Research On Analysis Method Of Reliability Of Drilling String In Vertical Well

By analyzing the data of drilling string failure, 80 percent of failures are caused by fatigue. Using theoretical analysis method, and combined with data in site, the reliability of drilling string in vertical well was researched.The mechanism of drilling string failure is that vibration of drilling string is generated with rotating because of buckling, so the stress of drilling sting is alternating, under the condition of alternating stress, the crack of drilling sting is propagating. According to the energy principle, the model of relationship between pitch and drilling parameters including of pressure, rotary speed, torque is established, more parameters are considered and closer to the site. Considering fatigue and based on analysis of mechanics of drilling string, strength calculation method is build and increasing safety of drilling string comparing to conventional method that is under the condition of static load. Using data in site, the rule of fatigue strength coefficient with drilling parameters is analyzed, fatigue strength coefficient is decreasing with pressure, rotary speed, depth increasing, and considering fatigue is important. Based on the analysis of mechanics of connection, deformation coordinating equation is established under the condition of axial load and bending load, and according to boundary conditions, the model of load of thread is constructed. The result of analysis of strength calculation shows that when connection stress is constant, stress concentration factor is decreasing and strength coefficient is increasing with the chamfer radius of thread bottom increasing.According to S ? N curve of drilling string and the Miner fatigue accumulative damage theory, calculation method about fatigue accumulative damage of drilling string is build, which can help to weight fatigue damage. The analysis of an example shows that drilling string damage is decreasing with depth increasing and increasing with rotary speed increasing. The method of replacing drilling string with drilling string above it is better, after replacing of six times, damage of all drilling string is same. And S ? N curve of drill collar is derived, which can help to analysis of fatigue life of drill collar.Based on analysis of fatigue crack life, according to character of drilling string mechanics, the assumption that fatigue crack of drilling string is I, III complex crack is suggested, and that is fit for drilling string, and the propagation life of it is researched.According to distribution rule of drilling string size, material character, drilling parameter based on reliability theory, the method of calculating fiduciary level of drilling string is analyzed, which is advanced relativity.