High Bending Bearing Drill Pipe Thread And Fatigue Behavior Research

Recently, directional wells, horizontal wells, multilateral wells, highly deviated wells and other advanced drilling technology have been widely used. Such well-type structure can significantly reduce drilling costs and improve reservoir development rate. However, in directional wells, horizontal wells, multilateral wells are present curved borehole section, which is the deflecting section. In the curved borehole, due to the wall of the drill string by force, will have a greater deflection. Drilling string rotates in the wellbore most of the working time. So the promise of a curved borehole drill string with the fatigue life is far less than vertical wells'condition. Drill pipe joint threads are the weaknesses throughout the drilling string. With the increasing of borehole's dogleg, bending loads during drilling process is getting worse. And the drill pipe joint thread strength has been become one of the main factors limiting the well bore trajectory design. Therefore, the research of increasing drill pipe joint thread bending streng is significantly. Based on the above, the research work is carried out as following:(1) For curved portion of the drill string suffered complexity loads, the author proposed high bending tool joint thread and high bending double shoulder tool joint thread. Based on orthogonal optimization method, we optimizated the key structural parameters (thread angle, taper gage, length of the thread, thread pitch and bevel angle) of the two kind of drill pipe joint. Obtained that the weights of the key parameters of high bending tool joint in order is thread pitch, thread angle, bevel angle, length of the thread and taper gage. The weights of the key parameters of high bending double shoulder tool joint in order is thread pitch, thread angle, length of the thread, bevel angle and taper gage. The better combination of the two structures is thread angle 45°, taper gage 1:12, length of the thread 133.35mm, thread pitch 3teeth/inch and bevel angle45°. High bending tool joint thread and high bending double shoulder tool joint thread bengding strength increase 25%.(2) Research the effection of load carrying properties by the deviation of taper gage, tooth type size, bevel angle and inclined shoulder clearance. The research show that taper deviation has a greater impact on the drill pipe torsional performance and compression performance, in the machining process, we should be strictly controlled the taper deviation, if not, we should relax the box's negative bias, and as much as possible to ensure that the external thread taper deviation does not occur. Tooth top negative deviation has less impact on the performance of the two structures carrying the tool joint thread, but overall positive tooth type and negative deviations are torsional performance and compression performance will have a greater impact, which is extremely sensitive parameter; negative deviation of the angle of inclined shoulder has a greater impact on its torsional performance and compression performance, and less impact on the bending performance and compression performance; the clearance of inclined shoulder could effect the torque and compression performance, if it can be controlled in the range of 0?0.02mm, we should try to reduce the clearance, if not, we should loss some of the torsional strength and compression strength, and control the clearance in the range of 0-0.02mm. And also computed the effection of many deviations of the load bearing strength, provide a reasonable basis for determining its tolerance range.(3)For the processing error during machining process, the author based on the method of CAE co-simulation, established 3-D drill pipe thread lathing model to research its effection. And deduce the computation formula of compensation due to thread turning cutting force by the principle of cutting radial force equal to the workpiece deformation force.(4)Studied the bending moment loads of the drill pipe in crooked hole, and it can be refer basic datas for the computation of drill pipe fatigue life. We researched the bending moment of SM1 and SM2 with whole well drillstring finite element model, and analysised the bending moment changing law while the drillstring rotate in the curved length, and even changing with borehole bending rate. Obtain the drill pipe 3D section bending moment of SM1 direction and SM2 direction with the method of back to overlay. And define the bending moment is bending unsteady section in the range of 0?25m, high bending load area in the range of 25m?170m, steady bending area from 170m to the point to horizontal section.(5)Propose theoretical model which is applicable to the calculation of drill pipe thread multiaxial fatigue by analyze the theory of multiaxial fatigue. And compute the drill pipe multiaxial fatigue life through finite element model. Research the affection law of API and high bending drill pipe thread of borehole bending rate. Obtain that the fatigue mainly occur during the bending rate lagger than 12°/30m. Set the range 25m-170m as an example, and based on the fatigue life equal principle, simulated the loading spectrum with sine function, deduced the formula of load spectrum. And then verify the accuracy of the fit formula with the computation results.Research work of this paper is to improve the bending bearing strength of the drill pipe joint, refer mechanical analysis, structural optimization and tolerance parameters selection method for the development and design of new drill pipe thread. A new scheme was proposed to improve the machining precision. The fatigue life analysis also could refer new accordance to scrap drill pipe joint.