Why vessels are different
Specifying ATEX enclosures for offshore vessels, FPSOs, and fixed platforms is more demanding than for onshore petrochemical plant. The combination of hazards is uniquely challenging:
- Continuous salt spray and humidity, chloride-laden air accelerates corrosion of inadequately specified materials
- Persistent vibration, engine and machinery vibration loosens terminals and fatigues gland seals over time
- Compact, congested decks, maintenance access is constrained; enclosures in inaccessible locations receive less inspection
- Dynamic zone classification, zones can change as the vessel's operational state changes (loading, offloading, maintenance)
- Class and flag-state requirements, marine classification societies (Lloyd's, DNV, Bureau Veritas) impose requirements beyond ATEX
Each of the seven mistakes below is compounded by these factors. On a vessel, a mistake made at the specification stage is extremely expensive to correct once the vessel is built and in service.
Mistake 1: Specifying for corrosion after the fact
The most common vessel enclosure failure is corrosion, not of the enclosure body (which is typically specified correctly), but of the glands, fasteners, earth clamps, and mounting hardware.
Standard 316L stainless steel enclosures are correctly specified for the topsides environment, but the full system must be consistent:
- Glands, marine-grade nickel-plated brass or 316 stainless; standard brass corrodes rapidly in chloride atmospheres
- Lid fasteners, A4 (316 grade) stainless fasteners; A2 (304) fasteners pit and seize in marine environments
- Earth clamps and bonding straps, 316 stainless or tinned copper; dissimilar metals create galvanic cells
- Mounting brackets and hardware, if mild steel is used for mounting and the enclosure is stainless, galvanic corrosion attacks the mild steel first
- Cable tray and conduit, the enclosure cannot be isolated from the wider corrosion picture
Specify the entire system, not just the enclosure body. The bill of materials for a vessel enclosure installation should specify gland material, fastener grade, and earth hardware alongside the enclosure model.
Mistake 2: Assuming zone classification is fixed
On an FPSO or production vessel, zone classification is dynamic. The zones around cargo tanks, manifolds, and process areas change during:
- Loading and offloading operations (tank venting creates Zone 1 around vents that may normally be Zone 2)
- Maintenance activities (opening process connections creates temporary Zone 1 or Zone 0 conditions)
- Purging and gas-freeing operations
- Tank cleaning with flammable solvents
Enclosures installed in areas where the zone can change to a more severe classification during operations must be specified for the worst-case zone they will ever experience, not just the normal operating zone. Specifying a Category 3G (Zone 2) enclosure in a location that becomes Zone 1 during loading is non-compliant during loading operations.
Review the operational profile with the vessel operator before finalising the zone classification basis for enclosure specification.
Mistake 3: Under-specifying cable sealing
Cable glands on vessels are exposed to movement, the vessel flexes, cables move, and gland seals are stressed cyclically in a way that onshore installations rarely experience. Gland failures, where the cable pull-out force is insufficient or the seal degrades, create two problems simultaneously: the IP rating is lost, and the Ex protection is compromised.
For vessel applications:
- Specify glands with armour clamps and inner seal retention, not just compression glands
- For SWA cable, ensure the armour clamp provides the mechanical retention force required (at least the cable weight × 20 in a vertical run)
- For flexible cables in dynamic locations (such as on swinging equipment), use flexible conduit connectors with rated IP and Ex protection
- Verify the gland seal material is compatible with the cable sheath material (EPDM seals on PVC cable sheaths can cause chemical attack in some configurations)
- Consider EMC-rated glands where inverter drives create interference
Mistake 4: Ignoring vibration
Vibration from propulsion machinery, generators, and thrusters is transmitted throughout the vessel structure. In ATEX terminal boxes, vibration causes:
- Terminal loosening, screw terminals that are correctly torqued at installation back off under vibration, creating high resistance connections that arc and heat
- Gland seal loosening, gland locknuts vibrate loose, destroying the IP seal and the mechanical retention
- Fastener back-off, lid screws and mounting bolts loosen, compromising IP and the Ex protection concept
Mitigation measures include:
- Spring-loaded or spring-washer secured lid fasteners
- Vibration-resistant terminals (spring-clamp terminals rather than screw terminals, particularly for smaller conductor sizes)
- Gland locknuts with nylon locking inserts or thread-locking compound (low-strength, not permanent, for maintainability)
- Inclusion of vibration testing in the FAT (Factory Acceptance Test) for critical enclosures on high-vibration plant
Mistake 5: Wrong temperature class
Vessels operating in tropical climates or in process areas with elevated ambient temperatures can exceed the standard ambient range of -20 °C to +40 °C. The ATEX certificate's temperature class assumes the enclosure is operating within its rated ambient range, outside that range, the surface temperature may exceed the T class.
Issues to check:
- High ambient temperature, machinery spaces and engine rooms on vessels regularly exceed 50 °C or 60 °C; enclosures must have an extended ambient range certificate (e.g., Ta = -20 °C to +60 °C), marked on the nameplate
- Solar gain, black or dark-painted enclosures on open decks in tropical sun can exceed 60 °C surface temperature even with moderate ambient temperatures; specify light-coloured or reflective coating, or move the enclosure to a shaded location
- Heat-generating contents, if the enclosure contains equipment that generates heat (VSD modules, power resistors), the internal temperature rise must be added to the ambient temperature and verified against the T class
- Cold environments, in Arctic operations, the lower ambient limit matters too; heating may be required to keep electronics within the rated range
Mistake 6: Poor maintenance access
ATEX inspections on vessels are required by IEC 60079-17 and by class society survey. If enclosures are not accessible for inspection, the inspection regime cannot be maintained, and class compliance is compromised.
Common access problems:
- Enclosures positioned behind pipework or structure added after the enclosure was installed
- Lid opening direction not considered, a lid that opens upward against a low deckhead cannot be fully removed for internal inspection
- Gland access obstructed by cable tray or adjacent equipment
- Enclosures requiring tools or scaffolding to access that are, in practice, never inspected
At the design stage, review the 3D model (or layout drawings) with the maintenance team to confirm inspection access for each hazardous area enclosure. This is particularly important for enclosures in cofferdams, void spaces, and machinery rooms where access is already constrained.
Mistake 7: Optimising first cost, not total cost
Vessel electrical systems are specified under significant cost pressure, and the enclosure is often seen as a commodity purchase. The consequences of choosing the lowest first-cost option are disproportionate on a vessel:
- Offshore replacement costs, replacing a failed enclosure on an operational FPSO requires a shutdown, potentially a platform supply vessel visit, and hot-work permit procedures. The cost of the enclosure is trivial compared to the cost of the operation.
- Survey failure consequences, a non-compliant installation found during class survey can lead to operational restrictions or required repair before departure. The downtime cost is many times the enclosure cost.
- Maintenance costs over vessel life, a vessel may operate for 25–30 years. An enclosure with a 5-year gasket life requires five replacement cycles; one with a 15-year gasket life requires one or two.
The correct comparison is total cost of ownership over the vessel lifetime, not purchase price. For the same specification (Ex eb IIC T6 Gb, IP66, 316L stainless), the price difference between a competently manufactured enclosure from a certified UK manufacturer and the cheapest import available may be £150–£400. The difference in lifetime cost, including replacements, inspection failures, and vessel downtime, is orders of magnitude larger.