Case Study: Replacing Carbon Steel with Duplex Steel Pipes Doubled Service Life in Water Injection Systems

Case Study: Replacing Carbon Steel with Duplex Steel Pipes Doubled Service Life in Water Injection Systems

How a simple material switch turned catastrophic monthly failures into reliable five-year service cycles

Executive Summary

A major offshore platform in the North Sea was experiencing persistent failures in its seawater injection piping system, with carbon steel components requiring replacement every 2-3 years despite cathodic protection and chemical treatment. After comprehensive analysis, the operator switched to duplex stainless steel (UNS S32205), resulting in a service life extension from 2.5 to 5+ years while reducing maintenance costs by approximately 70%. This case study examines the technical and economic factors behind this successful material upgrade.

The Challenge: Battling Multiple Corrosion Mechanisms

The seawater injection system represented a perfect storm of corrosive conditions:

Operating Parameters:

• Temperature: 15-25°C (seasonally variable)

• Chloride concentration: 19,000-21,000 mg/L

• Oxygen content: 50-200 ppb (despite deaeration efforts)

• Flow velocity: 2-3 m/s with occasional surges to 4 m/s

• System pressure: 120-150 bar

Failure Analysis of Carbon Steel System:
The original carbon steel piping (API 5L X52) suffered from three simultaneous degradation mechanisms:

1. General Corrosion: Wall thickness reduction of 0.8-1.2 mm/year

2. Localized Pitting: Isolated pits progressing at 2-3 mm/year, often leading to perforation

3. Erosion-Corrosion: Accelerated metal loss at bends, elbows, and flow disturbances

Despite implementing a comprehensive corrosion management program including:

• Sacrificial anode cathodic protection

• Oxygen scavenger injection (sulfite-based)

• Corrosion inhibitor treatment

• Biocide program

The system continued to experience unplanned shutdowns every 8-14 months for emergency pipe replacements, with associated production losses exceeding $500,000 per incident.

The Solution: Technical Justification for Duplex Steel

After evaluating multiple alternatives, the engineering team selected 2205 duplex stainless steel based on its balanced properties:

Material Properties Comparison:

The duplex microstructure—approximately 50% ferrite and 50% austenite—provided inherent advantages:

• Ferritic phase delivers chloride stress corrosion cracking resistance

• Austenitic phase provides toughness and fabricability

• High chromium (22%) and molybdenum (3%) content ensure robust passive film formation

• Nitrogen addition (0.15-0.20%) enhances pitting resistance and strength

Implementation: Phased Transition Strategy

The replacement program followed a carefully sequenced approach:

Phase 1: Pilot Section (Months 1-6)

• Replaced highest-failure-risk sections first

• Installed corrosion monitoring coupons and ER probes

• Conducted baseline performance assessment

Phase 2: Critical Path Replacement (Months 7-18)

• Prioritized sections with highest consequence of failure

• Implemented full system during planned maintenance turnaround

• Trained maintenance staff on duplex-specific procedures

Phase 3: System-Wide Rollout (Months 19-36)

• Completed remaining piping replacements

• Established new inspection protocols based on risk-based methodology

Performance Results: Exceeding Expectations

Quantitative Outcomes:

• Service Life: Increased from 2.5 years to 5+ years (projected 7-8 years)

• Maintenance Costs: Reduced from $280,000/year to $85,000/year

• Unplanned Downtime: Eliminated completely post-transition

• Inspection Intervals: Extended from 6 months to 24 months

Corrosion Performance Data:
After three years of continuous operation, inspection results revealed:

• No measurable wall thickness reduction

• Zero incidences of pitting or crevice corrosion

• Maintained passive film integrity even at weldments

• No evidence of stress corrosion cracking

Economic Analysis: Lifecycle Cost Justification

While duplex steel carried a 3.2× higher initial material cost compared to carbon steel, the total lifecycle economics told a different story:

Five-Year Cost Comparison (per meter of piping):

The analysis demonstrated a 64% reduction in total ownership cost and a payback period of just 14 months for the additional capital investment.

Lessons Learned and Best Practices

Fabrication Insights:

• Required controlled heat input during welding to maintain phase balance

• Essential to avoid intermetallic phase formation in heat-affected zones

• Proper pickling and passivation restored corrosion resistance post-fabrication

Operational Considerations:

• Eliminated oxygen scavenger injection requirement

• Reduced corrosion inhibitor dosage by 80%

• Maintained minimum flow velocity of 1.5 m/s to prevent marine growth

Inspection Protocol Updates:

• Implemented risk-based inspection focusing on high-stress areas

• Used advanced NDE techniques including eddy current array for pit detection

• Established baseline UT thickness mapping for future comparison

Conclusion: A Template for Success

This case study demonstrates that strategic material upgrades, even with higher initial costs, can deliver exceptional returns through extended service life and reduced operational expenses. The success of this transition hinged on:

1. Thorough technical evaluation matching material properties to service conditions

2. Phased implementation to manage risk and validate performance

3. Lifecycle cost analysis that considered both direct and indirect expenses

4. Adaptation of operational practices to leverage the new material's capabilities

For water injection systems operating in chloride-containing environments, duplex stainless steel represents a compelling alternative to carbon steel—transforming corrosion management from a constant battle into a controlled, predictable element of asset management.

Considering a similar material upgrade in your operation? The principles demonstrated in this case study can be adapted to various aggressive service environments. Share your specific application challenges in the comments for tailored recommendations.