As the demand for offshore wind energy continues to grow, the importance of robust welding qualification processes for monopile fabrication has never been more critical. In this series, we’ll explore practical strategies for developing and qualifying welding procedures and managing quality deviations. There will be 3 parts to this article.

Project Overview: Welding for Offshore Monopiles

This project involves the fabrication of wind turbine monopiles. At the onset, the contract is reviewed to ensure mutual agreement between the involved parties. Project requirements and technical specifications are carefully examined, including relevant project drawings and applicable standards.

For this project, welding activities are governed by BS EN ISO standards, particularly for the development and qualification of welding procedures.

Applicable BS EN ISO Standards (note: newer versions may exist and titles are summarised):

1. BS EN ISO 15609-1:2019: Welding procedure specification (WPS) for arc welding
2. BS EN ISO 15614-1:2017 + A1:2019: Welding procedure test for arc/gas welding of steels and nickel alloys
3. BS EN ISO 6892-1:2019: Tensile testing of metallic materials at room temperature
4. BS EN ISO 4136:2022: Transverse tensile test for welds
5. BS EN ISO 5173:2023: Bend tests for welds
6. BS EN ISO 148-1:2016: Charpy impact test method
7. BS EN ISO 6507-1: Vickers hardness test method
8. BS EN 10025-3:2019: Technical delivery conditions for normalized weldable fine-grain structural steels
9. BS EN 10204: Types of inspection documents

Material: S355NL and Its Properties

The selected construction material is S355NL, compliant with BS EN 10025-3:2019, offering:

1. Minimum Yield Strength: 355 MPa
2. Toughness: Specified minimum impact energy at −50 °C
3. Advantages:

• High strength and low-temperature toughness
• Excellent weldability (no preheat for thickness ≤30 mm)
• Suitable for multiple welding techniques

As per BS EN 10204, an inspection certificate 3.1 is issued. The mill certificate includes:

1. Plate dimensions: 100 × 2000 × 3000 mm
2. Normalising treatment: @ 900 °C for 5 min
3. Chemical composition and mechanical properties: (See Table 1)

The composition and mechanical properties meet the requirements of BS EN 10025-3.

Welding Procedure Specification and Qualification

Welding Procedure Specification (WPS) is developed per BS EN ISO 15609-1.

A preliminary welding procedure specification/welding procedure specification (pWPS/WPS) provides all the necessary information required to make the weld (see BS EN ISO 15609‑1, Pg 2.).

Weld Map and Joint Configuration

Fig. 1 shows the project Weld Map (not actual) with locations and assigned WPS.

Note that the Weld Maps show the location of welds and assign the appropriate WPS to each weld.

• Welding positions: PA, PC, PF  may apply.
• Welding processes:
  • Submerged Arc Welding (SAW – Process 121)
  • Flux-Cored Arc Welding (FCAW-G) – Process 136

Joint Design and Sequence

1. Fig. 2 shows the double-V butt joint preparation and welding sequence.
2. SAW is applied with solid wire electrode in PA position.
3. Welding parameters are provided in Table 2.

Weld Procedure Qualification Test

Test specimens were prepared per BS EN ISO 15614-1:2017, using full-penetration butt welds:

• Figs. 3(a) and 3(b): Show test piece layout and specimen extraction locations.

1 = joint preparation and fit-up for plate a = minimum dimension 150 mm b = minimum dimension 350 mm t = material thickness.

Fig. 3 Test piece and extraction location for a butt joint in plate with full penetration

Note: Butt joint plate: full penetration (pen) butt welds qualify full and partial pen butt welds and fillet welds in any type of joint (ISO 15614-1).

Fig. 4. Shows Butt Weld Procedure Plate and typical test pieces (not the actual).

Tensile Test (ISO 4136, ISO 6892-1) (Fig. 4 (b))

• Plate ID: P1-SAW-03
• YS (0.2% offset): 465 MPa, UTS(max): 581 MPa
• Elongation: 28%
  Complies with minimum mechanical property requirements.

Bend Test (BS EN ISO 5173:2023) Fig. 4 (c)

• Test piece: 10 mm wide, distance between rollers 66 mm
• Bend: 180°, former diameter = 40 mm
• Result: All four side bend tests passed

Charpy Impact Test (BS EN ISO 148-1:2016) Fig. 4 (d)

• Sample size: 55 x 10 x 10 mm (KV8)
• Test temperature: −50 °C
• Results in Table 3

Observation: Some test locations (e.g., Root FL + 2 mm and Root FL + 5 mm) fail to meet the minimum impact energy requirement.

Discussion and Outlook

The Charpy impact results show unacceptable variation across certain weld zones — particularly in the root fusion line. This suggests possible issues in weld metal composition, heat input control, or microstructure formation.

Key Questions:

1. What is the root cause of the low impact values in specific zones?
2. What corrective measures can be taken from a welding and metallurgical standpoint?

These questions will be addressed in Part 2 of this article.

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