HAZOPs, Bowties, LOPAs, and SFAIRP Assessments are often treated as finished products. In practice, they are a snapshot of a facility at a point in time. The plant keeps changing after the study is signed off. Some changes are captured through MOC. Some are not. Barriers age, procedures drift, people change roles, and operating practice slowly moves away from what was originally documented. Revalidation is how we check whether the documented risk picture still matches the facility as it is being operated today. It is not just a matter of putting a new date on an old study. 

Without periodic re-testing against current design, operating, and organisational conditions, it silently loses its validity as a basis for demonstrating that risk is controlled so far as is reasonably practicable. The "so what" is not compliance for its own sake; it is that stale studies function as a false negative on risk, and false negatives precede major incidents.

Triggers, Not Just Intervals

Most process safety guidance refers to a time-based HAZOP revalidation cycle, with five years commonly used in industry practice. That interval is useful, but it should not be treated as the only trigger. A facility can move a long way from its documented risk state within five years, particularly where changes are small, incremental, and individually low consequence. The five-year cycle should be the backstop. The real discipline is having clear triggers that prompt review when the risk picture changes. For Major Hazard Facilities, this is not just good practice. The obligation to review and revise safety cases and safety assessments is mandated by law.

Triggers need to include:

• Management of change triggers, e.g. a modification, recommissioning, or decommissioning, design-basis creep, and organisational changes
• Barrier-performance triggers, e.g. a control measure found to be insufficient
• Incidents and near miss triggers, e.g. how the investigation learnings inform understanding of causes, consequences and controls
• Outdated technical knowledge and identified new gaps with industry practice
• Increased or new risk; a newly identified risk or a significant rise in any major incident risk
• Activity divergence triggers, e.g. actual operations differing from those anticipated in the safety case
• Consultation/representation, e.g. worker consultation or an HSR request signalling a review is needed
• Regulator request

The point is simple: use the interval, but do not rely on it. A strong revalidation programme combines scheduled review with event-based triggers. All should support the same outcome: a current and defensible understanding of the facility's risk state.

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The MOC - Completeness Precondition

Revalidation only works if the MOC record is complete enough to be trusted. If modifications have not been formally captured, the team may be reviewing P&IDs and procedures that no longer represent the plant. That is not revalidation of the real facility; it is revalidation of the paperwork. Before the workshop starts, the MOC register should be reconciled against maintenance records, permit-to-work data, engineering change records, and, where needed, physical walkdowns. Always ask those operating and maintaining the facility what they are aware of that may have changed.

Bowtie and LOPA Specific Decay: Barriers, Not Just Hazards

For bowties and LOPAs, do not stop at the top event, threats, and consequences. Check the barriers closely. A barrier may still appear in the study as "in place" while its actual performance has degraded due to reduced testing, deferred maintenance, changes in operating practice, or loss of competency. Revalidation should therefore include a deliberate review of barrier effectiveness, ideally using critical control verification data rather than relying solely on document review. Challenge even the ‘presumed healthy’ barriers: whether they are as effective as originally assumed. Incident and near-miss data can provide insights here, too.

Challenge the validity of the original assumptions that informed enabling events and conditional modifiers.

Independent Re-Examination vs Line-by-Line Confirmation

A common weakness in revalidation is that the team works line by line through the previous worksheet and confirms what is already there. That can anchor the discussion to the old study. A better method is to independently re-derive causes and consequences for a sample of nodes or bowties before looking at the original worksheet or bowties. This helps test whether the current team would identify the same risk picture today.

Competency and Team Composition

The quality of revalidation depends heavily on who is in the room. Original operations personnel are valuable because they know why decisions were made. Where they are not available, that rationale needs to be recovered from records or transferred through a structured handover. This is a resourcing decision that should be made deliberately, not squeezed by schedule pressure.

Organisational Drift and Normalisation

Risk also drifts through normal work. A deferred maintenance task, a workaround, a temporary operating practice, or a local interpretation of a procedure may not, on its own, trigger MOC. Over time, these small shifts can change the assumptions that sat behind the original study. Revalidation needs to look for that drift, not just formal changes.

The MOC register is only one input. A useful currency check should also draw on:

• The incident and near-miss database.
• Changes to process safety information (PSI).
• Inspection and integrity data, for example, corrosion rates measured against original design-life assumptions.
• Trend data from process safety KPIs

Delta HAZOP: Revalidation Focused on Creeping Change

When a study needs revalidating, you have a choice. You can redo the whole HAZOP from scratch, or you can focus on what has actually changed since the last study. Delta HAZOP is the second option. It is a revalidation technique, not a HAZOP in the IEC sense, designed to find the subtle, creeping changes that a full ‘redo’ can miss.

The logic is straightforward. Once a unit has been through two or three good HAZOP cycles, a ‘redo’ tends to turn up less and less. Delta HAZOP works the other way around. Instead of walking every guideword again, the team starts from the changes, (MOCs, incidents and near misses, trend data on demands and alarms, procedure changes, and the quieter drift that never triggered an MOC), and asks whether any of it has introduced a new hazard or degraded a control that the original study relied on.

Delta HAZOP is not for every situation. Before you choose it, check that:

• There is at least one good operations-stage HAZOP to build on. A design-stage study on its own is not enough, because the operational assumptions had not yet been tested.
• The actions from that prior HAZOP have been closed out (properly).
• The MOC process is healthy and the P&IDs still reflect the plant.
• There has been no significant incident or near miss pointing to a gap in the earlier study.

If those conditions are not met, a ReDo HAZOP is usually the safer call. Delta HAZOP also does not suit every type of study. If the previous study was a "by exception" HAZOP applied to supposedly identical units, it is not the right tool. And whichever way you go, it remains the operator's job to confirm the technique meets the local regulatory requirements.

Advanced HAZOP

Understanding when a Delta HAZOP is appropriate, how to assess its prerequisites, and how it differs from a full ReDo HAZOP are important skills for experienced HAZOP leaders. These topics form part of the broader discussion on HAZOP revalidation practices covered in our Advanced HAZOP Training course. 

Conclusion

A defensible revalidation programme does more than meet a date in the calendar. These updated hazard identification and risk assessment studies must feed into updated critical control management, SFAIRP assessments and risk treatment actions. Revalidation must proactively ensure risks are identified, understood, and reduced so far as is reasonably practicable.

To explore these topics further, read Why Organisations Fail to Budget for SFAIRP to learn more about maintaining SFAIRP throughout the facility lifecycle. You can also read Process Safety Due Diligence for Executive Leadership to discover why effective leadership and due diligence are fundamental to strong process safety performance.

Ready to take your process safety strategy to the next level?

 Build stronger process safety capability with our Advanced HAZOP Training Course, which covers advanced facilitation techniques and HAZOP revalidation practices; our HAZOP Leader Training Course, designed to develop confident and effective HAZOP facilitators; and our Management of Change (MOC) Training Course, equipping teams to assess, manage, and implement change safely while reducing operational risk. 

Have questions or need guidance? Get in touch with our process safety consultants to discuss your facility risks and training needs