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Offshore engineering, as a specialized field characterized by high risk and high corrosion, places stringent demands on material performance. Duplex stainless steel pipe, with its excellent corrosion resistance, high strength, and good weldability, has become the material of choice for critical facilities such as offshore platforms, subsea pipelines, and seawater treatment systems. This article analyzes the practical application of different types of duplex steel pipe in offshore engineering through specific case studies.
1. Application of 2205 Duplex Steel Pipe in the Seawater Cooling System of an Offshore Oil Platform
Case Background: The seawater cooling system of an offshore oil platform in the Middle East has long faced challenges from high salt concentrations, high temperatures, and chloride ion corrosion. Traditional 316L stainless steel pipes, due to pitting and crevice corrosion, require frequent replacement, resulting in high maintenance costs.
Solution: 2205 duplex steel pipe (UNS S32205) was used to replace the original 316L pipes. 2205 steel contains 22% chromium, 5% nickel, and 3% molybdenum. Its ferrite-austenite duplex structure gives it a pitting resistance equivalent value (PREN) exceeding 33, significantly higher than the 25 for 316L.
Validation: After five years of operation, inspections of the platform revealed no visible corrosion on the inner wall of the 2205 pipeline, while the 316L pipeline, which had not been replaced during the same period, had localized pitting depths of up to 1.2mm. Furthermore, 2205 pipe is approximately twice as strong as 316L, allowing for thinner walls and approximately 15% weight reduction at the same pressure level, saving platform payload space.
II. Application of 2507 Super Duplex Steel in Deepsea Oil and Gas Pipelines
Case Background: A deepsea oil and gas pipeline project at an oil field in the Norwegian North Sea required pipelines that were resistant to high pressure and sulfide stress cracking (SSC). The water depth exceeded 1200 meters, and the medium contained H₂S (partial pressure 0.05 psi) and CO₂ (concentration 3%).
Solution: Super duplex steel 2507 (UNS S32750) was selected. It contains 25% chromium, 4% molybdenum, and 0.25% nitrogen, resulting in a PREN value exceeding 40 and excellent resistance to hydrogen-induced cracking (HIC). The pipeline passed HIC and SSC testing under API 5L standard and meets the DNVGL-OS-F101 specification for submarine pipelines.
Actual Performance: Monitoring data within three years of commissioning shows that the 2507 pipe exhibited no corrosion or mechanical degradation in high-pressure (12 MPa) and low-temperature (4°C) environments. Compared to conventional duplex steel 2205, 2507 exhibits approximately three times the lifespan in H₂S-containing environments, significantly reducing maintenance risks in deepwater operations.
III. Application of Economical 2304 Duplex Steel in Ship Ballast Water Systems
Case Background: The ballast water system of a 180,000-ton bulk carrier originally utilized a carbon steel + coating solution. However, coating failure led to localized corrosion, necessitating annual downtime for maintenance. The shipowner sought a more reliable metal piping solution.
Solution: 2304 duplex steel (UNS S32304) was used to replace carbon steel. 2304 has a lower nickel content (approximately 4.5%) and is approximately 15% less expensive than 2205 steel, while still offering sufficient resistance to chloride stress corrosion cracking (CLSCC) and suitable for low- to medium-corrosion environments.
Implementation Results: Five-year follow-up data after the retrofit showed that the 2304 piping remained intact even without coating protection, with no erosion corrosion observed at velocities up to 3 m/s in the ballast tanks. The shipowner estimated that the lifecycle cost (including maintenance) was 28% lower than the original carbon steel solution, and the risk of port state inspection detentions due to corrosion was avoided.
IV. Challenges and Improvement Directions
Despite the significant advantages of duplex steel pipes, welding process control (for example, the ferrite content in the heat-affected zone must be controlled between 30% and 70% to prevent embrittlement) and localized corrosion susceptibility (for example, the need to avoid chloride ion concentration in crevice environments) remain key concerns within the engineering community. For example, a Southeast Asian desalination plant once experienced reduced corrosion resistance in the weld zone of its 2205 pipeline due to improper welding parameters. This problem was subsequently resolved through optimized multi-layer, multi-pass welding and post-weld solution treatment.
From shallow to deep waters, from conventional corrosive environments to extreme operating conditions, duplex steel pipes, through customized designs with varying alloy ratios (e.g., 2205 for balanced cost-effectiveness, 2507 for high-corrosion performance, and 2304 for cost-effectiveness), have become a key component of the marine engineering materials system. With the advancement of deep-sea development and green ship technologies, new-generation duplex steels (such as LDX 2101) with higher corrosion resistance and lower nickel content are expected to further expand their application scenarios.
