Seamless vs ERW Pipe for Oil & Gas Water Lines: When It Matters

2026-10-22 · Pipe & Tube · 5 min read

On an oil and gas site, "seamless only" is often written into a water-line enquiry out of habit, not because the service needs it. Modern high-frequency ERW pipe is a fully qualified product under API 5L and ASTM A53, and for most firewater, utility and low-pressure produced-water lines it does the job at a lower cost and with more uniform walls. Seamless earns its premium in a narrower set of cases. This guide sorts them out so you can quote, or challenge, a specification with confidence.

Two ways of making the same pipe

Seamless (SMLS) pipe starts as a solid round billet that is heated, pierced and rolled over a mandrel into a hollow. There is no longitudinal weld. Because the hole is formed by piercing, wall thickness can vary slightly around the circumference.

ERW pipe is formed from hot-rolled strip, curled into a tube and welded along its length by electric resistance. Current API 5L practice uses high-frequency welding (HFW), and the standard requires the full length of the weld seam to be inspected by ultrasonic or electromagnetic testing. Because the starting strip is rolled to tight gauge, ERW walls are usually more even than seamless.

The design code decides more than the mill process

The most useful single fact for a buyer: whether ERW is penalised depends on which piping code the line is designed to.

Under B31.3, an ERW pipe needs roughly 18% more pressure-design wall than seamless of the same grade (1 ÷ 0.85 ≈ 1.18). For small plant water lines that often means one schedule heavier, which can erase the price gap. Under the pipeline codes, ERW and seamless of the same grade and wall carry the same pressure rating, so ERW is the normal choice for long water pipelines.

Grades and standards you will see on the drawing

Note that A106 is a seamless, high-temperature standard. If a drawing calls for A106 on an ambient-temperature firewater line, ask whether A53 Grade B or API 5L Grade B would be acceptable; the strength is the same. For plain and galvanized water pipe to A53 and BS 1387, see our BS 1387 vs ASTM A53 comparison.

Where ERW is the sensible default

• Firewater ring mains and hydrant lines on onshore plants, depots and terminals, typically API 5L Grade B or A53 Grade B, often internally lined or galvanized.
• Utility, cooling and service water at moderate pressure and ambient temperature.
• Low-pressure produced-water transfer where the fluid is not classed as sour and a corrosion allowance or internal lining is specified.
• Long water pipelines designed to B31.4, where ERW has a joint factor of 1.00 and larger diameters are readily available.

When seamless is really needed

High-pressure injection and wellhead lines

Water-injection lines near the wellhead run at high pressure and often call for heavy walls in small diameters. Seamless mills cover heavy wall-to-diameter ratios that ERW lines cannot form, and B31.3 piping avoids the 0.85 factor.

Sour produced water

If the water contains H₂S, the line falls under NACE MR0175 / ISO 15156 and pipe is ordered as API 5L PSL2 with Annex H. Both seamless and HFW can qualify, but the welded seam adds a zone that must meet the hardness limit (22 HRC for carbon steel) and pass HIC testing to NACE TM0284. Many operator specifications simply require seamless for sour service in smaller sizes. Follow the project specification here; do not substitute.

High temperature

Hot water and steam lines in process units are normally A106 seamless. Galvanized pipe should not be used in hot water service.

Older pipe and seam corrosion

Some older low-frequency ERW pipe suffered selective corrosion along the weld line in aggressive water. Modern HFW pipe with a heat-treated seam (mandatory for API 5L PSL2 welded pipe) is far less prone to this, but some clients remain cautious for corrosive produced water. Ask the end user before quoting ERW for that service.

Quick decision table

Buying checklist

1. State the design code (B31.3, B31.4 or B31.8); it tells the mill and you whether ERW wall must be increased.
2. Give the full specification: standard, grade, PSL level, any annex (H for sour, J for offshore), OD and wall or schedule.
3. Write the manufacturing type explicitly: SMLS, or ERW/HFW. "Seamless or ERW" invites the cheapest option.
4. Specify end finish (bevelled for welding, plain, or threaded and coupled), length (single or double random) and coating or lining.
5. For ERW, require full-length weld seam NDT records; for all pipe, hydrostatic test records and an EN 10204 3.1 mill certificate showing heat number, chemistry and mechanicals.
6. For sour service, add hardness and HIC test reports to the document list before ordering, not after shipment.

Weights per metre for galvanized water pipe are in our BS 1387 weight chart. For quotations, see our ERW and seamless steel pipe and seamless alloy pipe pages and send your line list with design code and service.

Frequently asked questions