DNV 2.7-1 vs DNV 2.7-3 for Mud Containers and Chemical Tanks: A Practical Guide

The core distinction in one sentence

DNV 2.7-1 certifies cargo carrying units — empty containers designed to transport equipment, materials, or fluids. DNV 2.7-3 certifies portable offshore units — modules that contain active equipment, electrical systems, or both, and may be intended for human occupancy during operation.

A mud skip carrying drilling fluid from the derrick to the mud pits? That is a DNV 2.7-1 unit. A mud mixing unit with agitators, pumps, and a control panel inside? That is a DNV 2.7-3 unit. The fluid is the same. The certification standard is not.

What DNV 2.7-1 covers for mud skips

DNV 2.7-1 (formally DNV-ST-E271) is the standard for offshore containers — including mud skips, chemical drums, baskets, and standard cargo containers. For mud skips specifically, it covers:

• Structural frame and lifting set — the outer framework, corner castings, and lifting eyes must be certified to withstand twice the Safe Working Load (SWL) during dynamic offshore lifting
• Floor design — mud skips carry dense, heavy fluids. The floor must be designed for concentrated point loads from the fluid mass under dynamic motion, not just static stacking
• Containment — for mud skips, the unit must be liquid-tight under all operating angles during vessel motion; this is verified by prototype testing
• Drainage and valving — DNV 2.7-1 mud skips typically include bottom drainage valving; the valve installation must not compromise the structural integrity of the floor
• Corrosion protection — the internal and external coating system must be specified for the fluid media being carried (oil-based mud, water-based mud, brine, chemical)
• Marking and identification plate — SWL, tare weight, MGW, container ID, DNV certification date

The critical point for mud skip operators: DNV 2.7-1 certification covers the container structure, lifting set, and fluid containment. It does not certify any equipment installed inside the container — pumps, valves, instrumentation, or fluid management systems are outside the standard's scope.

What DNV 2.7-3 covers for chemical tanks and service modules

DNV 2.7-3 (formally DNV-ST-E273) is the standard for portable offshore units — modules that contain active equipment and are certified as complete functional units. For chemical tanks and mud service modules, it covers:

• Structural design including equipment loads — the structural calculations account for the weight and dynamic loads of all installed equipment — pumps, agitators, heat exchangers, pipework — not just the container frame
• Electrical systems — all electrical installations within the module must comply with the applicable hazardous area classification (Zone1 or Zone 2 for oil and gas environments); ATEX or IECEx certification is typically required
• HVAC and pressurisation — if the module is intended for human occupancy or contains pressurised systems, HVAC design, pressurisation control, and breath通风 requirements are covered
• Fire detection and suppression — DNV 2.7-3 modules that contain active equipment or are occupied require fire detection and, in many configurations, fixed fire suppression systems
• Functional testing — prototype testing under DNV 2.7-3 includes not only structural proof load testing but functional testing of all installed systems under simulated operating conditions
• Personnel safety systems — emergency stops, escape routes, hazardous area signage, and PPE storage are within scope for occupied modules

For chemical tanks, the distinction is critical: a chemical tank certified to DNV 2.7-3 is certified as a complete system — tank, containment bund, valving, heating or cooling coils, agitation systems, and instrumentation. A DNV 2.7-1 chemical tank is only certified as a liquid-tight container; the internal equipment is not covered.

Side-by-side: DNV 2.7-1 vs DNV 2.7-3

Which standard applies to mud skips?

Most mud skips — including bulk mud containers, trip mud drums, and standard mud transfer skips — are certified under DNV 2.7-1. The key criteria:

• The unit is an empty container carrying mud or drilling fluid
• No active equipment is installed inside the unit
• MGW is below 25,000 kg
• The unit is lifted, transported, and set down — it does not operate while offshore

If all four conditions are met, DNV 2.7-1 is the correct standard. The certification covers the structural frame, lifting set, and liquid containment. Drainage valving and internal coating for the specific mud type are within scope.

Which standard applies to chemical tanks?

Chemical tanks straddle the line more than mud skips. The answer depends on what the tank does, not just what it carries.

DNV 2.7-1 applies when:

• The tank is an empty liquid-tight container for transporting chemical drums, bulk liquid chemicals, or kill fluid
• MGW is below 25,000 kg
• No heating, agitation, pumping, or instrumentation is installed inside the tank
• The tank is received, stored, and used as a container — not as a process unit

DNV 2.7-3 applies when:

• The tank includes integrated heating elements (e.g., for viscosity control of heavy crude or OBM)
• Agitation or mixing systems are installed inside the tank
• Pumping and flowmetering equipment is integrated into the tank structure
• The unit is a complete chemical injection or treatment module with active process control
• MGW exceeds 25,000 kg

Decision framework

What happens if you specify the wrong standard?

The consequences are not theoretical. Operators and drilling contractors who accept DNV 2.7-1 certified units for applications that require DNV 2.7-3 face real operational and legal exposure.

For mud skip operators: consequences of wrong certification

A mud skip operating under a DNV 2.7-1 certificate that should be DNV 2.7-3 is a unit with active equipment — pumps, flowmeters, or control systems — that was never included in the structural calculations or the certification scope. The DNV surveyor never reviewed those systems. The certificate does not cover them. If the pump mounting fails during a lift because the structural calcs never accounted for dynamic pump loads, the certificate provides no protection — and neither does your insurer.

For chemical tank operators: consequences of wrong certification

A chemical tank with integrated heating elements operating under DNV 2.7-1 is not certified for the temperatures, pressures, or hazardous area classification that DNV 2.7-3 would require. If that tank is in a Zone 1 area and the heating element sparks, the DNV 2.7-1 certificate is irrelevant — it never covered the heating system. The operator who accepted the unit is carrying the risk.

What to ask your supplier before procurement

Whether you are procuring a mud skip or a chemical tank, these are the questions that matter before you sign the purchase order:

• What is the Maximum Gross Weight? If it is above 25,000 kg, DNV 2.7-3 applies regardless of content. If below, continue to the next question.
• Is any active equipment installed or integrated inside the unit? Pumps, agitators, heaters, control panels, or instrumentation inside the container mean DNV 2.7-3 applies.
• What is the hazardous area classification for the installation? If Zone 1 or Zone 2, DNV 2.7-3 is required — with ATEX or IECEx certification for the electrical systems.
• Is the unit intended for human occupancy? Accommodation, workshop, or control modules require DNV 2.7-3.
• Can I see the DNV certificate scope? The certificate should clearly state whether it covers DNV-ST-E271 (2.7-1) or DNV-ST-E273 (2.7-3). If the supplier cannot produce it, walk away.
• Has the periodic inspection date lapsed? Even a correctly certified unit with a lapsed periodic inspection is not currently compliant.
• What media is the internal coating rated for? For mud skips, confirm the coating system matches the mud type. For chemical tanks, confirm compatibility with the chemical being carried.

Can you upgrade from DNV 2.7-1 to DNV 2.7-3?

Yes — but it is not a simple paperwork exercise. To upgrade an existing DNV 2.7-1 unit to DNV 2.7-3 certification:

• The unit must be re-presented to DNV with the new equipment scope included
• Structural calculations must be updated to include equipment loads, pipework, and any additional structural penetrations
• Electrical systems must be designed, installed, and certified to the applicable hazardous area standard (ATEX or IECEx)
• HVAC and pressurisation must be designed and certified if the module is intended for human occupancy
• Functional testing of all installed systems must be completed under DNV witness
• A new DNV 2.7-3 certificate is issued upon satisfactory completion

The upgrade cost is significant — often 40–70% of the original certification cost, depending on the complexity of the equipment integration. It is almost always cheaper to procure the correct certification from the start than to retrofit and re-certify.

Quick-reference summary

• DNV 2.7-1 is for offshore containers — mud skips, bulk mud containers, chemical drums — that carry fluids or materials without active equipment inside. MGW must be below 25,000 kg. Human occupancy is not permitted.
• DNV 2.7-3 is for portable offshore units — chemical tanks with heating or agitation, mud service modules, kill fluid units — that contain active equipment and may be intended for human occupancy. MGW is above 25,000 kg.
• The MGW threshold of 25,000 kg is a hard boundary, not a guideline. Units above this weight must be certified to DNV 2.7-3 regardless of content.
• Active equipment inside the container — pumps, agitators, heaters, control panels — is the clearest indicator that DNV 2.7-3 applies, not DNV 2.7-1.
• Converting a DNV 2.7-1 unit by adding equipment voids the original certificate. The unit must be re-certified under DNV 2.7-3 before going offshore.
• Periodic inspection is required under both standards. A lapsed certificate means the unit is not currently compliant.
• Always check the certificate scope before accepting a unit. The certificate number, standard reference, and periodic inspection date are the three things that matter.