Spiral Welded Pipe (SSAW): How It Is Made, Standards and Where It Fits

Quick Answer

Spiral welded pipe, also called SSAW or HSAW pipe and designated SAWH in API 5L and ISO 3183, is made by feeding hot-rolled steel strip into a forming line at an angle, so that it winds into a cylinder with a continuous helical seam, which is then submerged-arc welded inside and outside. Strip width B, pipe diameter D and the helix angle α between the strip and the pipe axis are linked by B = π × D × cos α. That is why one coil width can make many diameters, and why spiral pipe reaches very large sizes, up to around DN3000, from relatively narrow coil. The trade-off is weld length: a spiral pipe carries π × D / B metres of seam per metre of pipe, typically about 1.3 to 4 m, against 1 m for a single-seam longitudinal pipe. It is covered by API 5L / ISO 3183 for pipelines, AWWA C200 and EN 10224 for water, EN 10217 for pressure purposes, EN 10219 for structural sections and ASTM A252 for piles. It is the workhorse of large-diameter water, sewer, piling and structural work, and is used for onshore oil and gas lines where the project specification accepts helical-seam pipe.

A typical large-diameter water main tender ends with two kinds of steel pipe on the bid sheet: longitudinally welded (LSAW) and spiral welded (SSAW), with the spiral offer usually the cheaper of the two. The engineer then has to decide whether that is the same pipe at a lower price or a different product with different limits.

The answer is some of both. Spiral pipe is a mature, standardised product, and in many duties it is the better choice rather than just the cheaper one. It also has real differences — a longer seam, a different stress picture and a different reception in oil and gas — that should be understood before it is specified.

What Spiral Welded Pipe Is

Spiral welded pipe is formed from a continuous steel strip wound into a cylinder at an angle, like the cardboard tube inside a roll of kitchen paper, so the joint between turns runs round the pipe as a helix. That seam is welded by submerged arc welding (SAW) from the inside and the outside. The product goes by several names:

• SSAW — spiral submerged-arc welded

• HSAW — helical submerged-arc welded

• SAWH — the designation API 5L and ISO 3183 use for helical-seam submerged-arc welded line pipe; SAWL is the longitudinal-seam equivalent

• DSAW — double submerged-arc welded, a trade term for SAW pipe welded from both sides. It is not an API 5L designation and is used for both straight-seam and spiral pipe

How Spiral Pipe Is Made

1. Coil preparation. Hot-rolled coil is uncoiled and levelled. To keep the line running, the end of one coil is butt welded to the start of the next; standards and purchase specifications control whether such coil-end welds are accepted in finished pipe and how they are inspected.

2. Edge preparation. The strip edges are milled or trimmed to width and bevelled, so the inside and outside weld beads meet in a sound joint.

3. Forming. The strip is fed in at the helix angle and bent, commonly in a three-roll bending system, into a continuous, rotating pipe strand. The angle and the strip width together set the diameter.

4. Welding. In one-step (online) production, the helical seam is finish-welded by SAW inside and outside while the pipe is being formed. In two-step (offline) production, the forming line only tack-welds the seam at high speed, typically with gas metal arc welding; the strand is cut, and each length is finish-welded inside and outside by SAW on separate stations. Forming then runs at full speed while the slower, quality-critical welding is done under controlled conditions.

5. Cutting. A flying cutter travelling with the strand cuts it to length.

6. End finishing. Ends are squared and bevelled for field welding, or left plain for piles and mechanical joints.

7. Testing and inspection. Pressure pipe, such as line pipe and water pipe, is hydrostatically tested length by length; piling and structural standards such as ASTM A252 do not call for a hydrostatic test unless the order adds one. Typical mill practice adds automatic ultrasonic testing of the full seam, radiography to confirm indications and check pipe ends and repairs, and dimensional checks of diameter, out-of-roundness, straightness, length and wall. The product standard and the order decide which checks are mandatory, and to what acceptance level.

8. Coating and lining, where ordered.

The geometry: strip width, diameter and helix angle

Unroll a spiral pipe on paper and the seams become parallel straight lines, which makes the geometry simple:

B = π × D × cos α

where B is the strip width, D the pipe diameter and α the helix angle, the angle between the strip (and therefore the seam) and the pipe axis. Two useful results follow:

• Seam length per metre of pipe = π × D / B (which equals 1 / cos α)

• Axial advance per turn of the seam = π × D / tan α

Worked example. A 1,500 mm wide coil making 1,219 mm (NPS 48) pipe:

• cos α = 1,500 / (π × 1,219) = 0.392, so α ≈ 66.9°

• Each turn of the seam advances about 1.63 m along the pipe

• The pipe carries about 2.55 m of weld per metre of length

The same coil can make many diameters by changing the angle:

This is spiral pipe's main commercial strength: one coil width and one forming line cover a wide range of diameters, including sizes no plate mill could supply as a single-seam pipe. A single-seam longitudinal pipe of 2,032 mm OD needs a plate as wide as its circumference, about 6.4 m, which is why very large LSAW pipe has two seams.

These are geometric figures. In production the mill adjusts the angle continuously for springback and small variations in strip width, which is how the diameter is held within tolerance.

Why the Helical Seam Carries Less Hoop Stress

In a pressurised pipe the hoop stress is the largest stress, and a longitudinal seam carries all of it at right angles to the weld. A helical seam is inclined to the hoop direction, so the stress acting across it is lower:

σn = σh × cos²α + σa × sin²α

where σh is the hoop stress, σa the axial stress and α the helix angle to the pipe axis. For the 1,219 mm example (α ≈ 66.9°):

Two cautions apply. The reduction only covers pressure loading: axial stress from bending, ground movement or temperature change acts more directly across a helical seam than across a longitudinal one. And design codes do not credit it; the pressure rating is calculated on the hoop stress in the wall, whatever the seam orientation.

Standards for Spiral Welded Pipe

The service decides the standard, not the pipe type: a water utility calls for AWWA C200 or EN 10224, a pipeline operator for API 5L, a piling designer for ASTM A252 or EN 10219. The same mill line can produce to all of them; the differences are in chemistry, strength, testing, tolerances and documentation. For API 5L the product specification level matters most, since PSL2 adds tighter chemistry and carbon-equivalent limits, mandatory notch-toughness testing, maximum strength limits and more non-destructive testing. See our guides to API 5L grades, PSL1 and PSL2 and carbon steel pipe grades and standards.

SSAW vs LSAW vs ERW vs Seamless

LSAW pipe made by the UOE or JCOE route is mechanically expanded at the end of the line, which rounds it and sets the diameter closely. Spiral pipe gets its diameter in forming, so its roundness and residual stresses depend on how well forming is controlled. For field-welded pipelines, specify diameter and out-of-roundness tolerances at the pipe ends: they decide how well the girth welds fit up.

For oil and gas, check the project specification before offering spiral pipe. API 5L covers SAWH pipe, but some operators restrict helical-seam pipe for sour service, offshore lines or the highest pressures, while others accept it with additional testing. Sour service also brings its own material requirements; see our guide to NACE MR0175 / ISO 15156. For the wider comparison, see seamless vs welded steel pipe.

Pressure Rating: How It Is Worked Out

Spiral pipe is rated like any other steel pipe, with the Barlow formula and a design factor set by the governing code:

P = 2 × S × t / D × F × E × T

where S is the specified minimum yield strength, t the wall, D the outside diameter, F the design factor, E the longitudinal joint factor and T the temperature derating factor. US gas pipeline rules use F = 0.72 for Class 1 locations down to 0.40 for Class 4, and give API 5L submerged-arc welded pipe a joint factor of 1.00. Water pipe designed to AWWA M11 limits working stress to 50% of minimum yield, with up to 75% for working pressure plus surge.

Worked example: OD 1,219 mm × 10 mm wall

Three points matter more than the formula:

• The code sets the factor. The same pipe is rated very differently under a gas code, a water manual or a process piping code; process piping codes such as ASME B31.3 apply their own weld joint quality factors to welded pipe.

• Large D/t ratios bring other limits. At a diameter-to-thickness ratio of about 120, as here, soil load, vacuum and handling often govern the wall thickness rather than internal pressure.

• The mill hydrostatic test is a check, not a margin. It does not change what the code allows, and on spiral pipe it loads the seam less than service may, because most mill testers carry the end load. Full-length ultrasonic testing of the seam matters all the more.

Coatings and Linings

Large-diameter steel pipe is almost always protected inside and out.

External coatings

• Three-layer polyethylene (3LPE) — epoxy primer, adhesive and polyethylene, to ISO 21809-1 or DIN 30670; a common choice for buried pipelines outside North America, where FBE is more usual

• Fusion-bonded epoxy (FBE) — to ISO 21809-2; a thinner single layer, and the base layer of multi-layer systems

• Liquid epoxy and polyurethane — for fittings, specials, repairs and some water pipe

• Cement mortar coating — for water pipe, shop applied, for example to AWWA C205

Internal linings

• Cement mortar lining — the traditional lining for water mains, shop applied to AWWA C205 or EN 10298. The alkaline mortar keeps the steel passive and gives a smooth bore

• Liquid epoxy lining — for potable water, wastewater and some industrial fluids; for drinking water, check the approvals required in the country of use

Field joints need their own system, such as heat-shrink sleeves outside and mortar or epoxy inside, and the order should state the coating cutback at each pipe end. Buried pipelines with dielectric coatings normally also get cathodic protection; whether mortar-coated water pipe needs it depends on the soil. See our guides to pipeline coatings and cathodic protection.

Where Spiral Pipe Is Used

• Water — raw and treated water mains, pumping mains, intake and outfall lines

• Sewer and drainage — gravity and pressure sewers, culverts, stormwater outfalls and treatment plant pipework

• Piling and foundations — bearing piles, concrete-filled piles and combined walls for ports and bridges

• Structural — columns, trusses and other hollow members

• Hydropower — penstocks carrying water to the turbine; see our guide to Francis turbine runners

• Oil and gas — onshore lines where the project specification accepts helical-seam pipe

• Industrial — cooling water, slurry and process lines, and large ducts

At the smaller end of the range, compare steel with HDPE pipe on pressure, stiffness, jointing and installed cost.

How to Order Spiral Pipe

Put these on the enquiry:

1. Outside diameter and wall thickness in millimetres (or inches), not DN alone

2. Standard, grade and level — for example API 5L PSL2 L415M/X60M, EN 10224 L275 or ASTM A252 Grade 3

3. Length — exact or random, and the maximum length transport allows

4. End finish — bevelled for welding, plain, or prepared for a mechanical or socket joint

5. Coating and lining — system, thickness, standard and cutback length

6. Tolerances that matter for installation — end diameter and out-of-roundness for field welding, straightness for piles

7. Testing and inspection — hydrostatic test, NDT, impact testing where required, third-party inspection, and the certificate type (EN 10204 3.1 or 3.2)

8. Marking and documentation

9. Quantity and delivery schedule

A complete line reads like this: OD 1,219 × 12.7 mm, API 5L PSL2 L415M/X60M SAWH, 12 m lengths, bevelled ends, external 3LPE to ISO 21809-1 with 150 mm cutbacks, EN 10204 3.2 certificates.

For transport planning, the mass per metre of steel pipe is about (D − t) × t × 0.0247 kg, with D and t in millimetres. An 18 m length of 1,219 × 10 mm pipe weighs about 5.4 tonnes before coating.

Common Mistakes When Specifying Spiral Pipe

1. Ordering by DN only. DN is a nominal size, not a dimension.

   Prevention: State the outside diameter and wall in millimetres, and the standard that defines the tolerances.

2. Applying the wrong design rules. A gas design factor on a water main, or a water allowable stress on a gas line, gives the wrong wall.

   Prevention: Calculate the wall to the code that governs the project, and state that code on the enquiry.

3. Sizing thin walls for pressure only. A wall that is adequate for pressure can ovalise or buckle under soil load, vacuum or handling at a high D/t ratio.

   Prevention: Check ring stiffness, buckling and handling for buried and large-diameter pipe, not just internal pressure.

4. Offering spiral pipe into an oil and gas project without checking the specification. Some operators restrict helical-seam pipe for sour, offshore or high-pressure service.

   Prevention: Confirm that SAWH pipe is accepted, and what extra testing applies, before bidding.

5. Not asking about coil-end welds. A coil-end weld in a delivered pipe is a second seam the inspector needs to know about.

   Prevention: Ask whether coil-end welds may appear in delivered pipe, and how they are inspected and marked.

6. No end tolerances for field welding. Out-of-round ends cause high-low misalignment at the girth welds.

   Prevention: Specify end diameter and out-of-roundness tolerances, and agree end-matching where needed.

7. Coating without cutbacks or a field joint system. The joints are left to whatever the site has.

   Prevention: Specify cutback lengths and the field joint coating together with the pipe coating.

8. Treating the hydrostatic test as a substitute for NDT. A pipe can hold pressure and still contain weld defects.

   Prevention: Specify full-length ultrasonic testing of the seam and the acceptance standard, and the certificate type (EN 10204 3.1 or 3.2) that must report it.

Supply from Kasko Makine

Kasko Mak branded STS2 spiral pipes

• Spiral welded pipe: steel strip continuously formed into a helical seam, then submerged arc welded on the inside and outside

• Diameters DN200 to DN3000

• Wall thickness 5 mm to 25 mm

• Lengths up to 18 m per section

• Coating options: epoxy, polyethylene and cement mortar lining

• Each pipe hydrostatically tested, with ultrasonic weld inspection and dimensional verification

• Material certificates and inspection reports with every delivery

• For infrastructure, drainage and industrial projects

Engineering support: Send the outside diameter and wall thickness, the standard and grade your specification calls for, the lengths, end finish, coating and lining, and the quantity. We will check the pipe against your specification and quote.

Send your pipe schedule for a spiral pipe quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

• Seamless vs Welded Steel Pipe

• API 5L Line Pipe: Grades, PSL1 and PSL2

• Pipeline Coatings: 3LPE, FBE and How to Choose

• Cathodic Protection: Sacrificial and Impressed Current

• API 5CT Casing and Tubing: Grades, Sizes and Connections

Frequently Asked Questions

Q: What is spiral welded pipe?
A: Spiral welded pipe, also called SSAW or HSAW pipe and designated SAWH in API 5L and ISO 3183, is steel pipe made by winding hot-rolled strip into a cylinder at an angle so the joint forms a continuous helix, then welding that seam by submerged arc welding inside and outside. It is made in large diameters, up to around DN3000, and is widely used for water mains, sewers, piles and structural work.

Q: How is spiral welded pipe manufactured?
A: Hot-rolled coil is uncoiled, levelled and edge-milled, then fed into a forming section at the helix angle and bent into a continuous pipe. In one-step mills the helical seam is submerged-arc welded inside and outside during forming; in two-step mills it is tack welded at high speed, cut to length and finish-welded offline. Pressure pipe is then hydrostatically tested length by length, the seam is ultrasonically inspected, and each length is measured and, if ordered, coated and lined.

Q: What is the difference between SSAW and LSAW pipe?
A: LSAW pipe is formed from plate with one or two straight seams and is usually mechanically expanded, which gives close dimensional control; it is the usual choice for high-pressure and offshore pipelines. SSAW pipe is formed from coil with a helical seam, so one coil width can make many diameters at lower cost, but it carries more weld per metre. SSAW is widely used for water, sewer, piling and onshore lines.

Q: Which standards cover spiral welded pipe?
A: The main ones are API 5L and ISO 3183 for line pipe, where helical-seam pipe is designated SAWH; AWWA C200 and EN 10224 for water pipe; EN 10217 for welded tubes for pressure purposes; EN 10219 for structural hollow sections; ASTM A252 for pipe piles; and ASTM A139 for arc-welded pipe carrying liquids, gas and vapour. The service decides which standard applies, not the pipe type.

Q: Can spiral welded pipe be used for oil and gas pipelines?
A: Yes. API 5L covers helical-seam submerged-arc welded pipe, and spiral pipe is used for onshore gathering, transmission and distribution lines. US gas pipeline rules give API 5L submerged-arc welded pipe a joint factor of 1.00. Acceptance still depends on the project, because some operators restrict helical-seam pipe for sour, offshore or the highest-pressure service, so confirm the specification before ordering.

Q: How is the pressure rating of spiral welded pipe calculated?
A: With the Barlow formula, P = 2 × S × t / D, multiplied by the design factor of the governing code. For a 1,219 × 10 mm L360 pipe, a gas pipeline design factor of 0.72 gives about 42.5 bar, while the same pipe designed as a water main to AWWA M11, at 50% of minimum yield, gives about 29.5 bar. The code, not the pipe type, sets the factor.

Q: What coatings and linings are used on spiral welded pipe?
A: External coatings are usually three-layer polyethylene to ISO 21809-1 or DIN 30670, fusion-bonded epoxy, liquid epoxy or polyurethane, or cement mortar for water pipe. Internal linings are typically cement mortar to AWWA C205 or EN 10298, or liquid epoxy. The order should also specify the coating cutbacks at the pipe ends and the field joint coating system applied after girth welding.