Overlay Alloy Technology for FPSO Seawater System Penetration Fittings

Literature Overview

This paper by Xue Daliang from Dalian New Shipbuilding Heavy Industry Ship Design Research Institute, published in China Shipbuilding (2005, Vol. 46, B11, pp. 137-140), investigates the overlay welding of Alloy 625 on carbon steel pipes and flanges used in the seawater system of the BELANAK FPSO (FPSO-3) built for CONOCO. The study addresses the corrosion protection challenge of carbon steel penetration fittings exposed to seawater in offshore floating production systems, where long-term durability and reliability are paramount.

Technical Background and Corrosion Challenge

FPSO (Floating Production Storage and Offloading) systems operate in marine environments where the seawater system is continuously exposed to chloride-containing water at ambient to elevated temperatures. Carbon steel components in the seawater system are susceptible to:

• General corrosion from dissolved oxygen and chlorides
• Pitting corrosion and crevice corrosion at weld joints and flange gasket areas
• Corrosion under insulation (CUI) at insulated pipe sections
• Erosion-corrosion at high-velocity flow areas near fittings and valves

The overlay of Alloy 625 (UNS N06625) provides a nickel-chromium-molybdenum barrier that is highly resistant to pitting, crevice corrosion, and stress corrosion cracking in chloride environments. The alloy contains approximately 58-62% nickel, 22-23% chromium, 8-9% molybdenum, 3-4% iron, and 0.9-1.0% niobium, providing excellent corrosion resistance across a wide range of conditions.

Overlay Welding Process and Technical Challenges

The overlay welding of Alloy 625 on carbon steel presents several technical challenges that must be addressed through careful process design:

1. Dilution control: Excessive carbon steel dilution reduces the corrosion resistance of the Alloy 625 overlay by lowering the nickel and chromium content. The target dilution rate should be maintained below 10-15% to ensure adequate corrosion resistance.
2. Cracking prevention: The high thermal expansion coefficient of Alloy 625 (approximately 13.4 x 10^-6/°C) compared to carbon steel (approximately 12 x 10^-6/°C) creates thermal stresses that can lead to cracking if not properly managed.
3. Multi-pass welding: A multi-pass approach is typically required, with the first pass using a transition material (such as Alloy 309 or a nickel-based transition alloy) to buffer the composition change between carbon steel and Alloy 625.
4. Heat input management: Low heat input is preferred to minimize dilution and reduce the risk of cracking, but must be balanced against adequate fusion and penetration requirements.

The welding process typically involves:

• Surface preparation including grinding to a bright metal finish within the welding area
• Preheating to 150-250°C to reduce thermal gradients and minimize cracking risk
• First pass with a transition alloy (309L or similar) to establish a compatible metallurgical interface
• Subsequent passes with Alloy 625 consumables (wire or strip electrode)
• Post-weld inspection including visual examination, magnetic particle testing, and dimensional verification

Material Selection and Performance Verification

Alloy 625 was selected over alternative nickel-based alloys (such as Alloy 617, Alloy C-276, or Alloy 617) based on several factors:

• Superior resistance to pitting and crevice corrosion in chloride environments
• Excellent weldability with good resistance to hot cracking and solidification cracking
• Adequate mechanical properties including yield strength and elongation
• Proven track record in marine and offshore applications
• Availability of qualified welding consumables and established welding procedures

Performance verification of the overlay system includes:

• Metallographic examination of the overlay cross-section to verify dilution rate and microstructure
• Hardness testing to confirm adequate mechanical properties
• Corrosion testing including potentiodynamic polarization and immersion testing in simulated seawater
• Non-destructive testing including magnetic particle inspection for surface defects

Engineering Practice and Offshore Application Considerations

For offshore FPSO applications, the overlay welding of Alloy 625 on carbon steel seawater system components represents a cost-effective alternative to fabricating the entire system from nickel-based alloy materials. The carbon steel provides structural strength and cost efficiency, while the Alloy 625 overlay provides the necessary corrosion resistance for long-term seawater exposure. This hybrid approach is particularly advantageous for large-diameter piping and heavy-wall flanges where full alloy construction would be prohibitively expensive.

Key considerations for offshore implementation include:

• Compliance with classification society requirements (DNV, ABS, Lloyd's, etc.)
• Welding procedure qualification in accordance with ASME Section IX or ISO 15614
• Welder qualification specific to the overlay process and materials
• Environmental controls during welding to prevent contamination of the weld pool
• Post-weld inspection and documentation for classification society approval

The BELANAK FPSO project demonstrates that this overlay approach is technically feasible and commercially viable for large-scale offshore applications. The successful implementation requires careful attention to welding procedure development, welder training, in-process quality control, and post-weld inspection.

Study Insights and Implications

This paper provides valuable technical documentation for the overlay welding of Alloy 625 on carbon steel in marine applications. The approach of using carbon steel base materials with nickel alloy overlays is a well-established engineering practice that balances cost, performance, and reliability. For offshore engineering professionals, the key takeaway is that proper process qualification and quality control are essential to ensure the long-term performance of overlay systems in demanding marine environments. The dilution control challenge is particularly important, as even small variations in dilution rate can significantly impact the corrosion resistance of the overlay. Engineers should ensure that welding procedures are rigorously qualified and that in-process monitoring is maintained throughout the overlay welding operation to guarantee consistent quality across all components. The successful application of this technology to the BELANAK FPSO seawater system provides a reference case for similar projects in the offshore energy sector.