API 5CT Casing and Tubing

Casing and tubing strings are the main parts of the well construction. All wells drilled for the purpose of oil or gas production (or injecting materials into underground formations) must be cased with material with sufficient strength and functionality.


The casing is generally a steel pipe installed in the wellbore and is cemented in place to prevent Wellbore collapse, crossflow of fluids between different reservoirs, and to control abnormal formation pressures.

The casing is the major structural component of a well. The casing is needed to:

Maintain borehole stability
Prevent contamination of water sands
Isolate water from producing formations
Control well pressures during drilling, production, and workover operations

Steel casing pipe generally has no specific specifications, other than the need for the material to be extremely straight and round. In some areas, A.S.T.M. specifications may be required by project engineers. The specification most commonly called for is A.S.T.M. 139 Grade B. This specification gives parameters for minimum yield and tensile strength of the steel pipe being used for casing, and tolerances of straightness and concentricity.

Steel casing pipe is often specified as ASTM A-252 which is a structural grade material that does not require hydrostatic testing and the inspection requirements are not stringent and it usually costs less than other grades such as A-53, A-139, or API 5L. Used natural gas line pipe is also used as casing on many projects because it is often reclaimed in very good condition and can offer significant cost savings when compared to a new steel pipe. The used pipe is most likely to not have any testing data associated with it and is generally used when the only required specification is a given diameter and wall thickness of steel casing pipe.


Tubings are usually run inside the casing (cased hole) and serve as a conduit through which oil and gas is produced. It is basically a tube inside a casing which is used to produce reservoir fluids.

Tubing is the conduit through which oil and gas are brought from the producing formations to the field surface facilities for processing. Tubing must be adequately strong to resist loads and deformations associated with production and workovers. Further, the tubing must be sized to support the expected rates of production of oil and gas. Clearly, tubing that is too small restricts production and subsequent economic performance of the well. Tubing that is too large, however, may have an economic impact beyond the cost of the tubing string itself, because the tubing size will influence the overall casing design of the well.

API Spec 5CT – Specification 5CT/ISO 11960, Specification for Casing and Tubing, Eighth Edition, Petroleum and natural gas industries-Steel pipes for use as casing or tubing for wells.

Standard: API SPEC 5CT

API 5CT pipe Size (mm):
Outer Dimensions: 6.0mm – 219.0mm
Wall Thickness: 1.0mm – 30 mm
Length: max 12000mm
Application: gas, water, and oil transportation in both oil and natural gas.

The main steel grade of API 5CT:
API 5CT P110