Automated Pipeline Girth Welding in Alberta Oil Sands

Case Study 2: Automated Pipeline Girth Welding in Alberta Oil Sands

Scenario A pipeline construction crew is welding 36-inch OD, 25.4 mm wall X70 line pipe for a sour service (H₂S-containing) gathering system near Fort McMurray, Alberta. The environment imposes severe constraints: winter temperatures averaging −25°C require preheat maintenance (>100°C), limiting travel speed; strict HIC (Hydrogen Induced Cracking) mitigation mandates low heat input (<25 kJ/mm) and high deposition efficiency to minimize dilution of corrosion-resistant weld metal. Dual-wire GMAW-P (Pulsed) is used with solid ER100S-G wire. Schedule pressure demands ≥8.5 kg/h deposition rate per pass to complete 120 joints/week.

Given Data

• Welding Current: 480 A
• Welding Voltage: 28 V
• Travel Speed: 280 mm/min
• Wire Diameter 1: 1.2 mm (pulsed leading wire)
• Wire Diameter 2: 1.2 mm (cold trailing wire)
• Wire Feed Rate 1: 8.5 m/min
• Wire Feed Rate 2: 5.2 m/min
• Density of Welding Wire: 7.8 g/cm³

Calculation Using the same core formula:

Convert wire diameters: 1.2 mm = 0.12 cm → area = π/4 × (0.12)² ≈ 0.0113 cm²

Convert wire feed rates to cm/s:

• wfr₁ = 8.5 m/min = 850 cm/min = 14.17 cm/s
• wfr₂ = 5.2 m/min = 520 cm/min = 8.67 cm/s

Volume feed rate:

• Lead wire: 0.0113 cm² × 14.17 cm/s ≈ 0.160 cm³/s
• Trail wire: 0.0113 cm² × 8.67 cm/s ≈ 0.098 cm³/s
• Total = 0.258 cm³/s

Mass feed rate:

• 0.258 cm³/s × 7.8 g/cm³ = 2.012 g/s = 2.012 × 3600 / 1000 = 7.24 kg/h (theoretical)

Deposition efficiency for pulsed dual-wire GMAW-P under cold conditions is modeled as:

• η (%) = 82.5 + 0.021×I − 0.09×V − 0.0015×vₜ + 0.0008×(wfr₁ + wfr₂)
• η = 82.5 + 0.021×480 − 0.09×28 − 0.0015×280 + 0.0008×(8.5 + 5.2)
• η = 82.5 + 10.08 − 2.52 − 0.42 + 0.01096 ≈ 90.65%

Thus:

• Deposition Rate = 7.24 kg/h × 0.9065 ≈ 6.56 kg/h
• Deposition Efficiency = 90.65%

Result and Decision Initial run fell short of the 8.5 kg/h target (6.56 kg/h). Thermal modeling confirmed that increasing current beyond 520 A risked exceeding 25 kJ/mm heat input at 280 mm/min. Instead, engineers increased wire feed rate 1 to 10.2 m/min and wire feed rate 2 to 6.8 m/min—keeping current at 480 A and voltage at 28 V—to raise theoretical feed to 9.12 kg/h. Recalculating efficiency: η = 91.3%, yielding 8.32 kg/h, within 2% of target. Preheat was adjusted to 110°C to stabilize arc stability. This configuration passed qualification testing with zero HIC indications.

Lesson In sour service applications, deposition rate optimization must be bounded by heat input and metallurgical thresholds—not just equipment limits. Incremental, data-driven wire feed adjustments—validated by real-time efficiency estimation—are safer and faster than brute-force current increases.