Dropout Failure Analysis for Long Round Thread Connection at Mill End

Round thread casing connection may be dropout failure due to main reasons of material’s property, thread parameter, make-up position or make-up torque, field operation problems etc. Thread connection is easy to dropout at field end because of complex condition in oil field. In the present study the cause of thread connection dropout at mill end was analyzed by combining relative theory with test in this paper. The macroscopic analysis was done for failure sample. The material’s physical and chemical properties were checked. The measurement and comparative analysis was carried out for dropout coupling’s outside diameter and thread taper. Full scale tension test was done for two lots of casing connection simples. The finite element simulation analysis on interference thread connection was carried out under axial tensile loading according to calculation. The results showed that the casing material’s yield strength was lower than standard value. The thread connection’s make-up trace location at mill end was in accordance with corresponding standard. Dropout coupling’s outside diameter at different axial location had a good consistent and is little difference. The variation scope of dropout couple’s thread taper at mill end was high. The finite element force simulation result refer thread connection at mill end could not dropout at make-up trace location. The final results showed that dropout failure was ascribe to unqualified casing material’s yield strength and thread connection break-out because of float make-up in oil field.