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What is Oil tubing?
- May 31, 2018 -

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, 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.

Properties of casing and tubing

The American Petroleum Inst. (API) has formed standards for oil/gas casing that are accepted in most countries by oil and service companies. Casing is classified according to five properties:

  • The manner of manufacture

  • Steel grade

  • Type of joints

  • Length range

  • The wall thickness (unit weight)

Almost without exception, casing is manufactured of mild (0.3 carbon) steel, normalized with small amounts of manganese. Strength can also be increased with quenching and tempering. API has adopted a casing "grade" designation to define the strength of casing steels. This designation consists of a grade letter followed by a number, which designates the minimum yield strength of the steel in ksi (103 psi). Table 1 summarizes the standard API grades.


  •                                                                         Table 1-API Steel Grades

The yield strength , for these purposes, is defined as the tensile stress required to produce a total elongation of 0.5% of the length. However, the case of P–110 casing is an exception where yield is defined as the tensile stress required to produce a total elongation of 0.6% of the length. There are also proprietary steel grades widely used in the industry, which do not conform to API specifications. These steel grades are often used in special applications requiring high strength or resistance to hydrogen sulfide cracking. Table 2 gives a list of commonly used non-API grades.


  •                                                                      Table 2-NON-API Steel Grades

Pipe strength

To design a reliable casing string, it is necessary to know the strength of pipe under different load conditions. The most important mechanical properties of casing and tubing are:

  • Burst strength

  • Collapse resistance

  • Tensile strength


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