Connection Thread Strength Analysis And Optimization Of Srew Drill Shell

This thesis focus on screw drill failure and makes screw drill shell connection thread as the research object, we research the issue of thread type’s optimization choose based on the vibration of drilling tool. Screw drill also known as positive displacement motor (PDM), it’s a positive displacement downhole power drill including these parts:by-pass valve, motor assembly, cardan shaft assembly, transmission shaft assembly and so on. Input high-pressure drilling fluid into motor to make the motor rotate in order to drive the drill bit, in this progress the hydraulic pressure of high-pressure drilling fluid is transformed into mechanical energy, compare with other power drills, screw drill has its own structure characteristics and performance advantages that make it has wide range of applications, high efficiency and simple operation.At present, The failure probability of screw drill remains high. The vibration of drilling tool is large and The crack accidents of drills’ thread joint happen from time to time. So it’s very necessary to do researches on screw drill failure.The failure types of screw drill mainly include:shell crack, stator rubber failure, thread joint crack and so on. The failures are mainly caused by fatigue, nowadays, the researches on screw drill thread joint fatigue life and reliability still need to be completed such as in thread joint fatigue life calculation, we need to use environment load and material performance as constants instead of variables that obey the distribution and the result of the fatigue life is a fixed value; the researches on screw drill thread joint life’s reliability are limited to mathematical statistics and theoretical calculation, there is no research on vibration impact dynamic response. In summary, the researches on screw drill thread joint strength and life reliability need to be completed.A lot of drilling sites at home and aboard shows that screw drill thread joint fatigue damage has two main types; fatigue fracture and tooth shearing failure, generally speaking, maximum stress appears on male and female bucket joints’ last tread area and thread may fracture on these areas at first. Screw drill shell takes alternating bending stress due to internal fluid dynamic load and external shell vibration. Because drill shell’s rigidity is larger than the thread, the stress concentrates on connection thread it may easily cause the damage in connection thread. This thesis analyzes screw drill shell connection thread’s structure from these aspects:thread failure, pipe string dynamic, drill vibration, thread stress and so on.Consult the relevant literature at home and aboard, know the fatigue type and fatigue mechanism of thread, and get the reason of connection thread failure from screw drill failure, analyze the static and dynamic characteristics. From the literature of drill pipe dynamics and drill vibration, we can get that drill connection thread is under drill pipe dynamic load. So when we analyze the structure stress of thread, we must analyze drill pipe dynamics and vibration to get the initial condition of the thread stress.By analyzing drill pipe dynamics we simulate the complex actual stress state in drill pipe system, simplify dynamic and mathematic model in order to make assumption conditions, and it provides theoretical support for drill pipe modeling analyze.Drill vibration is the main reason that causes the failure, before research the thread structure we must research the drill vibration mechanism and it’s solving method, there are many solving methods in this thesis and offers reasonable assumption condition, emphasized that longitudinal vibration has the most effect on drill pipe life and build pipe longitudinal vibration model according to actual drill combination. With the help of FEA software we do forced vibration analysis (Harmonic response analysis), reach the regularity and characteristics of Drill string vibration and offer the initial dynamic condition for screw drill shell connection thread stress analysis.This thesis use φ244 screw drill as object of research, choose the best design which depends on different thread angle, pitch, taper for shell thread structure, by using finite element analysis to do thread geometry modeling, define reasonable boundary condition, loading solving analysis, abstract data result(all root position Equivalent stress, axial stress, dangerous point stress, maximum contact stress and so on). Faced with a large number of portfolio analysis of the results of a large amount of data, we use orthogonal design 3 factors and 3 level to reduce the detailed analysis scheme, and then use orthogonal data analysis method (Mean value,Range method) to do result data reduction, from effect graph we can see the design scheme from different group in dynamic and optimize the level of each factor. At last, do structure refinement modeling to thread angle and pitch, use orthogonal data analysis method (Mean value,Range method) to analyzethe result and do arrangement, analyze each factor that affects thread stress, choose the best thread type.Through the analysis of this thesis, we can know more about the stress effect regulation of drill thread and drill vibration.This thesis focus on the practical problem screw drill shell connection thread fatigue failure, do vibration response analysis, get the characteristics and regulation of dynamic loadings. Then, we do study on the strength analysis of screw pair to get screw pair dynamic stress variation characteristics. So that we can choose best thread type, reduce the failure rate of screw drill connection thread, which has great significance to improve the overall economic benefit of oil field development and the sustainable development of oil field.Though this thesis has made a lot program analysis, it is still in the initial exploration stage. Mainly reflected in:(1) Make a lot of assumption conditions in the method and the theory of the application.(2) We can’t take too much concern on influence factor and level choosing.(3) There are unclear evaluation criteria of the evaluation system. Therefore we need to do more in analysis so that we can conclude more accurate and reasonable theoretical issue.