There are many drilling programs in our industry today that stretch the limits of existing drill pipe and a drill pipe failure in these holes can be expensive in lost time and lost equipment. The primary failure mode of the drill pipe is fatigue. As with the drill pipe tube, the tool joints are also subject to fatigue failure and are, in many cases, the weak link in a drill string.
Rotary shouldered connections are very simple, very rugged mechanical devices that perform several functions during the drilling operations. These functions include transfer of torsional loads, transfer of tensile loads, transfer of compressive loads, transfer of bending loads, and containment of pressure. In spite of the variety of the functions, the connections must perform with zero tolerance for error and they must be made-up and broken-out under adverse working conditions.
The failure or success of the rotary shouldered connections for all these functions depends on the amount of make-up torque and shoulder load that is applied to the connection before being put into service. As in many other cases, more is not always better.
This paper presents considerations and recommendations to improve the fatigue life of rotary shouldered connections on drill pipe and other drilling tools. These recommendations are based on the stresses induced in the connection at make-up and the additional stresses resulting from the service loads. The analysis includes the effects of frictional properties of the thread compound and the possible adverse effects of drilling fluid being mixed with thread compound.
The Goodman diagram illustrates that the fatigue strength of a material is reduced when the static stress is increased. Generally, the higher the make-up torque for the connection the lower the allowable bending loads. The Goodman diagram can be used to evaluate the effect that the mean stress and fluctuating stresses has on pin fatigue resistance by plotting values on the diagram.
Fatigue failures in drill collar connections can occur in either the pin or box but fatigue failures in drill pipe tool joints almost always occur in the pin. These pin failures usually occur near the last engaged thread about 3/4 in. from the make-up shoulder. Cyclic loading from down-hole vibrations and rotating in doglegs causes the failures.
Tool joint pins are preloaded by make-up torque that produces a stress at the last engaged thread of about 60% of its yield strength. The preload and other unvarying service loads result in a static or mean stress that has a significant influence on the potential for fatigue failures.
Tool joint boxes don't generally experience fatigue failures because the cross sectional area and moment of inertia are great enough to prevent high stresses and because there is no preload to elevate the mean stress.
At first glance, tool joints appear to simply be a means for connecting joints of drill pipe to one another. This is true, but in doing so, they are called on to satisfactorily transmit torsional loads, tensile loads, compressive loads, bending loads and provide a seal for the drilling fluid being pumped down the pipe. It's not easy finding a mechanical device simpler than a tool joint that performs all of the above functions while carrying the required loads.