THREADED CONNECTIONS

The break-in procedure for new drill pipe recommends that the threaded connections be cleaned thoroughly with a solvent and wiped dry with a clean rag.  The treads and shoulders should then be carefully inspected for any burrs or damage.  A good grade of thread compound, containing powdered metallic zinc, in the amount of 40 to 60% by weight, should be applied to the threads and shoulders on both pin and box.  Drill pipe lubricants are not recommended because they normally are made with lead oxide which does not have sufficient body for the high shoulder loads necessary in proper joint make-up.

 

New joints should be carefully lubricated.  Any metal to metal contact can cause galling.  Application of the thread compound should be generous on shoulders, threads and in the pin and box stress relief grooves.  Extreme care should be used when stabbing the joints in order to avoid damage to the threads and shoulders of the connections.  Use the torque requirements that are proper for the threaded connections.  The connection should then be broken and inspeced for damage.  If any damage is found, it should be repaired or the threaded connection replaced.  The joint can then be lubricated again and made up with proper torque.

 

The drawings below show the effects of the stress concentration encountered at the last thread make up in a pin thread connection.  This stress concentration coupled with wobbling of the connection (due to loose make up) result in a high incidence of pin failures at the first engaged thread near the base.

The API standardisation committee has established a recommended practice of cutting stress-relief grooves in the joints 5-1/2 inches and above.  However, the use of stress related grooves is recommended by some drill pipe manufacturers on sizes less than 5-1/2 inches.  These are located near the base of the pin and in the bottom of the box just beyond the last engaged thread of the pin.  These relief grooves are intended to reduce stress concentrations in the critical bending areas of the pin and box.  They do this by removing unnecessary threads, which are notches, and replacing them with smooth, large radius contours.  It is essential that the surfaces of the stress relief areas be free of tool marks, stencil marks or other notches.


LOWER THREAD FAILURE

CAUSE: Box not made up against the shoulder of the pin.

SOLUTION: Check for proper make up.

 

This Top Sub which failed due to fatigue cracking at the root of the first enagaged thread shows no evidence of contact between the shoulder of the box thread and the pin,


Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

UPPER END THREAD FAILURE

Partial contact as shown by arrow

CAUSE: Worn Box Thread.

SOLUTION: Monitor wear OD on Drill Pipe

 

This happens when the thickness of the pin is large in comparison to the box or the make up shoulder of the box is thin, the box connection cannot support the pin in bending. 

Instead, the box bends with the pin at its mouth.  As you move towards the thicker section of the box, away from the shoulder, the box does begin to support the pin.

 

Under these conditions, pin failures can occur nearer the top end of the pin thread.  An examination (refer to image 1) of the shoulder area of the Top Sub that failed near the upper end of the thread reveals only partial contact with the box as a result of a worn box joint.

 

 

 

 

 

 

 

 

The fatigue failure of this pin thread was due to lack of support from an excessive chamfer on the outside diameter of the box.

 

 

 

 

 

 

 

 

 

 

 

 

 

In this failure, only the outer portion of the pin shoulder shows contact with the box as a result of an excessive chamfer on the inside diameter of the box joint.

 

 

 


 

This Top Sub failed due to fatigue cracking near the upper end of the pin thread.  An examination of the fractured surface shows conchoidal beach marks at the root of the pin thread in the three o'clock and ten o'clock positions.  These thumb prints, (like the impressions that receding sea water leaves on a sandy beach), marks the progression of the fatigue failure from its origin, (at the root of the thread), across the metal thickness of the pin.

 

At the ten o'clock position, a step can be seen in the middle of the beach mark.  This step is the meeting of two adjacent crack origins.  The mirror image of the crack origins on both sides of the pin thread indicate that the failure occurred concurrently on both sides of the pin thread and propagated side to side until the pin broke.  This side to side crack propagation is a good indication of the wobbling of the pin thread inside the box, that resulted from insufficient make-up torque.


Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

THREAD FAILURE

CAUSE: Too Much Torque

SOLUTION: Limit Hydraulic Pressure at Rotary Head

 

While insufficient make up torque can result in fatigue failures, too much make up torque will stretch the pin and cause tensile failures.  This is particularly true with small tool joints where the range of satisfactory makeup torque is narrow, or when overriding the limitation of the rotary drive unit attempting to free a hammer that is stuck in the hole.  This will generally lead to a ductile-type failure in the root of the last two threads near the pin shoulder or near the last thread in the box connection and can lead to a twist off of the pin in the box.  An examination of the resultant thread profiles shows the pin thread to have been stretched in tension prior to failure.


Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

LONGITUDINAL CRACK ON PISTON CASE

 

CAUSE: Loose joint

 

Problems with loose make up can also be encountered in the threaded connections used in the assembly of the down the hole hammer.  Insufficient make up torque can cause cracking in the threads of the piston cases..

 

This type of failure is always associated with excessive wear on the make up side of the threads.  Unfortunately, this wear is not easily observed in the field since it occurs on the back side of the threads.  Sectioning, as performed during factory evaluation can be used in the evaluation of this type of failure.


Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

DRIVER SUB BREAKAGE

cracked on base of shoulder

 

CAUSE: Loose Joint

 

With Driver Subs, failures due to loose make up can be encountered in the radius adjacent to the make up shoulder as well as the thread roots.  Most manufacturers shot peen these areas of stress concentration in order to reduce the incident of failure.  Excessive wear on the make up shoulder is quite often an indication of insufficient make up torque or that the driver sub has loosened in the hole.  This can occur due to hammering without rotation, in an effort to remove a hammer stuck in the hole.  Insufficient weight on the bit when drilling through soft and unconsolidated rock formations can also loosen the driver sub.

 


Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

SOLUTION

Recommended make up torque requirements for hammer threaded connections are given in the Operation and Maintenance Manual.  Similar to the torque requirements for API threads, these values are calculated to make the joint up tight enough to stretch the threads in the connections and to compress the ating shoulders elastically.  As with pin threaded connections, however, these torque values may not be possible to obtain when using the rotary drive units mounted on standard drilling rigs that are used for down the hole drilling.  The use of threaded connections with stress relief radius is, therefore, necessary.