Box OD Stability of Double Shoulder Tool Joints at Catastrophic Failure
Authors: Thomas Mac , Smith J.E. 
Published: December 2007

Abstract
Catastrophic failure of tool joints caused by over torque often results in box swelling severe enough to prevent fishing with an overshot. Double shoulder tool joints are stronger in torsion than conventional joints and swelling that does occur during catastrophic failure is small and would probably not prevent fishing with an overshot.

Introduction
Drilling or working in small diameter holes necessitates the use of small diameter tool joints. Because of their size, these joints may be torsionally weak compared to the pipe and may fail catastrophically.
Torsional loads that exceed a tool joint's rated capacity can result in damaging compressive and tangential stresses in the box. The stresses can exceed the elastic limit of the material and cause the box to swell as shown in figure 6. If the connection parts (twists off) and must be fished out of the hole, a swelled box may have to be milled away to allow the use of an overshot.
Certain double shoulder tool joints, denoted in this paper as "HT" connections, such as that shown in figure 7 offer two advantages over conventional joints: They usually have at least 40% greater torsional strength which decreases the chance of a torsional failure and, if the joint does part because of gross torsional overload, the box does not swell in the same manner as the box of conventional joints.

Box Swell
Conventional Tool Joints. In conducting the box swell tests and performing the analysis on the two types of connections, it became obvious that there were two issues to address:To show that box swell of double shoulder tool joints is usually less than that of conventional tool joints and will probably not prevent the use of an overshot in small clearance holes andThe inherent weakness of conventional tool joint make-up shoulders.
Figure 1 is a line drawing of a conventional tool joint in the bucked up condition, When the connection is made-up or over-torqued, a compressive force parallel to the axis of the joint acts on the make-up shoulder of the pin and box. The force is approximately equal to the average axial stress times the cross sectional area (CSA) of the pin or box at any plane perpendicular to the thread axis between the make-up shoulder and last engaged thread of the pin (LET). The force can be approximated using equation (1) taken from paragraph A.8.1 in the Fifteenth edition of API RP7G. The terms in the equation are defined in the appendix.
(1)
Yielding that precedes catastrophic failure usually begins at either the last engaged thread of the pin or the make-up shoulder of the box. Yielding is not necessarily confined to one member or the other.
The force on the make-up shoulder is counteracted by an axial force on the flank of each thread. The angle of the thread flanks of all commonly used tool joints is either 300 or 450 from a plane perpendicular to the axis; therefore, there is a radial component of the force at the threads that tends to swell the box.
Figure 3 shows the forces on one thread of the box. These forces act along the length of the thread, from the make-up shoulder end to the end or nose of the pin, and sum to equal the compressive force on the make-up shoulder. The graph in figure 5 shows the force distribution of a typical NC connection. The thread forces are greater at the last engaged thread, thread #1, where the box has a thinner wall, and they decrease toward the end of the pin.
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