**Differences Between Straight Seam Steel Pipe, Spiral Steel Pipe, and Seamless Steel Pipe**

**Welding Process**

In terms of the welding process, the welding methods for spiral welded pipes and straight seam steel pipes are the same. However, straight seam welded pipes inevitably have many T-shaped welds, which greatly increases the probability of welding defects. Moreover, the residual stress at the T-shaped welds is relatively large, and the weld metal is often in a triaxial stress state, increasing the likelihood of cracks forming.

Furthermore, according to the process specifications of submerged arc welding, each weld should have an arc start and arc extinguish point. However, when welding the circumferential seams of each straight seam welded pipe, this condition cannot be met, which may result in more welding defects at the arc extinguish points.

**Strength Characteristics**

When a pipe is subjected to internal pressure, two main stresses are usually generated on the pipe wall: the circumferential (hoop) stress \( \sigma_h \) and the axial (longitudinal) stress \( \sigma_a \). At the weld seam, a combined stress \( \sigma \) occurs, where \( \alpha \) is the helix angle of the spiral weld in the spiral welded pipe.

The helix angle \( \alpha \) of the spiral weld in a spiral welded pipe is generally [**data missing**] degrees. Therefore, the combined stress at the spiral weld is [**data missing**] times the principal stress in a straight seam welded pipe. Under the same working pressure, the wall thickness of a spiral welded pipe with the same diameter can be reduced compared to a straight seam welded pipe.

Based on the above characteristics, it can be concluded that:

– **When a spiral welded pipe experiences bursting**, due to the smaller normal stress and combined stress on the weld seam, the burst opening generally does not originate at the spiral weld seam, making it safer than a straight seam welded pipe.

– **When defects parallel to the spiral weld seam exist nearby**, the spiral weld seam is subjected to less stress, so the risk of defect propagation is not as significant as in straight weld seams.

– **Since the circumferential (hoop) stress is the maximum stress present in the steel pipe**, when the weld seam is perpendicular to the direction of this stress, it bears the maximum load. That is, the straight seam bears the largest load, the circumferential weld seam bears the smallest load, and the spiral weld seam falls in between the two.

**Static Pressure Burst Strength**

Relevant comparative tests have verified that the measured values and theoretical values of the yield pressure and burst pressure of spiral welded pipes and straight seam welded pipes basically match, with close deviations. However, both the yield pressure and burst pressure of spiral welded pipes are lower than those of straight seam welded pipes. Burst tests also show that the circumferential deformation rate at the burst opening of spiral welded pipes is significantly greater than that of straight seam welded pipes. This confirms that the plastic deformation capacity of spiral welded pipes is superior to that of straight seam welded pipes. The burst opening is generally confined within one pitch, due to the strong constraint effect of the spiral weld seam on crack propagation.

**Toughness and Fatigue Strength**

The development trend of pipelines is toward larger diameters and higher strength. As the diameter of steel pipes increases and higher-grade steel is used, the tendency for steady propagation of ductile fracture tips increases. According to tests by relevant American research institutions, although spiral welded pipes and straight seam welded pipes are of the same grade, spiral welded pipes have higher impact toughness.

Due to variations in transmission volumes, pipelines are subjected to random alternating loads during actual operation. Understanding the low-cycle fatigue strength of steel pipes is of great significance for assessing the service life of pipelines.

According to measurement results, the fatigue strength of spiral welded pipes is the same as that of seamless pipes and electric resistance welded pipes. The test data for spiral welded pipes are distributed in the same region as those for seamless pipes and resistance welded pipes, and are higher than those for general submerged arc straight seam welded pipes.