Difference Between LSAW and SSAW Pipe

In oil, gas, water transmission, and structural engineering, the selection of large-diameter welded steel pipes is crucial. LSAW (Straight Seam Submerged Arc Welded) pipes and SSAW (Spiral Seam Submerged Arc Welded) pipes are two of the most common large-diameter submerged arc welded pipe products. Although they both use the same welding process, they differ significantly in manufacturing methods, mechanical properties, and application scenarios.

This article will provide an accurate and detailed comparison from a technical perspective, correcting common misconceptions and helping you make the right choice.

1. Manufacturing Process Differences: Straight Seam vs. Spiral Seam

LSAW Pipe: First, the steel plate undergoes edge treatment, then it is gradually pressed into an open cylindrical shape using JCOE or UOE forming processes. The weld seam is a straight line along the longitudinal axis of the pipe, formed by double-sided submerged arc welding.
SSAW Pipe: The steel coil is continuously uncoiled and fed into the forming machine at a certain spiral angle. The welding head forms a spiral weld seam along the length of the pipe. This process can produce very large diameter steel pipes from narrower steel strips.

2. Weld Geometry and Stress Distribution

When a pipe is subjected to internal pressure, two main stresses are generated in the pipe wall: circumferential stress (σ_h) and axial stress (σ_a). The relationship between the weld direction and the principal stresses is crucial. In LSAW pipes, the circumferential stress intersects the weld at approximately 90°, and the weld bears higher direct tensile stress; while in SSAW pipes, the weld angle is helical, typically 30°~70°, decomposing the circumferential stress into two components, resulting in lower normal stress on the weld.

Because the helical weld simultaneously bears the combined effects of circumferential and axial stresses, under the same internal pressure, diameter, and material grade, SSAW steel pipes can theoretically use thinner wall thicknesses than LSAW steel pipes. This is a significant economic advantage of SSAW.

3. Comparison of Mechanical Properties

a. Hydrostatic Burst Strength
The measured and theoretical burst pressure values of both LSAW and SSAW steel pipes are highly consistent. However, controlled burst tests show that:

● SSAW steel pipes exhibit greater plastic deformation (expansion) before rupture. Because the geometry of spiral welds has a crack-arresting effect, the burst opening is typically limited to within a single pitch.
● LSAW steel pipes, due to the direct circumferential stress borne by the straight weld seam, have a slightly lower circumferential plastic deformation capacity than SSAW.

b. Toughness and Fatigue Strength
As pipe diameter increases and steel grades improve (X65, X70, X80), fracture toughness becomes crucial.

● Using the same base material and employing qualified welding processes, both LSAW and SSAW can achieve comparable Charpy V-notch impact values.
● However, under low-cycle, variable-amplitude loads (flow/pressure fluctuations common in oil and gas pipelines), SSAW steel pipes typically exhibit slightly higher average fatigue strength. Spiral welds distribute cyclic stress more evenly, delaying fatigue crack initiation.

Some field data and studies indicate that, for the same material, the fatigue strength distribution trend of SSAW steel pipes is higher than that of LSAW steel pipes—although this advantage diminishes with optimized welding processes and post-weld heat treatment.

4. Residual Stress and Weld Defect Sensitivity

Residual Stress:
LSAW Pipe: The JCOE/UOE forming process introduces significant residual stress at the longitudinal edges. Post-weld expansion helps reduce but cannot completely eliminate these stresses.
SSAW Pipe: Continuous spiral forming tends to produce a more uniform distribution of residual stress. However, complex triaxial residual stress exists near the weld toe of the spiral weld itself.

Defect Probability: 

A common misconception is that SSAW has a higher defect rate due to its longer weld length. For a given length of steel pipe, the weld length of SSAW is 2 to 5 times that of LSAW, statistically increasing the total number of potential defects. However, modern automated ultrasonic testing (AUT) and real-time weld monitoring make both pipe types highly reliable.

5. Size Range and Cost

SSAW pipe is more economical for large diameters, thin walls, and long lengths. LSAW pipe is suitable for thick-walled, high-pressure gas pipelines and acidic or polar service conditions where weld integrity is extremely important. Specific parameter comparisons are as follows:

Parameters	LSAW Steel Pipe	SSAW Steel Pipe
Diameter Range	Typically 16″~60″ (406mm~1524mm), maximum up to 72″	16″~120″ (406mm~3048mm) or even larger
Wall Thickness	Up to 2″ (50.8mm) or more	Typically no more than 0.75″~1″ (19~25mm)
Length	Typically 40~60 feet (12~18 meters)	Up to 100 feet (30 meters) or longer
Cost	Efficiency Higher cost per unit weight, suitable for thick walls	Lower material cost, allowing for thinner wall thicknesses

Common standards for LSAW and SSAW include: 

ASTM A53 (low pressure), API 5L (high pressure), ASTM A252 (piling pipe), EN 10219 (structural tube).

6. Apllication Selection

Suitable Scenarios for LSAW Steel Pipes:

● High-pressure oil and gas long-distance pipelines (Class 600 and above)
● Acidic service environments (including H₂S)
● Deepwater risers and marine pipelines
● Thick wall requirements (>25mm)
● Scenarios where specifications explicitly require specific stress calculations for longitudinal welds

Suitable Scenarios for SSAW Steel Pipes:

● Water conveyance and irrigation projects
● Medium and low-pressure oil and gas gathering and transportation pipelines
● Structural piles (bridge piers, foundation piles)
● General fluid transportation where cost is the primary driver
● Projects requiring ultra-large diameters (>60″)

Conclusion: 

Both LSAW and SSAW steel pipes are proven and reliable products when manufactured to API 5L or equivalent standards. LSAW steel pipes offer superior performance in high-pressure and thick-walled applications, while SSAW steel pipes provide cost-effectiveness and excellent ductility in large-diameter, thin-walled projects. The choice between LSAW and SSAW should be based on a comprehensive consideration of the required diameter, wall thickness, pressure rating, and budget. LSAW is typically specified for critical high-pressure gas pipelines; for water and structural applications, SSAW is often a more economical and technically sound choice.

Read more: Spiral Welded Pipe Material Selection or SSAW Steel Pipe Size Chart