What Is Line Pipe? Differences Between Line Pipe and OCTG Pipe

What Is Line Pipe?(API 5L)

Line pipe is a type of steel pipe specifically designed for the long-distance, high-pressure transportation of media such as oil, natural gas, and water. Based on manufacturing processes, it can be classified into seamless pipe, ERW pipe, LSAW pipe, and SSAW pipe.

Internationally, the most widely used standard is the American Petroleum Institute’s API 5L standard, while the corresponding domestic standard is GB/T 9711.

Steel grades are classified according to the API 5L standard, with common grades including X42, X52, X65, X70, and X80. The numbers represent the minimum yield strength of the pipe. The higher the number, the greater the strength of the steel pipe.

Typical applications of line pipe include oil and gas transmission pipelines, water conveyance projects, urban pipeline networks, and subsea pipelines.

What Is OCTG Pipe?

OCTG: Oil Country Tubular Goods, a category of specialized steel pipes used exclusively for the drilling and production of oil and gas wells.

The most widely adopted standards are API 5CT (casing and tubing) and API 5D (drill pipe).

Common steel grades for API 5CT include J55/K55, N80, L80, C90/T95, and others.

OCTG primarily consists of casing, tubing, and drill pipe.

The core function of casing in the entire process is to stabilize the wellbore, prevent wellbore collapse, and isolate different formations (oil and gas layers).

It accounts for the largest volume of OCTG.

Tubing is located inside the casing and is used to transport oil and gas; it serves as the actual conduit through which oil and gas travel from the reservoir to the surface.

Drill pipe connects the surface drilling rig to the drill bit, transmits torque, and pumps drilling fluid to the bottom of the well.

Difference Between Line Pipe and OCTG Pipe

Function: Transportation vs Drilling

The key difference between them lies in their intended applications.

Line pipe is used for the long-distance, large-scale transportation of media such as oil, gas, and water, serving as the “arteries” outside the oilfield.

OCTG pipe is used for the drilling, completion, and production of oil and gas wells, functioning as the “skeleton and blood vessels” within the well.

Standards: API 5L vs API 5CT

Common standards for line pipe include API 5L and GB/T 9711.

Under the API 5L standard, common steel grades include X60, X70, X80, and X100. Common steel grades under GB/T 9711 include L175, L210, L245, and L290, among others.

Common standards for OCTG pipes include API 5CT (casing/tubing) and API 5DP (drill pipe).

Common steel grades under API 5CT include J55, K55, M65, and C90. Common steel grades under API 5DP include E75, X95, and G105.

Mechanical Requirements and Testing

Line pipes are primarily used in above-ground, underground, or offshore environments, where they must withstand internal pressure and corrosion resulting from long-distance transportation, requiring excellent fracture toughness and safety.

OCTG pipes are mainly used in downhole environments, where they must withstand external compression from the formation, internal high pressure, and immense combined tensile, compressive, torsional, and bending stresses, while also facing highly corrosive environments containing H₂S, CO₂, and other substances.

Due to differences in operating environments, performance requirements vary, and the corresponding key test items are adjusted accordingly.

Line Pipe: Flattening test, hydrostatic test, non-destructive testing (NDT), drop weight tear test (DWTT)

OCTG Pipe: Tensile/impact/hardness tests, hydrostatic test, thread inspection, flattening test

Cost and Manufacturing Differences

Line pipe is connected via welding, so there are strict requirements for the material’s carbon equivalent (CEV). OCTG pipe is connected via threaded joints, which necessitate extremely high standards for machining precision, sealing performance, and strength—factors that directly impact the entire well’s production operations.

However, compared to OCTG pipe, line pipe is relatively less expensive. OCTG pipe has higher production costs and involves more complex technical requirements.

Line Pipe vs Casing and Tubing

The primary function of line pipe can be summarized as “transportation,” while casing and tubing are used for “well construction and production.”

Casing pipe acts as the “skeleton” of an oil well, providing structural support, and must extend thousands of meters underground.

Tubing acts as the “blood vessels” of an oil well, transporting oil and gas from the reservoir to the surface through the casing.

OCTG Standards and Applications Explained

The most important standard for OCTG is API 5CT, which applies to casing and tubing. It corresponds to the Chinese national standard GB/T 19830 and the international standard ISO 11960.

API 5CT classifies steel pipes into three performance levels (PSL-1, PSL-2, PSL-3), with stricter requirements at higher levels.

API 5CT classifies casing and tubing into 4 groups, comprising a total of 19 steel grades. The following are the most common steel grades:

Types and Functions of OCTG

1. Casing: Stabilizes the wellbore, prevents wellbore collapse, and isolates formations.

Casing can be further categorized as follows:

· Surface Casing: The first section run during initial drilling, primarily designed to seal out soft surface formations. This section does not require high-grade steel; J55 or lower grades are generally sufficient.

· Intermediate Casing: Designed specifically for complex formations. For example, in areas prone to collapse, mud loss, or salt-pudding layers, this casing layer has higher requirements for mechanical strength and sealing performance.

· Production Casing: The innermost layer is the production casing, which is installed last. It directly seals the oil and gas reservoir and must withstand production pressures over the long term, making it the most demanding in terms of specifications.

2. Tubing

Tubing serves as the conduit for transporting oil and gas from the reservoir to the surface and is located inside the casing.

It is classified by end configuration as follows:

NU (Non-Upset End tubing): The outer diameter of the tube ends is the same as that of the tube body

EU (External Upset End tubing): The outer diameter of the tube ends is increased, providing higher thread strength

IJ (Integrally Jointed Tubing): Thickened at both ends, used for special operating conditions

According to the API 5CT standard, common steel grades used for tubing include H40, J55, K55, N80, L80, C90, T95, P110, Q125, etc.

3. Drill Pipe

Drill pipe is the shaft used to connect the drill bit during drilling and transmit torque. Common grades of steel in the API 5DP standard:

E75: Lowest strength, used for shallow wells

X95: Medium strength

G105: Higher strength, widely used

S135: Highest strength, used for deep and ultra-deep wells

Applications of OCTG

1. Onshore Oil and Gas Wells

Cost is the primary consideration; H40 → J55 is recommended provided basic strength requirements are met.

2. Deep Wells / Ultra-Deep Wells

Extremely high strength requirements apply; P110 / Q125 is the most suitable choice.

3. Offshore Oil and Gas Platforms

In highly corrosive environments, high-grade steels such as L80, 13Cr, or Duplex are recommended.

4. H₂S-Containing (Acidic) Environments

In these conditions, steel pipes must possess sulfur resistance, and hardness must be strictly controlled. Recommended grades: L80-1 → C90 → T95.

How to Choose the Right Pipe for Oil & Gas Projects

When selecting casing for oil drilling, the decision ultimately isn’t based on which option is more expensive or which is sturdier.

Instead, it requires a comprehensive evaluation of three key factors: the actual conditions downhole, the expected oil production from the well, and the total cost over the entire lifecycle—from installation to decommissioning.

Choosing the wrong option can result in anything from premature failure of the casing to a major accident, and the consequences can be severe.

1. Determine the Basic Conditions of the Oil Well

The first principle in selecting casing is “consider the environment first, then strength.”

We need to determine the well depth: Measured Depth (MD) and True Vertical Depth (TVD).

The deeper the well, the greater the pressure and tension the casing must withstand, and the higher the steel grade required.

1.1.Internal Pressure and External Pressure (Collapse/Burst)

The Wall Thickness and steel grade of the tubing are determined based on internal and external pressures. If H₂S, CO₂, or acidic fluids are present, corrosion-resistant steel grades such as L80 or 13Cr must be selected.

2. Determine the steel grade

Select the steel grade based on the well’s pressure conditions and environment:

Note: Higher steel grades result in higher costs. The steel grade should be selected based on actual well pressure and the corrosive environment; there is no need to blindly prioritize strength.

3. Selecting Oil Pipe Size and Wall Thickness

Common outer diameters include 2-3/8“, 2-7/8”, and 3-1/2"; the selected size should match the casing diameter.

Wall Thickness determines the oil pipe’s resistance to pressure and tensile stress; the API 5CT Wall Thickness series is typically used.

3.1Common Wall Thickness Ranges for Tubing

3.2Common Wall Thickness Ranges for Casing

Lengths are categorized as Range 1, 2, and 3, with Range 3 being the longest (> 40 ft). Calculate the number of sections and fittings based on the well depth to avoid the increased risk of leaks caused by excessive joints.

4. Select the Threaded Connection Type

Tubing is connected via threaded connections, which are available in two types: API standard threads and Premium Connection (high-performance threads).

Premium Connection offers superior sealing performance and load-bearing capacity, and is typically used in demanding environments such as high-pressure wells, HPHT wells, or corrosive wells.

Why Choosing the Right Supplier Matters

Selecting the right supplier is crucial, as it impacts safety, production efficiency, and engineering risks throughout the oil drilling process.

1. Quality Assurance and Safety

During operation, oil pipes are subjected to high internal and external pressures, high temperatures, and corrosion. If quality is not assured, this may lead to engineering risks such as casing rupture and oil or gas leaks.

CORTEC STEEL strictly adheres to standards such as API 5CT, ISO 11960, and GB/T 9711, holds reliable international certifications, and can provide comprehensive quality documentation (Mill Test Certificate, MTC).

Our well-established production lines feature stable manufacturing processes and advanced steel pipe inspection technologies, ensuring the safety and reliability of construction projects.

2. Supply Capacity and Delivery Reliability

While maintaining high quality standards, we are able to deliver 98% of orders on time. This is made possible by our manufacturing system, which consistently produces 200,000 metric tons of steel pipes annually and maintains a standing inventory of 30,000 metric tons.

A supplier capable of delivering on time can help reduce downtime costs and risks.

3. Comprehensive Quality Traceability System

Each pipe is stamped with a unique mark that allows for traceability to its furnace number, batch, all process parameters, and inspection records. This ensures material traceability and compliance with global engineering standards.

4. Cost Control

We supply high-quality API 5L X70/X80 line pipes, API 5CT J55/N80/P110 casing and tubing for oil and gas operations, as well as API 5L steel pipes for long-distance transportation.

Furthermore, CORTEC STEEL has reduced total project costs by 15–25% through value engineering.

Conclusion

Line pipe is used for the long-distance, large-scale transportation of oil, gas, water, and other media, and complies with the API 5L standard. OCTG is used for oil well drilling and production operations, and complies with the API 5CT standard.

The selection of steel pipes requires a comprehensive evaluation based on the specific conditions of the project and the operating environment of the pipes.

Selecting a reliable supplier is just as important as choosing the correct steel grade, and this is a critical factor for the entire project timeline.

If you have any questions, please contact CORTEC STEEL; we are confident we will meet your expectations.