EN 1090-2 Execution of Steel Structures

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:

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):

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:

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:

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:

Selected tolerance limits for common measurements (Class 1 / Class 2):

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:

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

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