An Occupied Building Risk Assessment (OBRA) is widely recognised as an effective method of demonstrating that process hazard risks to personnel within buildings on high-hazard sites are As Low As Reasonably Practicable (ALARP).
For organisations operating under the requirements of the Control of Major Accident Hazards Regulations 2015, conducting an Occupied Building Risk Assessment is an important step in demonstrating that risks to personnel working on site are understood and properly managed.
OBRA studies can be applied to both existing buildings and new developments, helping operators make informed decisions about building location, occupancy, and risk mitigation.
Defining Occupied Buildings in an OBRA
The first stage of an Occupied Building Risk Assessment is identifying which structures on site qualify as occupied buildings. These are buildings intended for personnel occupancy and may include locations where:
- Personnel are assigned to work regularly
- Evidence of routine occupancy exists
- Buildings serve a recurring operational purpose
If a building does not immediately meet these criteria, it may still require assessment against a site-specific occupancy definition.
A typical definition used in OBRA studies is a building with more than two person-hours of occupancy within a 24-hour period, for example:
- One person present for more than two hours
- Two people present for more than one hour
Once occupied buildings are identified, the expected occupancy levels should also be documented. This information can play an important role in later stages of the risk assessment.
Hazard-Based Assessment in an Occupied Building Risk Assessment
Following identification of occupied buildings, the next stage of an Occupied Building Risk Assessment is a hazard-based assessment.
This stage examines potential process hazards that could impact personnel inside these buildings. Typical major accident hazard scenarios considered include:
- Fires and thermal radiation exposure
- Smoke exposure from fire events
- Vapour cloud explosions and overpressure effects
- Toxic gas releases
Information from previous process safety studies, such as HAZOP or HAZID, can be particularly useful in identifying credible hazard scenarios for evaluation.
Consequence Modelling and Threat Zones
Consequence modelling tools, such as DNV PHAST, can then be used to estimate the distances at which hazardous events may impact personnel.
These calculated areas of impact are known as threat zones.
If an occupied building falls within a threat zone for a potential major accident hazard, it may require further analysis within the risk-based or frequency-based stages of the OBRA.
Supporting ALARP Demonstration Through Occupied Building Risk Assessment
An Occupied Building Risk Assessment provides a structured and recognised method for demonstrating that risks to personnel located in buildings on high-hazard sites have been properly assessed and managed.
For facilities regulated under COMAH, OBRA studies can play an important role in supporting regulatory compliance and safety case demonstrations, while also helping operators identify opportunities to improve building safety and reduce risk exposure.
To learn more about how OTECSA supports clients with OBRA studies and process safety risk assessments, explore our latest case studies and insights.