ERW - high frequency resistance welding - straight seam welded pipe; LSAW - submerged arc welding - straight seam welded pipe; Both belong to longitudinal welded pipe, but they have different welding processes and different purposes, so they can not represent longitudinal welded pipe alone.

SSAW - spiral welding - pipe is also common.

According to different welding methods, straight seam high frequency (ERW) is divided into two forms: induction welding and contact welding. It uses hot-rolled wide coil as raw material and goes through pre bending, continuous forming, welding, heat treatment, sizing, straightening, cutting and other processes. Compared with spiral, ERW has the advantages of short weld, high dimensional accuracy, uniform wall thickness, good surface quality and high pressure, However, the disadvantage is that only small and medium-sized thin-walled pipes can be produced, and gray spots, incomplete fusion and groove corrosion defects are easy to occur at the weld. At present, it is widely used in urban gas, crude oil and refined oil transportation.

LSAW is produced by using a single medium thick plate as raw material, pressing (rolling) the steel plate in a die or forming machine, and expanding the diameter by double-sided submerged arc welding. The finished product has a wide range of specifications, good toughness, plasticity, uniformity and compactness of the weld, and has the advantages of large pipe diameter, pipe wall thickness, high pressure resistance, low temperature resistance and strong corrosion resistance. When building long-distance oil and gas pipelines with high strength, high toughness and high quality, most of the steel pipes required are large-diameter thick wall straight seam submerged arc. According to API standards, in large oil and gas transmission pipelines, when passing through class 1 and class 2 areas such as alpine zone, seabed and urban densely populated area, straight seam submerged arc is the only designated applicable pipe type. According to different forming methods, it can be divided into: UOE / JCOE / HME.

Spiral submerged arc welding (SSAW) is a process in which the advancing direction has a forming angle (adjustable) with the center line of the formed pipe. It is welded while forming, and its weld becomes a helix. The advantages are that steel pipes with multiple diameter specifications can be produced with the same specification, the raw materials can be used in a wide range, the weld can avoid the main stress, and the stress condition is good. The disadvantages are poor geometric dimensions, and the weld length is longer than that of straight seam pipe, It is easy to produce welding defects such as cracks, pores, slag inclusion and welding deviation, and the welding stress is in the state of tensile stress. The general design code for oil and gas long-distance pipeline stipulates that spiral submerged arc can only be used in class 3 and class 4 areas.