Brownfield modifications introduce a different risk profile from greenfield design. The project must work around operating equipment, existing safeguards, legacy drawings, constrained layouts and simultaneous operations. A technically small change can interact with old systems in ways that are not obvious on the new project P&IDs. Process safety review therefore needs to focus on interfaces as much as the new equipment itself.
Verify the existing plant basis
Before relying on existing drawings or safeguards, confirm that they represent the installed facility. Field verification may be necessary for isolation points, tie-ins, valve orientation, instrument connections and relief routing. Legacy facilities may contain undocumented modifications or equipment that no longer matches the original design assumptions. Hazard studies should identify where uncertainty remains.
Tie-in and isolation risk
Tie-ins create exposure during construction and commissioning. The review should consider positive isolation, trapped inventory, pressure sources, backflow, drain and vent arrangements, leak testing and reinstatement. Isolation philosophy should be consistent with the fluid, pressure and consequence. Where shutdown windows are limited, the project should avoid allowing schedule pressure to weaken isolation standards.
Simultaneous operations
Construction, maintenance and normal production can interact. Hot work, lifting, temporary power, scaffolding, open systems and vehicle movement may create new ignition sources or obstruct emergency access. SIMOPS risk should be considered during planning, not only immediately before field execution. A HAZID or focused operational risk review can help identify these interfaces.
Existing barrier capacity
New equipment can increase demand on flare, blowdown, firewater, drainage, electrical systems or control infrastructure. The project should verify that shared protection systems remain adequate. A new relief load or shutdown sequence may affect facilities outside the immediate modification boundary. These system-level effects are common sources of brownfield risk.
Control system integration
Adding trips or alarms into an existing control system can create dependencies that are not obvious on the P&ID. Shared I/O, logic solvers, communications and power supplies may affect independence. If the modification changes a credited IPL or SIF, the LOPA and functional safety basis should be reviewed.
Hazard study boundaries
A focused HAZOP should include enough upstream and downstream plant to capture interactions. Reviewing only the new skid can miss reverse flow, blocked-in sections, pressure propagation or control interactions with the existing facility. Brownfield nodes should be chosen around interfaces, not just project battery limits.
Commissioning and handover
Temporary configurations often exist during commissioning. Bypasses, blinds, temporary hoses and incomplete alarms should be controlled and visible. Before handover, confirm drawings, procedures, maintenance tasks, barrier registers and training are updated. The final risk basis should reflect the plant that operations actually receives, not only the approved design package.
Practical engineering checks
Review design pressure and temperature, relief loads, shutdown timing, depressurisation, minimum design metal temperature, materials compatibility, line sizing and operating limits where the modification changes process duty. Confirm that tie-in points, valves and instruments are correctly represented in controlled drawings. Where a shared system is affected, check cumulative demand from earlier modifications rather than assessing the new project in isolation.
Why brownfield experience matters
Brownfield process safety requires judgement about installed equipment, operations and real-world constraints. A technically correct new design can still create risk if interfaces with the existing facility are missed. Experienced facilitation helps the team identify those interactions and convert them into practical actions before field execution.
How Pro Engineers can assist
Pro Engineers provides process and process safety engineering support for brownfield projects, including HAZID, HAZOP, LOPA, relief and flare review, blowdown assessment and management-of-change support. Visit the Pro Engineers home page for an overview of services.
Legacy information and design assumptions
One recurring challenge in brownfield work is that original calculations may no longer represent current operation. Production rates, fluid composition, compressor duty, operating pressure and maintenance philosophy may all have changed. The project should distinguish between verified current data and historical design information. Where the modification depends on an old calculation, confirm that the assumptions remain valid before using it as the design basis.
Field verification can also uncover practical details that drawings miss, such as inaccessible valves, undocumented vents, common drain connections or shared utility supplies. These details may affect isolation, commissioning or the independence of protective functions. Integrating field observations into the hazard review can significantly improve the quality of the final design.
Relief, flare and blowdown interfaces
Brownfield modifications often connect to shared relief and flare systems. An additional PSV, larger control valve or revised emergency depressurisation case can increase header load and backpressure. The project should check the cumulative system rather than assess the new device in isolation. Our article on flare and blowdown systems in process safety studies explains these interactions in more detail.
Commissioning risks
Commissioning creates temporary process configurations that may never occur during normal operation. Systems may be partially complete, instruments may be bypassed, temporary hoses may be installed and isolation boundaries may change several times. A commissioning risk review should identify the safeguards required for each stage and confirm when normal plant protection becomes available. Operations should understand which temporary controls are still active at handover.
Maintaining traceability after start-up
Once the modification is operating, close-out should ensure drawings, alarm settings, trip set points, relief calculations, procedures and maintenance tasks match the installed plant. If the project changed a critical barrier, the change should be visible in the facility barrier management system. If new scenarios were identified, they should be incorporated into the appropriate HAZOP, LOPA or bowtie record.
Final brownfield review
Before start-up, the project should confirm that temporary isolations, bypasses and construction aids have been removed or formally controlled, that required safeguards have been function-tested, and that operations has the final marked-up information. A focused readiness review helps ensure the plant handed over is the plant that was actually assessed.
This final check also provides a clean baseline for future management of change and periodic revalidation.