Fatigue life evaluation: a key to avoid drillpipe failure due to die-marks

摘要

Slips and tongs produce permanent marks on pipe body and tool joints. Such marks develop high stress concentration that reduces strength of pipes. The remaining strength of pipes often falls below the pipe stresses which can lead to tubular failure. Slim pipes are most susceptables to failure due to diemarks. In many instances, slim pipes are handled suing double elevator system to reduce pipe failure. In this apper, results of a recent study of various die-mark related failures of drill pipes under different loading conditions, with particular emphasis on fatigue damage are presetned. Stress concentration due to die-marks is characterised by finite element analyses as a function of mark sizes to cover various gripping systems available in the market. Then a methodology is presented for the prediction of failure due to cumulative fatigue damage. Effect of stress concentration arising from die-markes is taken into account in the analysis. Results of this study suggest that the effect of stress concentration on the cumulative fatigue damage may be significant depending on particular gripping system in use. In mose cases, the fatigue life evaluation based on conventional assumption of smooth pipe surface is found to be very unsafe. Thus, a new approach is proposed in this paper for prediction of true safe life of marked drillpipes against fatigue failure. Fatigue damage results are presented in graphical forms. Calculation of cumulative fatigue damage of drillpipes used in a number of drilling events is then presented systematically in tabular form to assist drilling engineers in teh evaluation of actual remaining fatigue life of drillpipes for the target drilling event using a particular gripping system.