This chapter discusses OCTG materials and production process.
For OCTG, material “type” describes the composition of the steel used in manufacturing of the pipe, which impacts resistance to various types of corrosion.
The type of material for OCTG must be appropriate for the corrosiveness of the operating environment.
The six material types for OCTG are shown in Table CT-1.
OCTG pipe is manufactured by either a welded or seamless process.
|Table CT-1: The six materials used for OCTG manufacture|
|Material Name||Governing specification||Application|
|Carbon steels||API 5CT/ ISO 11960||Non-corrosive wells Sour service without CO2|
|13% Cr Martensitic||API 5CT/ ISO 11960||Sweet corrosion (CO2)|
|Super 13 Martensitic||API 5CRA/ ISO13680
|Sweet corrosion (CO2) and temperature|
|22% Cr or 25% duplex or super duplex||API 5CRA/ ISO13680
|Sour service + CO2|
|28% Cr Austenitic (Fe base alloys)||API 5CRA/ ISO13680
|Highly corrosive: Fit for purpose testing|
|Alloy 825, G3, C276.
Nickel base alloys
|API 5CRA/ ISO13680
|Extremely corrosive: Fit-for-purpose testing|
Welded tubulars are generally large diameter with relatively thin walls, suitable for structural pipe, conductors, surface casing, and marine risers.
Welded pipe is sometimes used for other applications such as intermediate casing, production casing, and tubing, though these applications are less common, especially in the smaller diameters. Welded pipes have generally good dimensional properties and are generally less expensive but have limitations:
Prohibited for API 5CRA tubulars;
Not suitable to 13 Cr;
Not suitable to sour service. Only accepted for L80, forbidden when higher strength properties are required;
Prohibited for couplings and accessories, per API 5CT;
Limited wall thickness because of weld limitation in wall.
- Seamless pipe is suitable for all types of material and grades, and is preferred when well conditions are severe. Per API 5CRA, all CRA tubulars are seamless. Because of manufacturing limitations, seamless pipe is generally only available in diameters of 18 in. or less.
Seamless tube manufacturing
There are multiple seamless steel-tube manufacturing processes that originated at the end of the 19th century. These include:
Continuous mandrel rolling process and push bench process: 21-178 mm (0.8-7.0 in.);
Continuous mandrel rolling process: 7 to 9 tandem rolling stands continuously mill and elongate the hollow shell of the tube over a floating mandrel bar to produce a final tube. Starting material is generally round rolled billets. First the material is heated, then pierced to produce a hallow shell. At this point the piece is elongated anywhere from 2 to 4 times its initial length. Finally the shell is rolled out in the continuous rolling mill to produce a continuous tube;
Push Bench: First billets are heated to rolling temperature, and then moved through the cylindrical dies of a piercing press, where they become thick-walled pierced billets (a.k.a., “hollow”) closed at one end.
Later the hollows are stretched using a 3-roll elongated, thereby leveling the wall thickness.
Once elongated the hollow is moved to a push bench, where a mandrel is inserted and it passes through a series of rollers.
The hollow passes roller to roller, resulting in smaller wall thicknesses.
Finally a hot saw removes the closed end from the hollow.