A Research On Failure Mechanism Of Drilling String In Interaction Of Stresses And Corrosion

Today the studies of mechanism of failure of drilling string are based on mechanics, aremainly fatigue failure. But investigation from fields indicate, drilling string failure often takeplace in a short time, and its service life are far less than its fatigue life, this indicates there areother factors effect on service life of drilling string. Corrosion trace is discovered at both insideand outside surfaces of the dissected drilling strings of oilfields, this also illustrate corrosion is avital reason. Working conditions of drilling string is complex underground, various factors ofteninteract, in the process of drilling, drilling string acts complex movement in drill fluid, drillfluid unavoidable corrupt drilling string because such as oxygen, hydrogen sulfide and manysalts are dissolved in it. At the same time, many cyclic stresses act on drilling pipe, such asflexural cyclic stress, torsional cyclic stress. So in the paper interaction of cyclic stress andcorrosion is considered as the main reason that causes drilling string failure.Through studying mechanism of drilling string failure in the action of interaction of cyclicstresses and corrosion, we find fracture source is accord with pitting corrosion inside surface ofdrilling string, crack from pitting corrosion lead to drilling string failure, the process of drillingstring failure is new drilling string → corrosion pit forming and crack initiation → corrosionfatigue crack propagation→piercement or fracture. Based on the study, a model of failure ofdrilling string in the action of interaction of cyclic stresses and corrosion is built up. In themodel, corrosion fatigue cracks form from corrosion pit, in load condition, the composite stressact on drilling string exceeds the critical fracture stress, corrosion fatigue crack propagates,produce activation metal surface and corrosion happen. Unload, crack-tip displacement is dueto corrosion, the corrosion of crack-tip dies out, crack-tip is inactivation, corrosion fatigue crackcontinuously propagate by repeating the process. The model can solve the problem of drillingstring failure in theory, and is of guiding significance for the safety evaluation of in servicedrilling string and the accidental analysis of drilling string failure.