Hazardous Area Classification for Offshore Topsides: Gas Group, Temperature Class and Zone Extent Practice

The problem: classification drawings that no longer match the deck

Most offshore hazardous area drawings are accurate on the day they are issued. Then a module gets a new wind wall, a laydown skid goes up beside a separator, a battery room is added, or a ventilation damper is re-set. The zone boundaries on the drawing stay where they were, but the real gas dispersion pattern has moved on. Ex inspectors find equipment that matches the drawing but not the hazard. Process safety engineers find a Zone 2 label on a spot where a small release can no longer disperse.

The consequences are practical. Either ignition-capable equipment sits in a location that is now Zone 1, or a deck that never needed heavy Ex equipment gets it anyway. Both are expensive. This article covers how release sources, ventilation and ventilation gradients set zone extents on typical offshore process decks, and how gas group and temperature class are chosen once the zone is known.

Standards and requirements context

The framework is the same wherever you work, in the North Sea or elsewhere:

• IEC 60079-10-1 covers classification of areas with explosive gas atmospheres. It is built on release grade, ventilation degree and availability, and the resulting zone type and extent.
• IEC 60079-0 sets general requirements for Ex equipment, including marking by group, temperature class and equipment protection level.
• IEC 60079-14 covers design, selection and erection of electrical installations in explosive atmospheres.
• IEC 60079-17 covers inspection and maintenance of electrical installations in explosive atmospheres.
• IEC 60079-20-1 provides material characteristics (gas group, autoignition data) for gas and vapour classification.
• API RP 505 covers classification of Class I, Zone 0, 1 and 2 locations at petroleum facilities. API RP 500 is the equivalent for the Division system.
• Directive 1999/92/EC addresses workplace explosion risk assessment and zoning. Directive 2014/34/EU addresses equipment placed on the market for explosive atmospheres.

In the IEC system, Group IIC (hydrogen, acetylene) is the most demanding and requires the most stringent equipment protection. In the NEC/API Division system, Group D is the most demanding. On offshore units with vendor skids from several regions, mixing the two standards is common, and it is a frequent source of selection errors.

How zones are derived

Release source and grade

Zoning starts with finding every release source and grading it:

• Continuous grade gives a release that is continuous or expected for long periods. This leads towards Zone 0.
• Primary grade gives a release expected periodically or occasionally in normal operation. This leads towards Zone 1.
• Secondary grade gives a release not expected in normal operation, and short-lived if it does happen. This leads towards Zone 2.

Ventilation then modifies this base zone. Typical offshore sources include:

• Process vent and drain terminations
• Sample points and sample-bottle connections
• Pig launcher and receiver closures
• Compressor seal vents
• Pump seals
• Instrument tubing connections
• Flanges and valve glands on hydrocarbon lines
• Open sumps and drains

Flanges and glands on pressurised hydrocarbon lines are usually primary grade. Vents and sample points that are opened in normal operation are primary grade or continuous. Closure doors and occasional connections are secondary grade.

Ventilation degree and availability

IEC 60079-10-1 treats ventilation in two dimensions. Degree describes how well the ventilation dilutes a release (high, medium or low). Availability describes how reliably it is there (good, fair or poor). A naturally ventilated open deck can offer high dilution, but its availability depends on the wind, which changes constantly. In practice, many assessments credit open offshore decks with good-availability ventilation only where the geometry is genuinely open. Enclosed modules depend on HVAC, so their availability is only as good as the fans, dampers and power feeding them.

Ventilation gradients

Real decks never have uniform ventilation. Wind direction, equipment congestion, deck plating, pipe racks and wind walls all create gradients. One side of a module may be well flushed while the lee side sits in a recirculation pocket. Drawing a zone as a symmetrical envelope around a source ignores all of that. Inspectors should look at where the air actually moves, using smoke tests, anemometer readings or CFD results where they exist, instead of trusting the plan view.

Gas density matters too, particularly in enclosed spaces and under solid canopies and deck soffits. Heavier vapours from condensate or LPG-rich streams settle low, into trenches and under skid frames. The correct zone shape follows the buoyancy of the release, not a standard sphere or cylinder.

Extent: geometry and judgement

Standards and codes give extent guidance, but offshore congestion often makes generic distances useless. The boundary is set by obstructions as much as by distance. Solid decks, bulkheads and plated walls stop or redirect a cloud. A zone commonly ends at a solid barrier, but it may carry through openings, doors and ducts into adjacent areas.

Gas group and temperature class selection

Gas group

Gas group is set by the most onerous substance that can be present, based on its ignition energy and flameproof gap characteristics.

Equipment marked IIC covers IIC, IIB and IIA applications, but not the reverse. For IEC 60079-20-1 classification, use the actual composition at worst-case process conditions, not the nominal gas analysis. A sour stream or a hydrogen-bearing off-gas can push the required group above the general hydrocarbon assumption.

Temperature class

Temperature class caps the maximum surface temperature of equipment so it stays below the autoignition temperature of the gas.

For hydrocarbon systems, pick the class from the lowest autoignition temperature among credible release fluids, including heavier condensate fractions and any additives. Don't go by the dominant component alone. Hot process surfaces such as exhausts, heat exchangers and heated lines are ignition sources in their own right, and they should be assessed against the same autoignition value.

Illustrative scenario: a compression module after a modification

This scenario is constructed for illustration and does not describe a real installation.

A gas compression module was classified as Zone 2 around flanged joints and seal vents, on the basis of open-sided natural ventilation. Later, wind walls went up on the weather side, and a plated mezzanine was added over part of the compressor train. The classification drawing was never revised.

An inspector walking the module finds that:

1. Junction boxes under the mezzanine are Ex e, suitable for Zone 2 but with no evidence of a Zone 1 assessment.
2. The mezzanine soffit forms a pocket where lighter-than-air gas from a seal vent can collect.
3. Smoke released at the seal vent stalls beneath the plate rather than clearing.

The right response is not simply to relabel the area. The process safety engineer checks the release source grade (is the seal vent really only a secondary release?), re-evaluates the ventilation degree in the pocket, and decides whether the zone near the soffit becomes Zone 1. Options include restoring openness, adding local ventilation, relocating the junction boxes, or upgrading equipment to whatever zone is now assigned. The classification drawing and equipment register are then revised, and the affected installation is re-inspected.

Checklist for inspectors and process safety engineers

• Confirm the classification drawing revision matches the deck as it stands: walls, plating, canopies, skids and ventilation arrangements.
• Check every modification since the last study for its effect on airflow and release sources.
• Verify release sources against the as-built P&IDs, including vents, drains, sample points and closures.
• Where practical, test ventilation behaviour in the field with smoke or airflow observation at dead zones, mezzanines and lee sides.
• Review loss-of-ventilation scenarios for enclosed spaces and confirm the zone assumptions hold for that case.
• Check density: lighter gases under soffits, heavier vapours in sumps, trenches and low enclosures.
• Confirm the assigned gas group covers the worst-case fluid, including hydrogen sources and sour streams.
• Confirm temperature class is based on the lowest credible autoignition temperature, and that hot surfaces have been assessed.
• Compare each item's marking (group, temperature class, equipment protection level) with the zone on the register.
• Record any mismatch as a deviation, with an owner, an interim measure and a date for re-classification.

Safety during inspection

Inspection near hydrocarbon sources runs under the installation permit-to-work system. If an inspection calls for opening, depressurising or working on hydrocarbon-containing equipment, make sure isolation, depressurisation, verification of zero energy, lockout/tagout and hazardous-area precautions are in place before work starts. Use calibrated portable gas detection, and vent safely to a designated location. Don't open enclosures of live Ex equipment in a hazardous area unless the permit and the equipment's certification allow it.

Conclusion and next steps

Offshore zone extents come from release grade, ventilation degree, availability and the actual airflow pattern. Gas group and temperature class then follow from the worst-case fluid. A classification drawing is only as good as its last review.

Practical next steps:

1. Pull the current classification drawings and compare them with the deck as it stands, starting with modules that have had wall, plating or ventilation changes.
2. Prioritise enclosed and partly enclosed areas, where ventilation gradients and loss-of-ventilation scenarios most often change the zone.
3. Reconcile each item's marking with its assigned zone, gas group and temperature class.
4. Feed management of change so that any structural or ventilation modification triggers a classification review before it is closed out.