Stainless Steel: The Basics
Seamless vs Welded Pipes: A Comprehensive Guide for the Oil & Gas Industry
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.
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.
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.
| 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.
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.
Low alloy steels (less than 8% alloy content) are commonly used in the oil and gas industry for:
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.
They are categorized as low, medium, or high carbon based on carbon content. Carbon steels are widely used in oil and gas for:
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.
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:
Stainless is used for critical components exposed to seawater, produced water, CO2, chlorides and mildly sour environments:
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 are used for the most demanding applications where stainless steels are inadequate:
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.
- 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.
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.
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.
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.
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.
The end finish is an essential consideration in the design of any piping system.
It affects the joining method, the cost, the installation time, and the maintenance requirements over the system's lifetime.
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.
The end finish is an essential consideration in the design of any piping system.
It affects the joining method, the cost, the installation time, and the maintenance requirements over the system's lifetime.
It's common for pipes to have different end finishes on each end, such as one bevelled end for welding and one threaded or grooved end for connecting to a valve or fitting.
This allows for flexibility in the design and construction of the piping system.
Here's an example of how a typical pipe order might be specified:
When ordering pipe, it's essential to be as clear and specific as possible to avoid confusion or delays.
If you have any questions or need assistance, don't hesitate to ask your supplier for guidance.
This allows for flexibility in the design and construction of the piping system.
How to Order Welded or Seamless Steel Pipe
When ordering steel pipe, several key pieces of information need to be specified to ensure you receive the right product for your application:- Material: Specify the material grade, such as carbon steel (A106 or A53), alloy steel (A335), stainless steel (A312), or nickel alloy (B444). If a specific grade is required, like 316L stainless or Grade B carbon steel, make sure to include that.
- Size: Specify the nominal pipe size (NPS) and the wall thickness as a schedule (Sch) or in inches or millimetres. For example, "NPS 6 Sch 40" or "NPS 12 with 0.375 inch wall."
- Length: Specify the length requirements, such as single random (SRL), double random (DRL), or specific cut lengths. If you need exact lengths, provide a cut sheet with the quantity of each length required.
- End finish: Specify how the pipe's ends should be finished, such as plain end (PE), bevelled end (BE), threaded (TE), threaded and coupled (T&C), or grooved. If different ends are needed, specify each, like "BE one end, TE other end."
- Quantity: Specify the total quantity required in pieces or feet. If you have multiple sizes or lengths, provide a breakdown of the amount for each item.
- Testing and certification: If any specific testing or certification is required, such as a mill test report (MTR), material test report (MTR), or positive material identification (PMI), make sure to include that in your order.
- Delivery: Specify the required delivery date and the shipping address. If you have any special offloading or receiving requirements, make sure to communicate those to the supplier.
Here's an example of how a typical pipe order might be specified:
- Material: A106 Grade B carbon steel
- Size: NPS 4 Sch 40 (4.5-inch OD x 0.237-inch wall)
- Length: Single random lengths (SRL)
- End finish: Beveled ends (BE) both ends
- Quantity: 2,000 feet
- Certification: Mill test report (MTR) required
- Delivery: Required by June 1st, 2023, to ABC Construction Site, 303-404 West 55th Avenue, Vancouver, BC V4C 5D6, Canada
When ordering pipe, it's essential to be as clear and specific as possible to avoid confusion or delays.
If you have any questions or need assistance, don't hesitate to ask your supplier for guidance.
Working with an experienced and reputable steel supplier can make ordering smoother.
They can help you select the right product for your application, ensure all the necessary information is included, and provide support throughout the delivery and installation process.
Whether you need seamless or welded, carbon steel or alloy, the key is to work with a knowledgeable and experienced supplier who can guide you through the selection process and ensure you get the best product for your specific needs.
At Unified Alloys, we have served the oil and gas industry for more than 40 years, providing high-quality steel pipes and expert guidance to customers throughout Canada and North America.
Our team of experienced product specialists can help you navigate the complex landscape of pipe specifications, grades, and standards and recommend the optimal solution for your application.
We are here to support you if you need help selecting the right material grade, determining the appropriate dimensions and tolerances, or specifying the best end finish for your joining method.
With our extensive inventory, global sourcing network, and state-of-the-art processing capabilities, we can deliver the pipes you need when and where you need them.
So, if you're looking for a trusted partner to help you optimize your oil and gas piping systems, look no further than Unified Alloys.
Contact us today to learn more about our products and services and to discuss how we can help you achieve your goals.
References:
They can help you select the right product for your application, ensure all the necessary information is included, and provide support throughout the delivery and installation process.
Final Thoughts
Selecting the right steel pipe for your oil and gas application is a critical decision that can significantly impact your operations' safety, efficiency, and profitability.Whether you need seamless or welded, carbon steel or alloy, the key is to work with a knowledgeable and experienced supplier who can guide you through the selection process and ensure you get the best product for your specific needs.
At Unified Alloys, we have served the oil and gas industry for more than 40 years, providing high-quality steel pipes and expert guidance to customers throughout Canada and North America.
Our team of experienced product specialists can help you navigate the complex landscape of pipe specifications, grades, and standards and recommend the optimal solution for your application.
We are here to support you if you need help selecting the right material grade, determining the appropriate dimensions and tolerances, or specifying the best end finish for your joining method.
With our extensive inventory, global sourcing network, and state-of-the-art processing capabilities, we can deliver the pipes you need when and where you need them.
So, if you're looking for a trusted partner to help you optimize your oil and gas piping systems, look no further than Unified Alloys.
Contact us today to learn more about our products and services and to discuss how we can help you achieve your goals.
References:
- Bird Stainless Steel: Steel For Oil And Gas Industry
- Ambica: Why to choose duplex for the oil and gas industry
- ESP Specialty Steel Products: Oil & Gas Grades
- Milfit Boru: Advantages of Seamless Steel Pipes
- Permanent Steel Manufacturing: Benefits and Drawbacks of ERW Steel Pipes
- MS: Seamless Steel Pipe Vs. Welded Steel Pipe
Unified Alloys will not be responsible for the accuracy or currency of any of the information contained herein. The specifications and information contained in the brochures are subject to change without notice. Unified Alloys expressly disclaims any liability for loss or damage caused by use of any information contained in this publication, including any special, incidental or consequential damages arising from such use. Nothing in this publication shall create or imply any warranty whether expressed or implied.