DNV-RU-SHIP Pt.4 Ch.6: Piping Systems on Classified Vessels

1. Scope and Application

DNV-RU-SHIP Part 4 Chapter 6 covers piping systems fitted on board vessels classed with DNV. It applies to all pressure-containing piping used for the conveyance of liquids and gases, including sea water, fresh water, fuel oil, lubricating oil, hydraulic oil, steam, compressed air, cargo, and bilge systems. The chapter does not apply to pressure vessels themselves (covered in other chapters) but governs all connecting piping, including fittings, valves, flexible hoses, and supports.

The requirements are tiered by piping class, which is assigned based on the design pressure, design temperature, and nature of the medium. Higher-class piping (Class I) carries the most stringent fabrication and inspection requirements; lower-class piping (Class III) allows simplified construction for low-risk services. Designers must assign the correct piping class at the outset — errors in class assignment propagate through material selection, welding procedures, and inspection scope.

2. Piping Pressure and Temperature Classes

DNV-RU-SHIP Pt.4 Ch.6 defines three piping classes based on design pressure (P_D) and design temperature (T_D):

Class	Design Pressure	Design Temperature	Typical Services	NDT / Inspection
Class I	PD > 16 bar, or any pressure for steam > 300°C, flammable fluids > flash point	TD > 300°C or cryogenic	Main steam, high-pressure fuel, hydraulic (high-P), LNG/LPG cargo	100% radiographic or ultrasonic testing of butt welds; full documentation
Class II	7 bar < PD ≤ 16 bar	TD ≤ 300°C	Auxiliary steam, fuel feed, compressed air, cooling water (high-P), hydraulic return	Random (10–20%) RT/UT of butt welds; visual + hydrostatic
Class III	PD ≤ 7 bar	TD ≤ 60°C for flammable, ≤ 200°C for non-flammable	Sea water cooling, bilge, ballast, sanitary, low-pressure fresh water	Visual inspection + hydrostatic test; no mandatory weld RT/UT

The design pressure is the maximum working pressure the system can reach under any operating condition, including pump shut-off head, thermal expansion, or surge. The design temperature is the highest temperature the fluid can reach under normal or transient conditions. Both must be established before class assignment.

3. Material Selection by Service

Material selection under DNV-RU-SHIP Pt.4 Ch.6 is driven by the fluid medium, operating temperature, and corrosion environment. DNV requires that materials be selected from approved grades; proprietary materials require type approval.

Seawater service

Seawater is the most corrosively demanding service for shipboard piping. Approved materials in approximate order of increasing performance and cost:

• Copper-nickel 90/10 (Cu-Ni, UNS C70600): The standard seawater piping material for classified vessels. Excellent corrosion resistance, good biofouling resistance, erosion velocity limit ~3 m/s. Minimum wall specified by DNV tables.
• Copper-nickel 70/30 (UNS C71500): Higher Ni content; for higher velocity services up to ~4.5 m/s and more aggressive biofouling environments.
• Aluminium brass: Suitable for seawater in moderate service. Not recommended for stagnant or polluted seawater.
• Duplex stainless steel (UNS S31803/S32205, PREN ≥ 35): For high-velocity or aggressive seawater. Requires cathodic protection assessment per DNV-RP-B401 to avoid HISC under CP overprotection.
• Super-duplex stainless steel (UNS S32750/S32760, PREN ≥ 40): For seawater injection, fire-water mains, and high-flow services. Preferred for offshore vessel seawater systems.
• GRP (glass-reinforced plastic): Permitted for seawater cooling and ballast piping with DNV type approval. Non-conducting — eliminates galvanic corrosion risk.

Fuel oil service

Carbon steel (e.g. EN 10216-1 P235TR1 or equivalent) is standard for fuel oil piping. Austenitic stainless steel is acceptable but not typical. Key requirements: pipe joints in fuel oil systems must be welded or flanged (no compression fittings); drip trays under all flanged joints in machinery spaces; Class I or II classification depending on pressure and flash point.

Cargo and chemical service

Material selection for cargo piping on tankers is governed by DNV-RU-SHIP Pt.5 Ch.4 (tanker rules) in conjunction with Pt.4 Ch.6. For chemical tankers, IACS UR P2 material tables apply. Austenitic stainless (316L for general service, 904L or 6Mo for aggressive chemicals), duplex stainless, and lined carbon steel are common. Material compatibility with cargo must be verified against published chemical resistance data.

Steam service

High-temperature steam piping uses seamless carbon steel (e.g. EN 10216-2 P235GH) for temperatures up to 300°C and low-alloy steels (P265GH, 15Mo3, 13CrMo4-5) for higher temperatures. Austenitic stainless (316H) for temperatures above 500°C. All high-temperature piping is Class I and requires full documentation of material certificates (EN 10204 Type 3.1 minimum).

Lubricating oil and hydraulic service

Carbon steel is standard. Galvanised steel is prohibited in contact with lube oil — zinc contamination causes oil degradation and bearing damage. Copper alloys may be used for hydraulic lines where Class III applies.

4. Wall Thickness and Pressure Containment

The minimum required wall thickness for straight pipe under internal pressure is calculated using the Barlow formula (modified for DNV safety factors):

Where:

• P_D = design pressure (bar)
• D_o = outside diameter (mm)
• σ_all = allowable stress = min(R_m/2.7, R_eH/1.8) at design temperature
• e₁ = joint efficiency factor (1.0 for seamless, 0.85 for ERW with confirmed weld quality)
• c₁ = corrosion allowance (typically 1.0–3.0 mm for carbon steel; 0 mm for stainless)
• c₂ = mill tolerance allowance (typically 12.5% of nominal wall per EN standard)

For commonly used materials at ambient temperature: the allowable stress for P235 carbon steel is approximately 86 MPa; for 316L austenitic stainless approximately 115 MPa. DNV provides tabulated minimum wall thicknesses for standard pipe sizes and pressure ratings in the chapter appendices, which are the practical basis for design.

5. Pressure Testing Requirements

All piping must be pressure tested before commissioning. DNV-RU-SHIP Pt.4 Ch.6 specifies hydrostatic testing as the standard method; pneumatic testing is permitted in specific cases where hydrostatic testing is impractical (e.g. very large systems where flooding is hazardous, cryogenic piping), subject to additional safety precautions.

Hydrostatic test pressure

The test pressure must be maintained for a minimum of 30 minutes (Class I and II) or 10 minutes (Class III) with no leakage or pressure drop. All welds, fittings, and connections must be inspected visually during the test. Pressure gauges must be calibrated and must be positioned at the lowest point to account for hydrostatic head.

Exceptions and special cases:

• Plastic (GRP/HDPE) piping: test pressure = 1.5 × P_D at ambient temperature, but limited by the material's rated pressure at operating temperature
• Pipelines integrated with machinery (heat exchangers, pumps): may be tested in situ at the machinery test pressure if agreed with the surveyor
• Cargo piping on tankers: tested to 1.5 × design pressure or 4 bar, whichever is greater, per MARPOL requirements

Shop vs. field testing

Prefabricated piping spools should be tested in the workshop where possible — easier leak detection and no risk of damage to installed insulation. Field testing is required for final assembled systems in all cases. DNV surveyors witness the pressure test for Class I systems; Class II and III may be self-certified by the shipyard with documented records.

6. Valve Requirements and Quick-Closing Valves

DNV-RU-SHIP Pt.4 Ch.6 specifies valve selection, isolation requirements, and quick-closing valve (QCV) arrangements for critical systems.

Isolation and safety valves

• Hull valves: All seawater connections to the hull must have a hull valve (sea cock) rated to the relevant piping class and operable from above the waterline. Remote operation required for valves in inaccessible locations.
• Safety (relief) valves: Required on all systems that can be isolated when hot or pressurised (steam, hydraulic, thermal expansion). Set pressure not more than 10% above design pressure for Class I; 15% for Class II.
• Non-return valves: Mandatory on all discharge lines from pumps that serve common headers, to prevent backflow between parallel pumps.

Quick-closing valves (QCVs)

SOLAS and DNV-RU-SHIP require QCVs on fuel oil storage tanks and settling/daily service tanks. QCVs must be operable from outside the machinery space where the tank is located — typically via a remote pull wire or hydraulic actuator terminated at a panel in the engine room vestibule or on deck. DNV requires QCVs to:

• Close in ≤ 5 seconds from actuation signal
• Fail closed on loss of actuating medium (fail-safe)
• Be testable without interrupting normal fuel service (bypass line arrangement)
• Be marked with flow direction and rated for the design pressure

7. Flexible Hoses and Expansion Bellows

Flexible connections are permitted in DNV-classed piping systems under controlled conditions. They are used to absorb vibration (machinery connections), compensate for thermal expansion, and accommodate relative movement between hull structure and piping.

Flexible hoses

Type approval is required for flexible hoses in Class I and II piping. DNV approval covers the hose construction, end fittings, pressure rating, and bend radius. Key constraints:

• Length: typically ≤ 1,000 mm (longer runs require engineering approval)
• Not permitted in fuel oil service at temperatures above the fluid's flash point, or in high-temperature steam
• Must be accessible for inspection and replacement
• Metal-clad hoses (SS braid over PTFE liner) are standard for lube oil and hydraulic service where rubber hoses are not acceptable

Expansion bellows and compensators

Axial and lateral expansion bellows are used in steam, high-temperature, and large-bore piping systems. DNV requirements: tied bellows must be designed for the full system operating pressure; untied bellows must be designed for the maximum pressure the piping can reach. Bellows in vibration service must be assessed for fatigue life over the design service period.

8. Piping Supports, Drainage, and Venting

Piping supports must be designed to carry the combined weight of the pipe, insulation, fluid, and any imposed loads (ship motion, thermal expansion). DNV requires:

• Maximum support spacing calculated using published span tables (function of pipe diameter, wall thickness, and fluid density)
• Slide supports at fixed intervals to allow thermal expansion while preventing lateral movement
• Anchor points at each side of expansion loops and at equipment nozzles to control thermal growth direction
• Dielectric insulation between carbon steel supports and stainless or copper-alloy piping to prevent galvanic corrosion

Drainage and venting

All piping that can trap water or condensate must have low-point drains. Steam piping requires steam traps at regular intervals (maximum 25–30 m on horizontal runs). All piping must have high-point vents for filling and de-aeration. Drain and vent valves must be capped (plugged) when not in use — open drain valves are a common finding in SOLAS surveys.

9. Fire Integrity and Hazardous Area Piping

Fire stops and penetrations

Piping penetrations through A-class fire divisions (main fire bulkheads, decks) must maintain the fire integrity of the division. DNV-RU-SHIP Pt.4 Ch.6 requires:

• Steel sleeves with fire-rated packing for all combustible pipe materials (GRP, plastic) through A-class bulkheads
• Metallic piping may penetrate directly with an annular seal; for ventilation piping, automatic fire dampers required
• Foam or intumescent fire stop compound between pipe and sleeve, rated to A-60 when required

Hazardous area (Ex zone) piping

Piping in ATEX/IECEx Zone 1 and Zone 2 hazardous areas (as defined by IEC 60092-502 for ships) must use metallic or anti-static materials. Plastic piping and rubber hoses are not permitted in Ex zones unless specifically type-approved for Zone 2 use. All flanged connections in Ex zones require bonding wires across the flanges to prevent static charge accumulation.

10. Seawater and Cooling Water Systems

Seawater systems are among the most critical and most failure-prone systems on classified vessels. DNV-RU-SHIP Pt.4 Ch.6 requirements for seawater service reflect the combination of corrosion, biofouling, and erosion challenges:

Sea chest and strainer arrangements

Vessels must have at least two sea chests (high and low suction) to allow operation in various loading conditions and to provide redundancy. Sea chests must have strainer/filter elements sized for maximum flow. DNV requires that the combined strainer area be at least 4× the pipe cross-section area to maintain adequate flow when partially blocked.

Cathodic protection in seawater systems

Internal cathodic protection of seawater pipework (sacrificial anodes in sea chests, zinc inserts in copper-nickel piping) prevents galvanic corrosion at dissimilar metal contacts. DNV requires that the anode material be compatible with the pipe material — zinc anodes are standard for steel and copper alloy systems; aluminium anodes for aluminium alloy piping. Impressed current cathodic protection (ICCP) is used for hull external systems and large sea water lift pumps.

Biofouling and chlorination

Seawater cooling systems are subject to biofouling (mussels, barnacles, biofilm) that can reduce flow capacity significantly over time. DNV permits electrolytic chlorination systems that dose a small amount of hypochlorite into the seawater flow to inhibit fouling. Dosing must be controlled to prevent chloride stress corrosion cracking in any stainless steel components downstream.

11. Cross-Standard Reference Table

Standard	Relationship to DNV-RU-SHIP Pt.4 Ch.6	When it applies
EN 13480 ISO	Metallic industrial piping — wall thickness calculation, materials, testing. DNV Pt.4 Ch.6 references EN 13480 as an acceptable standard for material and design calculations.	All Class I and II metallic piping on classified vessels
ASME B31.3 ASME	Process piping standard accepted by DNV as alternative to EN 13480 for process piping on FPSOs and offshore vessels. Separate documentation path — requires agreement with surveyor.	FPSO topsides, US-flagged vessels, vessels with ASME-based certification history
NORSOK P-100 NORSOK	Process piping on NCS offshore vessels — supplements DNV-RU-SHIP for topside process piping. More stringent material and inspection requirements. Governs where it exceeds DNV.	NCS-operated vessels with NORSOK project specs
NORSOK M-001 NORSOK	Material selection standard — PREN requirements for stainless steel in seawater, corrosion allowances, sour service requirements (H₂S). Must be read alongside Pt.4 Ch.6 for NCS projects.	All NCS seawater and sour-service piping
ISO 15649 ISO	Petroleum and natural gas industries — piping. Provides material selection and design guidance for offshore oil and gas piping systems, referenced by DNV for offshore vessel process piping.	Process piping on offshore production vessels
SOLAS Ch.II-2 IMO	Fire protection and fuel oil arrangements — QCV requirements, fire stop penetrations, and bilge pump arrangements. DNV-RU-SHIP Pt.4 Ch.6 is aligned with SOLAS but SOLAS governs as the statutory requirement.	All vessels operating under SOLAS flag-state requirements
DNV-RU-SHIP Pt.4 Ch.7 DNV	Rotating machinery — covers pump and compressor requirements that connect to piping systems. Nozzle loads on equipment must be within the limits specified in Ch.7.	All pump and compressor nozzle connections
DNV-RU-SHIP Pt.6 Ch.5 DNV	Additional piping class notation requirements — CLEAN, ECO, and other voluntary notations may impose additional piping standards for pollution prevention and environmental service.	Vessels seeking environmental notations

12. Common Engineering Errors

Based on drawing review findings and classification society survey reports, the following errors recur in piping design and fabrication on classified vessels:

1. Wrong piping class assignment: Assigning Class III to fuel oil lines where flash point triggers Class I/II. Always check flash point and system isolation conditions, not just operating pressure.
2. Galvanised steel in lube oil systems: Zinc contamination from galvanised fittings destroys oil quality and bearing surfaces. Prohibited — check all BOM items for finish specification.
3. Cu-Ni velocity exceedance: Designing seawater systems for 90/10 Cu-Ni at velocities above 3 m/s. Size up or switch to 70/30 Cu-Ni / duplex for higher-velocity sections.
4. No low-point drains on steam condensate lines: Steam lines without drains flood with condensate on start-up, causing water hammer and pipe failures. Required at every low point and before isolation valves.
5. Expansion bellows in straight runs without guides: Untied expansion bellows with inadequate pipe guides buckle under pressure thrust. Pipe guide spacing must be specified in the isometric drawing.
6. Hydrostatic test pressure calculated from working pressure, not design pressure: P_test = 1.5 × P_D (design pressure = relief valve set pressure or system maximum), not the normal operating pressure. Test at working pressure will under-test the pipe.
7. Missing dielectric insulation at stainless/carbon steel contacts: Direct contact between support clamps (carbon steel) and Cu-Ni or stainless piping creates galvanic couples. Use neoprene-lined or PTFE-insert clamps throughout.