Research On Failure Mechanism And Fatigue Life Prediction Of S135 Drill Pipe

S135 drill pipe is one of the most widely used tool in oil and gas drilling industry,but it also one of the most prone to failure tool.In recent years,a number of S135 drill pipes failed because of galling,cracking and piercing.Failure accidents seriously affected the normal drilling operation.Studying drill pipe's failure mechanism and service life prediction has important theoretical significance and engineering application value for controlling failure accident.S135 drill pipe's failure mechanism was researched based on the typical failure analysis combining with field operating environment and reproducibility test.Based on the fatigue test and reliability analysis,fatigue life of S135 drill pipe was studied systematically.The results show that galling of tool joint is mainly due to the plastic deformation and peeling under the effect of compressive stress caused by friction and compression effect among the metal surface in contact.Torque and thread compound has decisive influence on galling failure,while the influence of threaded connection's surface wear,rearranging,torque fluctuations and tensile load is not obvious.The cracking failure of tool joints is mainly due to the service environment.Box cracking failure is stress corrosion cracking.Longitudinal cracking and transverse cracking are two separate processes.The Longitudinal cracking is mainly due to organization deformation and phase transformation in thread crest caused by severe friction between the internal and external threads,while transverse cracking is mainly due to the great stress concentration in thread root.Pin fracture failure reason is mainly due to the dramatic friction between the tool joint and accumulated debris in the drill-string's rotating process.Accumulated debris blocked mud' circulation channel and leaded to poor circulation.Friction heat leaded tool-joint pin's local temperature to risen significantly,and reduced its carrying capacity.In the end,pin fractured under the effect of tensile and torsional loads.Drill pipe body's piercing failure research results show that stress concentration effect is formed in indentation bottom,and the stress concentration factor increases with the increasing of the indentation's depth.When indentation depth is up to certain value and stress of indentation bottom is greater than drill pipe's fatigue strength,it will significantly reduce drill pipe's fatigue life.Surface decarburized layer in drill pipe surface leads to great decrease of material hardness and promotes formation of corrosion pit,while its effect on tensile performance is not obvious.With the increasing of decarburized layer depth,fatigue crack's initiation life gradually decreases.When the decarburized layer depth is up to certain depth,fatigue crack's initiation life will decrease significantly.Drill pipe body's piercing failure mode includes elliptical piercing and slot piecing.Elliptical hole's length is mainly distributed in 20mm~50mm,and impact absorbed energy dispersed in 42J~156J.Slot hole's length is mainly distributed in 60mm~90mm,and impact absorbed energy concentrated in 76J~150J.There is certain correlation between hole length and impact absorbed energy.The impact absorbed energy formula of is proposed based on the fracture mechanics..The impact energy values of S135 drill pipe should be no less than 80 J in order to ensure drill pipe's safety.Calculation results show that bending and tensile stress on drill pipe body is significantly greater than that of tool joint during drilling process.Drill pipe body's fatigue strength is about 500 MPa and fatigue life S-N curve equation is a-=lg7655.137165.43 lgSN under the conditions that stress ratio is-1.Tool joint's fatigue strength is about 360 MPa and fatigue life S-N curve equation is a-=lg0534.347817.93 lgSN under the conditions that average tensile stress is 496 MPa.S135 drill pipe full-scale fatigue test results show that fatigue fracture location is in drill pipe body,and fatigue crack originated from the outer surface of tube.Compared with the fatigue life of material,drill pipe's full-scale fatigue life is significantly lower.Under the condition that confidence level is 95% and deviation is 5%,drill pipe fatigue life reliability research results show that fatigue life distribution is normal distribution while stress amplitude is 660 MPa,620MPa,580 MPa and 540 MPa separately.With the decreasing of stress amplitude,peak of logarithmic fatigue life' probability density distribution curve decreases gradually,and the dispersion of fatigue life gradually increases.Drill pipe's fatigue life S-N curve equation whose reliability is 50%,90%,99% and 99.9% separately is got.