The Effect Of Corrosive Environment On The Fatigue Fracture Resistance Of Drill Pipe Materials Against Low Load And Multiple Impact

Drill string is an important tool in oil drilling. Due to the long-term suffering of complex static and dynamic loads, the drill pipe stands a load of small energy and multiple impacts in the process of drilling. This kind of load often causes the drill pipe's frature failure where the local crack and gap exists. The traditional tests based on the API 5D standeard cannot simulate the actual failure of drill pipe properly. Repeated impact resistance is different to one-time resistance; it reflects the actual force environment of most machinery and equipments.By referring to the existing experimental methods and standards, this paper designed a method and device for dynamic fracture fatigue testing of drill string material which can simulate drilling conditions well. The damage mechanism of drill pipe and the calculation method of the energy under low load and multi impact are studied. According to the corrosion environment, the resistance of drill pipe material under different corrosion environment is analyzed. The following aspects are studied in this paper:(1) The influence factors on the crack initiation and fracture process of multi impact are studied. The cumulative damage model of multiple impact is established, and the process of crack such as initiation, initiation and propagation are analyzed. Combined with the stress intensity factor and the integral idea, a quantitative calculation method and formula of the multi impact cumulative energy is proposed.(2) Typical drill-pipe material's conventional mechanical properties are tested and compared with API 5D standard. The relationship between the single impact energy and the resistance of low load repeated impact is studied. The crack nucleation energy is calculated and compared with the experimental data of one-time impact test. Based on the analysis, the energy conversion efficiency of drill pipe materials is put forward. The breaking mechanism and fracture morphology is analyzed, and the fracture surface was compared with one-time impact fracture.(3) In the hydrogen sulfide corrosion environment, the rule of resistence to repeated impact with the corrosion time is studied. Through the observation and analysis of the fractography, typical fracture morphology is observed. According to the mechanism of hydrogen damage, the reason of the decline of the drill pipe material is explained by the microscopic view.(4) The influence of carbon dioxide corrosion environment to the resistance of repeated impact is investigated. The corrosion rate and the decrease of the impact resistance in different carbon dioxide corrosion environments are compared and analyzed. Through experiments, the effects of temperature increase and Cl- addition are explored. A brief analysis of the mechanism of impact resistance is proposed through the morphology of corrosion product.