A Comprehensive Guide to OCTG Pipe: Sizes, Casing, Tubing, and Materials

Types, Applications, Item Descriptions, and Alloys

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What is OCTG?

OCTG, or Oil Country Tubular Goods, refers to tubes used in oil and gas production, both onshore and offshore, as part of the industry’s upstream sector. PipeSearch specializes in two essential types of OCTG products: casing and tubing. These components are critical to the completion and production of oil, gas, and injection wells. Understanding OCTG products is essential for oil and gas sector professionals.

What are the Differences Between Casing and Tubing?

OCTG pipe sizes are generally denoted by their outer diameter (OD) measurements, ranging from 2-3/8” to 20” OD.

OCTG Casing

OCTG casing stabilizes the wellbore by lining the borehole drilled into the ground for oil extraction. Typically larger in diameter, it is secured in place with cement. Casing sizes range from 4.5” to 20” OD and are vital in ensuring operational stability.

There are five primary types of oil well-casing pipes:

• Conductor Casing: (18”–36” OD): Creates the initial barrier at the surface.
• Surface Casing: (13-3/8” OD): Protects freshwater zones, prevents blowouts, and supports wellhead equipment.
• Intermediate Casing: (13-3/8”–16” OD): Positioned between surface and production casing for added support.
• Production Casing: (varied sizes): Maintains structural integrity during hydrocarbon production.
• Production Liner: Suspended from production casing to save costs by reducing full-depth liner use.

OCTG Tubing

OCTG tubing, available in sizes ranging from 2-3/8” to 4-1/2” OD (or larger, up to 7” in offshore wells), is inserted through the casing to transport hydrocarbons to the surface. Tubing protects the casing from corrosive fluids, sand, and other wear-and-tear factors. Unlike casing, damaged tubing can be replaced, offering flexibility and enhanced durability.

Understanding an OCTG Pipe Item Description

An OCTG item description follows industry standards to denote key pipe characteristics, typically stated in this order:

1. Outer Diameter (OD)
2. Weight per Foot
3. Alloy/Grade
4. Yield Strength
5. End Finish/Connection
6. Range (Length)

Example Item Description:

For instance, a description like 5.5” 23# 13CR-85 Vam Top HC R3 translates to:

• Outer Diameter (OD): 5.5 inches
• Weight Per Foot: 23 lbs/ft
• Alloy/Grade: 13 Chrome (13CR)
• Yield Strength: 85 ksi (85,000 psi)
• End Finish/Connection Type: Vam Top HC
• Range Style (Length): Range 3 (R3)

Standardization of OCTG Pipe Sizes

Industry organizations like the American Petroleum Institute (API) and the International Organization for Standardization (ISO) standardize OCTG pipe sizes for compatibility and interchangeability. Standards like API Specification 5CT define technical casing, tubing, and line pipes requirements.

It’s important to note that these sizes are examples and not an exhaustive list. The exact range of standard outer diameters may vary based on specific API specifications or other industry standards. Additionally, there may be non-standard or custom sizes available based on project requirements.

What is Weight Per Foot in OCTG?

The weight per foot is a key parameter impacting OCTG product design, installation, and transportation. It is calculated using the following formula:

Weight per foot (lbs/ft) = [(OD – Wall Thickness) x Wall Thickness] x 10.69

In this formula, the outer diameter (OD) and wall thickness are usually expressed in inches. The resulting weight per foot will be in pounds.

It’s important to note that the weight per foot can vary for different sizes and grades of OCTG. The specific weight per foot values can be obtained from product specifications, manufacturers’ catalogs, or industry references for the specific OCTG casing or tubing you are interested in.

Overview of OCTG Alloys and Grades

OCTG casing and tubing are designed to withstand the downhole conditions of a well. The alloy/grade of the OCTG describes the actual chemical composition and mechanical properties of the material. Different alloys or grades are better suited to various downhole environments, or are designed to withstand more extreme conditions, such as high pressure, corrosive environments, and varying temperatures.

It is important to note that the American Petroleum Institute (API) and the National Association of Corrosion Engineers (NACE) which is now called the Association for Materials Protection and Performance (AMPP), have set standards and specifications for OCTG casing and tubing to ensure their quality and performance. The specific alloy selection depends on factors such as well depth, pressure, temperature, corrosiveness of the formation, and other operational requirements.

The commonly used alloys and grades for OCTG casing and tubing can be categorized into five general groups based on their resistance to environmental factors. These groups, listed in order of increasing environmental resistance, are Carbon Steel, Chromium-molybdenum (Cr-Mo) Alloys, Martensitic Stainless Steel, Duplex Stainless Steel, and Nickel-Based Alloys.

Carbon Steel Grades

Carbon Steel grades are generally the most cost-effective option and are suitable in non-corrosive environments. They are often used in shallow wells with relatively low pressure, downhole conditions where the corrosive potential is minimal, and temporary applications where components may be removed after the initial phase or long-term production is not expected, therefore the long-term corrosion resistance of higher-grade materials may not be necessary. Carbon steel alloys include J55, K55, N80, L80, and P110.

Chromium-Molybdenum Alloys

Chromium-Molybdenum (Cr-Mo) Alloys have a higher tensile strength and resistance to sulfide stress cracking under certain conditions. They are commonly used in mildly sour and slightly deeper wells. These alloys include C95 and C110.

Martensitic Stainless Steels

Martensitic Stainless Steels are used extensively in sweet (CO2) environments where carbon and low alloy steels experience general corrosion, and are acceptable for moderate sour service conditions. They range in yield strength from 80 to 125 ksi, achieved by quench and temper heat treatment. 13 Chrome is the least alloyed of the stainless steels and most common for OCTG. These alloys include 13 Chrome, Modified 13 Chrome, Super 13 Chrome, 15 Chrome, and 17 Chrome.

Duplex Stainless Steels

Duplex Stainless Steels are required in sweet (CO2) environments containing chlorides or dissolved ocygen, where pitting and crevice corrosion are a concern. Cold-worked tubes are available with with yield strengths up to 140 ksi.

Super Duplex Stainless Steels

Super Duplex Stainless Steels extend the useful range of temperature and chlorides. Defined by Pitting Resistance Equivalent Number (PREN) greater than 40, they are recommended for conditions with high chlorides or dissolved oxygen, such as seawater and water injection wells. Duplex Stainless Steel alloys include 22 Chrome Duplex, 25 Chrome Duplex, and Super Duplex.

Nickel-Based Alloys

Nickel-Based Alloys are used primarily in sour environments where the partial pressure of H2S exceeds 3 psi. Their useful range of temperature, H2S, and chlorides is extended by increasing additions of nickel, molybdenum, and other elements. Solid solution nickel based alloys are available as cold worked tube with yield strengths up to 160 ksi. In addition to excellent resistance to chlorides and H2S, they eliminate the risk of hydrogen embrittlement which may occur with heat treated alloys. Nickel based used in OCTG casing and tubing include 28 Chrome, 2535, 2035, 825, G3, G2, 2550, G50, C276, and C22.

What is Yield Strength in OCTG?

Yield strength, in relation to OCTG, refers to the maximum stress or load that a material can withstand before it undergoes deformation or starts to exhibit plastic behavior. It is an essential mechanical property that indicates the material’s ability to resist deformation under applied stress.

Yield strength is typically measured using a tensile test, also known as a tension test. This test involves subjecting a specimen of material, such as a small section of OCTG casing or tubing, to an applied force or lead in tension until it undergoes deformation.

During the tensile test, the specimen is placed in a testing machine, commonly known as a universal testing machine. The machine applies a gradually increasing tensile force to the specimen, causing it to elongate until it reaches the point of yield. The force and the resulting elongation or deformation are continuously measured throughout the test.

Once the yield strength is determined, it serves as a parameter useful in assessing its suitability for various well conditions and it’s ability to withstand applied loads and pressures in oil and gas wells. The yield strength value is typically reported in units of force per unit area, such as pounds per square inch (psi) or 1,000 pounds per square inch (ksi). Generally, the higher the yield strength the greater the ability to withstand applied loads, higher pressures, more severe downhole conditions.

What is an OCTG Connection?

The connection refers to the machined finish on the ends of each joint or piece of OCTG that allows the pieces to connect to one another to form a string of pipe in the well. OCTG pipe connections are leak-proof, strong, durable designs that fall into three general categories: API, Semi-Premium, and Premium.

API connections meet or exceed all American Petroleum Institute (API) specifications. Semi-Premium connections are similar to API but are engineered for enhanced torque. Premium connections provide performance beyond standard API connections and are engineered to produce a gas-tight metal-to-metal seal.

In terms of connection design, there are various types of connections. Thread manufacturers have engineered a number of proprietary threads within these types to best serve specific applications. The general types can be categorized as the following: Threaded and Coupled (T&C), Semi-Flush, and Flush.

A threaded and coupled connection uses a threaded coupling to join two threaded ends of pipe. A semi-flush connection has a slight upset and lacks a coupling so one threaded pin end fits into one threaded box end. A flush connection lacks an upset and a coupling where one threaded pin end fits flush into one threaded box end. This is desirable when clearances are limited downhole and there is nothing additional on the OD to accommodate a coupling or upset end.

OCTG can also be acquired with plain ends or without connections. Of course, it cannot be used downhole until it is threaded.

OCTG Pipe Ranges

OCTG length is typically categorized using Ranges. These designations differ slightly between casing and tubing.

• Tubing Ranges: Range 1 (R1) is 20-25 feet, Range 2 (R2) is 28-32 feet, and Range 3 (R3) is 38-45.
• Casing Ranges: Range 1 (R1) is 16-25 feet, Range 2 (R2) is 25-34 feet, and Range 3 (R3) is 34-48.

The range pipe required for adequate well completion is dependent upon various factors including rig capabilities, well depth and well design. If the rig can accommodate longer lengths, however, Range 3 pipe may be most desirable as it reduces the number of needed connections.

In conclusion, OCTG casing and tubing plays a vital role in the completion and production of oil, gas, and injection wells. Casing and tubing each have a specific purpose and general application. Item descriptions help to differentiate between products, and alloys and connections are vital to operation downhole.

Partnering with a Trusted OCTG Pipe Supplier

Choosing a reliable OCTG pipe supplier ensures consistent quality and adherence to API standards, which are essential for smooth oil and gas operations. PipeSearch provides a streamlined platform with carefully curated options, giving your operations a competitive edge.