used by people and to identify all process hazards that might affect them. Hazard screening at this stage is extremely efficient and cost-effective way of highlighting which and how many buildings require a more detailed assessment and which one’s 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:

Invariably there will be buildings that require a more rigorous assessment, including more detailed consequence modelling in conjunction with quantified or semi-quantified risk assessments. This is to provide you with realistic hazard levels to ensure buildings ensuring your providing sufficient protection to the occupants.


Demonstration of ALARP:

The impact of hazard levels is then assessed, and appropriate remedial measures are 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 will be fully documented in order to demonstrate to the regulatory authority that risks to building(s) occupants have been reduced to tolerable levels and are being managed and assessed on a regular basis.