ERW(Electric Resistance Welded) steel pipe technology has tended to mature and improve. Theory and practice have proven that ERW tubes have the characteristics of reliability, safety and economy in oil and gas transportation applications.
At present, the pipes used in domestic natural gas pipeline projects can be divided into three categories and eight types according to the production process: seamless steel pipes(SML pipe), high-frequency resistance welded steel pipes (HFW), and submerged arc welded steel pipes. The main ones used are SML pipes, spiral seam submerged arc welded pipe (SSAW), straight seam submerged arc welded pipe (LSAW), ERW pipe and other 4 types.
At present, the international high-quality ERW steel pipe not only cannot identify the weld, but also has a weld coefficient of 1, achieving a match between the weld area structure and the base material. The diameter of domestic ERW steel pipes is DN100~600, and the price is equivalent to that of spiral seam submerged arc welded steel pipes.
Simultaneously, the growing disparity between supply and demand arises from the ongoing reduction in production capacity of existing gas fields in the Chuanxi region, juxtaposed with the escalating demand for natural gas across the entire region. This not only makes it impossible for the region to supply gas normally but also exacerbates the supply and demand contradiction of natural gas in the entire Chuanxi region, relying on the allocation of the large pipeline network in Sichuan and Chongqing. Therefore, the construction of this pipeline project must not only meet economic rationality but also achieve rapid and efficient results.
The gathering and transportation pipeline of this project uses DN300L360 pipelines with a design pressure of 6.3MPa to transport natural gas from Lianhuashan, Zhangjiaping and other structures in the Ya'an area to Pingluo Dam for dehydration treatment over a distance of about 60km, and then supplies it to downstream users and imports it into the Sichuan and Chongqing areas. Large pipe network. Lianhuashan, Zhangjiaping and other structures produce gas, mainly the Xujiahe gas reservoir, which basically has low water content, less acidic components, and a low-acidic temperament containing only a small amount of carbon dioxide.
When selecting pipes, there are several pipes mentioned earlier for comparison. Compared with seamless pipes, ERW steel pipes have fewer appearance quality defects and the appearance quality is better than SML steel pipes. Due to advanced production technology and high production efficiency, the production cost of ERW steel pipes is much lower than that of seamless steel pipes.
Compared with SSAW, ERW steel pipes, because the length of SSAW weld is about 4-5 times that of ERW, there is no process capability to remove the internal weld burr, so the friction on the inner wall of the pipe is inevitably increased when transporting media. However, the ERW weld length is short and the internal weld flash can be removed. When the medium flows in the pipeline, the friction generated by the inner wall of the pipeline is relatively small compared to spiral submerged arc welded steel pipes.
According to the residual stress measurement results, it is shown that the circumferential forming internal stress of ERW welded pipe is smaller, which is beneficial to the pipe. For SSAW welded pipes, due to the large weld area, the stress within the weld is both tensile and compressive, and the stress state is more complex than that of ERW pipes. This is related to the forming process of spiral welded pipes. The wide range of tensile stress on the inner wall of the weld and heat-affected zone of SSAW welded pipes will have a negative impact on the initiation of cracking and stress corrosion of inner wall defects.
Since the SSAW weld length is about 4-5 times the weld length of high-frequency straight seam pipes, the anti-corrosion treatment of the finished pipe welds is more difficult; ERW steel pipes have short welds (compared to spiral seam steel pipes) and thicker steel pipe walls Uniform, stable length, high geometric and dimensional accuracy, anti-corrosion and easy to ensure the quality of assembly and welding. At the same time, since the temperament of this project contains a small amount of water and carbon dioxide and is slightly acidic, it is not suitable to use SSAW welded pipes.
In terms of construction, requirements for the impact performance of pipe welds and non-destructive testing of steel pipe welds were put forward to further ensure the quality of steel pipes. Shortly after the pipeline was put into operation, the world-shaking "Wenchuan Earthquake" occurred on May 12, 2008. The Lianhuashan structural test pipeline, which is also located in the main earthquake zone of the Longmen Mountains, experienced severe tests and emerged unscathed. In the two years since the earthquake, the pipeline has been operating safely and smoothly, and passed formal acceptance in May 2010.
1. Types of pipeline for natural gas transmission
How to choose appropriate pipe materials in some projects with tight time and heavy tasks is directly related to the project.Depending on the investment size, construction period and the social and economic benefits that the project can produce, ERW steel pipes are gradually becoming the first choice for pipes used in such projects.At present, the pipes used in domestic natural gas pipeline projects can be divided into three categories and eight types according to the production process: seamless steel pipes(SML pipe), high-frequency resistance welded steel pipes (HFW), and submerged arc welded steel pipes. The main ones used are SML pipes, spiral seam submerged arc welded pipe (SSAW), straight seam submerged arc welded pipe (LSAW), ERW pipe and other 4 types.
1.1 SML pipe
Seamless steel pipes for natural gas transportation are hot-rolled from killed steel. Since there are no welds, they exhibit high uniformity in quality and relatively consistent physical, chemical, and mechanical properties. The diameter of domestic hot-rolled seamless steel pipes is ≤ DN600, mostly below DN400. Seamless steel pipes are the most expensive due to their excellent performance. They are generally used in pipeline projects with smaller diameters or difficult control projects, and small and medium-diameter pipes in stations.1.2 SSAW pipe
Spiral submerged arc welded steel pipes are rolled from killed steel hot-rolled (controlled rolling) steel plates (strips) and formed by automatic submerged arc welding. The welds are filled with deposited filler metal. The strength of the welds is generally not lower than that of the base material; it has a wall thickness It has the advantages of uniformity, multiple specifications, small weld stress, better quality control, and lower price than seamless steel pipes. The spiral welded steel pipes of DN300~1000 produced in my country are widely used in domestic long-distance oil and gas pipelines.1.3 LSAW pipe
Longitudinal submerged arc welded steel pipes are made of killed steel hot-rolled (controlled rolling) steel plates (strips), formed by UOE, JCOE or HME methods, and welded by automatic submerged arc welding. The welds have deposited filler metal, and the weld strength is generally not less than Base material; it is characterized by good weld quality, high port size accuracy, and easy butt welding; The disadvantages include higher manufacturing costs. Domestic longitudinal submerged arc welded steel pipes with diameters of DN300~1000 are widely used in important projects.1.4 ERW pipe
Resistance welded steel pipes are formed by continuous rolling of killed steel hot-rolled (controlled rolling) steel plates (strips) with rollers. The welding heat is generated by resistance or electric induction. After heating, it is pressed together by mechanical force; there is no deposited filler metal, and the weld seam The strength is not as good as that of submerged arc welding, carbon segregation and slag inclusion are prone to occur, and the quality of pipe making is unstable. With the emergence of high-purity and high-quality steel, mature welding technology, the adoption of many new technologies and the improvement of strict quality control and testing systems, the quality of domestic ERW pipes has been greatly improved, the pressure-bearing grades and usage occasions have continued to expand, and they have been widely used in gas pipelines.At present, the international high-quality ERW steel pipe not only cannot identify the weld, but also has a weld coefficient of 1, achieving a match between the weld area structure and the base material. The diameter of domestic ERW steel pipes is DN100~600, and the price is equivalent to that of spiral seam submerged arc welded steel pipes.
2. Application of ERW steel pipes in oil and gas transportation projects
The main trunk line of the Lianhua Mountain structure, built in China in 2007 for trial production, is a typical internal gathering trunk line project of a gas field. Its purpose is to further clarify the resource reserves in key exploration areas such as Lianhua Mountain, Zhangjiaping, and Tiantai Mountain in the Chuanxi region of the Southwest Oil and Gas Field Branch of PetroChina.Simultaneously, the growing disparity between supply and demand arises from the ongoing reduction in production capacity of existing gas fields in the Chuanxi region, juxtaposed with the escalating demand for natural gas across the entire region. This not only makes it impossible for the region to supply gas normally but also exacerbates the supply and demand contradiction of natural gas in the entire Chuanxi region, relying on the allocation of the large pipeline network in Sichuan and Chongqing. Therefore, the construction of this pipeline project must not only meet economic rationality but also achieve rapid and efficient results.
The gathering and transportation pipeline of this project uses DN300L360 pipelines with a design pressure of 6.3MPa to transport natural gas from Lianhuashan, Zhangjiaping and other structures in the Ya'an area to Pingluo Dam for dehydration treatment over a distance of about 60km, and then supplies it to downstream users and imports it into the Sichuan and Chongqing areas. Large pipe network. Lianhuashan, Zhangjiaping and other structures produce gas, mainly the Xujiahe gas reservoir, which basically has low water content, less acidic components, and a low-acidic temperament containing only a small amount of carbon dioxide.
When selecting pipes, there are several pipes mentioned earlier for comparison. Compared with seamless pipes, ERW steel pipes have fewer appearance quality defects and the appearance quality is better than SML steel pipes. Due to advanced production technology and high production efficiency, the production cost of ERW steel pipes is much lower than that of seamless steel pipes.
Compared with SSAW, ERW steel pipes, because the length of SSAW weld is about 4-5 times that of ERW, there is no process capability to remove the internal weld burr, so the friction on the inner wall of the pipe is inevitably increased when transporting media. However, the ERW weld length is short and the internal weld flash can be removed. When the medium flows in the pipeline, the friction generated by the inner wall of the pipeline is relatively small compared to spiral submerged arc welded steel pipes.
According to the residual stress measurement results, it is shown that the circumferential forming internal stress of ERW welded pipe is smaller, which is beneficial to the pipe. For SSAW welded pipes, due to the large weld area, the stress within the weld is both tensile and compressive, and the stress state is more complex than that of ERW pipes. This is related to the forming process of spiral welded pipes. The wide range of tensile stress on the inner wall of the weld and heat-affected zone of SSAW welded pipes will have a negative impact on the initiation of cracking and stress corrosion of inner wall defects.
Since the SSAW weld length is about 4-5 times the weld length of high-frequency straight seam pipes, the anti-corrosion treatment of the finished pipe welds is more difficult; ERW steel pipes have short welds (compared to spiral seam steel pipes) and thicker steel pipe walls Uniform, stable length, high geometric and dimensional accuracy, anti-corrosion and easy to ensure the quality of assembly and welding. At the same time, since the temperament of this project contains a small amount of water and carbon dioxide and is slightly acidic, it is not suitable to use SSAW welded pipes.
In terms of construction, requirements for the impact performance of pipe welds and non-destructive testing of steel pipe welds were put forward to further ensure the quality of steel pipes. Shortly after the pipeline was put into operation, the world-shaking "Wenchuan Earthquake" occurred on May 12, 2008. The Lianhuashan structural test pipeline, which is also located in the main earthquake zone of the Longmen Mountains, experienced severe tests and emerged unscathed. In the two years since the earthquake, the pipeline has been operating safely and smoothly, and passed formal acceptance in May 2010.
