DSAW — Double Submerged Arc Welded — is the welding variant that defines most large-diameter, heavy-wall line pipe used in gas transmission, offshore pipelines, and major crude oil trunklines. The name refers to the two-pass weld sequence: an inside weld first, then an outside weld, ensuring full through-thickness fusion on pipe where a single pass cannot reach the centreline. In project specifications and commercial documentation, DSAW and LSAW are often used interchangeably, but understanding the distinction matters when reviewing mill data books and qualifying weld procedures.
ZC Steel Pipe manufactures DSAW pipe to API 5L PSL1 and PSL2 in grades X52 through X80, using JCOE and UOE forming processes. Diameters range from 16 inches (406 mm) to 64 inches (1626 mm) with wall thicknesses from 6 mm to 40 mm. All PSL2 pipe is supplied with full-length automated ultrasonic testing of the weld seam and hydrostatic testing as standard.
1. What Is DSAW Pipe?
DEFINITION
DSAW (Double Submerged Arc Welded) pipe is a large-diameter longitudinally welded steel pipe manufactured using two sequential submerged arc weld passes — one applied to the inside seam (IW) and one to the outside seam (OW). The submerged arc process buries the arc beneath a layer of granular flux, shielding the weld pool from atmospheric contamination and producing a dense, consistent weld with low hydrogen content and excellent mechanical properties.
The "double" in DSAW refers specifically to the two-pass weld sequence, not to any doubling of the pipe wall. In single-pass LSAW — used on lighter-wall pipe — only the outside weld is applied, and the weld metal must fuse the full joint thickness from one side. As wall thickness increases above roughly 10–14 mm, achieving full penetration from a single outside pass becomes unreliable. DSAW resolves this by welding from the inside first, laying down a root and fill pass from within the bore, then completing the joint from outside.
In the industry, DSAW and LSAW are frequently used as synonyms on data sheets, purchase orders, and inspection certificates. This is commercially acceptable because virtually all LSAW pipe produced to API 5L above moderate wall thickness is DSAW. Engineers reviewing mill documentation should confirm the weld procedure qualifies both IW and OW passes.
Procurement Note — DSAW vs LSAW on POs
If your project specification says "LSAW pipe" and the supplier quotes "DSAW," there is no contradiction. The two terms describe the same physical pipe at heavy wall. Where you do need clarity is in the forming process: JCOE and UOE produce different residual stress profiles and slightly different OD tolerances. Confirm which forming route the mill uses before committing to a purchase order for offshore or sour service applications.
2. DSAW vs LSAW — The Weld Pass Distinction
The table below clarifies the relationship between LSAW (the manufacturing category) and DSAW (the weld technique) and where the terms apply:
| Feature | Single-Pass LSAW | DSAW (Double-Pass LSAW) |
| Weld passes | 1 — outside only | 2 — inside (IW) then outside (OW) |
| Typical wall thickness | ≤ 14 mm | 6–40+ mm (standard for ≥ 10 mm) |
| Joint penetration | Full from OD side | Full through-thickness fusion, both faces |
| Application | Lighter-wall distribution lines | Gas transmission, offshore, heavy-wall |
| NDE scope | OW seam UT | IW + OW seam UT, RT at ends |
| Weld procedure qualification | Single WPS (OW) | Separate WPS for IW and OW passes |
| Commercial interchangeability | Often listed as "LSAW/DSAW" on mill certs; confirm wall thickness triggers DSAW |
Engineering Insight — Why the Inside Weld Is Made First
The inside weld is always the first pass in DSAW. This is not arbitrary: laying the IW root establishes the joint geometry and back-gouges any imperfections at the inner surface. Once the IW is complete, the pipe is inspected visually and by UT before the OW is applied. The OW then fills the groove from outside, fusing into the IW metal and completing the joint. This sequence ensures that any lack-of-fusion defects introduced by the OW are caught before they become buried — the IW acts as a verifiable backstop.
3. Manufacturing Process (JCOE & UOE)
DSAW pipe is produced by two main forming methods. The choice of forming process determines the dimensional tolerances, residual stress profile, and maximum size available from a given mill.
JCOE Process
The JCOE (J-forming → C-forming → O-forming → Expanding) process uses a series of press-forming steps to shape a flat plate into a round pipe before welding. The plate is first bent into a J profile at the edges (J-forming), then progressively pressed to C and O shapes using hydraulic dies. After forming, a pre-weld tack pass is applied, followed by the inside SAW pass, then the outside SAW pass. The pipe is then cold-expanded (E step) to achieve final OD tolerance.
JCOE Key Parameters
OD range: 406–1626 mm (16–64 in)
Wall thickness: 6–40 mm
Length: 6–18 m (std 12 m)
OD tolerance: ±0.5% (min ±1.6 mm)
Roundness: ≤ 0.5% OD
Forming: Press-forming (incremental)
UOE Key Parameters
OD range: 406–1626 mm (16–64 in)
Wall thickness: 6–38 mm
Length: 6–18 m (std 12 m)
OD tolerance: ±0.5% (min ±1.6 mm)
Roundness: ≤ 0.5% OD (superior)
Forming: U-press + O-press (single-stroke)
UOE Process
The UOE process uses two large hydraulic presses — a U-press to create the initial curved profile and an O-press to close it to a round cross-section — producing the pipe shape in fewer steps than JCOE. UOE delivers marginally superior roundness and OD consistency, making it preferred for offshore applications where joint fit-up tolerances are tight. After welding (IW then OW), the pipe is expanded on a mandrel expander to relieve residual stresses and set the final OD.
Field Note — JCOE vs UOE on Offshore Projects
On deepwater projects (S-lay or J-lay), tight end-to-end OD match is critical for automatic welding girth weld stations. UOE pipe generally achieves better joint-to-joint consistency in OD. JCOE-produced pipe is fully acceptable for most onshore and shallow-water work and is more widely available from Chinese DSAW mills. If the laying contractor specifies UOE, confirm with the mill — not all JCOE facilities can certify to deepwater offshore project specs.
4. API 5L Specifications & Grades
DSAW pipe for oil and gas service is manufactured primarily to API 5L (Specification for Line Pipe). Two product specification levels apply:
| Requirement | PSL1 | PSL2 |
| Chemistry limits | Standard (C, Mn, P, S) | Tighter + CE (carbon equivalent) limits |
| Tensile requirements | Minimum UTS only | Min + max UTS, Y/T ratio ≤ 0.93 (most grades) |
| Impact testing (CVN) | Not required | Required, temperature per grade |
| Weld seam UT | Optional (if specified) | Mandatory, full length |
| Pipe body UT | Not required | Required |
| Hydrostatic test | Required | Required |
| Sour service option | No | Yes (PSL2+) |
| Offshore option | No | Yes (PSL2+) |
Grade Chemical & Mechanical Summary
| Grade | SMYS (MPa) | SMTS (MPa) | Max C (%) | Max CE (IIW) | Typical Application |
| X52 | 359 | 455 | 0.24 / 0.22 | 0.43 / 0.43 | Gathering lines, distribution |
| X56 | 386 | 490 | 0.24 / 0.22 | 0.43 / 0.43 | Moderate-pressure gas lines |
| X60 | 414 | 517 | 0.24 / 0.22 | 0.43 / 0.43 | Gas transmission, trunk lines |
| X65 | 448 | 531 | 0.22 / 0.18 | 0.43 / 0.43 | High-pressure gas, offshore risers |
| X70 | 483 | 565 | 0.22 / 0.18 | 0.43 / 0.43 | Long-distance gas transmission |
| X80 | 552 | 621 | 0.22 / 0.18 | 0.43 / 0.43 | Ultra-high-pressure gas pipelines |
Values shown as PSL1 / PSL2 where they differ. CE = Carbon Equivalent per IIW formula: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Confirm to the applicable edition of API 5L for full chemistry tables.
Critical Engineering Point — PSL1 vs PSL2 for Gas Service
Many national pipeline codes (ASME B31.8 in the US, EN 14161 in Europe, AS 2885 in Australia) require PSL2 for high-pressure gas transmission above certain design pressures or operating temperatures. Do not default to PSL1 on a gas transmission project without confirming the applicable code and design basis. The absence of CVN impact testing under PSL1 can be a non-conformance when the pipe is reviewed against project specifications.
5. Size Range & Dimensions
| OD (mm) | OD (inches) | Wall Thickness Range (mm) | Common Grades | Typical Use |
| 406.4 | 16" | 7.9–25.4 | X52–X65 | Gathering, distribution |
| 457.2 | 18" | 7.9–28.6 | X52–X70 | Trunk lines |
| 508.0 | 20" | 7.9–31.8 | X52–X70 | Gas transmission |
| 558.8 | 22" | 9.5–31.8 | X60–X70 | Gas transmission |
| 609.6 | 24" | 9.5–38.1 | X60–X80 | High-pressure gas |
| 660.4–762.0 | 26"–30" | 9.5–38.1 | X65–X80 | Major trunk lines |
| 813.0–914.4 | 32"–36" | 12.7–40.0 | X65–X80 | Long-distance transmission |
| 1016.0–1219.2 | 40"–48" | 14.3–40.0 | X70–X80 | National grid pipelines |
| 1422.4–1626.0 | 56"–64" | 16.0–40.0 | X70–X80 | Mega-project transmission |
Standard pipe length is 12 metres (range 6–18 m). Random lengths (R1: 6–8.5 m; R2: 8.5–11.6 m; R3: 11.6–14.6 m) and double random (DR: ≥ 11.6 m average) are available to API 5L definitions. Bevel ends are standard at 30° ± 5° per API 5L for girth welding. Square cut and plain ends can be specified for specific applications.
Procurement Note — Specifying Wall Thickness
Always confirm whether the wall thickness in your purchase order is nominal or minimum. API 5L permits a wall thickness undertolerance of 12.5% on seamless pipe but for DSAW pipe the tolerance is typically −0/+mill tolerance per agreement. On high-pressure designs, the difference between nominal and minimum wall can affect MAOP calculations. Request mill MTCs that clearly state measured wall thickness at the pipe body, not just at the weld seam.
6. NDT & Testing Requirements
DSAW pipe requires a more comprehensive testing programme than ERW or seamless pipe of comparable size, reflecting the critical nature of the longitudinal weld in large-diameter applications.
| Test | PSL1 | PSL2 | Notes |
| Hydrostatic test | Required | Required | Hold pressure per API 5L Table 44/45 |
| Weld seam UT | If specified | Full length, mandatory | AUT per API 5L Annex E |
| Pipe body UT | Not required | Required | Lamination detection |
| Weld end RT | If specified | Required | 50 mm from each end minimum |
| CVN impact test | Not required | Required | Freq. and temp per grade |
| Tensile test | Required | Required | Pipe body + weld seam |
| Bend test | Required | Required | Guided bend at weld seam |
| DWT (drop weight) | Not required | If specified (sour, offshore) | DWTT per API 5L Annex G |
| Dimensional check | 100% visual + end gauging | 100% | OD, WT, length, straightness, bevel |
Engineering Insight — Automated UT vs Radiography for DSAW Seams
API 5L PSL2 mandates automated ultrasonic testing (AUT) of the full-length weld seam. AUT detects volumetric flaws (porosity, inclusions) and planar flaws (cracks, lack of fusion) with high sensitivity and speed. Radiographic testing (RT) is required at weld ends because the AUT beam geometry cannot resolve flaws within ~50 mm of the pipe end. For critical sour service or offshore projects, some project specifications additionally require phased-array UT (PAUT) with ToFD (Time-of-Flight Diffraction) for enhanced flaw sizing. Specify this at order stage — it cannot be retrofitted to a standard PSL2 inspection programme.
7. DSAW vs SSAW — When Each Is Specified
DSAW (longitudinal seam) and SSAW — Spiral Submerged Arc Welded, also called HSAW — are the two principal SAW welding routes for large-diameter pipe. They have different performance profiles and their selection is largely driven by application pressure, diameter requirements, and project specification.
| Parameter | DSAW (LSAW) | SSAW (HSAW) |
| Seam orientation | Straight (longitudinal) | Spiral (helical) |
| OD range | 406–1626 mm | 219–3048 mm (more flexible) |
| Wall thickness | 6–40 mm (superior at heavy wall) | 5–25.4 mm |
| Dimensional accuracy | Superior — tighter OD, roundness | Moderate — larger tolerances |
| Residual stress profile | Lower (cold expanded) | Higher (spiral forming) |
| High-pressure gas (X70+) | Widely specified | Limited — check project spec |
| Offshore / subsea | Standard | Generally excluded |
| Sour service (HIC) | Available (PSL2+) | Less common, check mill qual |
| Cost at same OD | Higher | Lower |
| Water transmission / piling | Suitable but over-specified | Preferred (cost-effective) |
Critical Engineering Point — SSAW Exclusions in Major Project Specs
Many international gas transmission project specifications (DNV-ST-F101 for offshore, various national operator standards) explicitly exclude SSAW pipe for high-pressure service above a stated design pressure or pipe grade. If a project spec says "SAW pipe," do not assume SSAW is permitted. Confirm with the client or engineer-of-record. Supplying SSAW where DSAW is specified is a material non-conformance that can trigger full pipe replacement — a commercially catastrophic outcome.
8. Applications
Onshore Gas Transmission
DSAW pipe is the standard choice for long-distance, high-pressure natural gas transmission in diameters from 20 inches to 56 inches. Grades X65 and X70 are the workhorses of modern gas trunk systems, providing the combination of wall efficiency (thinner walls at higher yield strength) and weld quality needed for design pressures above 70 bar. The straight longitudinal seam simplifies pipe tracking, coating inspection, and in-service intelligent pigging.
Offshore Pipelines & Risers
Deepwater and shallow-water export pipelines, J-lay and S-lay installations, and rigid steel risers use DSAW pipe to API 5L PSL2, typically with supplementary requirements per DNV-ST-F101 or project-specific annexes. The tight OD and roundness tolerances from UOE forming are critical for automatic orbital girth welding and for controlling buckle propagation in deepwater installations. Wall thicknesses of 20–38 mm in X65/X70 are common for deepwater gas export lines.
Crude Oil Trunk Lines
Major onshore crude export pipelines — export lines from oilfields to loading terminals, cross-border pipelines, and national grid crude systems — use DSAW pipe in diameters from 30 to 48 inches. The consistency of the double-pass weld and the availability of heavy walls in large diameters make DSAW the practical choice for these systems, which are designed for decades of uninterrupted service.
Structural Piling (Large Diameter)
Where structural piling requirements call for large diameters with relatively heavy wall — harbour structures, offshore jacket foundations, bridge piers — DSAW pipe manufactured to ASTM A252 or EN 10219 is used. These applications tolerate wider dimensional tolerances than API 5L pipeline pipe and are often produced on the same JCOE mills using the same SAW welding sequences.
Field Note — DSAW in African and Middle East Pipeline Projects
ZC Steel Pipe has supplied API 5L DSAW pipe to pipeline projects across West Africa and the Middle East, where X65 and X70 heavy-wall pipe in 24–48 inch diameters is standard for new-build export systems. In these markets, third-party inspection at the mill (typically by SGS, Bureau Veritas, or TÜV) is routinely required by the engineering contractor. ZC's PSL2 production programme includes full MTR traceability, heat-by-heat chemistry verification, and serialised weld seam UT records that can be delivered to the client's EDMS. Contact Mandy for project-specific qualification packages.
9. Frequently Asked Questions
What does DSAW stand for?
DSAW stands for Double Submerged Arc Welded. It describes a two-pass weld sequence in which a submerged arc weld is first applied to the inside of the pipe seam (inside weld, IW) and then to the outside (outside weld, OW). The "submerged arc" part means the arc is buried beneath a layer of granular flux, shielding the weld pool from air contamination. DSAW is the standard technique for heavy-wall LSAW pipe — effectively the same physical product, described from the welding process angle rather than the pipe geometry angle.
What is the difference between DSAW and LSAW pipe?
LSAW (Longitudinal Submerged Arc Welded) is the broad category covering all large-diameter pipe with a straight longitudinal seam made by the SAW process. DSAW is the specific weld technique within LSAW where both an inside and outside weld pass are applied. Single-pass LSAW (outside weld only) is used on lighter walls; DSAW is standard for wall thicknesses typically above 10 mm because full-penetration fusion from a single outside pass becomes unreliable at heavier walls. In commercial practice and on mill certificates, the two terms are used interchangeably for heavy-wall line pipe.
What sizes does DSAW pipe come in?
DSAW pipe is produced in outside diameters from 406 mm (16 inches) up to approximately 1626 mm (64 inches) depending on the mill's forming capability (JCOE or UOE). Wall thickness ranges from 6 mm to 40+ mm. Standard delivery length is 12 metres, with a range of 6–18 m available. Common API 5L grades are X52, X56, X60, X65, X70, and X80. ZC Steel Pipe can supply DSAW pipe in all of these grades and sizes — contact us with your OD, WT, grade, and quantity for a project-specific quotation.
What NDT is required for DSAW pipe under API 5L?
Under API 5L PSL2 — the level required for most gas transmission and offshore applications — DSAW pipe requires: full-length automated ultrasonic testing (AUT) of the weld seam; radiographic testing (RT) at both weld ends (minimum 50 mm); pipe body ultrasonic testing for laminations; hydrostatic pressure testing; and Charpy V-notch impact testing at the specified temperature. PSL1 requires hydrostatic testing and mechanical testing but does not mandate full-length weld seam UT unless project-specified. For sour service or offshore applications, project specifications typically layer additional requirements on top of API 5L minimums.
When should I specify DSAW instead of seamless pipe?
Specify DSAW pipe when the required OD exceeds the practical seamless range — generally above 24 inches (610 mm) — or when the combination of large diameter and heavy wall is needed (seamless mills cannot economically produce large-diameter heavy-wall pipe). DSAW is also appropriate for long-distance transmission pipelines where consistent weld quality across thousands of joints is critical, and where budget and lead time favour plate-based manufacturing. Seamless pipe is preferred for smaller diameters in high-pressure sour service, where the absence of a weld seam eliminates the preferential corrosion risk at the HAZ.
What is the difference between DSAW and SSAW pipe?
DSAW pipe has a straight longitudinal weld seam; SSAW (spiral/helical SAW) has a helical seam produced by rolling strip at an angle to the pipe axis. DSAW offers superior dimensional accuracy, better roundness, lower residual stress (after cold expansion), and is the required choice for offshore, sour service, and high-pressure gas transmission. SSAW is more cost-effective for large-diameter lower-pressure applications such as water transmission and structural piling. Major project specifications typically exclude SSAW for high-pressure gas above a stated design pressure or pipe grade.
Request DSAW Pipe — ZC Steel Pipe
ZC Steel Pipe manufactures DSAW pipe to API 5L PSL1 and PSL2 in grades X52 through X80, OD 406–1626 mm, WT 6–40 mm. JCOE and UOE forming. Full NDE, hydrostatic testing, and serialised MTRs as standard. Third-party inspection (SGS, BV, TÜV) accommodated. We have completed DSAW line pipe supply to projects in West Africa, the Middle East, and South America.
mandy.w@zcsteelpipe.com
WhatsApp: +86-139-1579-1813
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