Tool Joint for Drill Pipes

The invention refers to a joint for a drill string. It is well known that the joints used presently, with threaded pin and box, pressure welded to the drill pipe, allow variable stress conditions to arise in the pin and box system, resulting in fatigue, pin failures and drill pipe failure at the critical point (the transition from upset ends to the pipe body).

The present invention eliminates these disadvantages by using a tool joint which removes the variable bending moments induced in the pin-and-box system, under the influence of external loads, and associated strain phenomena, reduces the bending moments in the critical zones and increases the tensile load bearing strength. The tool joint consists of an internally threaded box having a cylindrical internal recess in the unthreaded zone, an externally threaded pin also having an internal cylindrical recess, and a convergent-divergent nipple pressure fitted in the recesses, the nipple resisting the bending moments occurring in the pin-and-box thread and in the critical zones, at the same time permitting self-balancing of these moments, reducing and equalizing their values across the contact faces at the nipple ends.

An example of the invention is described below and illustrated in the accompanying drawing in which

FIGS. 1 and 2 are views showing a longitudinal cross section of the joint.

According to the present invention the tool joint consists of the internally threaded box 1, said box having an internal cylindrical recess a, a pin 2 also having an internal cylindrical recess b, in which a convergent-divergent nipple 3 is pressure fitted, the nipple resisting the bending moments arising in the pin-and-box thread and in the two critical zones c of the drill pipe 4. The nipple 3 permits self-balancing of said bending moments, the reduction and equalization of their values across each contact face of the nipple ends d.

The particularly sensitive zones c of the drill pipe 4 are, as can be seen from the drawing, the regions immediately behind the thickened or upset portions to which the box 1 and the pin 2 are affixed.

As is also apparent from FIGS. 1 and 2, the threaded connection between the box 1 and the pin 2 is a conventional tapered thread and below the internal thread, the box 1 is formed with a recess a. The recess b of the pin 2 can extend beyond the length of its external thread.

The nipple 3 is termed a convergent-divergent nipple because its interior initially converges from its upper end face, at which it is flush with the interior of the pin 2, to a central region and then diverges to its lower-end face d to which it is flush with the interior of the box 1. The tool joint according to the present invention has the following advantages over the existing drill pipe joints:

it eliminates the variable bending moments arising in the pin-and-box thread under the influence of the external loads, and the resulting strain,

it reduces bending moments by about 30% in the critical zones of the drill pipes, where failures are most likely to occur,

it increases the tensile load bearing strength of the thread in that the thread length is increased within the same API range.

All these advantages result in the reduction of drill-pipe failures, which are known to be expensive to solve.