LSAW vs SSAW Pipe: Differences, Applications, and When to Specify Each

When an EPC procurement specification reads "welded line pipe per API 5L," it does not specify which welding process. That distinction — longitudinal or spiral — matters more than most procurement teams realize, and getting it wrong in the purchase order can result in a delivery that the project specification rejects on arrival.

ZC Steel Pipe supplies API 5L LSAW line pipe for oil and gas transmission projects in West Africa, the Middle East, and South America, typically in the OD range of 16" to 56" for high-pressure service. The comparison below is based on what we see on project specifications, not on mill brochures.

What Is LSAW Pipe?

Longitudinal Submerged Arc Welded pipe has a single weld seam that runs parallel to the pipe axis, from one end to the other. The manufacturing process starts with a flat plate cut to the required width. The plate is formed progressively — either by the JCO process (press forming in three stages: J shape, C shape, O shape) or by the UOE process (U press, O press, mechanical expansion). The open seam is then closed by submerged arc welding, typically with one interior pass and one exterior pass.

The result is a pipe with one straight seam, low weld volume relative to the pipe body, and very tight dimensional control. OD tolerance for LSAW pipe under API 5L Table 10 is typically ±0.5% of the specified OD, and most LSAW mills hold even tighter tolerances in practice. Wall thickness capability reaches approximately 50 mm, making LSAW the standard for thick-wall applications such as deepwater pipelines and high-pressure transmission lines at X65 and above.

LSAW pipe is manufactured to API Specification 5L, 46th Edition, ISO 3183, and for offshore applications, DNV-ST-F101. Available grades run from X42 through X80 PSL2, with sour service (HIC-tested, NACE-qualified) grades available at X52 through X70.

What Is SSAW Pipe?

Spiral Submerged Arc Welded pipe is formed by pulling a continuous narrow coil strip — called skelp — through a forming head that curves it into a helix, with each revolution overlapping and welded to the previous one by submerged arc welding on both the inside and outside. The weld seam follows a continuous spiral path around and along the pipe.

The principal advantage of SSAW is size flexibility: the pipe diameter is determined by the helix angle of the forming head, not by a forming die. By adjusting the angle, the same mill can produce a range of diameters from the same skelp width. SSAW can produce diameters from approximately 219 mm (8") up to 3,048 mm (120") — well beyond the practical LSAW range. For very large diameter pipes used in water transmission or drainage, SSAW is often the only cost-effective production route.

SSAW is also produced to API 5L and ISO 3183. However, the spiral geometry imposes practical limits: wall thickness is constrained by skelp availability (typically 20–25 mm maximum), and the weld seam orientation means dimensional tolerances are slightly wider than LSAW for equivalent sizes.

LSAW vs SSAW: Comparison Table

The table above shows structural differences, not a quality ranking. SSAW produced under API 5L PSL2 to a rigorous project specification can be high-quality pipe. The choice is about fit for application, not inherent superiority.

When NOT to Use SSAW

Sour service pipelines. Most operator and EPC specifications for H2S-containing service — lines classified under NACE MR0175 / ISO 15156 — specify seamless or LSAW and exclude SSAW. The spiral weld creates a complex three-dimensional stress state at the weld toe when the pipe is under combined axial, hoop, and bending load. This complicates the fracture mechanics assessment that sour service qualification requires. In practice, no major West African or Middle Eastern operator we work with accepts SSAW for sour gas transmission without explicit qualification testing, and most exclude it outright.

HPHT offshore pipelines. DNV-ST-F101 (Submarine Pipeline Systems) permits SSAW in principle but sets supplementary requirements that, in practice, most offshore contractors find easier to satisfy with LSAW. Deepwater reel-lay and J-lay operations subject the pipe to high bending strain during installation; tight OD tolerance and weld quality are critical. The additional qualification burden for SSAW is rarely worth the cost saving offshore.

Automated girth welding. Mechanized orbital welding systems — CRC-Evans, Lincoln, Colfax — rely on tight OD tolerance to maintain consistent arc gap as the head travels around the joint. SSAW's slightly wider OD variation increases the risk of hi-lo at the girth weld bevel, which causes incomplete root fusion. For cross-country pipelines using automated welding spreads, most project specifications require LSAW.

Long-distance high-pressure gas transmission at X65 and above. For API 5L X65 PSL2 and X70 PSL2 high-pressure gas pipelines, project specifications from major operators and national oil companies almost universally restrict welded pipe to LSAW. The combination of wall thickness requirements (often 12–22 mm), pressure class, and Charpy impact requirements at the weld centreline and HAZ makes SSAW impractical.

When NOT to Use LSAW

Very large diameter, low-pressure service. Water transmission mains, drainage culverts, and structural piling in diameters above 1,500 mm (60") are typically SSAW territory. At these sizes, LSAW plate forming becomes economically and technically difficult. SSAW is the standard production method and is well proven in these applications.

Odd diameters not in the LSAW product range. LSAW requires a forming die matched to the pipe diameter. Standard LSAW mills run established OD sizes — 16", 18", 20", 24", 30", 36", 42", 48", 56". An unusual diameter (say, 850 mm for a specific project requirement) may be producible by SSAW but not available from LSAW mills without a tooling order.

Short lead time for sizes where LSAW mill order books are full. LSAW plate availability and mill capacity are tighter than SSAW at some points in the commodity cycle. For non-critical service (domestic water, low-pressure distribution), SSAW from an alternative supplier can shorten delivery when LSAW lead times are extended.

SSAW in API 5L PSL2 — Is It Allowed?

Yes. API Specification 5L, 46th Edition does not restrict SSAW to PSL1. SSAW pipe can be produced and certified to PSL2, including with Charpy V-notch impact requirements at the weld centreline, weld heat-affected zone (HAZ), and pipe body. The PSL2 requirements apply regardless of manufacturing method.

The restriction on SSAW in high-pressure service does not come from API 5L — it comes from the operator's project specification or supplementary requirements document (SRD). Many major operators (Shell MESC, Saudi Aramco SAES, BP GIS) have supplementary requirements that restrict welded pipe to LSAW for specific service categories. If your project has an operator-issued SRD, check it before assuming that API 5L PSL2 certification is sufficient.

Dimensional Tolerances: A Practical Comparison

Dimensional tolerance matters most at the girth weld joint. The key parameters are OD tolerance, wall thickness tolerance, and straightness.

Under API 5L Table 10, OD tolerance for pipe with D > 508 mm is ±0.5% of the specified OD for pipe body and ±1.6 mm at the pipe ends. In practice, LSAW mills typically supply to the ±0.5% level for the body. SSAW mills can meet API 5L's table tolerance, but achieving ±0.5% consistently requires tight control of the forming head angle and skelp width — variation here translates directly to OD ovality.

Wall thickness tolerance under API 5L is +20% / −10% for seamless, and +15% / −10% for welded (both LSAW and SSAW). In this respect, LSAW and SSAW are specified identically by API 5L. The difference in practice is that LSAW plate thickness is controlled before forming, while SSAW skelp thickness may vary slightly along the coil length — a minor effect that does not typically produce rejectable material under API 5L but is worth noting for design calculations where you are using minimum wall.

For the full dimensional reference, see the API 5L specification tables → and use the Pipeline Design Calculator → to check wall thickness adequacy against your operating pressure.

Named Failure Modes

Spiral weld toe fatigue cracking. In cyclic service — offshore flowlines, pig-launched lines that see repeated pressure cycling — the spiral weld toe is a stress concentration at an angle to the primary hoop stress. Fatigue crack initiation at the weld toe is the primary failure mode for SSAW in dynamic service. LSAW's straight seam runs parallel to the pipe axis, where hoop stress is perpendicular to the seam — a more benign orientation for fatigue. For any offshore or cyclic-service application, LSAW's seam geometry is inherently more favourable.

Lamination in SSAW skelp. SSAW relies on coil skelp, which is produced by hot rolling and coiling. Laminar defects within the skelp — flat voids parallel to the surface — are carried into the finished pipe. Under transverse loading or in hydrogen-charging environments (wet H2S, HIC conditions), laminations become nucleation points for stepwise cracking. API 5L PSL2 requires HIC testing for sour service pipe, but the lamination risk in coil-derived material is why sour service LSAW and seamless pipe are sourced from plate stock, which has better through-thickness ultrasonic inspection history than coil.

Hi-lo at the girth weld. When SSAW pipe joints are aligned for welding, OD variation between joints creates a step — "hi-lo" — at the bevel. For manual welding, experienced welders compensate; for automated welding, hi-lo above approximately 1.5 mm begins to cause incomplete root fusion. Severe hi-lo requires grinding of the bevel and re-cutting, adding time at the field joint. LSAW's tighter OD tolerance means hi-lo is less frequent and less severe.

Which to Specify in Your Project Documents

For high-pressure oil and gas transmission, the safest purchase order language is explicit:

"Pipe shall be manufactured by the Longitudinal Submerged Arc Welded (LSAW) process per API Specification 5L, 46th Edition. Spiral welded (SSAW) pipe is not acceptable."

If sour service is involved, add:

"Pipe shall meet the requirements of [Operator SRD reference] for sour service (H2S-containing) service. HIC test results per NACE TM0284 shall be included in the MTC."

If the project permits SSAW for certain line classes (non-critical, low-pressure), specify this separately and clearly by pipeline segment or service classification. Mixed-process pipe on the same project is manageable, but the inspection protocol at the receiving yard needs to distinguish between the two types — their NDT records look different and should not be filed interchangeably.

Purchase Order Guidance

Procurement trap: A purchase order that reads "welded pipe, API 5L X65 PSL2" does not restrict the manufacturing process. If the project specification requires LSAW but the PO does not reference it or state it explicitly, the mill is compliant supplying SSAW. We have seen this create a hold at third-party inspection when the inspector checks the MTC and notes "SSAW" in the manufacturing record against a project spec that says LSAW only. Re-procurement at that stage delays the project and shifts the cost to the buyer. Specify the manufacturing method in the PO, not just in the project spec filing.

MTC verification checklist for large-diameter welded line pipe:

1. Confirm manufacturing process designation — "LSAW" or "SSAW" must appear explicitly in the MTC header or body.
2. Verify heat number traceability to plate (LSAW) or skelp coil (SSAW) chemistry.
3. For PSL2: confirm Charpy impact test records at weld centreline, HAZ, and pipe body. Check test temperature matches the project specification.
4. For sour service: confirm HIC test results per NACE TM0284 are included — C.A.R., C.T.R., and C.S.R. all below the project limits (typically API 5L Annex H: CLR ≤ 15%, CTR ≤ 5%, CSR ≤ 2%).
5. Check OD and wall thickness mill measurements — confirm they fall within API 5L Table 10 tolerances for your size.
6. For LSAW: confirm weld seam NDT records include both internal and external UT plus radiographic inspection at the pipe ends.