Engineering and Project Management
 
Compliance

Hazardous Area Classification (HAC) Study

Hazardous Area Classification Study

A Hazardous Area Classification (HAC) study is a systematic, engineering-driven assessment that identifies every location on a facility where flammable gas or combustible dust concentrations could reach levels that require specially rated electrical equipment. It is the foundational layer of explosion safety for every process facility, offshore topsides installation, and vessel handling flammable cargo — and it is mandatory for all new builds, major modifications, and vessel conversions under UKCS, NCS, and most international jurisdictions.

Getting the classification right matters at every level. Under-specify the zones and you expose the facility to the risk of ignition. Over-specify them and you mandate unnecessary explosion-proof equipment at significant cost premium, with longer procurement lead times and ongoing maintenance overhead that is difficult to justify to a certifying authority. The study is not a compliance exercise — it is an engineering decision that shapes capital expenditure and operational safety for the lifetime of the facility.

What Our Services Cover

Our HAC study service applies the full scope of IEC 60079-10-1 (gas), IEC 60079-10-2 (dust), and the operator-specific standards that sit above them — including Shell DEP 34.91.10.10-Gen, Equinor EQ-001, TotalEnergies GS EP STP EC 001, and the Norwegian NORSOK S-001 framework where those apply. The service covers three interconnected phases: source identification and release rate calculation, zone extent determination, and equipment selection and inspection scheduling. Each phase produces a distinct deliverable, and together they form a complete, defensible area classification package that will withstand regulatory scrutiny from the UK HSE, the Norway PSA, or any other applicable authority.

Source identification begins with your P&IDs and process data sheets. Every vessel, tank, pump, compressor, flange, valve stem, and vent connection is reviewed against the fluid inventory to determine which ones constitute a release point. Each source is then classified by duration and frequency: continuous (Zone 0), primary (Zone 1), or secondary (Zone 2), following the definitions in IEC 60079-10-1. This classification drives everything else in the study, so it is done carefully and documented source-by-source.

Release rate calculation follows the IEC 60079-10-1 Annex B, C, and D methodology depending on the ventilation configuration at each source. For sources with natural ventilation, the quasi-gas concentration method calculates the distance from the release point at which gas concentration falls below the Lower Explosive Limit (LEL). For mechanically ventilated areas, the calculation accounts for the dilution effect of forced airflow. The fluid properties — gas group (IIA, IIB, or IIC), flashpoint, boiling point, LEL, UEL, and molecular weight — are taken from IEC 60079-20-1 and the process data sheets, and the gas group assignment is documented in a properties table that forms part of the calculation register.

For LNG facilities and bunkering stations, the methodology extends beyond the standard gas calculation paths. Cryogenic LNG releases behave differently from warm gas releases: the cold dense vapour cloud does not disperse in the same way as a lighter-than-air gas, and it accumulates in low points — which means zone extents around LNG equipment can extend further and in different directions than the standard IEC model predicts. We also address rapid phase transition (RPT) — the physical phenomenon where LNG contacts water and flashes instantaneously — as a separate hazard input that requires its own classification logic. LNG tank annular spaces are classified as Zone 0 by design, and the boil-off gas (BOG) compressor area requires Zone 1 classification around the seal interfaces. These are not edge cases; they are standard scope for any LNG HAC study.

The zone extents are then drawn onto P&ID-based area classification drawings — plan view and elevation view for each relevant area, with Zone 0, Zone 1, and Zone 2 boundaries clearly marked. Where ventilation is directional — for example, mechanical extraction fans creating a pressure gradient — the zone shape is elongated in the extraction direction. Every drawing carries a notes field explaining the basis for each zone boundary, referencing the specific calculation sheet and assumption set that produced it. This is what makes the drawings defensible when a certifying authority reviews them.

Outputs and Deliverables

Our HAC study produces four distinct deliverables, each serving a different audience and purpose:

  • Area Classification Drawings — P&ID-based drawings showing Zone 0, Zone 1, and Zone 2 boundaries for gas atmospheres (and Zone 20, 21, 22 for dust). Each drawing is revision-controlled, with a drawing register linking every zone boundary to its corresponding calculation sheet. These drawings are the procurement specification basis for all electrical equipment on the facility.
  • Zone Extent Calculations — A calculation register containing one sheet per release source, documenting fluid properties, gas group determination, release rate, ventilation rate and type, calculation method (with IEC 60079-10-1 Annex reference), resulting zone extent in metres, and a full statement of assumptions. Where assumptions are uncertain — for example, an assumed ventilation rate that may vary seasonally — the calculation is run at bounding cases (minimum ventilation, maximum release rate) to confirm the zone extents are conservative.
  • Equipment Selection Schedule — Room-by-room or area-by-area schedule listing every piece of electrical equipment, its required zone classification, gas group, temperature rating (T1 through T6), protection concept (Ex d flameproof, Ex e increased safety, Ex i intrinsic safety, Ex p pressurised, etc.), applicable standard (IEC 60079 series), and minimum IP rating for mechanical protection. This schedule is the document the procurement team uses to specify and purchase Ex-rated equipment. It also flags where existing equipment may be non-compliant with the updated classification — a common finding when a modification study reveals that previously classified areas have expanded.
  • Ex Inspection Register — Based on IEC 60079-17, this register lists every Ex-rated item on the installation with its equipment ID, location, zone, protection concept, IECEx certificate number, date of last inspection, date of next inspection, and the inspector qualification required for each level of inspection (visual, close, detailed). The register is structured for import into maintenance management systems — SAP, AVEVA, Ellipse — and includes the IEC 60079-17 Table 1 mapping of inspection level by equipment type and zone. It is the ongoing compliance document that demonstrates the installation is maintained in accordance with the IEC 60079-17 inspection regime.

The study also includes a HAC Study Report that documents the full methodology, regulatory context, fluid inventory, gas group determination table, assumptions and limitations, drawing register, and recommended review frequency. For UKCS facilities, the report references the Energy Institute IP 15 guidance alongside IEC 60079-10-1. For NCS facilities, it is structured to align with the PSA's area classification expectations and NORSOK S-001. The report is the document that goes to the certifying authority as part of the safety case or consent package.

Sector Application

The service applies across the full range of oil and gas facilities where flammable media are handled:

Offshore topsides and fixed platforms: Drill floor mud gas vents and shaker rooms (Zone 1 around shakers), mud pits containing oil-based mud (Zone 1), wellbay production headers and Christmas tree leak points (Zone 1 at wellhead), separator areas (Zone 1 around separator vessels and relief valves), hydrocyclone and de-sander areas (Zone 2 general, Zone 1 at vent points), flare and vent systems (Zone 1 around flare tip, Zone 2 at flare boom level). Each area requires its own source-by-source calculation; the classification is not uniform across the platform deck.

FPSO and marine vessels: Cargo tanks (Zone 0 within the ullage space, Zone 1 at tank vents and pressure relief valves), pump rooms (Zone 1 around pump seals and valve stems), inert gas generator rooms (Zone 1), fuel gas valve units and manifolds (Zone 1). Vessel conversions are a particular focus — a newbuild has a HAC study done as part of design, but a conversion frequently does not, and the classification for the modified zone must be recalculated against the new fluid inventory rather than inherited from the original design.

LNG bunkering stations and land-based LNG facilities: Transfer arm and hose connection points (primary release sources, Zone 1), LNG storage tank annular spaces (Zone 0 by design), boil-off gas compressor areas (Zone 1 around seal interfaces), vaporizer exhausts (continuous Zone 1), cryogenic spill bunds (Zone 1/2 based on spill volume and ventilation), and LNG tank cleaning operations (Zone 0 inside the tank during cleaning). The cryogenic and RPT considerations are addressed explicitly — these are not covered by the standard gas calculation paths in IEC 60079-10-1.

Onshore process facilities and refineries: Tank farms (Zone 1 around tank vents and fill connections), process unit vents and drains, compressor buildings (Zone 1 around seal vents), fired equipment (Zone 1 around burner management system components), loading and unloading bays (Zone 2 general, Zone 1 at loading arms and connections).

Regulatory Context

The study is governed by the IEC 60079-10 series as the primary technical standard, with operator-specific standards applied where the client operates under a particular operator's framework. The mandatory nature of HAC is established through different instruments depending on jurisdiction:

  • UKCS: The Pipelines Safety Regulations 1996, the Offshore Safety Case Regulations 2015, and the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) all require area classification for facilities handling flammable fluids. The HSE's guidance note HSG 71 and the Energy Institute's IP 15 provide the methodology expected in practice.
  • NCS (Norway): The Activities Regulations (aktivitetsforskriften) and the Facilities Regulations (facilitiesforskriften) require area classification as part of the safety case. NORSOK S-001 is the sector-specific standard that Norwegian operators typically reference, and PSA Norway's expectations are calibrated to it.
  • International: IEC 60079-10-1 and IEC 60079-10-2 are the globally recognised technical standards, and most international jurisdictions that have adopted IEC standards as their national framework treat HAC as mandatory for new process facilities, topsides modifications, and vessel conversions handling flammable cargo.

Engagement Model

The service is scoped to the specific facility and the specific modification or study type. A new build or major modification requires the full study: P&ID review, source-by-source calculation, full drawing set, equipment schedule, inspection register, and study report — typically delivered over 8 to 16 weeks depending on process complexity and drawing set size. A topsides modification affecting a single area is scoped to that area plus the interface check on adjacent zones — a more focused deliverable that still produces a complete and defensible classification for the modified area.

A vessel conversion HAC is a distinct engagement type, because converting a vessel to a new cargo service changes the fluid inventory, ventilation configuration, and potentially the equipment layout — the previous HAC was calculated against a different set of flammable media and must be fully recalculated. A gap analysis of an existing HAC is available for operational facilities where the current classification drawings may no longer reflect the as-built configuration or where the study has not been updated following process modifications.

For LNG bunkering stations, the study includes the cryogenic and RPT methodology as standard scope — not as an optional extra. These considerations are part of the correct application of IEC 60079-10-1 to cryogenic facilities, and any study that does not address them is not a complete HAC.

Following delivery of the study package, the inspection register is maintained as a living document — updated after each periodic inspection, after any equipment change, and after any process modification that alters the fluid inventory or ventilation configuration. Annual or biennial review is recommended for operational facilities, with a mandatory full update within five years or following any incident, whichever comes first.

If you are planning a new facility, a topsides modification, a vessel conversion, or an LNG bunkering operation, the HAC study should be scoped at the earliest design stage — before equipment is specified, before procurement packages are issued, and before the safety case is finalised. Getting the area classification right at that stage is orders of magnitude less expensive than correcting it later.

What We Do

We provide comprehensive Hazardous Area Classification (HAC) study services for oil and gas facilities, offshore platforms, FPSO vessels, and LNG bunkering operations. The service applies IEC 60079-10-1 (gas), IEC 60079-10-2 (dust), and operator-specific standards including Shell DEP, Equinor EQ-001, TotalEnergies GS EP STP EC 001, and NORSOK S-001.

A. Source Identification and Release Rate Calculation

  • Review P&IDs and process data sheets to identify every release point: vessels, tanks, pumps, compressors, flanges, valve stems, vents
  • Classify each source by duration and frequency: continuous (Zone 0), primary (Zone 1), secondary (Zone 2) per IEC 60079-10-1
  • Calculate release rates using IEC 60079-10-1 Annex B, C, and D methodology appropriate to natural or mechanical ventilation
  • Determine gas group (IIA, IIB, IIC) from MESG data in IEC 60079-20-1; document in a properties table as part of the calculation register
  • For LNG facilities: apply cryogenic and rapid phase transition (RPT) methodology beyond standard gas calculation paths

B. Zone Extent Determination and Drawing Production

  • Calculate zone extents for every release source using LEL-based dispersion methodology from IEC 60079-10-1
  • Produce P&ID-based area classification drawings: plan view and elevation view, Zone 0/1/2 boundaries marked
  • Account for directional ventilation: zone shapes elongated in extraction direction where mechanical ventilation applies
  • Run bounding case sensitivity analysis where ventilation rates or release rates are uncertain
  • Link every zone boundary to its corresponding calculation sheet — making drawings defensible under regulatory review

C. Equipment Selection Schedule Development

  • Produce room-by-room or area-by-area schedule listing every electrical item: zone, gas group, temperature rating (T1–T6)
  • Specify protection concept: Ex d (flameproof), Ex e (increased safety), Ex i (intrinsic safety), Ex p (pressurised), etc.
  • Reference applicable standard (IEC 60079 series) and minimum IP rating for mechanical protection
  • Identify non-conformances: equipment already installed that does not meet the updated zone requirements
  • Use schedule as the procurement specification basis — preventing incorrect Ex-rated equipment from being ordered

D. Ex Inspection Register and Ongoing Compliance

  • Build inspection register per IEC 60079-17: every Ex-rated item, equipment ID, location, zone, protection concept, IECEx certificate number
  • Define inspection intervals by zone and equipment type: visual, close, and detailed inspection levels per IEC 60079-17 Table 1
  • Structure register for CMMS import (SAP, AVEVA, Ellipse)
  • Record last inspection date, next inspection date, and inspector qualification required for each item
  • Maintain as a living document: update after every inspection, equipment change, or process modification