OCTG pipes are tubular products used for onshore and offshore oil & gas upstream operations. The OCTG pipe family includes casing pipes (to stabilize the wellbore), tubing pipes (to transport hydrocarbons from the reserve to the surface) and drill pipes to perforate the soil. The dimensions, tolerances, and materials for “Oil Country Tubular Goods” are covered by the API 5CT specification (that can be purchased on the API store).


Three types of pipes belong to the OCTG family:


Casing pipes are used to stabilize the wellbore. A casing pipe is subject to axial tensions and to internal pressures generated by the pumped oil or gas, by their heavyweight and the external pressures coming from the surrounding rocks

Tubing pipes are tubular goods through which the oil or gas is transported from the wellbore to the surface. Tubing segments are generally around 30′ long with a threaded connection on each end (standard or premium connections are available)

Drill pipes
are heavyweight seamless tubular that rotates the drill bit and circulates the drilling fluid. Pipe segments of 30′ are coupled with tool joints. Drill pipe is subject to high torque by drilling, axial tension by their weight, and internal pressure due to the purging of the drilling fluid. Additionally, alternating bending loads due to non-vertical or deflected drilling may be superimposed on these basic loading patterns


(Source: Eng. Mohammed Aboholi – Youtube)



The API 5CT specification covers seamless and welded casing and tubing pipes for upstream operations (pipes that belong to the OCTG family, as illustrated above).


casing pipes octg

Casing pipes scope

Casing pipes are a key structural component for an oil & gas well and have the following scope:

  • Keep the borehole stability in the well
  • Prevent the bore contamination from water sands
  • Prevent water from producing formations
  • Exercise a tight control of the wellbore pressures during the drilling, production, and repair operations

Casing pipes are used to install:

  • Blowout Preventers (BOP)
  • Other wellhead equipment necessary to extract hydrocarbons
  • Production tubing and packers

Casing pipes are also one major individual component of the overall cost of the well,  therefore the correct selection of the casing size, materials, connectors, and depth shall be at the top of design engineers (for cost and efficiency reasons).

The six basic types of casing strings are:

  • Conductor Casing
  • Surface Casing
  • Intermediate Casing
  • Production Casing
  • Liner
  • Liner tieback casing

Oilfield casing pipes are positioned into the wellbore and cemented in place to secure both subsurface formations and the wellbore from collapsing, also to enable drilling mud to circulate and extraction to take place. The strict quality requirement for steel is due to the harsh working conditions of the casing. The steel product should be produced and checked in accordance with special standards or specifications. ISO 11960 and API Spec 5CT have specified the steel product standards of the casing.


Casing pipes sizes and materials

Casing pipes are available in a diameter range 4 1/2  to 20 inches (the dimensions of API5CT casing pipes are shown in this article), and in the following materials: H-40, J-55, K-55, N-80, L-80/C, 90/T, 95, P110, Q-125. Consult this article to learn more about the materials for API5CT pipes.


Casing pipes connections

The main types of connections for casing pipes are STC (short threads), LTC (long threads), BTC (buttress threads) and premium gas-tight connections.



tubing pipes OCTG

Tubing pipes scope

Tubing pipes are used to bring oil and gas from the underground reserves up to the field for further processing.

Tubing pipes need resistance to mechanical stress as they are subject to very high loads and deformations during the production operations.

In addition, tubing pipes sizes should be properly calculated to support the expected oil and gas flow from the ground to the surface (a too small diameter would decrease the production rate and the return on investment on the wellbore licenses, whereas too large tubing would generate non-recoverable costs due to the greater amount of steel used for the bore construction vs. the actual requirement (steel for the casing and tubing pipes).


Tubing pipes sizes and materials

Tubing pipes are manufactured in seamless and welded execution, in the size range of 1.050 to 5 1/2 inches (consult this article to see the AP5CT tubing pipes sizes) and in the following material grades: H-40, J-55, K-55, N-80, L-80, C-90, T-95, P-110, Q-125 (more details about API 5CT tubing materials are in this article).


Tubing pipes connections

The main types of connections for tubing pipes are NUE (non-upset), EUE (external upset) and premium. Corrosion resistance under sour service conditions is a very important OCTG characteristic, especially for casing and tubing.



The manufacturing processes of casing and tubing pipes include:

  • Continuous mandrel-rolling process and the push bench process for sizes between 21 and 178 mm OD.
  • Plug mill rolling for sizes between 140 and 406 mm OD.
  • Cross-roll piercing and Pilger rolling for sizes between 250 and 660 mm OD.

These processes typically do not allow the thermomechanical processing customary for the strip and plate products used for welded pipes. Therefore, the high-strength seamless pipe must be produced by increasing the alloying content in combination with a suitable heat treatment such as quench & tempering.

Meeting the fundamental requirement of a fully martensitic microstructure even at large pipe wall thickness requires good hardenability. Chrome (Cr) and Manganese (Mn) are the main alloying elements used to produce good hardenability in conventional heat-treatable steel. However, the requirement for good sulfide stress cracking (SSC) resistance limits their use. Mn tends to segregate during continuous casting and can form large MnS inclusions that reduce hydrogen induced cracking (HIC) resistance. Higher levels of Cr can lead to the formation of Cr7C3 precipitates with coarse plate-shaped morphology, which acts as hydrogen collectors and cracks initiators. Alloying with Molybdenum can overcome the limitations of Mn and Cr alloying. Mo is a much stronger hardener than Mn and Cr, so it can easily recover the effect of a reduced amount of these elements.

Traditionally, OCTG grades were carbon-manganese steels (up to the 55-KSI strength level) or Mo-containing grades up to 0.4% Mo. In recent years, deep well-drilling and reservoirs containing contaminants that cause corrosive attack have created a strong demand for higher strength materials resistant to hydrogen embrittlement and SCC. Highly tempered martensite is the structure most resistant to SSC at higher strength levels, and 0.75% is the Mo concentration that produces the optimum combination of yield strength and to SSC resistance




API 5CT Tubing OD Coupling OD Length Unit Weight
Inches Inches Inches lbs
API EUE Tubing Couplings 1.315 1.66 3.25
1.66 2.054 3.5
1.9 2.2 3.75
API NUE Tubing Couplings 2 3/8 3.063 4.875
2 7/8 3.668 5.25
3 1/2 4.5 5.75



API 5CT Casing OD Short Thread
Coupling OD
Coupling Length Unit Weight
Inches Inches Inches
API Casing Couplings Short Thread 41/2 5.000 6.250
51/2 6.050 6.750



API 5CT Casing OD Long Thread
Coupling OD
Length Weight per Coupling
Inches Inches Inches
API Casing Couplings Long Thread 41/2 5.000 6.250
51/2 6.050 6.750



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