TOP SUB THREAD FAILURE

 

CAUSE: Corrosion at the root of the thread

SOLUTION: Regular replacement of thread lubricant

 

Failure in a API Regular Top Sub thread (in the thread roots) near the base of the pin due to the threads being exposed to corrosive drilling fluids.  Resulting corrosion which pits in the thread roots become very sharp notches which are stress risers.  The effect of these risers can be seen by the steps around the outer periphery of the fractured surface.


Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

BROKEN RIGID VALVE

 

CAUSE: Corrosion Fatigue

 

SOLUTION: Immediately after drilling with foam or water injection, clean and oil hammer by blowing dry lubricated air through the system.

 

This rigid valve has failed due to cracking in the under cut radius.  The steps around the outer periphery of the fracture surface indicates that the failure was the result of corrosion fatigue.  The severity of the rust pits on the surface of the part and the large number of steps on the fracture surface gives a good indication that proper precautions in order to prevent corrosion were not taken with the foam used in drilling application.

 

See images below


Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

BROKEN PISTON

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CAUSE: Corrosion in piston undercut

 

SOLUTION: Immediately after drilling with foam or water injection, clean and oil hammer by blowing dry lubricated air through the system

 

 

The failure of this piston was the result of corrosion pitting at the start of the radius joining the under cut diameter to the lower guide diameter.  Part configurations which contain changes in section sizes are particularly affected by corrosion pitting.  The change in section size, itself, represents a stress concentration.  The addition of corrosion pitting multiplies the effect of the stress concentration.


Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

BROKEN PISTON

 

CAUSE: Corrosion in foot valve counter bore.

 

SOLUTION: Immediately after drilling with foam or water injection, clean and oil hammer by blowing dry lubricated air through the system.

 

This piston has failed due to cracking occurring between the striking face and the first oil groove.  An examination of the fractured surface shows the conchoidal beach marks to originate on the surface of the foot valve counterbore at the 7 o'clock position and propagate outwardly in concentric ring towards the outside diameter of the part.  Severe corrosion pitting present in the foot valve counterbore resulted in the piston failure.

 

See images below.

 


Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

CIRCUMFERENTIAL CRACKING IN PISTON

 

CAUSE: Corrosion in thread roots

SOLUTION: Regular replacement of thread lubricants

 

Circumferential cracking was encountered in the upper end of the piston case.  A section of buttress thread shows the fractured surface to contain steps indicative of a corrosion fatigue failure at the thread root.

 

Failures due to corrosion fatigue are encountered more frequently in the upper threads of the piston cases, since thread lubricants are generally not applied as frequent of an interval as on the driver sub end.  When drilling in limestone, on the other hand, corrosion fatigue failures are sometimes encountered on the driver sub end because of longer bit life and again less frequent application of lubricant.

Note: Most mission piston cases have a surface treatment to reduce the risk of corrosion in threads.  See images below


Reference:                       

Trouble shooting manual

Driltech Mission, LLC

Sandvik

SHANKED BIT

 

CAUSE: Corrosion in shoulder radius

SOLUTION: Clean and coat all surfaces with a more viscous drill oil prior to storage.

 

In this image shanking of a bit head has occurred at the radius joining the head to the shank due to corrossion fatigue.  The steps resulting from the joining of cracks originating at sites of corrosion pitting are evident around the outer periphery of the shoulder radius.  Normally, the fractured surface detail of shank failures are not generally easily seen because of continued hammering after failure has occurred.

 

Note: Corrosion in the shoulder radius is critical when using oversize bit heads.  It is important to keep the shoulder radius well oiled and corrosion free when the bit is not in use.


Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik