Experimental Study on Drill Pipe Stress-Fatigue Life Curve

Drill pipe failure due to fatigue is the main cause of the oilfield drilling accidents, so it is necessary to study the stress-fatigue life curves of drill pipes. The fatigue performances of S135 and G105 drill pipe materials were studied by using the rotating bending fatigue testing machine. The logarithmic normal distribution model was adopted to obtain the stress-fatigue life curves corresponding to any survival rates. The direct extrapolation and 2m-1 index method were adopted to extend the fatigue curve into long-life area and the extrapolated fatigue results were compared with the up-and-down method results. The up-and-down method showed that the fatigue strengths of S135 and G105 drill pipes (survival rate was 99% and confidence level was 95%) were 550.19 MPa and 454.79 MPa respectively. The corresponding strengths of drill pipes S135 and G105 were 510.23 MPa and 402.01 MPa respectively when the fatigue life obtained by the curve with direct extrapolation survival rate of 99% was 1.0×107, while the 2m-1 method showed that when the curve of survival rate 99% obtained a fatigue life of 1.0×107, the drill pipe strengths were 532.95 MPa and 428.98 MPa respectively. The tests turned out that the fatigue strengths extrapolated by these two methods were both less than the test values. The fatigue strength value measured by the 2m-1 index method was closer to the test value by up-and-down method, so the 2m-1 index method was recommended to extrapolate the probability-stress-fatigue life curves so that a complete fatigue life curve could be obtained.