Visual, non-destructive failure analysis on a failed 6⅝” oil field casing collar coupling was requested by a J.E.I. Metallurgical, Inc. client. The casing collar had failed in approximately the 15th thread from the cut end. The fracture surface had visible areas of washout, metal fatigue and mechanical damage. The mechanical damage was most likely due to rubbing against the mating fracture surface following complete separation of the collar.
Photograph 1 Overall view of failure 6 5/8” casing collar
The areas of washout, metal fatigue and mechanical damage are noted in Photograph 1. Metal fatigue and mechanical damage are understandable terms. Washout is erosion, usually of a fracture or through wall perforation of a crack. It occurs where high pressure fluids flow from inside the pipe or casing through or over the crack or fracture to the outside of the pipe. The high pressure fluid flow will literally wash or erode away the steel surface. The outside surface of the collar exhibited at least two sets of tong marks. That is, tong marks made at different times. See Photograph 2
Photograph 2 Tong marks on the external surface of the failed casing coupling.
A close-up view of the tong marks at the edge of the fracture surface is shown in Photograph 3.
Photograph 3 Tong marks which have deformed or nippled up the edge of the fracture.
It was evident from the microscopic view of the tong marks in Photograph 3 that the set of tong marks at the edge of the fracture surface have deformed or nippled up the fracture surface. Therefore, these tong marks are post failure. They probably occurred while removing, or attempting to remove the failed coupling from the subject casing joint.
A close-up view of the washed out segment of the subject collar fracture is shown in Photograph 4. The surface literally appears as if it was “washed” and eroded.
Photograph 4 Close-up view of washout on casing collar fracture.
A close-up view of the fracture surface adjacent to the washout area is shown in Photograph 5.
Photograph 5 Casing coupling fracture adjacent to washout.
A series of fatigue ratchet marks are noted initiating or emanating from the groove at the bottom of the coupling threads. Ratchet marks are created when several, in this case numerous, small fatigue cracks initiate in close proximity to one another. As each tiny fatigue crack grows into the wall of the coupling, they merge together and form a united fatigue crack front. The ratchet marks are small shear steps located between each tiny fatigue crack before they merge and form a united fatigue crack front. The fatigue crack origins are located midway between each ratchet mark and can usually only be visually observed with higher magnification using a digital microscope, stereomicroscope or a scanning electron microscope. J.E.I. Metallurgical, Inc. provides a Keyence Digital Microscope, which is transportable for field or location investigations.
A close-up view of an area where the coupling fracture surface has been mechanically damaged is shown in Photograph 6.
Photograph 6 Mechanically damaged section of coupling fracture surface.
Several opinions and conclusions result from the visual failure analysis. Those opinions and conclusions are:
1) Failure of the subject 6 5/8” casing coupling was the result of metal fatigue.
2) The metal fatigue began at the groove at the bottom of the coupling threads.
3) The metal fatigue appeared to extend fully, circumferentially (360°) around the coupling, indicating a long-term, low stress environment.
4) An extended area of washout in conjunction with large amounts of fatigue fracture indicates that the coupling operated on this well, with a through wall crack or leak for a significant time.
5) Strength, hardness, and/or steel type, i.e., N-80, P-110, etc., was not determined because destructive testing was not authorized.
6) Thread form, form conformity or non-conformity and machining errors were not evaluated as that would have required destructive testing.
7) Because of the apparently low stress environment in which the metal fatigue occurred, a slightly stronger collar material may have prevented the subject casing failure.