Steel pipes are the backbone of the oil and gas industry, and they are used in every stage, from extraction to processing to transportation.
Selecting the correct type of pipe is critical for ensuring safe, efficient, and reliable operations.
The two main categories of steel pipes are seamless and welded, each with unique characteristics and applications.
In this article, we will provide a comprehensive comparison of seamless and welded pipes in the context of the oil and gas industry.
We will cover the manufacturing processes, each type's essential properties and advantages, and the most common applications.
We will also discuss the various material options, including carbon steel, alloy steel, stainless steel, and nickel alloys, and guide on selecting the correct grade for different environments.
Finally, we will delve into the practical aspects of specifying and ordering pipe, including the relevant standards, dimensions, and end finishes.
By the end of this article, you will thoroughly understand the differences between seamless and welded pipes and be equipped to make informed decisions for your oil and gas projects.
Pipe Types Explained
The oil and gas industry uses three main steel pipe types: Seamless, ERW (Electric Resistance Welded), and LSAW (Longitudinal Submerged Arc Welded).
Seamless pipes are made from solid steel billets that are heated and pushed or pulled over a form to create a hollow tube without any welded seams.
They are commonly used for high-pressure applications in upstream operations like drilling and exploration, midstream fluid transmission, downstream refining, and utility services.
ERW pipes are made using cold-forming, shaping steel coils into cylindrical forms, and the seam is welded together using an electric current.
The weld seam runs along the length of the pipe. ERW pipes are cost-effective for low-to-medium-pressure applications, such as transporting water, oil, and gas.
LSAW pipes are made by bending and welding steel plates along the length, with the seam running straight (longitudinal seam) or in a spiral.
They are used for large-diameter pipes (16-60 inches) in critical applications, such as long-distance oil and gas pipelines crossing through cities or underwater.
The table below summarizes the key differences between these pipe types and their typical applications in the oil and gas industry:
| Pipe Type | Manufacturing | Pressure Rating | Typical Applications |
|---|---|---|---|
| Seamless | No weld seam | High | Upstream, midstream, downstream |
| ERW | One straight seam | Low-medium | Water, oil & gas transport |
| LSAW | 1-2 straight or spiral seams | Low-medium | Water, oil & gas transport |
Manufacturing Process for Seamless, ERW, and LSAW Pipes
Seamless Pipe Production
Seamless pipes are made through a hot working process without welding. The main steps are:
A solid round steel billet is heated in a rotary hearth furnace to around 1204C/2200F.
The red-hot billet is then pierced through the center using a piercer point to create a hollow tube.
The hollow tube passes through a plug mill, where a mandrel is inserted. The tube is then rolled to reduce the wall thickness and increase the length.
The pipe is then passed through sizing stands to achieve the final dimensions.
After cooling, the pipe is straightened, cut to length, tested, and shipped.
Seamless pipes can be further cold drawn to produce pipes with smaller diameters and thinner walls.
ERW Pipe Production
ERW pipes are made by cold-forming steel coil and welding the seam:
The appropriate width and gauge steel coil is selected and loaded onto an uncoiler.
The strip passes through rollers, progressively forming it into a round shape.
The longitudinal edges are heated by passing a high-frequency current between them.
Rolls press the edges together to create a fusion weld without adding filler metal.
The welded pipe passes through sizing stands to achieve the final diameter.
The pipe is cut to length, the ends are formed (plain, bevelled, threaded, etc.), and the pipe is tested before shipping.
The entire ERW process is continuous, with the pipe moving at a constant speed through the forming and welding stands.
LSAW Pipe Production
LSAW pipes are made from steel plate that is formed and welded:
Steel plates are selected, and the edges are milled to prepare them for welding.
The plate is pre-bent on a press brake to start forming it into a round shape.
A U-press forms the plate into a U-shape, and an O-press closes the U into a tube.
The longitudinal seam is initially tack welded on the inside to hold the edges together.
The pipe is then passed through automatic welding stations that lay down multiple passes of submerged arc welds inside and outside the seam.
A mechanical expander expands The welded pipe slightly to achieve a proper round shape.
The pipe is cut to length, bevelled, tested hydrostatically, and inspected before shipping.
LSAW pipes are generally used for larger diameters (16" and above) and have thicker walls than ERW pipes.
Pros and Cons of Seamless, ERW, and LSAW Pipes
Seamless Pipes
Pros:
No weld seam eliminates potential weak points and allows for higher pressure ratings
A smooth interior surface reduces friction and turbulence for better flow characteristics
Superior strength, durability and resistance to bending and impact forces
Excellent corrosion resistance due to lack of weld seam
Handles high temperatures and pressures well, making it ideal for critical oil & gas, chemical, and power applications
Cons:
More expensive than welded pipes due to complex manufacturing process
Limited size range, with challenges producing very small or very large diameters
Longer lengths are complicated, so size options may be more limited than welded pipes
Thicker walls and heavier weight than welded pipes
ERW Pipes
Pros:
Lower cost than seamless due to a more straightforward and more automated welding process
Available in longer lengths since there are no sizing restrictions like seamless
Thinner walls and lighter weight than seamless while still maintaining good strength
Smooth interior and exterior surfaces
Suitable for general low to moderate-pressure applications like utility and construction
Cons:
Weld seam is a potential weak point vs seamless, lowering pressure ratings
Weld is more susceptible to corrosion than seamless body
Extensive testing of weld seam required to ensure integrity
Not suitable for critical high-pressure, high-temperature, or severe service applications
LSAW Pipes
Pros:
Handles higher pressures and mechanical loads than ERW due to heavier walls and multiple weld passes
Sizing flexibility-a common choice for large diameter pipes over 16"
Tight tolerances and good dimensional control
Suitable for critical applications like long-distance oil & gas transmission lines
Cons:
It is more expensive than ERW due to heavier materials and specialized welding process
Potential for weld defects if process not adequately controlled, requiring rigorous inspection and testing
Thick and heavy walls increase material costs vs ERW
Size and length limitations vs ERW
In summary, seamless pipes provide the highest strength, pressure capacity, and corrosion resistance, making them ideal for demanding oil & gas, chemical, and power plant applications.
The tradeoffs are higher cost and more limited size options.
ERW is a cost-effective choice for general utility and construction applications where the weld seam is not a major integrity concern.
It offers a wide range of sizes and long lengths.
LSAW is a heavy-duty welded option for large-diameter, high-pressure pipelines requiring the utmost welding quality and inspection.
The multi-pass welds provide strength approaching seamless.
The key is selecting the right product based on a thorough application evaluation and the environment's pressure, temperature, corrosivity, and criticality.
Choosing the Ideal Steel Types for Oil & Gas Pipes
Alloy Steel
Alloy steels contain alloying elements beyond iron and carbon to enhance strength, toughness, corrosion resistance, or high-temperature performance.
Low alloy steels (less than 8% alloy content) are commonly used in the oil and gas industry for:
Drill pipes and drill collars
Casing and tubing
Valves, wellheads, and Xmas trees
Fasteners like nuts, bolts, and studs
The alloying elements impart higher strength, allowing thinner walls to be used, which reduces weight and cost. Some alloys also resist hydrogen sulphide (H2S) cracking and corrosion.
Carbon Steel
Carbon steels contain mainly iron and carbon, with only residual amounts of other elements.
They are categorized as low, medium, or high carbon based on carbon content. Carbon steels are widely used in oil and gas for:
Flowlines and pipelines
Storage tanks and pressure vessels
Structural components like beams, plates, and tubular
Low-pressure piping systems
Carbon steels are strong, tough, and relatively inexpensive.
However, they have poor corrosion resistance and are susceptible to H2S cracking in sour service. Inhibitors or coatings are often required.
Stainless Steel
Stainless steels contain at least 10.5% chromium, which forms a protective oxide layer on the surface, providing excellent corrosion resistance.
Grades used in oil and gas include:
Austenitic (300 series): 304, 316, 317, 321, 347
Ferritic: 405, 430
Martensitic: 410, 420, 431
Duplex: 2205, 2507 super duplex
Stainless is used for critical components exposed to seawater, produced water, CO2, chlorides and mildly sour environments:
Subsea equipment
Wellhead and Xmas tree components
Heat exchangers and pressure vessels
Piping, valves, and instrumentation
The high strength of duplex grades allows thinner walls, while super austenitic and super duplex provide resistance to aggressive conditions.
However, stainless has a higher initial cost than carbon steel.
Nickel Alloys
Nickel alloys contain high levels of nickel and other elements like chromium and molybdenum for maximum corrosion resistance, even in hot, acidic, and highly sour environments. Common grades include:
Nickel-copper: Monel 400, K-500
Nickel-chromium-iron: 825, 625, 718
Nickel-chromium-molybdenum: C-276, 2550, 686
Nickel-iron-chromium: 800, 800H, 800HT
Nickel alloys are used for the most demanding applications where stainless steels are inadequate:
Sour gas wells and flowlines
Subsea manifolds and jumpers
Downhole tools and safety valves
Vessels and piping in refineries and gas plants
The excellent corrosion resistance and stability at high temperatures come at a significantly higher cost than stainless steels.
Nickel alloys are reserved for severe environments that would rapidly degrade other alloys.
Carbon steel is an economical choice for many oil and gas applications, while alloy steels provide enhanced strength and toughness when needed.
Stainless steels resist corrosion in aqueous environments, while nickel alloys extend performance in hot, acidic, and sour conditions.
The fluid composition, temperature, and pressure must be carefully evaluated to select the optimum alloy for long-term reliability and safety.
Understanding Seamless and Welded Pipe Dimensions and Sizes
Nominal Pipe Size (NPS)
Pipe size is typically specified by the nominal pipe size (NPS), which is a dimensionless designator. For sizes 1/8 to 12 inches, the NPS corresponds to the pipe's approximate inside diameter (ID) in inches, using common fractions. For example, a NPS 2 pipe has an ID of roughly 2.1 inches.
For sizes 14 inches and above, the NPS is the approximate outside diameter (OD) in inches. So, a NPS 20 pipe has an OD of about 20 inches.
It's important to note that the exact OD and ID will vary depending on the wall thickness specified.
Wall Thickness
The wall thickness determines the pipe's pressure rating and weight. It is typically specified by the schedule (Sch) number or the actual thickness in inches or millimetres.
Standard schedule numbers are 10, 20, 30, 40, 60, 80, 100, 120, 140, and 160. The higher the number, the thicker the wall. For example, an NPS 6 Sch 40 pipe has a wall thickness of 0.280", while an NPS 6 Sch 80 has a wall thickness of 0.432".
For a given NPS, pipes with different schedules will have the same OD but different IDs. The OD remains constant, and the ID decreases as the wall thickens.
Relevant ASME Standards
The ASME (American Society of Mechanical Engineers) publishes several standards that cover the dimensions of pipes made from various materials:
ASME B36.10: Welded and seamless carbon steel and alloy steel
ASME B36.19: Stainless steel
ASME B36.20: Metallic material for general applications
ASME B36.21: Nonmetallic material for general applications
These standards provide dimensions tables for NPS 1/8 to 80, specifying the OD, ID, wall thickness, weight per foot, and other critical parameters for each combination of NPS and schedule or wall thickness.
The standards ensure consistent dimensions across manufacturers, although some variation is allowed within the specified tolerances.
Referring to the relevant standard when specifying or ordering pipe is crucial to ensure the dimensions meet the application requirements, piping code, and regulations.
Seamless and Welded Pipe Lengths and End Finishes
Pipe Lengths
Steel pipes are commonly available in three length options:
Single random lengths (SRL) are the most common and economical option. The exact lengths will vary but are typically 16 to 24 feet long. The mill optimizes the lengths to minimize scrap during production.
Double random lengths (DRL): These are twice as long as single random, ranging from 32 to 48 feet. They are less common and may have limited availability, but they can reduce the connections required in a long pipeline.
Specific cut lengths: Pipes can be ordered to specific lengths per the customer's requirements. This is more expensive as it may create more scrap for the mill and require additional setup and handling. Cut lengths are usually only specified when the exact length is critical, such as for spool fabrication or offshore installations where joints must fit precisely.
The available lengths may also vary depending on the size and wall thickness of the pipe. Larger diameters and heavier walls are more difficult to handle and transport, so that they may be limited to shorter lengths.
End Finishes
The ends of the pipe can be finished in several ways depending on how it will be joined:
Plain end (PE): The ends are cut square and left unfinished. This is the most basic option suitable for welding or mechanical couplings.
Bevelled end (BE): The ends are cut at an angle, typically 30° or 37.5°, to create a V-groove for welding. The bevel allows for complete penetration of the weld metal into the joint. Bevelled ends are the most common choice for welded construction.
Threaded end (TE): The ends are threaded either externally (male) or internally (female) to allow the pipes to be screwed together. Threaded connections are commonly used for low-pressure applications like water or air lines and temporary or portable installations. The threads can be tapered (NPT) or straight (BSPP or BSPT).
Threaded and coupled (T&C): One end is externally threaded, and a coupling (sleeve) is screwed onto it. The other end of the coupling is internally threaded to accept the next pipe. This allows for quick assembly and disassembly in the field.
Grooved end: A groove is cut around the circumference of the pipe near the end to accept a mechanical coupling. Grooved couplings allow for fast installation and some flexibility in the joint to accommodate thermal expansion or contraction.
