Occupied Buildings Risk Assessment (OBRA)

Arthian’s consultants carry out Occupied Building Risk Assessments (OBRA) to evaluate risks from hazardous events, strengthen workplace safety, and ensure compliance with COMAH and industry standards.

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Process Safety & Blast-Resistant Design

Arthian delivers Occupied Building Risk Assessments (OBRA) to help operators of high-hazard sites evaluate and mitigate risks to personnel in offices, control rooms, temporary buildings and other occupied structures. Our approach combines process safety risk modelling with structural engineering expertise—ensuring buildings are appropriately located, designed and protected against fire, explosion and toxic gas hazards.

We support:

Upper and Lower Tier COMAH sites
Distilleries and drinks manufacturing
Chemical and pharmaceutical facilities
Energy from waste and renewables
Industrial gases, fuels and bulk storage
High-risk manufacturing and processing sites

Our assessments follow the Chemical Industries Association (CIA) Guidance for the location and design of occupied buildings on major hazard sites.

What is an OBRA and Why is it Required?

An OBRA evaluates whether building occupants are exposed to avoidable risks from Major Accident Hazards (MAHs). It considers:

Building location relative to hazard sources
Potential exposure to fire, explosion or toxic release
Emergency response and shelter-in-place capability
Structural resilience and survivability

While not a statutory requirement, OBRA is considered industry best practice and is expected by regulators as part of COMAH safety report submissions. It helps demonstrate that risks are as low as reasonably practicable (ALARP).

Our OBRA Services

We offer a full suite of OBRA services alongside comprehensive structural engineering expertise. Our structural engineers specialize in blast-resistant building design and retrofit, structural response modelling using single- and multi-degree-of-freedom methods, cladding and envelope assessment, layout optimization and siting analysis, as well as design support for both new and temporary buildings. This integrated approach ensures effective risk reduction while enhancing engineering resilience.

Hazard-Based and Risk-Based OBRA Studies

Consequence Modelling (Overpressure, Thermal, Toxic Gas)

Risk Ranking and Mitigation Strategy

Building Vulnerability Assessment

Site-Wide Occupied Building Strategy

Integration with COMAH, DSEAR and Permitting

Arthian Occupied Building Risk Assessment OBRA

Hazard-Based and Risk-Based OBRA Studies

We assess building vulnerability through both hazard-based and risk-based Occupied Building Risk Assessments (OBRA), ensuring alignment with regulatory expectations and industry best practice.

Our services include:

Hazard-based OBRA to identify credible external threats (e.g. fire, explosion, toxic release)
Risk-based OBRA to evaluate likelihood, consequence, and effectiveness of existing safeguards
Support in defining building siting, occupancy, and protection strategies
Integration with COMAH, DSEAR, and functional safety frameworks

Our balanced approach helps clients protect personnel, justify building locations, and meet regulatory scrutiny with confidence.

Consequence Modelling (Overpressure, Thermal, Toxic Gas)

We use advanced modelling tools to simulate hazardous scenarios and support robust, defensible risk evaluations.

Our services include:

Overpressure modelling to assess blast impacts and structural vulnerability
Thermal radiation modelling to evaluate fire scenarios and exposure risks
Toxic gas dispersion modelling to predict concentration profiles and affected zones
Integration of results into risk assessments, OBRA studies, and emergency planning

Risk Ranking and Mitigation Strategy

We help clients protect occupied buildings by identifying key risks and recommending proportionate, targeted mitigation measures.

Our services include:

Ranking risks based on consequence severity, likelihood, and proximity to occupied structures
Prioritising threats that pose the greatest exposure to personnel and assets
Recommending tailored mitigation strategies—such as siting adjustments, structural upgrades, or occupancy controls
Ensuring alignment with OBRA methodology and regulatory expectations (e.g. COMAH, DSEAR)

Our structured approach supports defensible decision-making and helps clients reduce vulnerability across high-risk sites.

Building Vulnerability Assessment

We evaluate the resilience of occupied buildings to major accident hazards, supporting safe siting and protection strategies.

Our services include:

Assessment of structural integrity against overpressure, thermal radiation, and toxic release scenarios
Evaluation of occupancy levels, usage patterns, and exposure duration
Identification of vulnerability factors and potential escalation risks
Integration with OBRA studies and regulatory frameworks such as COMAH and DSEAR

Site-Wide Occupied Building Strategy

Our OBRA services support the development of integrated strategies to manage risks to occupied buildings across complex, high-hazard sites.

Our services include:

Mapping of building locations relative to major accident hazard sources
Aggregated risk evaluation across multiple structures and occupancy types
Development of site-wide mitigation strategies including siting, shielding, and occupancy controls
Alignment with COMAH, DSEAR, and corporate safety standards

This holistic approach ensures consistent protection, regulatory compliance, and informed decision-making across your facility.

Integration with COMAH, DSEAR and Permitting

Our OBRA outputs are designed to integrate seamlessly with wider regulatory frameworks, supporting efficient compliance and consistent safety management.

Our services include:

Alignment of OBRA findings with COMAH safety report requirements
Integration with DSEAR risk assessments and hazardous area classification outputs
Support for environmental permitting submissions, including risk justification and mitigation evidence
Ensuring traceability and consistency across all regulatory deliverables

This integrated approach streamlines regulatory engagement and strengthens the defensibility of site-wide safety decisions.