An oil well casing with an ultra flush coupling was being set. The lift plug was in place and the casing joint was lifted. During the lift, 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" and that the coupling mouth was flared by approximately 9/64" or 0.140 inches.
Photograph B Measurement of flared coupling mouth and surface deformation.
The magnitude of the casing mouth enlargement is further illustrated in Photograph C
Photograph C 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, which is an increase in casing outside diameter, at the threaded end of the subject casing is the result of radial force being exerted from the interior of the casing. This inside force was most probably the result of radial pressure from the tooling used to cut the casing internal threads. If excessive tooling pressure is applied, or if insufficient support is not given to the casing outside surface during machining, the result is permanent, plastic deformation of the casing mouth. The permanent plastic deformation which occured during machining resulted in flaring or lipping of the casing mouth.