Overlay Alloy Technology for FPSO Seawater System Penetration Parts

Literature Overview

This paper, published in Shipbuilding of China (Vol. 46, Suppl. B11, 2005, pp. 137-140) by Xue Daliang from Dalian New Ship Heavy Industry (DNS), addresses the overlay welding technology applied to seawater system penetration parts for the BELANAK FPSO (FPSO-3) built for Conoco. The study focuses on the application of Alloy 625 overlay welding on carbon steel pipes and flanges to provide corrosion resistance in seawater service.

Core Technical Approach

Floating Production Storage and Offloading (FPSO) vessels operate in marine environments where seawater corrosion poses a significant challenge. The seawater system, which includes pumps, valves, piping, and penetration parts, must withstand continuous exposure to saltwater at temperatures of 20-40°C with high chloride concentrations (typically 19,000-21,000 ppm Cl⁻).

Application Scope

The overlay welding technology was applied to:

Alloy 625 Overlay Specifications

Alloy 625 (UNS N06625) is a nickel-chromium-molybdenum alloy known for its excellent resistance to pitting, crevice corrosion, and stress corrosion cracking in chloride-containing environments.

Overlay Welding Process

The overlay welding process for internal pipe surfaces and flange faces requires specialized techniques:

For internal pipe overlay, the challenge is accessing the pipe interior for welding. This typically requires:

1. Internal TIG welding: Using a specialized torch that fits inside the pipe, with external backing for root pass.
2. Rotational welding: Rotating the pipe while the torch remains stationary, ensuring uniform overlay coverage.
3. Multiple passes: Building up the overlay in multiple thin layers to control dilution and achieve the required thickness.

Standards and Quality Requirements

The overlay welding work must comply with applicable marine and offshore standards:

Connection with Engineering Practice

FPSO Seawater System Design Considerations

The seawater system on an FPSO serves multiple functions:

• Cooling water supply: For jacket cooling, heat exchangers, and condensers.
• Ballast water: For vessel stability and trim control.
• Fire water: For fire protection systems.
• Deck washing: For deck and equipment cleaning.

The seawater contains dissolved oxygen, chlorides, and suspended solids, creating a highly corrosive environment. Carbon steel components would suffer rapid corrosion (potentially 0.5-1.0 mm/year) without protection. Alloy 625 overlay provides a durable barrier against this corrosion while maintaining the structural integrity of the carbon steel base material.

Practical Challenges and Solutions

Cost-Benefit Analysis

The use of Alloy 625 overlay on carbon steel components offers significant cost advantages compared to using solid Alloy 625 components:

The overlay approach provides a balance between cost, weight, and corrosion resistance that is particularly suitable for FPSO applications where the seawater system is extensive but the budget is constrained.

Key Questions and Reflections

A significant concern in FPSO seawater overlay applications is the long-term reliability of the overlay under cyclic loading conditions. FPSO vessels experience significant dynamic loads from wave action, which can cause fatigue cracking at the overlay-base metal interface. The thermal expansion mismatch between Alloy 625 (CTE: ~13×10⁻⁶/°C) and carbon steel (CTE: ~12×10⁻⁶/°C) is relatively small, which is favorable, but the residual stresses from the welding process must be carefully managed.

Another important consideration is the quality of the overlay surface finish. For flange faces, the overlay must be machined to a precise finish (typically Ra ≤ 6.3 μm) to ensure proper gasket sealing. The machining process can expose the overlay to mechanical damage, and the residual stresses from machining must be considered in the overall stress analysis.

The paper also raises the question of inspection and maintenance. Overlay welding on internal pipe surfaces creates a situation where the overlay quality is difficult to verify after installation. Pre-installation NDT (preferably UT) is essential, and any repair welding must be performed with the same qualification level as the original overlay.

Study Insights and Implications

This paper documents a successful application of Alloy 625 overlay welding technology in FPSO construction, demonstrating that carbon steel components can be effectively protected against seawater corrosion through overlay welding. The key insights for engineers include:

1. Material selection: Alloy 625 provides superior corrosion resistance compared to 316L or duplex stainless steel overlays in seawater, particularly for high-temperature service.
2. Process control: The internal overlay welding process requires careful parameter control to ensure uniform coverage, adequate dilution control, and acceptable residual stress levels.
3. Quality assurance: Comprehensive NDT and mechanical testing are essential to verify overlay quality before installation.
4. Design integration: The overlay approach must be integrated into the overall design from the beginning, considering factors such as access for welding, post-weld machining, and inspection requirements.

For offshore engineering projects, this technology represents a cost-effective solution for protecting critical seawater system components while maintaining the structural efficiency of carbon steel construction. The experience gained from the BELANAK FPSO project provides valuable reference data for future FPSO and offshore platform designs that require seawater corrosion protection.