Offshore Platform Structural Welding in North Sea

Case Study 1: Offshore Platform Structural Welding in North Sea

Scenario A major EPC contractor is fabricating jacket leg sections for a new fixed-bottom offshore wind support structure in Stavanger, Norway. The project demands high-integrity, full-penetration SAW (Submerged Arc Welding) girth welds on 42 mm-thick API 5L X80 steel. Environmental constraints include strict carbon footprint targets and tight schedule windows—welding must achieve ≥12 kg/h deposition rate to meet the 3-week module assembly deadline. Limited crane availability restricts rework tolerance; therefore, deposition efficiency must exceed 92% to minimize dilution-related defects and post-weld NDT call-backs.

Given Data

• Welding Current: 720 A
• Welding Voltage: 34 V
• Travel Speed: 360 mm/min
• Wire Diameter 1: 1.6 mm (lead wire)
• Wire Diameter 2: 1.6 mm (trail wire)
• Wire Feed Rate 1: 11.2 m/min
• Wire Feed Rate 2: 11.2 m/min
• Density of Welding Wire: 7.8 g/cm³

Calculation The tool computes deposition rate using the formula:

Deposition Rate (kg/h) = (π/4) × [(d₁² × wfr₁) + (d₂² × wfr₂)] × ρ × 60 / 1000

Where:

• d₁, d₂ = wire diameters in cm (1.6 mm = 0.16 cm)
• wfr₁, wfr₂ = wire feed rates in cm/s → convert m/min to cm/s: 11.2 m/min = 1120 cm/min = 1120/60 ≈ 18.67 cm/s
• ρ = 7.8 g/cm³
• Factor 60 converts per-minute to per-hour; /1000 converts g/h → kg/h

First, compute cross-sectional area per wire:

• Area = π/4 × (0.16)² ≈ 0.0201 cm²

Material feed volume rate per wire:

• 0.0201 cm² × 18.67 cm/s = 0.375 cm³/s
• For two wires: 2 × 0.375 = 0.75 cm³/s

Mass feed rate:

• 0.75 cm³/s × 7.8 g/cm³ = 5.85 g/s = 5.85 × 3600 / 1000 = 21.06 kg/h (theoretical wire consumption)

Deposition efficiency is estimated empirically from current/voltage/travel speed using industry-validated regression calibrated for dual-wire SAW:

• η (%) = 85.2 + 0.012×I − 0.18×V + 0.004×vₜ (where vₜ in mm/min)
• η = 85.2 + 0.012×720 − 0.18×34 + 0.004×360
• η = 85.2 + 8.64 − 6.12 + 1.44 = 89.16%

Thus:

• Deposition Rate = 21.06 kg/h × 0.8916 ≈ 18.78 kg/h
• Deposition Efficiency = 89.16%

Result and Decision Although the calculated deposition rate (18.78 kg/h) exceeds the 12 kg/h target, the deposition efficiency (89.2%) falls below the 92% threshold required to limit dilution and avoid rework. The team trialed increasing travel speed to 420 mm/min while holding current at 720 A and voltage at 35 V—this raised efficiency to 92.3% (per recalibration) but reduced deposition rate to 16.4 kg/h, still sufficient. Final parameters locked: I=720 A, V=35 V, vₜ=420 mm/min, wfr₁=wfr₂=11.5 m/min.

Lesson Deposition efficiency—not just rate—is the critical constraint for high-integrity structural welds; optimizing for speed alone risks metallurgical compromise. Real-time efficiency estimation enables proactive parameter tuning before NDT reveals dilution-related flaws.