What is the difference between seamless and welded line pipe?

Seamless line pipe is formed from a solid steel billet that is pierced and rolled into a tube — there is no weld seam anywhere in the pipe wall. Welded line pipe is formed by rolling steel plate or strip into a cylinder and welding the seam, either longitudinally (LSAW, ERW) or spirally (SSAW). The absence of a weld seam in seamless pipe gives it more uniform mechanical properties around the circumference and eliminates the weld-related defect risk, but limits the size range and increases cost versus welded alternatives in larger diameters.

Which is stronger — seamless or welded line pipe?

For the same grade and wall thickness, seamless and welded line pipe have the same minimum yield and tensile strength requirements under API 5L. The practical difference is consistency and defect risk. Seamless pipe has uniform mechanical properties with no seam — the entire circumference has the same microstructure. Welded pipe has a weld zone with different mechanical properties than the parent metal, and the weld introduces a potential defect site. However, modern LSAW production with submerged arc welding and full-length weld NDE produces weld quality that meets or exceeds the pipe body in most mechanical property measures.

When should I specify seamless instead of ERW or LSAW?

Specify seamless when: the application requires high reliability with no weld seam risk (subsea spool pieces, riser connections, compressor station piping, high-pressure small-bore gathering); the OD is below 16 inches where seamless is cost-competitive with LSAW; the pipe will see fatigue loading from cyclic pressure or vibration where weld fatigue is a concern; or the project specification explicitly requires seamless. For large-diameter (above 16 inches) pipeline applications, LSAW is more economical than seamless and produces equivalent quality under proper QC.

What is the difference between ERW, LSAW, and SSAW?

ERW (Electric Resistance Welding) forms pipe from strip steel by cold-forming and welding the seam using electrical resistance heating — no filler metal is used. Suitable for diameters up to approximately 24 inches and moderate pressures. LSAW (Longitudinal Submerged Arc Welding) forms pipe from plate by pressing into a U and O shape (UOE) or JCO process, then welding with submerged arc welding — produces high-quality welds suitable for large-diameter, high-pressure applications. SSAW (Spiral Submerged Arc Welding) forms pipe by helically winding strip steel and welding the spiral seam — economical for large diameters at lower pressure, but the spiral seam is less suited to high-pressure gas transmission than LSAW.

Is LSAW better than SSAW for high-pressure gas pipelines?

Yes — LSAW is the preferred manufacturing method for high-pressure gas transmission pipelines above approximately 16 inches OD. LSAW's longitudinal weld produces a more consistent weld geometry and allows better NDE coverage than the spiral seam in SSAW. LSAW pipe also has better dimensional consistency — OD, wall thickness, and straightness — which is important for automated field welding. SSAW is suitable for lower-pressure applications such as water transmission, drainage, and low-pressure gas distribution, where its cost advantage over LSAW justifies its use.

What size range does each pipe type cover?

Seamless covers ½ inch to approximately 24 inches OD (21.3 mm to 609.6 mm) for line pipe applications. ERW covers approximately 2 inches to 24 inches (60.3 mm to 609.6 mm). LSAW covers approximately 16 inches to 60 inches (406.4 mm to 1524 mm). SSAW covers approximately 16 inches to 100 inches (406.4 mm to 2540 mm), though above 60 inches is rare for oil and gas applications. In the overlap zones — particularly 16 to 24 inches — seamless, ERW, and LSAW all produce valid pipe, and the choice depends on pressure rating, quality requirements, and project economics.

Seamless vs Welded Line Pipe — Guide

The choice between seamless and welded line pipe — and between ERW, LSAW, and SSAW within the welded category — is driven by a combination of OD and wall thickness, operating pressure and fluid type, quality and NDE requirements, and project economics. There is no universal answer, and specifying the wrong manufacturing method for an application either adds cost unnecessarily or introduces risk that better manufacturing control would eliminate.

ZC Steel Pipe supplies seamless, ERW, LSAW, and SSAW line pipe in API 5L grades X52 through X80. This guide covers the technical basis for choosing between manufacturing methods, the size and pressure ranges each method covers, and the applications where each type is the correct specification.

How Each Type Is Made

Seamless — a solid steel billet is heated and pierced on a mandrel mill or push bench to form a hollow shell, then rolled to the target OD and wall thickness. No weld seam anywhere in the pipe wall. The seamless process limits practical OD to approximately 24 inches for line pipe grades, though specialist mills can produce seamless up to 28 inches.

ERW (Electric Resistance Welding) — strip steel is cold-formed into a cylinder and the abutting edges are welded using high-frequency electrical resistance — the heat generated by the electrical resistance fuses the edges without filler metal. ERW produces a narrow, straight weld with good mechanical properties when properly executed and 100% weld seam NDE is applied. Practical range: 2 to 24 inches OD.

LSAW (Longitudinal Submerged Arc Welding) — steel plate is pressed into a U-shape, then an O-shape (UOE process) or formed by the JCO process, and the longitudinal seam is welded inside and outside with submerged arc welding — a high-heat-input, filler-metal process that produces a deep, narrow weld bead with excellent mechanical properties. Practical range: 16 to 60 inches OD. The standard manufacturing method for large-diameter, high-pressure pipelines.

SSAW (Spiral Submerged Arc Welding) — strip steel is fed at an angle into a forming head that wraps it into a helix, with the spiral seam welded by submerged arc welding. The spiral seam geometry means that for any given wall thickness, the seam is longer than an equivalent LSAW seam — and the angle of the seam to the pipe axis affects the weld's resistance to hoop stress. SSAW is cost-effective for large diameters at lower pressures. Practical range: 16 to 100 inches OD.

Size and Pressure Range by Manufacturing Method

Method	OD Range	Wall Range	Max Grade	Best Application
Seamless	½″ – 24″	2.1 – 40 mm	X80	High-pressure, small-bore, no seam required
ERW	2″ – 24″	1.8 – 19.1 mm	X70	Moderate-pressure gathering and distribution
LSAW	16″ – 60″	6.4 – 50.8 mm	X80	High-pressure large-diameter transmission
SSAW	16″ – 100″	5.0 – 25.4 mm	X70	Low-to-moderate pressure, large diameter

When Seamless Is the Right Choice

Seamless line pipe is specified when the application requires the highest reliability and the absence of a weld seam is an engineering or commercial requirement:

Compressor and pump station piping — high-pressure, high-temperature, cyclic loading. Weld fatigue risk makes seamless the standard specification.

Riser and jumper connections — subsea tie-in spools and jumpers see combined pressure, bending, and thermal cycling. Seamless is standard for sizes where it is available.

Small-bore high-pressure gathering — 2 to 8 inch OD gathering at high pressure (above 100 bar) where seamless cost is competitive with welded alternatives.

Project specifications — some operating company standards require seamless for specific service categories regardless of OD.

When LSAW Is the Right Choice

LSAW is the standard for large-diameter, high-pressure gas transmission pipelines. The submerged arc welding process produces weld quality that — with full-length NDE and proper QC — matches or exceeds seamless in mechanical property consistency for large-diameter applications where seamless is impractical or uneconomical.

LSAW is specified for: 16 to 60 inch OD high-pressure gas transmission; offshore pipeline main line pipe above 16 inches; and any large-diameter application where dimensional consistency for automated field welding is required.

When ERW Is the Right Choice

ERW is economical for moderate-diameter, moderate-pressure applications where the absence of a fusion-welded seam is not required:

Gas distribution — 4 to 16 inch OD distribution lines at moderate pressure.

Liquid gathering — onshore oil and produced water gathering at moderate pressure.

Low-pressure gas gathering — where MAOP is below 50 bar and the pipeline code permits ERW.

ERW is not suitable for offshore or high-pressure gas transmission above approximately 70 bar where LSAW provides better weld quality assurance.

When SSAW Is the Right Choice

SSAW is economical for large-diameter, low-to-moderate pressure applications:

Water transmission — 24 to 60 inch OD municipal water transmission at low pressure.

Low-pressure gas distribution — large-diameter trunk mains at MAOP below 40 bar.

Structural and casing applications — large-diameter pipe used for foundation casing, HDD, or microtunneling.

SSAW is not appropriate for high-pressure gas transmission or offshore applications.

API 5L Quality Requirements by Manufacturing Method

Requirement	Seamless PSL2	ERW PSL2	LSAW PSL2	SSAW PSL2
Body NDE	Full-length UT/EMI	Full-length UT/EMI	Full-length UT	Full-length UT
Weld NDE	N/A	Full-length UT	Full-length UT	Full-length UT
Weld seam tensile	N/A	Mandatory	Mandatory	Mandatory
Charpy impact	Mandatory	Mandatory	Mandatory	Mandatory
Hydrostatic test	Mandatory	Mandatory	Mandatory	Mandatory

LSAW and SSAW also require radiographic or ultrasonic testing of weld seam repairs under API 5L PSL2.

How to Specify Manufacturing Method on a Purchase Order

State the manufacturing method explicitly:

API 5L, Grade X65, PSL2, LSAW, 457.2 mm OD × 12.7 mm wall...

Do not leave manufacturing method unspecified if it matters to your application. An unspecified purchase order may be filled with the manufacturing method most economical for the supplier, which may not be appropriate for the application.

References

API Specification 5L — Specification for Line Pipe
ISO 3183 — Steel Pipe for Pipeline Transportation Systems
DNV-ST-F101 — Submarine Pipeline Systems

Seamless vs Welded Line Pipe — How to Choose