Defective Oilfield Casing Coupling

An oil well casing with an ultra flush coupling was being set. The lift plug was set, the casing joint was lifted, the lift plug dislodged, and the casing fell to the rig floor, injuring a rig hand. The casing coupling is shown in Photograph A.

Photograph A: Subject casing coupling with expanded and flared mouth.

The measurement of the coupling mouth, as shown in Photograph B indicated that the casing outside diameter was 8 13/16". Thus, the coupling mouth was flared by approximately 9/64" or 0.140 inches.

Photograph B: Close-up profile view of measurement of flared coupling mouth and surface deformation.

The magnitude of the casing mouth is further illustrated in Photograph C

Photograph C: Overall view of flared casing coupling mouth.

Visual examination revealed that the subject casing was flared as a result of improper machining when the subject 8 5/8" casing coupling was manufactured.

The flare (increase in casing outside diameter) at the threaded end of the subject casing is indicative of radial force being exerted from the interior of the casing. This inside force is most probably the result of radial pressure from the tooling used to cut the casing internal threads. If excessive tooling pressure is applied and/or if insufficient support is not given to the casing outside surface during machining, the result is permanent, plastic deformation of the mouth of the casing. This permanent plastic deformation will result in flaring or lipping of the casing mouth.

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