Casing Steel Grades
There are twenty different steel grades for oil and casing pipes, namely H40, J55, K55, M65, N80, L80, C90, T95, C95, P110, Q125. Different color codes and symbols are used to distinguish various steel grades and thread types of the oil and casing pipes. The two or three digits following the color codes and letters represent the minimum yield strength of the oil and casing pipes. For example, J55 indicates a minimum yield strength of 55,000 psi (379 MPa) and a maximum of 80,000 psi (552 MPa), while P110 indicates a minimum yield strength of 110,000 psi (758 MPa) and a maximum of 140,000 psi (965 MPa). The letters H, J, K, N represent general strength oil and casing pipes, while C, L, M, T represent limited yield strength oil and casing pipes, which have some resistance to sulfur corrosion.
The steel grade of oil and casing pipes refers to the material’s yield strength; for instance, H40 represents a strength of 40 * 1000 / 145 MPa = 275.86 MPa.
Commonly, J55 is used for local surface casing, N80 for oil layer casing, and P110 for high-pressure layers or deeper wells where the upper part uses P110.
Pressure Trends in Pipeline Transportation
There is a trend towards increasing the conveying pressure in pipelines, especially evident in gas pipelines. This is because increasing the conveying pressure within a certain range can improve economic benefits. Using gas pipelines as an example, under constant flow conditions, as the conveying pressure increases, the density of the gas increases and the flow rate decreases, thus reducing frictional resistance.
Within the distance between stations on a gas pipeline, the pressure gradually decreases from the inlet to the outlet, while the flow rate gradually increases, causing the frictional resistance to also increase. Approximately three-quarters of the length from the station consumes half of the outlet pressure drop Δp, while the remaining quarter consumes the other half. The primary difference between gas and oil pipelines is that the flow rate increases from the inlet to the outlet due to the compressibility of the medium. Oil, being essentially incompressible, maintains a constant flow rate even though the conveying pressure decreases along the pipeline, resulting in constant frictional resistance before and after.
For gas pipelines, raising the pressure can reduce frictional resistance and, consequently, lower energy consumption. It should be noted that the energy consumption of gas pipelines is significantly higher than that of oil pipelines. For example, the West-to-East Gas Pipeline in China operates at a conveying pressure of 10 MPa, with an annual throughput of 12 billion cubic meters over a pipeline length of 4000 kilometers. Roughly estimating, the energy consumption is approximately 1.2 billion cubic meters per year, which is about one-tenth of the throughput consumed along the route.
Due to concerns over reducing energy consumption, there has been a gradual increase in conveying pressures. Initially, natural gas pipelines in Sichuan Province operated at 2.5 MPa, later increased to 4 MPa, and the Shaanxi-Beijing line was raised to 6 MPa, while the West-to-East Gas Pipeline was increased to 10 MPa. In economically developed countries abroad, many gas pipelines operate at 12 MPa.
The pressure ratio in gas pipelines also shows a decreasing trend. The pressure ratio refers to the ratio of inlet pressure to outlet pressure. A decrease in the pressure ratio means that the entire pipeline operates at higher pressures, thereby reducing energy consumption. Historically, this ratio was often 1.6 but has since decreased to 1.4, and some recent foreign gas pipelines operate at a pressure ratio of 1.25. However, a reduction in the pressure ratio necessitates more compressor stations, leading to increased investment. Optimization calculations must be performed for pipe diameter, pressure, and pressure ratio.
If the flow rate is determined, after optimizing the pipe diameter, pressure, and pressure ratio, selecting a higher pressure with too low a steel grade would result in excessively thick walls, creating difficulties during pipe manufacturing, field welding, and transportation, even making it impractical. Production needs have promoted the improvement of steel grades.
API published the API 5L standard in 1926, initially including only A25, A, and B steel grades with minimum yield strengths of 172, 207, and 251 MPa, respectively. In 1947, the API 5LX standard was introduced, adding X42, X46, X52 steel grades with minimum yield strengths of 289, 317, and 358 MPa, respectively. Starting in 1966, X56, X60, X65, X70 were added, with minimum yield strengths of 386, 413, 448, and 482 MPa, respectively. In 1972, the API introduced U80 and U100 standards with minimum yield strengths of 551 and 691 MPa, respectively, which were later renamed to X80 and X100. Globally, prior to 2000, the usage of X70 was around 40%, while X65 and X60 fluctuated around 30%. Smaller-diameter refined oil pipelines often used X52 grade, primarily ERW steel pipes.
Regarding X80 grade, extensive discussions have taken place both domestically and internationally. Despite significant investment by international steel giants in the development of X80 and even X100, X80 remains in the “experimental section” phase, with a total length of only about 400 kilometers. No currently constructed pipelines use X80 grade steel. Based on discussions with technical personnel from multiple international pipeline engineering companies responsible for pipeline design, the consensus is:
1. With the increase in operating pressure and the completion of preparations, X80 will undoubtedly develop in the future.
2. Currently, major oil companies are hesitant to adopt X80 for the following reasons:
– There is a period of high defect rates during initial production of new steel grades, and adopters would bear this risk.
– There is a learning curve during field welding processes, which also involves risks.
– The adoption of X80 may require changes in equipment such as cold bending machines, welding rods, automatic circumferential welders, and thermal bending processes, increasing project costs.
– Higher steel grades imply thinner walls when the operating pressure is not sufficiently high, reducing pipeline rigidity and increasing vulnerability to third-party damage.
Considering China’s national circumstances, despite rapid economic growth over the past decade, we remain a developing country. Therefore, it is suggested that we do not be the first to adopt X80 but rather take a “hidden brightness” approach, which benefits both pipeline owners and the domestic metallurgical industry.
In recent years, China’s metallurgical industry has made great efforts to develop pipeline steels, achieving commendable results. Currently, efforts are being made to develop wide plates for X70 grade straight seam submerged arc welded pipes and to stabilize the quality of X70 hot-rolled coil plates. If X80 were to be widely adopted now, our metallurgical industry, lacking experience and track record, would struggle to compete internationally. While I believe in the capabilities of our metallurgical industry, it is advisable not to rush into adopting X80 extensively. However, conducting limited exploratory research on X80 is still necessary, but the focus should remain on the main issues.