If you manufacture, specify, or operate portable offshore units — and that includes mud service modules, workshop containers, instrumentation cabins, and pressurised work pods — and they are certified under DNV 2.7-1, you need to read this. DNV 2.7-1 and DNV 2.7-3 are not the same standard applied at different weight classes. They are different standards for different engineering problems. Confusing them is not a minor technicality — it is a compliance failure.
What is a Portable Offshore Unit (POU)?
The term "Portable Offshore Unit" is DNV's own classification for a category of equipment that sits between a standard offshore container and a fixed offshore installation. Formally, DNV-ST-E273 (DNV 2.7-3) defines a POU as a unit that is designed to be lifted and transported offshore, contains active equipment or systems, and may be intended for human occupancy during operation.
The critical word is "contains." A DNV 2.7-1 container carries cargo. A DNV 2.7-3 POU contains functioning systems — pumps, electrical panels, HVAC, control instrumentation, pressurisation equipment — that are part of the unit's purpose, not just its payload.
Formal definition — DNV-ST-E273
The mis-certification problem
Here is what happens constantly in the offshore industry: a mud service unit is built with a pump, a flowmeter, a control panel, and a ventilation system inside. It is certified under DNV 2.7-1. The certificate is real. The DNV surveyor signed it. But the certificate never covered the pump, the control panel, or the ventilation system — because DNV 2.7-1 does not require those systems to be in the structural calculations, does not require hazardous area electrical certification, and does not require functional testing of the equipment inside.
The unit goes offshore. It works. Until it does not. And when something goes wrong — a pump failure during a lift, a spark in a Zone 1 area, a structural deformation under equipment loads that were never accounted for — the certificate provides no protection, because the certificate was never for that configuration.
A DNV 2.7-1 certificate for a mud service unit does not certify the pump, the control panel, the ventilation, or the pressurisation system. It certifies the empty container structure, the lifting set, and the liquid containment. The equipment inside is not covered. If your unit has active equipment installed inside it, you need DNV 2.7-3 — not DNV 2.7-1 with a more detailed data sheet.
DNV 2.7-1 vs DNV 2.7-3: the fundamental difference
The two standards differ not just in their requirements but in what they are trying to achieve. DNV 2.7-1 is a structural and lifting certification for empty containers. DNV 2.7-3 is a systems certification for functional modules. This distinction drives every difference in the requirements.
| Aspect | DNV 2.7-1 | DNV 2.7-3 |
|---|---|---|
| Certification object | The empty container structure | The complete functional module, including all equipment |
| Structural calculations cover | Frame, corner castings, floor, lifting set | Frame + all equipment loads, pipework, HVAC, structural penetrations |
| Electrical systems | Basic lighting only — not in scope | Full scope — hazardous area classified, ATEX/IECEx certified |
| Equipment functional testing | ✗ Not in scope | ✓ Required — all systems tested under load |
| Human occupancy | ✗ Not permitted | ✓ Permitted — with HVAC, pressurisation, fire systems |
| Maximum Gross Weight | Below 25,000 kg | Above 25,000 kg |
| Fabrication surveillance intensity | Standard DNV surveyor visits | Enhanced — multiple discipline coverage |
| Certification complexity | Moderate | High — multi-discipline engineering |
| Typical lead time | 12–20 weeks | 20–36 weeks (multi-system integration) |
| Certification cost | Lower (single-discipline) | Higher (multi-discipline survey team) |
Who actually needs DNV 2.7-3
The question is not what the unit looks like — it is what the unit does. If any of the following conditions apply, DNV 2.7-3 is the correct standard, regardless of what the supplier's quotation says.
1. Active equipment is installed inside the container
Pumps, agitators, compressors, blowers, heaters, cooling units, generators, or power packs mounted inside the container structure — even if they are partially installed — mean the unit is a POU, not a container. The structural calculations must account for equipment loads. The certification must cover the equipment. DNV 2.7-1 does not do either.
2. The unit has electrical systems in a hazardous area
If the unit will operate in a Zone 1 or Zone 2 area — which means almost any offshore oil and gas installation — all electrical equipment inside the module must be ATEX/IECEx certified. DNV 2.7-1 does not require this. DNV 2.7-3 does. The two standards have different hazardous area requirements built in by design.
3. The unit is intended for human occupancy
Workshop containers, accommodation modules, control cabins, mud logging units, and any other module where personnel are expected to work inside the unit during operation require DNV 2.7-3. The standard mandates HVAC, pressurisation to prevent ingress of hazardous gases, fire detection, and emergency egress. DNV 2.7-1 has none of these requirements — because it explicitly does not permit human occupancy.
4. Maximum Gross Weight exceeds 25,000 kg
This is the hard MGW boundary. If the loaded unit — including all equipment, fluids, and contents — exceeds 25,000 kg, DNV 2.7-3 applies regardless of content type. Many mud service units and chemical injection modules, when fully loaded with fluid and equipment, cross this threshold. Check the fully loaded MGW, not the empty tare weight.
5. The unit contains pressurised systems
Pressurised mud systems, nitrogen pods, hydraulic power units with pressurised reservoirs, and any module with vessels or pipework rated above atmospheric pressure require DNV 2.7-3 certification. The structural design must account for pressure vessel loads and the functional testing must include pressure hold tests.
6. The unit is a complete functional system, not a container
If the unit's purpose is to perform an operation — not just carry materials — it is a POU. Mud mixing units, cement mixing units, chemical injection skids, filtering units, and de-sanding units that perform a process function, not just a transport function, are POUs. DNV 2.7-1 certifies transport containers. DNV 2.7-3 certifies operational modules.
You commissioned a standard DNV 2.7-1 mud skip. After delivery, you added a pump, a flowmeter, and a control panel to make it a mud service unit. The supplier said that was fine. The DNV 2.7-1 certificate is still valid, they said. It is not. Adding active equipment to a DNV 2.7-1 certified container changes the certification scope. The structural calculations no longer cover the equipment loads. The electrical systems are not certified. The certificate does not cover the modification. To operate this unit offshore, you need it re-certified under DNV 2.7-3. This is one of the most common — and most costly — certification errors in the offshore drilling industry.
Common POU types that are frequently mis-certified
Mud service units and mud mixing units
These are the most commonly mis-certified POU type. A mud service unit typically contains: one or more centrifugal or progressing cavity pumps, a flowmeter, a control panel with variable frequency drive (VFD), a heating system for viscosity control, and a ventilation system. All of these are active systems. None of them are in scope under DNV 2.7-1. Yet many are certified under DNV 2.7-1 because the supplier had a DNV 2.7-1 certification on file and used it as a default. Correct standard: DNV 2.7-3.
Workshop containers and tool store modules
A standard tool container is a DNV 2.7-1 item. A workshop container with welding equipment, compressed air distribution, electrical tool circuits, and extraction ventilation is a POU — because the electrical systems, even if not in a Zone 1 area, are not covered by DNV 2.7-1. If the workshop is in a hazardous area, DNV 2.7-3 with full ATEX/IECEx scope is required. If it is in a non-hazardous area, the question is whether the electrical installation was included in the structural and certification scope — if not, DNV 2.7-3 or a separate electrical certification is needed.
Cable basket modules with junction boxes
A simple cable basket with junction boxes for power distribution is borderline. If the junction boxes are ATEX/IECEx certified and the cable routing is mechanically protected within the basket structure, the hazardous area electrical scope may be handled separately. But if the module includes active power distribution equipment — generators, UPS units, switchboards — it is a POU and DNV 2.7-3 applies.
Instrumentation and MWD/LWD cabins
Measurement-while-drilling (MWD) and logging-while-drilling (LWD) cabins are classic DNV 2.7-3 units. They contain active electronic instrumentation, power supplies, telemetry equipment, and typically pressurised enclosures for use in Zone 1 areas. They are also frequently occupied by personnel during operation. DNV 2.7-3 is the correct standard. DNV 2.7-1 is not an acceptable substitute.
Accommodation modules
Personnel accommodation containers for offshore use — even single-person refuge stations — require DNV 2.7-3 if they include HVAC, electrical systems, and life-safety equipment. The structural certification under DNV 2.7-1 does not cover the HVAC design, fire suppression, or emergency power systems that human occupancy requires.
Kill fluid and bullwork brine tanks
As noted in the DNV 2.7-1 vs 2.7-3 mud skip article: kill fluid tanks with integrated pumping systems, valving manifolds, and flow instrumentation are POUs. The pumping system and manifold are active equipment. The structural design must account for the dynamic loads from the pump and the vibration loads from the pipework. DNV 2.7-3 applies. A DNV 2.7-1 kill fluid tank is only correct if it is an empty transport container for kill fluid — not a functional kill fluid system.
"The test for whether DNV 2.7-3 applies is not the container's dimensions or its MGW alone. It is whether the unit contains active systems that perform an operational function. If the answer is yes — if there is a pump, a panel, a ventilator, a pressuriser, or a heater inside it — the answer is DNV 2.7-3."
Decision framework: DNV 2.7-3 or DNV 2.7-1?
| Condition in your unit | Implication | Correct standard |
|---|---|---|
| Active equipment installed inside (pumps, agitators, heaters, VFDs) | Structural calcs must include equipment loads; electrical cert required | DNV 2.7-3 |
| Electrical systems in Zone 1 or Zone 2 hazardous area | ATEX/IECEx certification mandatory; DNV 2.7-1 does not require this | DNV 2.7-3 |
| Unit intended for human occupancy during operation | HVAC, pressurisation, fire detection, and egress required | DNV 2.7-3 |
| Maximum Gross Weight above 25,000 kg | Hard boundary in DNV 2.7-1; DNV 2.7-3 applies regardless of content | DNV 2.7-3 |
| Pressurised vessel or pipework system inside | Pressure vessel loads in structural calcs; pressure hold test required | DNV 2.7-3 |
| Unit performs an operational process function (mixing, injection, filtration) | Functional testing of process systems required; not in DNV 2.7-1 scope | DNV 2.7-3 |
| Unit is an empty container for transport only — no active equipment | Transport container; structural calcs for frame and lifting set only | DNV 2.7-1 |
| Unit is a simple basket or open cargo carrier — no equipment, no electrics | Lightweight certification; DNV 2.7-2 or DNV 2.7-1 as appropriate | DNV 2.7-1 / 2.7-2 |
| MGW below 25,000 kg, no active equipment, no human occupancy, non-hazardous area | Standard offshore container scope; DNV 2.7-1 is correct | DNV 2.7-1 |
| You are unsure — unit has mixed equipment scope | Submit scope to DNV for pre-assessment before committing to certification route | DNV pre-assessment |
What DNV 2.7-3 certification involves
Understanding the DNV 2.7-3 certification process helps procurement teams plan realistic timelines and budgets. It is more complex than DNV 2.7-1 — and it should be.
Design appraisal
Unlike DNV 2.7-1, where the structural design review is the primary design appraisal task, DNV 2.7-3 requires review of multiple engineering disciplines: structural, electrical (hazardous area), mechanical (process equipment and pipework), HVAC, and fire detection. Each discipline's calculations and drawings must be submitted and approved before fabrication. DNV will typically assign a multi-discipline team for the review.
Equipment certification
All equipment installed inside the POU must have its own valid certification: pumps must have hydraulic and performance certificates, electrical equipment must have ATEX/IECEx certificates, pressure vessels must have valid test certificates. These are not optional add-ons — they are prerequisites for the DNV 2.7-3 certification to proceed.
Fabrication surveillance
DNV surveyors visit the fabrication facility multiple times during production, covering structural, mechanical, and electrical disciplines. Open non-conformances in any discipline are a block to certification. The surveillance intensity is higher than DNV 2.7-1 because the certification scope is wider.
Functional testing
DNV 2.7-3 requires functional testing of all installed systems under simulated operating conditions. This includes: running the pump at design flow and pressure, testing the ventilation system, verifying hazardous area pressurisation, testing fire detection and alarm circuits, and confirming that emergency stops de-energise the correct systems. These tests are witnessed by DNV surveyors and documented in the test report.
Certificate scope
The DNV 2.7-3 certificate is issued for the complete functional unit as built — not for the container structure alone. The certificate schedule lists all equipment included in the certification scope, their serial numbers, and their certification references. Any subsequent change to the equipment configuration requires a certificate amendment or re-certification.
A straightforward DNV 2.7-3 certification for a mud service unit with a single pump system and basic electrical scope typically takes 20–28 weeks from design submission to certificate issuance. Complex modules with multiple process systems, pressurisation, and full hazardous area scope can take 36 weeks or more. Build this timeline into your project schedule — not as a target, but as a minimum. Attempting to rush DNV 2.7-3 certification typically results in non-conformances that extend the timeline further.
What happens when DNV 2.7-3 applies but DNV 2.7-1 is used
The consequences are real and they fall on the operator, not just the manufacturer.
- Certificate invalidity — The DNV 2.7-1 certificate does not cover the active equipment inside the unit. If the equipment causes an incident, the certificate provides no evidentiary protection. Insurers will not accept it as proof of compliance for the POU scope.
- Operator liability — An operator who accepts a DNV 2.7-1 certified mud service unit on a DNV-classed installation is accepting a unit without valid certification for its actual configuration. If an incident occurs, the certificate gap is a liability exposure, not a technicality.
- Project rejection — Major operators and drilling contractors conduct pre-mobilisation checks. A unit with a DNV 2.7-1 certificate that clearly contains active equipment will be flagged. The unit will be grounded until the correct certification is obtained — which means the unit goes back to the manufacturer for re-certification, not a quick paperwork fix.
- Retrofitting cost — To bring a mis-certified unit into DNV 2.7-3 compliance, the unit must be stripped, have the structural calculations updated, have all equipment removed and re-certified, have the electrical systems redesigned to ATEX/IECEx, and be re-presented to DNV for full re-certification. This routinely costs 60–100% of the original unit price.
The manufacturer sold a mud service unit certified to DNV 2.7-1. The operator accepted it on the basis of that certificate. The unit went offshore. During a lift, a pump mounting bracket — which was never included in the structural calculations because DNV 2.7-1 does not require it — failed. The crane operator was injured. The DNV 2.7-1 certificate was not void because it was fraudulently obtained — it was void because it was never for the configuration that was built. The manufacturer, the operator, and the certification body all have exposure. This is not a hypothetical scenario. It is the kind of incident that drives regulatory investigations.
How to verify a supplier's certification scope
Before signing a purchase order for any portable offshore unit, ask these questions:
- What is the DNV certificate number? The certificate should reference DNV-ST-E273 (DNV 2.7-3), not DNV-ST-E271 (DNV 2.7-1). If the supplier cannot produce a DNV 2.7-3 certificate, they are not certifying the unit correctly.
- What is on the certificate schedule? The certificate schedule lists the equipment that is within the certification scope. If the pump, the control panel, and the ventilation system are not on the schedule, they are not covered.
- What is the MGW? If it is above 25,000 kg, DNV 2.7-3 applies regardless of what the supplier says. Get the fully loaded weight confirmed in writing.
- Has the periodic inspection date lapsed? Even a correctly certified DNV 2.7-3 unit with a lapsed periodic inspection is not currently compliant.
- Can DNV confirm the certificate directly? DNV maintains a public certificates database. Verify the certificate number, the scope, and the issuing DNV office before accepting the unit.
Can you upgrade from DNV 2.7-1 to DNV 2.7-3?
Yes — but it is not a simple re-certification. The process involves: re-engaging DNV with the complete equipment scope, updating structural calculations to include all equipment loads, having all equipment removed and re-certified with current ATEX/IECEx certificates where applicable, redesigning the electrical systems to hazardous area standards, completing functional testing under DNV witness, and issuing a new DNV 2.7-3 certificate.
The practical reality: for most mud service units and similar POUs, the cost of upgrading from DNV 2.7-1 to DNV 2.7-3 approaches the cost of a new build. The structural modifications required — new base frame calculations, new lifting set for the heavier loaded unit, new pad-eye welds — typically require the unit to be substantially disassembled for the structural work to be verified.
The right answer is to specify DNV 2.7-3 from the start of the project — before the purchase order is placed, before the manufacturer starts designing, before the first piece of steel is cut.
Quick-reference summary
- DNV 2.7-3 (DNV-ST-E273) applies to Portable Offshore Units — modules that contain active equipment, electrical systems, or are intended for human occupancy
- DNV 2.7-1 (DNV-ST-E271) applies to empty offshore containers for transport — it does not cover active equipment, hazardous area electrical systems, or functional process testing
- The most common mis-certification: mud service units, workshop containers, and pressurised modules certified under DNV 2.7-1 when DNV 2.7-3 is required
- The key indicators that DNV 2.7-3 applies: active equipment inside, hazardous area operation, human occupancy, MGW above 25,000 kg, pressurised systems, or a process function
- Adding equipment to a DNV 2.7-1 certified unit voids the original certificate; the unit must be re-certified under DNV 2.7-3
- Verify the certificate number and schedule before accepting any POU — the schedule must list the equipment, not just the container structure
- Certification timeline for DNV 2.7-3 is 20–36 weeks; plan it at project start, not after fabrication
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