Oil Country Tubular Goods (OCTG)
Oil country tubular goods (OCTG) is a family of seamless rolled products consisting of drill pipe, casing, and tubing subjected to loading conditions according to their specific application. (see Figure 1 for a schematic of a deep well):
The Drill Pipe is a heavy seamless tube that rotates the drill bit and circulates drilling fluid. Pipe segments 30 ft (9m) long are coupled with tool joints. The drill pipe is simultaneously subjected to high torque by drilling, axial tension by its dead weight, and internal pressure by purging drilling fluid. Additionally, alternating bending loads due to non-vertical or deflected drilling may be superimposed on these basic loading patterns.
Casing pipe lines the borehole. It is subject to axial tension from its dead weight, internal pressure from fluid purging, and external pressure from surrounding rock formations. The pumped oil or gas emulsion particularly exposes the casing to axial tension and internal pressure.
Tubing is a pipe through which oil or gas is transported from the wellbore. Tubing segments are generally around 30 ft [9 m] long and have a threaded connection on each end.
Corrosion resistance under sour service conditions is a crucial OCTG characteristic, especially for casing and tubing.
Typical OCTG manufacturing processes include (all dimensional ranges are approximate)
Continuous mandrel-rolling and push bench processes 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 the welded pipe. Therefore, high-strength seamless pipe must be produced by increasing the alloying content in combination with a suitable heat treatment, such as quench and tempering.
Figure 1. Schematic of a deep thriving completion
Meeting the fundamental requirement of a fully martensitic microstructure, even at large pipe wall thickness, requires good hardenability. Cr and Mn are the main alloying elements that 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 act as hydrogen collectors and crack 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 quickly 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 attacks 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% Mo concentration produces the optimum combination of yield strength and SSC resistance.
