What is MHHHRA? New Guidance for COMAH Risk Assessment

What Is MHHHRA? New Guidance for COMAH Risk Assessment

RAS was a key contributor to the industry working group responsible for developing the new Understanding Major Hazard Human Harm Risk Assessment (MHHHRA) for COMAH guidance, working alongside regulators and industry representatives through the Chemical and Downstream Oil Industries Forum (CDOIF).

The publication of the guidance represents a significant development in major hazard risk assessment across the UK. It provides operators with a structured framework for identifying, analysing and assessing major accident hazards, and for demonstrating that risks have been reduced As Low As Reasonably Practicable (ALARP).

For COMAH operators, the key question is: what is MHHHRA, and what does the guidance mean for existing risk assessments, Quantitative Risk Assessments (QRAs) and COMAH Safety Reports?

As organisations begin to implement the guidance, many are assessing whether their existing methodologies, scenario selection processes and ALARP demonstrations align with the principles set out within MHHHRA. Understanding those expectations is becoming increasingly important as the guidance becomes established as an industry benchmark for good practice.

What Is Major Hazard Human Harm Risk Assessment?

Major Hazard Human Harm Risk Assessment, commonly abbreviated to MHHHRA, is a framework for assessing risks to people arising from major accident hazards at COMAH establishments.

MHHHRA is not a standalone modelling technique or a replacement for established process safety assessments. Instead, it provides an overarching structure that draws together evidence from:

• HAZOP and HAZID studies
• Major accident hazard identification and screening
• Consequence modelling
• Quantitative Risk Assessment (QRA)
• Layers of Protection Analysis (LOPA)
• Occupied building and facility siting studies
• Human factors assessments
• Incident learning and operational experience
• COMAH Safety Reports
• ALARP assessments and supporting cost-benefit analysis

The objective is to create a consistent and traceable demonstration of how major accident hazards have been identified, how representative scenarios have been selected and analysed, and how the resulting risks have been assessed and reduced to ALARP.

The Four Stages of MHHHRA

The MHHHRA guidance establishes a structured four-stage process:

1. Identification of major accident hazard scenarios
2. Analysis of those scenarios
3. Assessment of risk
4. Demonstration that risks have been reduced to ALARP

These stages are interconnected. A technically detailed consequence model or QRA may not provide a complete risk demonstration if the basis for scenario selection is unclear, relevant harm outcomes have not been considered or there is no transparent link between identified hazards and the final ALARP conclusions.

The guidance was developed to improve consistency across industry and provide greater transparency in how major accident hazards are identified, assessed and demonstrated to be controlled. MHHHRA therefore places importance on the complete assessment process, rather than focusing only on the outputs of individual technical studies.

A Clear Line of Sight from Hazard Identification to ALARP

A central principle of MHHHRA is the need for a clear and auditable line of sight through the risk assessment.

Historically, elements of major hazard assessment have sometimes been undertaken separately. For example, consequence modelling may have been completed using a selection of representative loss-of-containment scenarios without a sufficiently clear record of how those scenarios were derived from the establishment’s full major accident hazard inventory.

Under the MHHHRA framework, operators should be able to demonstrate:

• How credible major accident hazards were identified
• Which records, studies and operational experience informed the hazard inventory
• How hazards and scenarios were screened
• Why particular scenarios were selected for detailed analysis
• Whether selected cases are representative of the establishment’s risk profile
• How consequence and frequency inputs were derived
• How uncertainty and conservatism were addressed
• How individual and societal risk were assessed where relevant
• How further risk reduction measures were identified and evaluated
• How the final ALARP conclusion is supported by the evidence

The intention is not necessarily to generate more scenarios or undertake more modelling. It is to ensure that the assessment is complete, proportionate and traceable.

How MHHHRA Defines Human Harm

MHHHRA also reinforces the need to consider the full meaning of human harm under the COMAH Regulations.

The requirement to consider serious danger to human health is not new. COMAH has always encompassed fatalities, serious injuries and serious ill health. However, some historical assessments have concentrated primarily on fatality risk or treated serious injuries conservatively by assuming that they result in fatalities.

The MHHHRA guidance promotes explicit consideration of both fatality and serious permanent injury outcomes. This can provide a more representative understanding of the consequences of fires, explosions and toxic releases, particularly where an event could cause life-changing harm without necessarily resulting in fatality.

For some COMAH operators, this may affect:

• Major accident hazard screening
• Harm criteria and consequence endpoints
• Scenario selection
• Vulnerability assumptions
• Risk calculations
• Interpretation of QRA results
• ALARP demonstrations
• The presentation of risk within COMAH Safety Reports

Existing assessments may therefore require review to confirm that the full spectrum of relevant human harm has been considered.

Does MHHHRA Replace Existing COMAH Risk Assessments?

In most cases, MHHHRA should not mean discarding existing studies and starting again.

Existing COMAH risk assessments, HAZOPs, HAZIDs, consequence assessments, QRAs and ALARP demonstrations may continue to provide valuable evidence. The key issue is whether those studies collectively satisfy the principles of MHHHRA and form a coherent, transparent demonstration of major accident risk management.

A review may identify that the underlying technical analysis remains suitable but requires better documentation, clearer scenario mapping or additional justification. In other cases, gaps may require further analysis, particularly where serious permanent injury outcomes, scenario representativeness or the link between hazard identification and risk assessment are not sufficiently clear.

Implementation of the MHHHRA Guidance

Although the MHHHRA guidance creates no new legal duties, it establishes an industry-recognised benchmark for good practice in major hazard human harm risk assessment.

The Competent Authority is developing a delivery guide to support inspectors in applying MHHHRA principles during regulatory interventions. COMAH operators should therefore be prepared to explain how their current risk assessment arrangements align with the guidance or provide a robust technical justification for using an alternative approach.

A proportionate MHHHRA gap analysis can help duty holders identify:

• Missing or incomplete major accident hazard inventories
• Weaknesses in scenario selection and screening
• Gaps in the consideration of human harm outcomes
• Inconsistencies between hazard studies, modelling and QRA inputs
• Limited traceability through to ALARP conclusions
• Assessments that no longer reflect current operations or site layouts
• Areas in which additional documentation or technical analysis is required
• Potential implications for new or revised COMAH Safety Reports

Reviewing these areas early allows operators to prioritise work according to risk and regulatory significance, rather than commissioning unnecessary wholesale reassessment.

How RAS Can Support MHHHRA Implementation

Unlike organisations that are only now beginning to interpret the guidance, RAS was directly involved in the CDOIF working group responsible for its development. This provides us with a detailed understanding of the intent behind the guidance, the challenges it was designed to address and the practical considerations involved in its implementation.

Our team can help organisations apply MHHHRA principles proportionately, identify and close gaps within existing assessments, and develop robust demonstrations that major accident risks have been identified, assessed and reduced to ALARP.

Our support can include:

• MHHHRA gap analysis
• Major accident hazard identification and scenario development
• Review of HAZOP and HAZID outputs
• Consequence assessment and QRA strategy
• Review of harm criteria and assessment endpoints
• Scenario screening and representative case selection
• Review and development of ALARP demonstrations
• Integration of technical studies into COMAH Safety Reports
• Development of proportionate programmes for closing identified gaps

The aim is not to create additional analysis for its own sake. It is to help operators develop a clear, technically robust and defensible demonstration that major accident risks are understood and reduced to ALARP.

To discuss MHHHRA, arrange a gap analysis or review your existing COMAH risk assessment methodology, contact [email protected]