Comparison of characteristics between ERW and SSAW steel pipe

Straight seam electric resistance welded steel pipe (ERW pipe) and spiral submerged arc welded steel pipe (SSAW pipe) can better meet the requirements of oil and natural gas pipelines. For steel pipes with medium diameter (Ф630mm) and below, ERW steel pipes are mostly used abroad. Spiral submerged arc welded pipes are generally only used for low-pressure fluid transportation, civil buildings, structures and piling pipes. At present, SSAW steel pipes are mainly used in such long-distance pipelines in my country. This is mainly because there is no ERW steel pipe in my country at the beginning, so the variety of steel pipes used in design is limited; secondly, it is related to the understanding of ERW steel pipes. However, the universal application of medium-diameter ERW steel pipes in the world, especially the successful use of submarine pipelines, has fully demonstrated the advantages of ERW steel pipes. It is the future development trend to promote the use of ERW steel pipes in medium-diameter oil and gas pipelines in my country.

Electric resistance welded steel pipe (Electric-Resistance-Welded) steel pipe, referred to as ERW steel pipe, has many advantages such as high productivity, low cost, high dimensional accuracy and beautiful appearance. In recent years, countries are actively studying the use of ERW steel pipes to replace seamless steel pipes and spiral seam welded steel pipes. In industrially developed countries, the proportion of ERW has reached about 70% when the diameter is below Ф610mm. In the past, due to the poor reliability of the welding seam, the scope of use was greatly limited. In the past 10 years, with the improvement of production technology and the continuous improvement of product quality, the welding seam quality of ERW steel pipe can be compared with that of UOE steel pipe. Therefore, it is widely used in oil and gas pipelines in different regions and harsh conditions.

1. Features of ERW steel pipe

(1) The research on the welding mechanism of ERW steel pipe has made gratifying achievements, and the optimization of the welding process is carried out under the guidance of theory, and computer control is realized for all optimization parameters, which improves the quality of the weld.

(2) The welding seam annealing adopts the fluorescent line positioning technology, so that the annealing is carried out strictly in the specified position, so that the quality of the welding seam is further improved.
(3) El valor FATT de la temperatura de torneado dúctil y frágil en la zona de soldadura de los primeros tubos de acero ERW es mucho más alto que el del metal base, con una diferencia máxima de 40-50 grados. Debido al avance de la tecnología metalúrgica, el valor FATT del metal base de la tubería soldada puede ser tan bajo como -70-80 grados. De esta manera, incluso si hay una cierta diferencia entre los dos, aún se pueden cumplir los requisitos del usuario.
(4) Due to the application of automation and ultrasonic testing technology, ultrasonic automatic tracking and online testing can be realized, thereby further reducing the probability of welding seam forming defects due to missed inspection.

2. Comparison of characteristics between ERW steel pipe and spiral submerged arc welded steel pipe

Due to the different forming, welding and process of ERW steel pipe and spiral submerged arc welded pipe in the pipe making process, there are certain differences in the performance of the two steel pipes.
(1) The surface quality of ERW steel pipe is better than that of spiral submerged arc welded pipe. The surface of ERW steel pipe is smooth, flat and beautiful, and the spiral submerged arc welded pipe has internal and external weld excess height.
(2) ERW steel pipe welds are shorter than spiral submerged arc welded pipes, ERW steel pipe welds are as long as steel pipes, and spiral submerged arc welded pipe welds are generally 1.4-1.7 times the length of the steel pipe. The longer the weld, the higher the chance of weld defects.
(3) The geometrical requirements for ERW steel pipes and spiral submerged arc welded pipes are basically the same in the standard, but in terms of physical objects, ERW steel pipes have sizing and straightening procedures, so the geometric dimensions are more accurate than spiral submerged arc welded pipes.
(4) The types of weld defects of ERW steel pipe are less than that of spiral submerged arc welded pipe. Spiral submerged arc welded pipe adopts deposition welding, and there are many types of defects. In addition to volume defects, there are also line depressions. The welding seam of ERW steel pipe has only linear defects, and the welding seam is straight, which is easy to track during the inspection process, and the defect is at the maximum reflection point. Once there is a defect, the ultrasonic reflection is strong and easy to detect.
(5) The residual stress of ERW steel pipe is lower than that of spiral submerged arc welded pipe. Due to the sufficient deformation of ERW steel pipe during the deformation process, and the subsequent changes of sizing flat ellipse, vertical ellipse and diameter reduction, the residual stress of ERW steel pipe becomes smaller. The physical test shows that after the ERW steel pipe of the same specification and the same material is cut along the axis, the axial and transverse incision displacement is smaller than that of the spiral submerged arc welded pipe.
(6) The overall performance of the ERW steel pipe is consistent. When the steel pipe is defective, the steel pipe is judged to be scrapped, and repair welding is not allowed. The spiral submerged arc welded pipe allows repair welding, but the performance of the repair welding is inconsistent with the overall performance of the steel pipe, and stress corrosion cracking is prone to occur at the repair welding. Experiments show that the stress corrosion fatigue test life of ERW steel pipe is longer than that of spiral submerged arc welded pipe.
(7) The coating quality of ERW steel pipe is higher than that of spiral submerged arc welded pipe.

The ERW production unit has a large investment and many processes, and the processing cost of the steel pipe is relatively high. Since ERW steel pipe has no repair welding, it is not allowed to face the head pipe, and the relative yield is low. Therefore, the direct price of ERW steel pipe is about 300 yuan/Ton higher than that of spiral submerged arc welded steel pipe. The overall cost of pipeline construction is sub-board and does not increase. First, the ERW steel pipe is of good quality and has a long service life; second, the ERW steel pipe has good surface quality, uniform coating, saving a lot of coating materials, and low anti-corrosion cost of the steel pipe; third, the ERW steel pipe has high geometric dimensional accuracy, fast butt welding speed, and on-site construction. High efficiency, in addition to reducing the cost of spinning.