Abstract
Corrosion fatigue occurs in drill stems during drilling operations, cracks developing as a result of simultaneous action of cyclic stressing and corrosion pitting. Fatigue failures will occur without the presence of corrosion, but they occur quicker with corrosion present. Efforts to reduce corrosion fatigue failures were made by Hughes in 1932 when it was recommended a protective coating be applied or that a zinc wire be wrapped around the pipe under the counterbore of the tool joint. This problem was solved by welding the tool joint to the pipe, preventing the mud from reaching threads in the vulnerable zone. Zones of various drill stem members with greatest susceptibility to corrosion are illustrated. Failures in zones adjacent to unengaged threads in bottom of drill collar are vastly accelerated under corrosive conditions.
It is possible, by inspection, to anticipate corrosion fatigue failures by detection of cracks. Preliminary tests indicate that under corrosion fatigue conditions depth of a crack for a given length is proportional to the severity of corrosion.
Corrosion of drill collar boxes is frequent and serious. Practically all failures of drill stem members result in difficult fishing jobs.
