Executive Summary
A petrochemical complex planning a capacity expansion required a full Quantitative Risk Assessment to support land-use planning approval, as residential development had progressively encroached toward the site boundary since the facility's original establishment. The planning authority required demonstrated individual and societal risk contours, benchmarked against tolerability criteria, before approving the expansion. This representative example illustrates QRA methodology and findings typical of land-use planning submissions at facilities where surrounding development has changed materially since original siting.
Facility Background
The complex operated multiple interconnected process units handling large flammable hydrocarbon inventories, with the proposed expansion adding a new unit and associated storage adjacent to the existing site boundary closest to a growing residential area. Since the facility's original establishment, residential development had extended significantly closer to the boundary than existed at the time of original siting approval, a common pattern where land-use planning around industrial facilities has not kept pace with facility risk profiles. The planning authority's approval process required updated risk contours reflecting both the existing facility and the proposed expansion, rather than relying on any original siting-era risk assessment.
Hazard Profile
- —Large flammable hydrocarbon inventories across multiple interconnected process units, with credible loss-of-containment scenarios ranging from small leaks to major vessel failures
- —Domino effect potential given the facility's unit interconnection and equipment density
- —New unit and storage inventory introduced by the proposed expansion, requiring assessment alongside existing facility risk rather than in isolation
- —Residential development proximity requiring risk contours extending to the site boundary and beyond
Study Methodology
- 1.Hazardous inventory review across existing and proposed units, identifying representative loss-of-containment scenarios for each
- 2.Failure frequency assignment using recognised industry databases, informed by equipment type, size, and service conditions
- 3.Consequence modelling across multiple weather categories for each scenario, covering jet fire, pool fire, flash fire, and vapour cloud explosion outcomes as applicable
- 4.Individual risk integration across all scenarios and weather conditions, producing risk contours overlaid on the site plan and surrounding land use
- 5.Societal risk calculation using population data for the surrounding residential area, expressed as an F-N curve against tolerability criteria
- 6.ALARP demonstration reviewing candidate risk reduction measures against the residual risk identified
Key Findings
- —Individual risk contours from the existing facility alone already approached the tolerability boundary criterion at the nearest residential development, before accounting for the proposed expansion
- —The proposed expansion's incremental contribution to individual risk at the residential boundary was modest relative to the existing facility's baseline contribution, given the new unit's smaller relative inventory
- —Societal risk F-N curve results fell within the tolerable (ALARP) region rather than the broadly acceptable region, requiring a documented ALARP justification rather than a straightforward acceptance
- —Several candidate risk reduction measures were identified but judged grossly disproportionate in cost relative to the incremental risk reduction achievable, supporting an ALARP case without requiring inventory reduction
Risk Reduction Measures
- —Enhanced isolation valve automation on the new unit's highest-consequence scenarios, reducing credible release duration without requiring inventory reduction
- —Additional gas detection coverage at the boundary-facing perimeter of the expansion area to reduce detection and response time
- —Formal ALARP report documenting the cost-benefit reasoning for measures considered and not implemented, supporting the land-use planning submission
- —Revised individual risk contours and F-N curve submitted with the planning application, reflecting both existing facility and expansion risk
Lessons Learned
Land-use encroachment around an existing facility can force a full QRA refresh even without any change to the facility itself.
The existing facility's risk contours had not been reassessed since original siting, despite residential development having materially changed the risk picture at the boundary — a reminder that QRA currency depends on surrounding land use, not only on facility changes.
An expansion's incremental risk contribution can be smaller than the baseline risk it is being measured against.
Land-use planning discussions can mistakenly focus entirely on the new unit's risk in isolation; presenting the existing facility's baseline contribution alongside the expansion's incremental contribution gave the planning authority the full picture needed for a defensible decision.
ALARP demonstration, not risk contour results alone, is often what unlocks approval in the tolerable region.
Falling within the ALARP band rather than the broadly acceptable region did not block approval — a well-documented cost-benefit case for the measures considered was what the planning authority needed to grant approval without requiring disproportionate inventory reduction.
Technical Takeaways
- —Reassess QRA currency whenever surrounding land use changes materially, not only when the facility itself changes
- —Present incremental expansion risk alongside existing facility baseline risk in land-use planning submissions
- —Prepare a formal, documented ALARP report whenever results fall in the tolerable band, rather than relying on risk contour numbers alone
- —Consider isolation and detection improvements as risk reduction measures before defaulting to inventory reduction discussions