Offshore Gas Export Pipeline Tie-In Using P91 Clad Spools

Scenario

An offshore gas platform in the North Sea required tie-in of two 16" × 38 mm wall P91-clad carbon steel spools (clad thickness 3.5 mm, base material A694 F65) into an existing export line. The work occurred on a floating vessel with ambient temperatures of 4°C and wind speeds averaging 32 km/h. Critical constraints included limited deck space for heating equipment, strict 24-hour weather window availability, and mandatory DNV-OS-F101 requirements for hydrogen-induced cracking (HIC) mitigation in sour service environments.

Given Data

• Material Factor: 1.3 (increased due to cladding interface complexity and higher residual stress risk)
• Thickness: 38 mm (base material thickness; clad layer excluded per DNV guidance for preheat calculation)
• Hydrogen Content: 5.1 mL/100g (elevated due to marine transport humidity exposure and surface rust requiring aggressive grinding — verified by ASTM E1443)
• Heat Input: 15.4 kJ/mm (GTAW+SAW hybrid procedure optimized for clad integrity)
• Base Preheat Temperature: 100°C (per ASME BPVC Section IX, unchanged for clad configurations when base metal governs)

Calculation

Using the same empirical formula:

• Base: 100.0°C
• Thickness contribution: 1.3 × 38 × 0.8 = 39.52°C
• Hydrogen contribution: 5.1 × 15 = 76.5°C
• Heat Input contribution: 15.4 × 2.5 = 38.5°C
• Total = 100.0 + 39.52 + 76.5 + 38.5 = 254.52°C → 254.5°C (per tool’s precision)

Result and Decision

The tool recommended 254.5°C, but vessel safety protocols capped preheat at 230°C to prevent insulation degradation and fire risk near hydraulic systems. To compensate, the engineering team mandated: (1) vacuum-box drying of joint surfaces for 2 hours pre-weld, (2) use of low-hydrogen E9015-B9 electrodes (certified ≤2.5 mL/100g), and (3) immediate post-weld heat treatment (PWHT) within 15 minutes of weld completion (vs. standard 2-hour max). All welds passed 100% phased-array UT and delayed-crack testing per NACE TM0284.

Lesson

In constrained offshore environments, preheat temperature is only one lever — combine it with rigorous hydrogen source control (electrode selection, surface prep, environmental shielding) and accelerated thermal management (tight PWHT timing) to achieve equivalent crack resistance when nominal preheat cannot be met.