Currently, the steel grade of international natural gas pipelines on land has developed from X70 to X80. Since 2003, three new X80 long-distance natural gas pipelines have been constructed in China and North America, greatly promoting the application of X80 steel in land-based natural gas pipelines. The newly constructed X80 long-distance natural gas pipelines have adopted a large number of spiral welded pipes, improving the economy and competitiveness of the pipelines.

In less than 10 years, the steel used for long-distance natural gas pipelines in China has advanced from X70 to X80, reaching an international advanced level. Among the high-pressure long-distance natural gas pipelines that have been built or are under construction worldwide, China's West-East Gas Pipeline II project is among the best in the world in terms of steel grade, length, pipe diameter, wall thickness, and transmission pressure. A comparison of several X80 long-distance natural gas pipelines worldwide is shown in Table 1.

Table 1 Comparison of Current X80 Natural Gas Long-Distance Pipelines Worldwide

 

Submarine natural gas pipelines continue to advance into deeper waters. Currently, the submarine pipeline with the greatest depth is the ITP pipeline in the Gulf of Mexico, with a maximum water depth of 2,753 meters. Petrobras and others are studying the construction of submarine pipelines with depths ranging from 3,000 to 6,000 meters. Currently, the steel grade of submarine pipeline pipes has been raised to X70, and the gas transmission capacity of these pipelines has been continuously increased. The Nord Stream pipeline, currently under construction, extends from Russia to Western Europe and uses double-pipe laying, with a gas transmission capacity of 2 × 275 billion m³/year. Modern submarine pipelines are characterized by high working pressure, thick steel pipe walls, stringent toughness requirements, and strict dimensional tolerances. For example, the working pressure of the Nord Stream pipeline reaches 22MPa, with steel pipe wall thicknesses ranging from 26.8 to 41.0 mm. The Charpy impact energy at a test temperature of -10°C must be at least 150 J, the DWTT shear area must be ≥85%, and the steel pipe ovality must not exceed 5 mm. Additionally, a high-performance external anti-corrosion and cement weighting layer is applied.
 

The recently launched natural gas pipeline project from the North Slope of Alaska to North America will elevate the construction of X80 natural gas long-distance pipelines to a new level. The pipeline is designed to transmit 430 to 590 billion m³/year of gas and will adopt higher working pressure (greater than 17MPa), thicker walls (greater than 23 mm), and stricter toughness requirements.
 

Russia plans to build a natural gas pipeline from the Pavnekovo gas field on the Yamal Peninsula to Ucha. The pipeline is designed to transmit 1,150 to 1,400 billion m³/year, more than twice the capacity of the Alaska pipeline. The pipeline is designed to be 1,074 km long, with double-pipe laying, an outer diameter of 1,420 mm, a transmission pressure of 11.8MPa, and K65 (X80) steel with wall thicknesses of 23.0 mm, 27.7 mm, and 33.4 mm. Because it is located in a northern cold region, the steel pipe's Charpy impact energy must be ≥180 J, and the DWTT shear area must be ≥85% at a temperature of -20°C. The lowest temperature during pipeline construction may reach -60°C. The Russian Gas Industry Corporation Research Institute conducted 10 full-scale gas explosion tests to evaluate the crack-arresting ability of the steel pipe. The pipeline will pass through 385.6 km of frozen soil and 158 km of swampy areas, necessitating the use of high-strain steel pipes. These two giant natural gas long-distance pipelines have much higher transmission capacity, steel pipe wall thickness, and harsher working environments than China's West-East Gas Transmission Line 2. This will pose a new challenge to long-distance pipeline technology.
 

The manufacturing and application technology of high-strain steel pipes is becoming increasingly mature. The successful development of dual-phase microstructure pipeline steel has improved the plastic deformation capacity of high-grade pipelines and reduced strain aging. X80/X100 high-strain steel pipes have been successfully developed and are now capable of mass production. China’s West-East Gas Pipeline II successfully adopted strain-based design in specific areas and applied X80 high-strain steel pipes in batches. Strain-based design and X100 high-strain steel pipes have been applied in small batches in North American test sections.

Extensive research, analysis, and physical testing have been conducted on the axial tensile and compression limits of steel pipes, yielding data on the ultimate deformation capacity of high-strain steel pipes and establishing a more accurate analysis method. Numerous girth weld and wide plate tensile tests have also been conducted, leading to an understanding of the tolerance of girth welding seam defects and longitudinal strain. Field test section welding has been conducted to ensure the quality of the girth welding seam.
 

Currently, international research and development activities on high-grade steel pipes are very active, with a focus on X100 steel pipes. The development of ultra-high-strength pipelines may achieve significant breakthroughs at the X100 level. The development of X100 steel pipes has progressed from simple trial production to the laying of test sections over 5 km in length. Simultaneously, girth welding has been a key development focus, and the process has been largely resolved. Extensive research has been conducted on the strain aging and tensile/compressive strain capacity of X100 steel pipes, leading to significant progress. The currently developed X100 steel pipes have been utilized in test sections based on strain design. Another key issue in the application of X100 steel pipes, fracture control, has also shown significant progress and is nearing practical application. The development of X100 spiral welded pipes has also achieved significant progress and is approaching practical requirements.
Although several X100 test sections have been successfully constructed, the design coefficient of all X100 pipeline test sections thus far does not exceed 0.6, and their hoop stress is less than the 0.8 design coefficient level of X80 pipelines. The purpose of constructing test sections is to focus on assessing and improving pipeline design and construction technology. The evaluation of steel pipe strength, toughness, and reliability primarily relies on laboratory tests and field test assessments (including full-scale blasting tests and test section evaluations).
 
Since 2005, China has started developing ultra-high strength (X90, X100, and X120) pipelines. In 2006, the X100-grade JCOE straight seam submerged arc welded steel pipe was successfully trial-produced. In 2007, the X90- and X120-grade JCOE straight seam submerged arc welded steel pipes were successfully trial-produced. In 2010, the X90- and X100-grade spiral submerged arc welded steel pipes were successfully trial-produced.

The development of ultra-high strength pipelines in China has significantly narrowed the gap between China and advanced Western countries in this field. However, there remains a gap between our research on the applicability of X100—such as strain-based design, on-site girth welding, and systematic research on fracture control—and the international advanced level. For example, although a small number of X100 steel pipes have been developed, X100 steel pipes with dual-phase microstructure, lower yield strength ratio, and improved strain aging behavior have not yet been successfully developed. So far, China has only conducted a full-scale blasting test on X80, and a blasting test on X100 has not yet been conducted. There are still few research results on fracture control for X100. The wide plate tensile and full-scale bending test equipment required for research on the strain capacity of X100 steel pipes is not yet available. New pulse welding processes, equipment, and welding materials for X100 girth welding, as well as on-site cold bending and coating technologies for X100 steel pipes, need to be developed.