Zhuojin Pipe Fitting Co., LtdDevelopment of X65QO Deep-Sea Coil Pipe Seamless Steel Pipe
Technical Requirements and Design Philosophy
The paper by Zang Qifei, Li Liwei, Niu Yongwang, Zuo Guofeng, Hou Jing, Cheng Xianglong, and Luo Shiyong, published in the journal Metal Materials and Metallurgical Engineering (Volume 53, Issue 6, 2025, pp. 20-25), documents the development of X65QO deep-sea coil pipe seamless steel pipe. This research was conducted collaboratively by Hunan Hengyang Steel Pipe Group Co., Ltd., CNOOC Research Institute Co., Ltd., and Bureau Veritas International Inspection Group.
Deep-sea coil pipe laying represents a significant advancement in offshore pipeline installation technology. Unlike traditional rigid pipe laying methods, coil pipe is manufactured in long, continuous lengths that can be stored on a reel and unspooled during installation. This method offers advantages in terms of installation speed, cost efficiency, and flexibility in challenging seabed conditions. However, the deep-sea environment imposes stringent requirements on the pipe material, including resistance to large plastic deformation during coiling and uncoiling, excellent weldability, corrosion resistance, and fatigue resistance.
The design philosophy for X65QO deep-sea coil pipe seamless steel pipe encompasses four key elements:
- Low carbon equivalent composition design: The carbon equivalent (CE) value is carefully controlled to ensure adequate weldability and toughness. The CE value, calculated per ISO 4063 or similar standards, should typically be below 0.45% for good weldability.
- Clean steelmaking: The steel is produced through advanced steelmaking processes that minimize impurities such as sulfur, phosphorus, and non-metallic inclusions. This is achieved through practices such as vacuum degassing, argon stirring, and continuous casting with electromagnetic stirring.
- High-precision forming through hot rolling combined with cold drawing: The combination of hot rolling and cold drawing processes enables precise control of the pipe's dimensional accuracy and surface quality, which are critical for coil pipe applications.
- Quenching and tempering heat treatment: The quench-and-temper (QT) treatment provides the required combination of strength, toughness, and ductility for deep-sea service conditions.
Material Properties and Performance Verification
The produced X65QO deep-sea coil pipe seamless steel pipe demonstrated excellent performance across multiple property categories:
| Property Category | Test Condition | Requirement | Achieved Result |
|---|---|---|---|
| Wall thickness deviation | Manufacturing tolerance | Within ±3% | Within ±3% |
| Ductile-to-brittle transition temperature | Charpy V-notch impact | Below -80°C | Below -80°C |
| Weld joint performance | Tensile, impact, hardness | Meet X65QO standard | Satisfactory |
| Parent pipe tensile strength | Room temperature | Per X65QO standard | Compliant |
| Strain-aged tensile properties | 1.5% strain aging | Per X65QO standard | Compliant |
| Strain-aged tensile properties | 2.5% strain aging | Per X65QO standard | Compliant |
| CTOD (Crack Tip Opening Displacement) | Various conditions | Per X65QO standard | Compliant |
| Corrosion resistance | Various test media | Per X65QO standard | Compliant |
| Fatigue resistance | BS 7608 (2014) | Cycles after 1.5% strain cycling | Meets requirement |
The fatigue test results are particularly noteworthy. After subjecting the parent pipe to 1.5% strain amplitude for two positive and negative bending cycles (simulating the coiling and uncoiling process), fatigue testing was conducted at a stress level of 172.4 MPa. The number of cycles to failure satisfied the requirements of BS 7608 (2014 edition), demonstrating that the pipe retains adequate fatigue resistance even after experiencing the plastic deformation associated with coil pipe installation.
The strain aging tests at both 1.5% and 2.5% strain levels are critical for coil pipe applications. During the coiling and uncoiling process, the pipe material experiences significant plastic strain, which can lead to strain aging effects that reduce toughness and ductility. The fact that the material maintains satisfactory properties after strain aging indicates that the composition design and heat treatment effectively mitigate this degradation mechanism.
Manufacturing Process and Quality Control
The manufacturing process for X65QO deep-sea coil pipe seamless steel pipe involves several critical steps, each requiring precise control and quality assurance:
- Steelmaking and casting: The steel is produced through basic oxygen furnace (BOF) or electric arc furnace (EAF) steelmaking, followed by ladle refining and vacuum degassing. The chemical composition is carefully controlled to achieve the target mechanical properties while maintaining low carbon equivalent for weldability.
- Hot rolling: The steel billet is hot rolled into a pipe blank with controlled dimensions and microstructure. The rolling schedule must be optimized to achieve the desired grain size and phase composition.
- Cold drawing: The hot-rolled pipe is cold drawn to achieve the final dimensions with high precision. The cold drawing process introduces work hardening, which increases strength but must be balanced against the requirement for adequate ductility.
- Quenching and tempering: The cold-drawn pipe is subjected to quenching and tempering heat treatment to achieve the target mechanical properties. The quenching medium, quenching rate, tempering temperature, and tempering time are all critical process parameters that must be carefully controlled.
- Non-destructive testing: The finished pipe undergoes comprehensive NDT inspection, including ultrasonic testing (UT) for internal defects, eddy current testing (ECT) for surface and near-surface defects, and hydrostatic testing for pressure integrity.
From a welding perspective, the X65QO grade designation indicates a minimum yield strength of 450 MPa (65 ksi) with the QO suffix indicating suitability for coil pipe applications. The weldability of this grade is critical for field installation, where girth welds and tie-in welds must be performed under potentially challenging environmental conditions. The low carbon equivalent composition design ensures that pre-heating requirements are minimized and that the heat-affected zone (HAZ) maintains adequate toughness.
Study Insights and Industry Implications
This research represents a significant advancement in deep-sea pipeline technology, demonstrating that seamless steel pipes can be manufactured to meet the demanding requirements of deep-sea coil pipe applications. The combination of low carbon equivalent composition design, clean steelmaking, high-precision forming, and quench-and-temper heat treatment provides a robust technical framework for producing high-performance pipeline steel.
The verification of properties after strain aging and fatigue testing after coiling simulation demonstrates a comprehensive approach to material qualification. This level of testing and verification goes beyond conventional pipeline steel qualification, reflecting the unique challenges of deep-sea coil pipe applications. The involvement of Bureau Veritas International Inspection Group in the research and qualification process adds credibility to the material assessment and ensures compliance with international standards and best practices.
For the steel pipe manufacturing industry, this work highlights the importance of integrating materials science, metallurgical processing, and mechanical testing in the development of advanced pipeline products. The seamless manufacturing route, while more expensive than welded alternatives, offers superior material homogeneity and resistance to manufacturing defects, which is particularly valuable for deep-sea applications where failure consequences are severe. The successful development of X65QO deep-sea coil pipe seamless steel pipe opens new possibilities for deep-water pipeline installation, contributing to the exploitation of offshore hydrocarbon resources in increasingly challenging environments.