Offshore Wind Tower Fabrication in North Sea

Case Study 1: Offshore Wind Tower Fabrication in North Sea

Scenario A European EPC contractor fabricating S355NL steel tower sections (40 mm wall thickness) for a North Sea offshore wind farm faced strict heat input limits to prevent excessive grain growth and loss of toughness in the heat-affected zone (HAZ). Environmental constraints included limited on-site rework capability, mandatory third-party NDT, and a maximum allowable heat input of 2.2 kJ/mm per WPS qualification. Ambient temperatures ranged from −5 °C to 12 °C, requiring preheat control and real-time thermal monitoring.

Given Data

• Welding Current: 245 A
• Welding Voltage: 28.5 V
• Travel Speed: 420 mm/min

Calculation The tool uses the standard heat input formula:

$$ \text{Heat Input} = \frac{\text{Voltage} \times \text{Current} \times 60}{\text{Travel Speed}} \quad \text{(in kJ/mm)} $$

Step-by-step:

• Numerator: $28.5 , \text{V} \times 245 , \text{A} = 6982.5 , \text{W}$
• Multiply by 60 (to convert seconds to minutes): $6982.5 \times 60 = 418,950 , \text{J/min}$
• Divide by travel speed: $418,950 , \text{J/min} \div 420 , \text{mm/min} = 997.5 , \text{J/mm} = 0.9975 , \text{kJ/mm}$
• Rounded to two decimal places: 1.00 kJ/mm

Result and Decision Calculated heat input (1.00 kJ/mm) fell well within the qualified WPS limit (≤2.2 kJ/mm) and aligned with the target range for S355NL at 40 mm (0.8–1.2 kJ/mm). The team confirmed no preheat adjustment was needed beyond the standard 50 °C, and proceeded with production welding using the same parameters—validated via thermocouple-mapped interpass temperature logs.

Lesson Even when voltage and current appear moderate, low travel speed can disproportionately increase heat input; always verify the combined effect—not individual parameter compliance—against material-specific thresholds.