Performance of JCOE Longitudinally Submerged Arc Welded Steel Pipe Manufactured from Rolled Plate

Literature Overview

This technical paper by Yang Yanhua, Yang Zhuanzhao, Ding Yi, Li Yunlong, Ma Qiurong, and Wei Yaqiu from Xi'an Aeronautical Technical College, the China Petroleum Pipe Research Institute, and PetroChina Tarim Oilfield Corporation presents a comprehensive study of the manufacturing process and performance characteristics of longitudinally submerged arc welded (LSAW) steel pipes produced from hot-rolled coil plate using the JCOE forming process. The research was published in the journal "Oil and Gas Storage and Transportation" in 2008. The study covers the entire manufacturing process, from coil plate preparation to final pipe testing, and evaluates the chemical composition, tensile properties, impact toughness, hardness, and drop weight tear test (DWTT) results against the requirements of DNV-OS-F101 (2000) and API 5L standards.

Core Technical Findings

Manufacturing Process Overview

The JCOE (Joule, C, O, and E) process is a multi-stage forming process for large-diameter LSAW pipes. The process involves the following stages:

Stage	Description	Key Quality Control Points
Coil plate preparation	Slitting, shearing, and edge preparation of hot-rolled coil	Flatness, edge quality, surface cleanliness
J-shape forming	Partial roll forming to create a J-shape	Springback control, dimensional accuracy
C-shape forming	Further roll forming to create a C-shape	Ovality control, stress distribution
O-shape forming	Final roll forming to create a near-circular shape	Circularity, residual stress
E-shape welding	Final expansion and welding of the longitudinal seam	Weld quality, residual stress, dimensional accuracy

Performance Test Results

The comprehensive testing programme included:

Chemical composition: Verified conformity with DNV-OS-F101 and API 5L requirements for carbon equivalent, manganese, phosphorus, sulfur, and other alloying elements.
Tensile properties: Yield strength, tensile strength, and elongation all met the specified requirements for the pipe grade.
Impact toughness: Charpy V-notch impact test results demonstrated adequate toughness at the required test temperatures, including the minimum test temperature specified by DNV-OS-F101 for offshore applications.
Hardness: Hardness measurements confirmed the absence of excessive hardness in the heat-affected zone (HAZ) of the weld, which is critical for resistance to hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC).
DWTT: The drop weight tear test results demonstrated adequate fracture resistance in the through-thickness direction, which is essential for preventing catastrophic fracture in offshore applications.

Standards Compliance

The pipe met the requirements of both DNV-OS-F101 (2000) and API 5L, which is significant because:

DNV-OS-F101 is the primary standard for submarine pipelines in the oil and gas industry, with stringent requirements for toughness, fracture resistance, and resistance to corrosion mechanisms.
API 5L is the widely used standard for line pipe in onshore and offshore applications, covering chemical composition, mechanical properties, and manufacturing requirements.

Engineering Practice Implications

Welding Process Control

The longitudinal seam welding in the JCOE process is typically performed using submerged arc welding (SAW), which is well-suited for thick-section welding due to its high deposition rate, deep penetration, and low spatter. Key welding parameters and quality control measures include:

Welding current and voltage: These parameters must be optimised for the pipe diameter, wall thickness, and material grade. Too high a current can lead to excessive heat input and HAZ softening, while too low a current can result in incomplete fusion.
Travel speed: The travel speed must be controlled to ensure adequate penetration and fusion without excessive heat input. Typical travel speeds for LSAW range from 200 to 600 mm/min, depending on the thickness and material.
Flux selection: The flux composition must be compatible with the base metal and filler wire to ensure adequate weld metal chemistry and avoid contamination. Low-hydrogen fluxes are typically used for offshore applications to minimise the risk of hydrogen-induced cracking.
Preheating and interpass temperature: Preheating is often required for thicker sections or higher-strength materials to control the cooling rate and prevent HAZ cracking. The interpass temperature must be maintained to prevent excessive cooling between weld passes.

Residual Stress Management

The JCOE forming process introduces significant residual stresses in the pipe, which can affect the mechanical properties, dimensional accuracy, and long-term performance of the pipe. Residual stress management strategies include:

Controlled forming sequence: The sequence of forming operations (J, C, O, E) is designed to minimise residual stresses by balancing the plastic deformation in each stage.
Post-weld heat treatment (PWHT): For high-strength materials or thick sections, PWHT may be required to relieve residual stresses and improve toughness. The PWHT parameters (temperature, duration, and cooling rate) must be carefully controlled to avoid grain growth or embrittlement.
Shot peening: In some applications, shot peening of the weld area can introduce compressive residual stresses that improve fatigue resistance and crack initiation resistance.

Key Questions and Reflections

The use of hot-rolled coil plate for JCOE pipe manufacturing is an attractive option because it offers cost advantages over plate steel and is readily available in large quantities. However, the quality of the coil plate is critical to the final pipe performance. In my experience, the flatness, edge quality, and surface condition of the coil plate can significantly affect the forming process and the final pipe quality. Any defects in the coil plate, such as surface inclusions, edge cracks, or thickness variations, can propagate through the forming process and become critical defects in the final pipe.

The DWTT results are particularly important for offshore applications, where the pipe is subjected to complex loading conditions, including bending, axial compression, and internal pressure. The DWTT measures the resistance to through-thickness fracture, which is a critical failure mode for pipelines. The results reported in this study demonstrate that the JCOE process, when properly controlled, can produce pipes with adequate fracture resistance for offshore applications.

The compliance with both DNV-OS-F101 and API 5L standards is a strong indicator of the quality of the manufacturing process. However, standards compliance does not guarantee optimal performance. The actual performance of the pipe in service depends on many factors, including the installation conditions, operating environment, and maintenance practices. Therefore, a comprehensive quality management system, encompassing all stages from raw material to final installation, is essential for ensuring long-term pipeline integrity.

Study Insights and Conclusions

This paper provides a comprehensive technical report on the manufacturing and performance of JCOE LSAW pipes produced from hot-rolled coil plate. The study demonstrates that the JCOE process, when properly controlled, can produce pipes that meet the stringent requirements of DNV-OS-F101 and API 5L standards for offshore applications. The comprehensive testing programme, including chemical analysis, mechanical testing, and fracture resistance testing, provides a thorough evaluation of the pipe performance. For the steel pipe manufacturing industry, this study reinforces the importance of process control, material quality, and rigorous testing in ensuring the reliability of large-diameter LSAW pipes for critical applications.