Cladding Alloy Technology for FPSO Seawater Pipeline Components

Literature Overview

This 2005 paper by Xue Daliang from Dalian New Ship Heavy Industry Ship Design Research Institute, published in China Shipbuilding, documents the development and application of cladding alloy technology for seawater pipeline components on a Floating Production Storage and Offloading (FPSO) vessel — specifically the FPSO-3 project. Offshore seawater systems present unique corrosion challenges due to the combination of high-chloride marine environment, elevated flow velocities, biofouling, and cathodic protection requirements that distinguish them from inland water applications.

The FPSO-3 project required cladding of seawater piping, valves, and fittings with corrosion-resistant alloys to achieve a design life of 20-25 years in tropical marine environments where chloride concentrations exceed 19,000 ppm and water temperatures range from 25-35°C.

Material Selection and Technical Requirements

The paper evaluates several cladding alloy systems for seawater service on the FPSO:

Service Condition Analysis

The seawater system on FPSO-3 operates under the following conditions:

• Temperature: 25-35°C (tropical offshore environment)
• Chloride concentration: 19,000-22,000 ppm
• Flow velocity: 3-8 m/s in process lines, up to 12 m/s in fire water systems
• pH: 7.5-8.5 (natural seawater)
• Dissolved oxygen: 5-8 mg/L
• Cathodic protection potential: -850 mV vs. Ag/AgCl (required for carbon steel structure)
• Biofouling: Moderate to severe in tropical waters

The combination of high chloride, elevated temperature, and cathodic protection creates a severe environment for standard austenitic stainless steels. The paper demonstrates that 316L experiences pitting corrosion at flow velocities above 5 m/s under these conditions, while 2205 duplex stainless steel maintains integrity up to 8-10 m/s.

Cladding Process Development

The paper describes the development of a multi-pass cladding procedure using submerged arc welding (SAW) with flux-cored wire for the base passes and gas metal arc welding (GMAW) with solid wire for the cap pass:

Process Parameters:

The cladding thickness specification is 3.0 ± 0.5 mm for process lines and 2.0 ± 0.5 mm for fire water systems. The minimum thickness requirement ensures that the corrosion allowance is maintained even after accounting for machining and surface finishing operations.

Key Technical Challenges and Solutions

Challenge 1: Cathodic Protection Compatibility

The carbon steel pipe body is protected by cathodic protection at -850 mV, which can cause hydrogen-induced cracking in certain cladding alloys. The paper demonstrates that 2205 duplex stainless steel is resistant to hydrogen cracking at these potentials, while Alloy 625 requires special precautions. The recommended solution is to maintain the cladding potential above -950 mV by using mixed potential monitoring.

Challenge 2: Flow Velocity Erosion

At flow velocities above 8 m/s, even corrosion-resistant alloys experience erosion-corrosion. The paper recommends increasing cladding thickness to 4.0 mm for high-velocity sections and specifying a maximum surface roughness of Ra 0.4 μm to reduce turbulence-induced erosion.

Challenge 3: Dissimilar Metal Welding

When connecting clad pipes to uncladed components (valves, flanges), dissimilar metal welds can create galvanic corrosion cells. The paper specifies the use of transition pieces with matching alloy composition and recommends isolation flanges with non-conductive gaskets at all dissimilar metal joints.

Quality Assurance and Inspection

The paper establishes a comprehensive inspection protocol for the cladded seawater components:

Study Insights and Engineering Implications

This paper provides a practical roadmap for implementing cladding technology in offshore marine applications, where the consequences of failure are particularly severe due to the remote location and difficulty of repair. The selection of 2205 duplex stainless steel as the primary cladding alloy represents an optimal balance of corrosion resistance, mechanical strength, cathodic protection compatibility, and cost — a finding that has been validated by subsequent industry practice in offshore projects worldwide.

The paper's emphasis on cathodic protection compatibility is particularly important for FPSO applications where the entire vessel structure is protected by impressed current or sacrificial anode systems. Engineers must ensure that the cladding alloy maintains passive film integrity at the imposed cathodic potential, which requires careful attention to the alloy's immunity potential (Eimm) relative to the protection potential.

The quality assurance framework described in this paper, with its combination of destructive and non-destructive testing methods, provides a model for qualification procedures that can be adapted to other marine cladding applications. The requirement for simulated seawater corrosion testing of production coupons is particularly valuable, as it verifies the actual performance of the specific heat of material under realistic service conditions rather than relying solely on generic material data.

For future FPSO projects, the paper suggests that advanced alloys such as Alloy 625 or Alloy C-276 may be justified for critical components (e.g., ballast water systems with high flow velocities and aggressive chemistry) where the extended service life and reduced maintenance requirements offset the higher material cost. The emerging technology of laser cladding with fine-grained microstructures offers the potential for even thinner cladding layers with equivalent corrosion performance, which could reduce weight and cost for weight-sensitive offshore applications.

Concluding Summary

These five literature studies collectively represent the breadth and depth of cladding technology applications across multiple industrial sectors — from high-temperature wear-resistant cladding in power generation to corrosion-resistant marine applications in offshore energy production. Each paper addresses a specific technical challenge within the broader field of bimetallic component manufacturing, and together they illustrate the discipline's evolution from empirical practice to scientifically guided engineering. The common thread across all five studies is the fundamental importance of understanding the relationship between microstructure, processing parameters, and service performance — a principle that continues to guide cladding technology development in the 21st century.