What a Large Diameter Thick Wall Spiral Welded Steel Pipe Is
A spiral welded pipe, also called SSAW pipe, starts as hot rolled steel coil rather than as a solid billet. The coil is uncoiled, levelled and edge milled, then fed at an adjusted forming angle into the spiral forming unit, where the strip is bent helically into a cylinder. The two edges of the strip are joined by automatic double sided submerged arc welding, one pass on the inside and one on the outside, which is why the product is also described as double side submerged arc welded pipe. Because the finished diameter is set by strip width and forming angle rather than by a die, the process is economical for large outside diameters and heavy walls that would be difficult to produce by other routes.
In practice a large diameter thick wall spiral welded steel pipe means an outside diameter from about 508 mm upwards, with wall thickness from roughly 8 mm to 25 mm. The same line can produce pipe from about 219 mm up to 3620 mm outside diameter by changing strip width and forming angle, which is what makes the process attractive for transmission mains, water and slurry lines and structural piling.
Standards and Steel Grades
The governing standard is agreed at the enquiry stage, because it fixes the grade range, the product specification level and the inspection regime that will be applied.
| Standard | Typical use of spiral welded pipe | Grades commonly ordered |
|---|---|---|
| API 5L | Oil, gas and water transmission lines, PSL1 and PSL2 | L245 up to L555, equivalent to Grade B up to X80 |
| GB/T 9711 | Chinese line pipe specification aligned with API 5L | L245 up to L555 |
| SY/T 5037 | Water supply, drainage and lower pressure service | Q235B and Q355B class structural steels |
| ASTM A252 | Structural and foundation piling | Grade 1, Grade 2 and Grade 3 |
| ASTM A139 | General purpose fluid and structural service | Electric fusion welded grades from Grade B upwards |
| EN 10219-1 | Cold formed welded structural hollow sections | S235JRH, S275J0H and S355J2H |
Chemical composition and carbon equivalent are not quoted from memory. They are read from the tables of the ordered standard for the specific grade and product specification level. For PSL2 pipe, carbon equivalent limits and the mandatory notch toughness requirements of API 5L apply in addition to the tensile requirements, and the impact test temperature is stated in the order.
Dimensional Range, Tolerances and Testing
| Item | Typical spiral welded range | Controlled by |
|---|---|---|
| Outside diameter | 219 mm to 3620 mm | Strip width and forming angle |
| Wall thickness | 5 mm to 25 mm | Coil thickness and edge preparation |
| Length | 6 m to 12 m, fixed or random | Ordering length and cutting plan |
| Pipe ends | Plain, bevelled or prepared weld end | End finish stated in the order |
| Hydrostatic test | Every pipe held at the pressure calculated from its size under the standard | API 5L or the ordered specification |
| Weld examination | Continuous non destructive examination of the spiral seam | API 5L with ISO 10893 series methods |
Dimensional tolerances for outside diameter, wall thickness, ovality, straightness and length are taken from the tolerance clause of the applicable standard. Wall thickness tolerance for submerged arc welded pipe is expressed as a band around nominal thickness, and the designer should note that the minus tolerance affects the calculated design pressure, so pressure design is normally carried out on the minimum wall rather than on the nominal wall.
Where Spiral Welded Pipe Is Used
Water transmission and drainage mains, including large diameter trunk lines for municipal supply.
Oil, gas and product pipelines at moderate pressure, where a line pipe grade and PSL2 toughness are specified.
Slurry, tailings and dredging lines in mining and mineral processing.
Structural piling for bridges, docks, buildings and foundation tubes.
Large diameter closed sections used as piles, poles and building structures.
In each case the corrosion protection system is chosen together with the pipe: three layer polyethylene, fusion bonded epoxy, or an internal cement mortar lining for water service. Pipe is normally supplied with the coating already applied and with the weld ends left bare for field girth welding.
Ordering and Quality Control Points
A complete enquiry states outside diameter, wall thickness, length, steel grade and standard, product specification level, pipe end finish, coating system, hydrostatic test pressure or reference to the standard calculation, required non destructive testing level, and marking and traceability requirements. The mill then matches coil to the order and issues a mill test certificate for the finished pipe.
Points worth reviewing before shipment include coil quality, strip end and edge preparation, forming angle control, welding parameters for the inside and outside passes, the continuous seam examination records, hydrostatic test records, dimensional records and coating thickness measurements. Third party inspection by a nominated agency is common on transmission projects and is best arranged before production begins rather than at the end of the run.
FAQ
Q: How is spiral welded pipe different from straight seam welded pipe?
The strip is fed at an angle so the weld runs as a helix around the pipe, instead of running parallel to the axis. This allows a much larger diameter to be produced from a given coil width and distributes the seam load differently around the circumference.
Q: Which wall thickness should be ordered for a given design pressure?
Calculate the required minimum wall from the design pressure, diameter and allowable stress in the applicable standard, then add the manufacturing minus tolerance to arrive at the nominal wall to be ordered.
Q: Can spiral welded pipe be used for high pressure gas transmission?
Yes, provided the pipe is ordered to API 5L PSL2 with the required toughness, weld seam examination and hydrostatic testing, and provided the project specification accepts the spiral seam configuration for the design pressure involved.
Q: What length can be supplied?
Standard lengths run from 6 m to 12 m. Fixed lengths are cut to the project plan, which reduces field waste, while random lengths are cheaper to produce and are used where the line is cut and fitted on site.
Q: What coating options are available?
Three layer polyethylene and fusion bonded epoxy are the usual external coatings for buried transmission lines, and internal cement mortar lining or an epoxy lining is used for water and drainage service.
Q: How is the spiral seam examined on the production line?
The seam is examined continuously while the pipe is being made, using ultrasonic and radiographic methods as agreed in the order, with the sensitivity set from a reference standard and verified at defined intervals.
