Drill string stresses

Calculations for the minimum performance properties of drill pipe are based on formulas given in Appendix A of API RP 7G. It must be remembered that numbers in Tables 4-80-4-83 have been obtained for the uniaxial state of stress, e.g., torsion only or tension only, etc. The tensile stress resistance is decreased when the drill string is subjected to both axial tension and torque a collapse... 

If the drill string is thought to be a thin wall cylinder with closed ends, then the drill pipe pressure produces the axial stress and tangential stress given by the following formulas ... 

In particular, makeup torque induces a tensile state of stress within the pin and compression stress in the box. Thus, when the tool Joint is exposed to the additional axial load due to the weight of the drill string suspended below the Joint, the load capacity of the tool Joint is determined by the tensile strength of the pin. 

The best performance of the individual members of the drill string is obtained when the bending stress ratio of subsequent members is less than 5.5 [38]. Bending stress ratio (BSR) is defined as a ratio of the bending section moduli of two subsequent members, e.g., between the drill collar and the pipe right above it. 

Software analysis programs based on stress-wave theory (Wang et al., 1990) or finite-element methods (Kalsi etal.,1987) can be used to predict and optimize jar placement in the drill string and physical performance measures. From jar input including size and overpuU, these programs can produce output on impact... 

Corrosion due to mechanical and metallurgical problems also exist. Metal tools used in drilling wells are often softer than the formation being penetrated. The abrasiveness of formation solids can easily erode protective films from drilling equipment, leaving metal exposed to corrosion-erosion attack. Mechanical and chemical separation of abrasive solids helps control this attack. It is difficult, however, to control stress concentrations in a string of driU pipe that may reach many kilometers into the earth. 

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