Seamless vs Welded Line Pipe — How to Choose

The choice between seamless and welded line pipe — and between ERW, LSAW, and SSAW within the welded category — is driven by OD and wall thickness, operating pressure, fluid type, and what the applicable pipeline code actually permits for that service. Preference does not enter into it. A procurement team that specifies ERW for a 20-inch gas transmission line and an engineer who specifies SSAW for a high-pressure offshore trunkline have both made the same type of error: they have selected a manufacturing method without reading what the code requires for that combination of diameter, pressure, and fluid.

We supply seamless, ERW, LSAW, and SSAW line pipe at ZC Steel Pipe in API Specification 5L, 46th Edition, grades X52 through X80, PSL1 and PSL2. The orders we receive for West African and Middle Eastern gas transmission projects are almost entirely LSAW PSL2 X65 or X70 in the 20- to 48-inch OD range. Seamless orders come predominantly from compressor station piping packages and subsea spool fabricators working in sizes from 2 to 16 inches.

How Each Type Is Made

Seamless — a solid steel billet is heated to rolling temperature, pierced on a mandrel mill or push bench to form a hollow shell, and then elongated and rolled to the target OD and wall thickness. No weld seam exists anywhere in the pipe wall. The piercing process introduces circumferential variation in wall thickness that seamless mills control through tight rolling pass schedules; API 5L permits a wall thickness tolerance of +20% / −12.5% for seamless, which is wider than the tolerance for welded pipe formed from plate of known thickness. Practical seamless OD for line pipe grades runs from ½ inch to approximately 24 inches; above that, production yield drops sharply and the commercial case for seamless disappears.

ERW (Electric Resistance Welding) — strip steel is cold-formed into a cylinder and the abutting edges are fused by high-frequency electrical resistance — the current flowing across the interface generates heat that welds the edges without filler metal. The ERW weld is narrow, fully solid-state, and runs the full length of the pipe. When 100% weld seam ultrasonic testing is applied and the strip material is clean, ERW produces good mechanical properties. The seam's solid-state character is also its weakness: hook cracks and lack-of-fusion defects can form at the seam edges if the forming geometry or power parameters drift, and these defects can be invisible to inadequate NDE. Practical range: 2 to 24 inches OD, wall up to approximately 19 mm.

LSAW (Longitudinal Submerged Arc Welding) — steel plate is pressed into a U-shape then an O-shape (the UOE process) or formed progressively by the JCO process, and the longitudinal seam is welded inside and outside by submerged arc welding — a high-heat-input process using a flux-covered arc and filler wire that produces a deep, narrow, fusion weld with excellent mechanical properties and a large HAZ that subsequent PWHT or controlled cooling manages. LSAW is the engineering standard for large-diameter, high-pressure gas transmission pipelines. Practical range: 16 to 60 inches OD, wall up to approximately 50 mm.

SSAW (Spiral Submerged Arc Welding) — strip steel is fed at an angle into a forming head that wraps it into a helix; the spiral seam is welded by submerged arc welding, also inside and outside. The SSAW process produces pipe of any diameter from a single strip width by adjusting the helix angle, which makes it flexible and cost-effective for large diameters. The seam orientation — angled to the pipe axis — has consequences discussed in the weld quality section below. Practical range: 16 to 100 inches OD, though above 60 inches is rare for oil and gas service.

Size and Pressure Ranges

The table below covers the practical production range for each manufacturing method. Overlap zones — particularly 16 to 24 inches OD — are where the selection decision requires engineering judgment rather than a simple size rule.

In the 16-to-24-inch overlap zone, seamless, ERW, and LSAW can all produce pipe to the same API 5L grade. The practical guide: above 70 bar MAOP for gas, specify LSAW or seamless and confirm which the project code requires. Below 50 bar for liquid service, ERW in this size range is commercially reasonable where the project code permits it.

For the complete grade tables with PSL1 and PSL2 mechanical properties and chemistry limits, see the API 5L specification tables and the PSL1 vs PSL2 selection guide.

Weld Quality — ERW vs LSAW vs SSAW

The weld quality differences between the three welded manufacturing methods matter most in two scenarios: high-pressure gas service and cyclic pressure loading. For low-pressure water service, all three methods produce adequate weld quality under proper QC.

ERW seam failure modes the industry has documented include hook cracks — elongated inclusions at the weld line that open under tensile or pressure loading — and lack-of-fusion defects at the seam edges, which result from insufficient heat input or poor edge preparation. Both defect types can run parallel to the seam, making them difficult to detect with transverse UT alone. The API 5L PSL2 requirement for full-length, multi-angle ultrasonic testing of the weld seam addresses this, but seam quality in ERW is only as good as the strip cleanliness and the process control at the weld station.

LSAW seam — the submerged arc process deposits filler metal into a prepared groove and produces a metallurgically sound weld with a large fusion zone. Full-length UT of the inside and outside weld passes, combined with weld repair NDE and end-area radiography, gives confidence that LSAW weld quality under PSL2 is consistent. The failure mode in LSAW is not typically a seam defect missed at production — it is weld repair quality. Repairs performed to access a defect and re-weld must be re-tested to the same standard as the original seam, and repair records must be traceable on the MTC.

SSAW seam failure in cyclic pressure service is the documented concern: the spiral seam introduces a weld orientation relative to the applied hoop stress that is less favourable for fatigue crack initiation and propagation than LSAW's longitudinal seam. The mechanism is geometric — the spiral seam is longer per unit pipe length, and the HAZ runs at an angle to the principal stress direction. In sustained steady-pressure liquid service, this is rarely critical. In high-pressure gas service with start-stop cycling or pressure fluctuation, SSAW's seam geometry is one reason most project codes exclude it from mainline gas transmission.

When NOT to Use Each Type

Do not use SSAW for high-pressure gas transmission mainlines. Above approximately 40 bar MAOP, most national gas pipeline codes and major operator project specifications exclude SSAW from mainline service. The combination of spiral seam fatigue sensitivity and the dimensional variability inherent in the spiral forming process (OD and wall tolerance is wider for SSAW than LSAW) makes SSAW unsuitable for this application regardless of the grade specified. SSAW has a legitimate role in water transmission, structural casing, and low-pressure gas distribution trunk mains — use it there.

Do not use ERW above 24 inches. ERW mills do not produce pipe at this OD; the cold-forming process is not practical at large diameters. More relevant is the threshold where project codes stop permitting ERW for high-pressure service: most gas pipeline codes cap ERW at 70 bar or require additional testing above that pressure. If the MAOP is above 70 bar for a gas pipeline in the 8-to-24-inch range, check the applicable code before specifying ERW — the code may require LSAW or seamless.

Do not specify seamless above 24 inches for volume orders. Seamless production above 24 inches OD is limited to specialist mills and the commercial quantities available are small. If a project requires 100 km of 30-inch pipe, seamless is not a viable manufacturing method regardless of budget. The correct specification for large-diameter, high-pressure pipe is LSAW PSL2.

Do not use seamless where LSAW is the engineering standard for that size. Specifying seamless X65 PSL2 at 20-inch OD instead of LSAW X65 PSL2 is not a conservative decision — it is an unnecessary cost premium (seamless at 20 inches, where it exists, costs significantly more than LSAW) without a corresponding reliability gain. For large-diameter mainline gas transmission, LSAW with full-length NDE under PSL2 is the engineering correct specification, not a compromise.

Wall Thickness Worked Example (ASME B31.8)

A 20-inch (508 mm OD) LSAW X65 PSL2 pipeline with a maximum allowable operating pressure (MAOP) of 10 MPa in a Class 1 location. Minimum wall thickness under ASME B31.8 (Gas Transmission and Distribution Piping):

Formula:

t = (P × D) / (2 × SMYS × F × E × T)

Variables:

• P = 10 MPa (MAOP)
• D = 508 mm (nominal OD)
• SMYS = 450 MPa (X65 minimum yield per API 5L, 46th Edition)
• F = 0.72 (design factor, Class 1 location per ASME B31.8 Table 841.114A)
• E = 1.0 (longitudinal joint factor for LSAW per ASME B31.8 Table 841.115A)
• T = 1.0 (temperature derating factor, ambient temperature)

Calculation:

t = (10 × 508) / (2 × 450 × 0.72 × 1.0 × 1.0)

t = 5,080 / 648

t = 7.84 mm

Round up to the nearest commercially available wall: 8.0 mm, or confirm against the project's standard wall schedule. For LSAW at 20 inches, 9.5 mm and 12.7 mm are common stocked walls in the Middle East and West African supply chain; the 8.0 mm minimum wall leaves a margin that most operators accept.

The same formula applies identically to seamless X65 PSL2 — the SMYS is the same and the calculation does not change. The practical difference is that seamless at 20 inches OD is not commercially available in the quantities required for a mainline pipeline in most markets. The specification for a 20-inch high-pressure gas line in these markets is LSAW X65 PSL2, not seamless — not because the formula is different but because the supply chain does not support seamless at this size for volume orders.

To run this calculation for your specific pipeline parameters, use the Pipeline Design Calculator.

Procurement Trap — Contradictory Specification

A PO or project datasheet that specifies "seamless LSAW X65 PSL2" is not a conservative specification — it is a contradictory one. Seamless and LSAW are mutually exclusive manufacturing methods. Seamless pipe has no weld seam; LSAW pipe has a longitudinal submerged arc weld seam. A pipe cannot be both.

When contractors receive a PO with contradictory manufacturing method terms, they resolve the ambiguity in their favour — which typically means the cheaper option. If LSAW is less expensive at the required OD (which it usually is above 16 inches), the mill produces LSAW and ships it against the PO. The MTC will read "LSAW" because that is what was made. If the purchaser's receiving inspection then flags that the MTC does not say "seamless," the dispute begins.

The correct PO language is one of these — not both:

• "API 5L, Grade X65, PSL2, Seamless, [OD] × [wall]..." — for small-bore, high-pressure, or fatigue-sensitive applications below 16 inches
• "API 5L, Grade X65, PSL2, LSAW, [OD] × [wall]..." — for large-diameter gas transmission mainlines

We see this contradiction most often on specifications drafted from a previous project datasheet that mixed requirements from different line items. Carry the manufacturing method review through to the purchase order explicitly — do not leave it to the mill to resolve.

A second common trap: specifying "seamless or equivalent" without defining what constitutes an equivalent. Mills will interpret "equivalent" as any welded manufacturing method, including SSAW for large diameters. If seamless is required, the PO must say "seamless" only.

Supply

We supply seamless line pipe from ½ inch through 16 inches OD, LSAW from 16 through 48 inches OD, ERW from 2 through 16 inches OD, and SSAW from 20 through 60 inches OD for water service and structural applications. PSL2 is our default for all gas transmission inquiries in any manufacturing method — PSL1 is available on request for lower-specification applications.

Most LSAW orders for West African and Middle Eastern gas projects ship in X65 or X70 PSL2 with Charpy V-notch testing at −20°C and EN 10204 3.2 MTC (third-party witnessed). For compressor station piping packages, seamless X65 or X70 PSL2 in 6 through 16 inches OD is a standard procurement. SSAW we supply for water transmission projects and structural casing where the project specification explicitly permits spiral pipe.

For further reading on PSL1 vs PSL2 requirements and which delivery condition is appropriate for your project, see the PSL1 vs PSL2 selection guide, X65 PSL2 specifications, and X70 PSL2 specifications.