EN 1090-2 is the fabrication execution standard for steel structures in Europe — it defines how structural steel is to be fabricated, not just designed. For offshore and subsea structures built under DNV or NORSOK rules, EN 1090-2 governs welding qualification, NDT extent, dimensional tolerances, and surface treatment. Understanding execution classes and what they demand is essential before any weld procedure or inspection plan reaches the shop floor.
1. Scope and Position in the Standards Stack
EN 1090-2:2018 covers the execution of steel structures — plates, sections, hollow profiles, and their welded or bolted assemblies. It sits directly below EN 1090-1 (the conformity assessment and CE marking standard) and is the primary reference for fabrication quality on structural steel components supplied under CE marking.
In the offshore context, EN 1090-2 is often applied in conjunction with:
- DNV-ST-0377 / DNV-OS-C101 — structural design requirements that set the execution class
- NORSOK M-101 — additional offshore fabrication requirements that supplement EN 1090-2
- EN 10204 — material certificate requirements (3.1 or 3.2 per product class)
- EN 1090-1 — CE marking and factory production control
2. Execution Classes (EXC1 to EXC4)
The execution class is the single most important parameter in EN 1090-2 — it controls welding qualification, NDT extent, tolerance class, and documentation requirements across the entire fabrication scope. There are four classes, with EXC1 being the least demanding:
| Class | Consequence Level | Typical Applications | Weld Quality Level |
|---|---|---|---|
| EXC1 | CC1 — Low | Purlins, stairs, minor secondary steel | ISO 5817 Class D |
| EXC2 | CC2 — Normal | General building frames, industrial platforms | ISO 5817 Class C |
| EXC3 | CC3 — High | Offshore topsides, bridges, primary structural steel | ISO 5817 Class B |
| EXC4 | CC3+ — Very high | Special/critical connections in high-consequence structures | ISO 5817 Class B+ |
For offshore structures under DNV-ST-0377, Special structural members typically require EXC3 or EXC4, Primary members EXC3, and Secondary members EXC2.
2.1 Determination of Execution Class
EN 1090-1 Annex B defines the method: EXC is determined by combining consequence class (CC) from EN 1990 with service category (SC) and production category (PC):
| SC | PC | CC1 | CC2 | CC3 |
|---|---|---|---|---|
| SC1 | PC1 | EXC1 | EXC2 | EXC3 |
| SC1 | PC2 | EXC1 | EXC2 | EXC3 |
| SC2 | PC1 | EXC2 | EXC3 | EXC3 |
| SC2 | PC2 | EXC2 | EXC3 | EXC4 |
SC2 (fatigue-loaded or dynamically loaded structures) with PC2 (cold-formed or complex welded components) at CC3 consequence yields EXC4 — the maximum class.
3. Welding: WPS, WPQR, and Welder Qualification
3.1 Welding Procedure Specification (WPS)
EN 1090-2 §7.4 requires that all welding be carried out to a qualified Welding Procedure Specification (WPS) qualified by a Welding Procedure Qualification Record (WPQR) per ISO 15614-1 (arc welding of steels).
The WPS must specify:
- Base material grade and thickness range
- Welding process (SMAW/111, GMAW/135, FCAW/136, SAW/121, etc.)
- Joint design: groove geometry, root gap, backing
- Pre-heat and interpass temperature requirements
- Heat input range (kJ/mm) — critical for HAZ toughness
- Post-weld heat treatment (PWHT) if required
- Filler material designation per EN ISO 14341 or equivalent
3.2 Pre-Heat Requirements
Pre-heat temperature is determined from the carbon equivalent (CE) of the base material per EN ISO 13916. The most common formula:
CE < 0.35 → typically no pre-heat required for t ≤ 25 mm
CE = 0.35–0.45 → 50–75°C pre-heat for t > 25 mm
CE > 0.45 → 100–150°C+ pre-heat; PWHT may be required
S355J2 (CE ≈ 0.39–0.43 for thicker plates) is on the boundary — pre-heat requirements depend on thickness and heat input.
3.3 Welder Qualification
All welders must be qualified per ISO 9606-1 (fusion welding of steels). The qualification scope (process group, material group, thickness range, position) must cover the actual production welds. EN 1090-2 §7.3.2 requires:
- Welder qualification records available at the fabrication site
- Re-qualification if welder has not welded in the relevant process for >6 months
- For EXC3/EXC4: welding coordination per EN ISO 14731 (Responsible Welding Coordinator, RWC)
4. NDT Extent Requirements
EN 1090-2 Table 24 (and EN ISO 17635 for reference) defines minimum NDT extent by execution class and joint type. These are minimum requirements — project specifications (NORSOK, DNV) typically increase them for offshore applications:
| Weld Type | EXC2 | EXC3 | EXC4 |
|---|---|---|---|
| Transverse butt welds, full penetration (fatigue-loaded) | 10% UT/RT | 100% UT/RT | 100% UT/RT + TOFD |
| Transverse butt welds, full penetration (non-fatigue) | 5% UT/RT | 10% UT/RT | 20% UT/RT |
| Longitudinal butt welds | 5% UT/RT | 10% UT/RT | 20% UT/RT |
| T-joints and cruciform joints (full penetration) | 5% UT | 10% UT | 20% UT |
| Fillet welds (structural) | 5% MT/PT | 10% MT/PT | 20% MT/PT |
| All welds — visual (VT) | 100% | 100% | 100% |
Note: UT = ultrasonic testing; RT = radiographic testing; MT = magnetic particle testing; PT = penetrant testing; TOFD = time-of-flight diffraction.
4.1 Acceptance Criteria
Weld acceptance criteria depend on the execution class:
- EXC2: ISO 5817 Quality Level C (intermediate)
- EXC3: ISO 5817 Quality Level B (stringent)
- EXC4: ISO 5817 Quality Level B+ (special requirements)
Common imperfections: lack of fusion (LoF), root concavity, undercut, porosity, cracks. All cracks are rejectable at all execution classes.
5. Dimensional Tolerances
EN 1090-2 §11 defines two tolerance classes:
| Tolerance Class | Application | Tighter Than Class 1? |
|---|---|---|
| Class 1 (Essential) | EXC1/EXC2 — standard fabrication | — |
| Class 2 (Functional) | EXC3/EXC4 — close-fit or precision applications | Yes, typically 50–60% tighter |
Selected tolerance limits for common measurements (Class 1 / Class 2):
| Measurement | Class 1 | Class 2 |
|---|---|---|
| Straightness of a member (length L) | L/1000, max 12 mm | L/1500, max 6 mm |
| Squareness of flange to web | b/100, min 2 mm, max 5 mm | b/200, min 1 mm, max 3 mm |
| Overall cross-section height (h) | ±3 mm (h ≤ 900 mm) | ±2 mm |
| Plate flatness (over 1 m gauge) | δ ≤ 3 mm | δ ≤ 1.5 mm |
| Hole position | ±2 mm | ±1 mm |
6. Surface Preparation and Coating
EN 1090-2 §10 specifies surface preparation requirements before painting or coating. The preparation grade is per ISO 8501-1:
| Preparation Grade | Description | Typical Offshore Requirement |
|---|---|---|
| Sa 1 | Light blast — loose mill scale removed | Not used offshore |
| Sa 2 | Thorough blast — most scale removed | Non-critical internal |
| Sa 2½ | Very thorough blast — no scale, grey/white appearance | Standard external offshore |
| Sa 3 | Blast to visually clean steel | Submerged zone, high-performance coatings |
Surface profile (anchor pattern) for offshore coatings is typically 50–85 μm Rz (ISO 8503). EN 1090-2 §10.2 references the coating specification — for offshore structures, NORSOK M-501 governs the coating system selection and application requirements.
7. Bolted Connections
EN 1090-2 §8 covers bolted connections. Key points for offshore fabricators:
- Bolt grade: 8.8 is standard; 10.9 used for high-strength friction grip (HSFG) connections
- Preloading: For slip-resistant connections (Category B/C), preloading must be verified by torque wrench, turn-of-nut, or direct tension indicator
- Assembly: Bolts tightened by snug-tight first (all bolts in group), then final torque applied in a star pattern from the centre
- Inspection: 5% of bolts per batch verified at EXC2; 10% at EXC3
- Coating: Hot-dip galvanised bolts (HDG) are not pre-loadable unless explicitly specified — HDG changes friction coefficient
8. Documentation and Quality Plan
EN 1090-2 §4.2 requires a quality plan for EXC3 and EXC4 that includes:
- Inspection and test plan (ITP)
- List of applicable WPS/WPQR and welder qualifications
- NDT procedure references (per EN ISO 17635 and method standards)
- Material traceability (heat numbers linked to EN 10204 certs)
- Non-conformance procedure and corrective action records
For offshore, the ITP is reviewed and approved by the client and verification body (e.g., DNV as MWS surveyor) before fabrication commences. Weld maps linking each weld seam to its WPS, welder ID, NDE result, and certificate are a mandatory deliverable.
9. Cross-Reference Map
| Standard | Relationship to EN 1090-2 | KB Status |
|---|---|---|
| EN 1090-1 | Part 1 of the same series — defines CE marking, factory production control, and how the execution class is determined from consequence class × service category × production category | ✅ Ingested |
| EN 10204 | Defines material test certificate types (2.1/2.2/3.1/3.2). EN 1090-2 §5.2 requires 3.1 for EXC2+ and 3.2 for EXC3/EXC4 on primary structural steel | ✅ Ingested |
| EN 10025-2 | Hot-rolled structural steels (S235/S275/S355/S420/S460) — the primary plate material standard for EN 1090-2 fabrications | ✅ Ingested |
| DNV-ST-0377 | References EN 1090-2 for execution class assignment based on structural category (Special/Primary/Secondary) and sets additional offshore requirements beyond minimum EN 1090-2 | ✅ Ingested |
| DNV-OS-C101 | Structural design standard that specifies execution class requirements and references EN 1090-2 for fabrication quality control | ✅ Ingested |
| NORSOK M-101 | Offshore fabrication supplement — higher NDT extent, additional WPS qualification, PWHT and pre-heat rules that override or supplement EN 1090-2 minimums on North Sea projects | 🟡 In HIGH backlog — not yet ingested |
10. Common Non-Conformances and Pitfalls
- Execution class not specified on drawings or in the execution specification — fabricator defaults to EXC2, which is non-conservative for offshore primary members
- WPS qualified for one joint geometry applied to a different groove design without re-qualification — EN ISO 15614-1 essential variables are strictly defined and changes require a new WPQR
- NDT performed immediately after welding on susceptible steels — EN 1090-2 §12.4.2 requires 16 h delay for hydrogen crack-sensitive materials; immediate UT can miss delayed hydrogen cracking
- Using the same NDT extent for all welds regardless of fatigue loading — fatigue-loaded transverse butt welds in EXC3 require 100% UT; reducing this to 10% because the non-fatigue row was used is a common oversight
- ISO 5817 acceptance class not tied to execution class — reviewers often see EXC3 fabrication submitted for acceptance per Level C (the EXC2 requirement) rather than Level B
- Hot-dip galvanised bolts pre-loaded without verifying friction coefficient — HDG surfaces have a different μ than bare steel; using the bare-steel torque table will give under-preloaded joints
- Surface preparation grade Sa 2 used under high-performance offshore coatings — the required anchor profile and cleanliness for most offshore coating systems is Sa 2½ or Sa 3; paint adhesion failures and early corrosion follow
- Weld maps submitted without welder ID traceability — EN 1090-2 requires each weld to be traceable to a qualified welder; weld maps missing this information will be rejected at FAT
Ask Leide Navigator about EN 1090-2
EN 1090-2:2018 is ingested in the Navigator knowledge base (149 chunks), together with EN 1090-1 and EN 10204. Ask about execution class determination, WPS qualification scope, NDT extent requirements by weld type, or dimensional tolerance limits for a specific member type.