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The most important mechanical properties of casing and tubing are burst strength, collapse resistance and tensile strength. These properties are necessary to determine the strength of the pipe and to design a casing string. If casing is subjected to internal pressure higher than external, it is said that casing is exposed to burst pressure loading. Burst pressure loading conditions occur during well control operations, casing pressure integrity tests, pumping operations, and production operations. The MIYP of the pipe body is determined by the internal yield pressure formula found in API Bull. This equation, commonly known as the Barlow equation, calculates the internal pressure at which the tangential (or hoop) stress at the inner wall of the pipe reaches the yield strength (YS) of the material.
Introduction Tubing is the normal flow conduit used to transport produced fluids to the surface or fluids to the formation. Its use in wells is normally considered a good operating practice. The use of tubing permits better well control because circulating fluids can kill the well; thus, workovers are simplified and their results enhanced. Flow efficiency typically is improved with the use of tubing. Furthermore, tubing is required for most artificial lift installations. Tubing with the use of a packer allows isolation of the casing from well fluids and deters corrosion damage of the casing. Multicompletions require tubing to permit individual zone production and operation. Governmental rules and regulations often require tubing in every well. Permission may be obtained for omission of tubing in special cases (tubingless completions). These special completions typically are flowing wells with relatively small casing. Tubing strings are generally in outside diameter (OD) sizes of 2 3/8 to 4 1/2 in. The proper selection, design, and installation of tubing string are critical parts of any well completion. See the chapter on inflow and outflow in this section of the handbook for more information. Tubing strings must be sized correctly to enable the fluids to flow efficiently or to permit installation of effective artificial lift equipment. A tubing string that is too small causes large friction losses and limits production. It also may severely restrict the type and size of artificial lift equipment. A tubing string that is too large may cause heading and unstable flow, which results in loading up of the well and can complicate workovers. The planned tubing must easily fit inside the installed casing.
This paper describes development of and the procedures and equipment described in API RP 7A1, Recommended Practice for Testing of Thread Compounds for Rotary Shouldered Connections, published in 1991.
Trouble-free rotary-shouldered-connection performance depends on proper joint makeup. Joints must be tight enough to prevent shoulder separation under bending and tensile loads but not so tight that their tensile capacity decreases or the pin or box is damaged. The preload in a connection from tightening depends on the makeup torque and frictional properties of the thread compound.
Why Thread Compound Is Important
Besides acting as a gall preventive and sealing aid, thread compound must provide a consistent, predictable friction coefficient. When a rotary-shouldered connection is bucked up, the frictional forces acting on the makeup shoulders and thread flanks create a resistance to tightening. With the same makeup torque, a connection with low friction resistance will be made up tighter than a connection with high friction resistance.
The induced axial stresses that preload a rotary-shouldered connection are proportional to the makeup torque. API RP 7G2 recommends that the makeup torque produce a tensile stress of 62,500 psi in a drill-collar connection and 72,000 psi in a tool joint.