DNV-OS-C401 (July 2023) – Fabrication & Testing of Offshore Structures

Overview and Objective of DNV-OS-C401

The document stipulates the minimum technical requirements for fabrication, welding, mechanical fastening, testing and corrosion protection of offshore units, installations and equipment. Its four explicit aims are: (1) furnish an internationally acceptable fabrication standard, (2) serve as contractual reference between manufacturers, suppliers and purchasers, (3) guide all stakeholders—including regulators, designers and fabricators—on safe practice, and (4) define DNV-specific criteria for certification, verification and classification.

Scope of Application

• Applies world-wide to welded fabrication of load-bearing hull structures, superstructures, crane pedestals, helideck foundations, machinery bedplates, pressure systems, pipework, etc.
• May, by agreement, be used for other structures (e.g., fish-farm units).
• Chapters 1 & 2 form a free-standing technical standard. Chapter 3 adds extra requirements whenever DNV certification / class is requested. Governmental regulations may, however, impose further demands.

Foreword and Liability

DNV states that the on-line PDF is the official edition; conflicting copies are invalid. Use by third parties is entirely at their own risk—DNV accepts no liability for any resulting loss.

July 2023 Revision – Key Changes (supersedes July 2021)

Updates highlighted in red in the original

1. Chapter 1 fully re-structured to DNV’s generic format; welding–operator qualification added.

2. Single-sided tubular welds now explicitly covered: ISO 19902 & API RP-2A referenced; extra qualification guidance.

3. Welding Procedure Qualification Test (WPQT) extensions:
   • Dissimilar-material WPQTs required.
   • Extra Charpy locations for wide-gap butt repairs (>16 mm up to $1.5\,t$).
   • NDT acceptance to consider special applications (ship hull, pressure vessels, LNG tanks…).
   • Notch positioning clarified for varying bevel angles.
   • Fracture-mechanics (CTOD) testing shall examine both Weld Metal (WM) and Grain-Coarsened HAZ (GCHAZ).

4. Category I/II inspection welds: additional butt-weld qualification subsection.

5. Deep-penetration fillet welds no longer limited to “automatic” process.

6. Carbon-equivalent (CET) allowed for WPS qualification; heat-input increases above Approval-of-Manufacturer (AOM) level trigger re-qualification (notably NV 550+).

7. Aluminium dissimilar-grade joints: new qualification range rules.

8. Forming: complete section with cold/warm/hot forming table, formulae, Figures, distinction between forming vs. fairing.

9. Non-conformance “spikes” may escalate verification scope. NDT plans must record % tested per weld category & process and be continuously updated.

10. Structural-tank testing methodology and pressure heights revised; COAT-PSPC class-notation clarified.

11. Bibliography removed; many editorial corrections.

Historic Change Log

Appendix lists July 2021, 2020, 2019, 2018 etc. alterations: e.g., ISO 9712 (2012+) mandatory, quality-level B for most WPQTs, deep-penetration fillet-weld criteria, IACS UR-W32 for welders, full adoption of IACS UR-W35 for NDT suppliers, etc.

Structural Layout of the Standard

• Chapter 1 – Introduction (objective, scope, references, definitions, procedural req.).
• Chapter 2 – Technical Provisions with 10 Sections: General, Contractors, Welders, Consumables, Procedures, Fabrication/Tolerances, NDT, Structural & Tightness testing, Corrosion Protection, Bolts/Fastening.
• Chapter 3 – Additional DNV classification/certification requirements.

Referenced Documents

DNV Internal

15 Rules/Standards (RU-OU-0101 … OS-D101), 6 Class Guidelines (e.g.
CG-0162 Robotic Welding), approval programmes CP-0069 (consumables) etc.

External

Over 60 ISO/EN/ASME/API/IACS/NORSOK/NACE references; latest editions to be used unless the verifier agrees otherwise.

Core Definitions

Key verbs: “shall” = mandatory, “should” = recommended, “may” = permissible.
Definitions span automatic welding, contractor, heat-input, hold-point, failure-rate, wettight/weathertight, WPQT/WPQR/WPS, etc.
Heat-input per pass: $E=\frac{U\,I}{\text{travel speed}}$ (adjusted for thermal efficiency $\eta$ when EN 1011-1 is invoked).

Symbols & Abbreviations

Tables list ≈40 Latin symbols (e.g.
$CET$, $P<em>{cm}$, $l</em>r$) & 40 Greek symbols (e.g.
$\alpha$ tubular-joint angle). 60 acronyms include CTOD, PAUT, TOFD, ALS, CO2 etc.

Section-wise Technical Highlights

1 General Requirements

• Workshops may require pre-qualification (AOM, WWA).
• Furnace heat-treatment: calibrated control & recording; uniformity essential.
• Carbon-equivalents: $C<em>{eq}=C+\frac{Mn}{6}+\frac{Cr+Mo+V}{5}+\frac{Ni+Cu}{15}$; $P</em>{cm}=C+\frac{Si}{30}+\frac{Mn+Cu+Cr}{20}+\frac{Ni}{60}+\frac{Mo}{15}+\frac{V}{10}+5B$; $CET=C+\frac{Mn+Mo}{10}+\frac{Cr+Cu}{20}+\frac{Ni}{40}$.

2 Contractor Requirements

• ISO 9001 QMS plus ISO 3834 (welding) & CP-0640 / ISO 17020 (NDT).
• Must supply Inspection/Test Plans, procedures, welding sequence, pre-assembly info.

3 Welder & Operator Qualification

• Welders certified per ISO 9606 / ASME IX / AWS D1.1; 6-monthly renewal.
• Welding-operator proficiency records per ISO 14732; training covers groove assessment, weather limits, consumable handling.

4 Consumables

• Grouped by parent-steel grade; hydrogen class H5/H10 per CP-0069.
• Shielding-gas families I (inert Ar/He) & M/C (active CO₂/O₂/H₂ blends)—see Table of compositions.

5 Welding Procedures

Qualification

• pWPS → WPQT → WPQR → final WPS.
• Record every pass (U,I,v,heat input, bead width). Instruments $\le5\%$ calibrated.

Joint Coverage

• Butt, T/Y/K, fillet; partial-penetration joints need bespoke approval. Wide-gap (>16 mm) repairs demand extra MT/PT and dual-side Charpy.

Mandatory Tests

– 100 % VT + RT/UT + MT/PT on procedure coupons.
– Tensile: fracture shall meet BM minimum $R_m$.
– Bend: mandrel $4t$ (NV 420↓), $5t$ (NV 500), $6t$ (NV 550-690).
– Charpy sets: WM, FL, FL+2 mm (extra FL+5/10 mm for >50 kJ/cm). Acceptance: NV A/B 27 J, others 47 J or 34/39 J per position/process.
– CTOD ≥ $0.15\,\text{mm}$; characteristic value by Table-3 statistical rule.

Essential Variables

Base-metal grade, thickness (Table-6), heat-input range (≤50 kJ/cm: ±25 %, >50 kJ/cm: +10/–25 %), preheat ↓, inter-pass ↑, polarity switch, weave→stringer, backing deletion, change to higher toughness, etc.

6 Fabrication & Tolerances

• Shop primers must be CP-0109-approved if welded over.
• Welding environment: no wind/damp; wet welding prohibited unless specially agreed.
• Cold forming: theoretical strain $\varepsilon$ calc. Cold-form ≤12 % needs BM tests; >20 % not allowed (except tested/heat-treated). For duplex SS root & cap must stay clean (back-purge O₂ < 50 ppm). • Post-weld heat-treat: $550–620\,^{\circ}!\text{C}$, >2\,\text{min}/\text{mm}; QT steels max 30 °C below tempering.
• Weld repairs: qualified WPS; excavate +50 mm each side; max two repairs in same spot.
• Straightness & alignment tolerances per IACS 47 or Table-2 (δ = 0.0015l etc.).

7 Non-Destructive Testing

• Delay before final NDT to detect hydrogen cracking: $48\,\text{h}$ (NV 420↓), 72 h (NV 690).
• Minimum extent (Table-2): Special Cat I 100 % MT/PT & RT/UT; Primary Cat II 20 – 100 %; Secondary Cat III 2 – 5 % spot.
• NDT plans must map weld IDs, category, % tested; failure-rate >5 % triggers scope extension.
• UT <10 mm thick needs advanced techniques (PAUT/TOFD).
• Acceptance: ISO 5817 level B (undercut etc. may use level C); planar flaws always reject unless Engineering Criticality Assessment shows adequacy.

8 Structural & Tightness Tests

• Hydrostatic or hydro-pneumatic: test head $h<em>{op}$ equals design filling height (min 2.4 m). Tanks below draught $T</em>E$: head = $T_E - z$.
• Tightness: 100 % hose or air/soap, before final coating; SAW continuous welds may omit if weld profile & NDT prove sound.

9 Corrosion Protection

• Seawater ballast tanks must at least meet IMO PSPC MSC.215(82).
• Sacrificial-anode studs on doubler plates $\ge10\,\text{mm}$, with r > 20 mm corners & continuous weld.

10 Bolts & Mechanical Fastening

• Structural bolts $\le10.9$ on deck; submerged bolts $\le8.8$ to avoid HISC. Alloy steel QT if R_e > 490\,\text{MPa}.
• Slip-critical surfaces prepared per DNV-OS-C101 (surface class A/B/C by grit-blast + coating).

Chapter 3 – Additional DNV Classification Rules

• Contractors unknown to DNV must prove capability; welding workshops may require WWA-C.
• Welders on hull work re-validated every 6 months; 3-year re-test or 2-year welded-joint evidence route.
• Consumables need Type Approval (CP-0069); non-approved require extra WPQT with all-weld-metal tensile.
• NDT suppliers must hold AOSS-C (CP-0640) or operate QMS to IACS W35.
• Vessel-specific submissions: NDT plans/procedures, tank-test plans, erection & inspection plans, hole/penetration plans, welding tables, WPS/WPQR etc. (Table-4).
• DNV surveyor witnesses WPQT, mechanical tests, structural/tank tests; may audit NDT work, review failure rate and escalate inspection.

Practical Implications & Best-Practice Connections

• Welding plans must bind heat-input to BM manufacturer’s AOM limits—critical for TM/QT steels in wind-farm jackets.
• Cold-forming strain auditing with formulas (1)–(4) avoids hidden strain-age embrittlement in thick Y-plates.
• Adoption of PAUT/TOFD needs formal procedure-qualification with POD demonstration; verifier may demand comparison RT/UT shots.
• PSPC compliance melds structural & coating QA: ensure peak-to-valley 30-75 µm blast profile, DFT > 320 µm, salt < 50 mg/m², 2 × stripe coats.
• COAT-PSPC notation invokes DNV survey of all coating work (otherwise outside class scope).

Ethical & Safety Considerations

Strict tracking of consumable hydrogen class, temperature delays, and NDT failure-rates is critical to prevent brittle fracture incidents—ethical duty parallels legal occupational-safety mandates. Documentation provides traceability for accident investigation and continual improvement.

Summary Cheat-Sheet Equations

$C<em>{eq}=C+\frac{Mn}{6}+\frac{Cr+Mo+V}{5}+\frac{Ni+Cu}{15}$ $P</em>{cm}=C+\tfrac{Si}{30}+\tfrac{Mn+Cu+Cr}{20}+\tfrac{Ni}{60}+\tfrac{Mo}{15}+\tfrac{V}{10}+5B$
$CET=C+\tfrac{Mn+Mo}{10}+\tfrac{Cr+Cu}{20}+\tfrac{Ni}{40}$
Heat input (single wire) $E=\eta\,\frac{U I}{v}$; multi-wire $E=\sum!E<em>i$ if T{interpass}>250^{\circ}!\text C.
Cold-forming strain: Cylinders $\varepsilon=100\,\ln!\Bigl(\frac{R}{R-t}\Bigr)$, Bends $\varepsilon=100\,(\tfrac{t}{2R_c})$ etc.

Concluding Remarks

The July 2023 edition consolidates OS-C401 as a modern, risk-based, globally referenced fabrication code. Stakeholders must particularly note the widened WPQT coverage (dissimilar, wide-gap, deep-penetration, robotic, hybrid-laser), explicit cold-forming qualification spectrum, and the tighter linkage between NDT performance metrics and verification scope.