复杂工况下4(2)/1 REG涡轮轴连接螺纹断裂分析

为预防和减少涡轮钻具的涡轮轴在解卡过程中连接螺纹发生断裂，需对其断裂原因进行分析。根据长庆油田A井所用涡轮钻具轴连接螺纹的结构，建立了带螺旋升角4 1/2 REG锥形螺纹的三维力学模型，基于连接螺纹弹塑性本构关系和有限元控制方程，分析了涡轮轴连接螺纹在压扭、拉扭、弯扭复合载荷工况下的应力分布。研究发现:涡轮轴连接螺纹部位在扭矩一定的情况下，随着钻压增大，涡轮轴外螺纹所受应力呈增大的趋势，且螺纹牙的应力分布不均匀，整圈螺纹牙上的应力波动较大；当涡轮轴受到的拉力一定，随扭矩增大，外螺纹台肩部位第一牙处应力增幅较大，存在疲劳失效的风险；在扭矩为25 kN·m条件下，当井眼曲率为10和20（°）/100m时，外螺纹的最大应力为700.0 MPa，涡轮轴螺纹连接部位比较安全，但当井眼曲率为30°/100m时，螺纹台肩内外螺纹牙最大应力为903.6 MPa，接近材料的屈服极限。研究结果表明，涡轮轴连接螺纹在弯扭载荷作用下易在外螺纹第一牙处出现应力集中现象，导致其失效，因此应避免在井眼曲率较大井段使用涡轮钻具。 

Fracture Analysis of 4(2)/1 REG Turbine Shaft Connection Thread under Complex Working Conditions

During the process of trying to free stuck pipe,it is common to have thread breakage in the turbine shaft of the drill. In order to avoid the conditions leading to thread breakage,we completed an in-depth analysis of the contributing factors.Based on the connection thread structure of turbine shaft used in Well A in Changqing Oilfield,the team developed a 3D mechanical model of 4 1/2 REG tapered thread with an established helix.They thenanalyzed the stress distribution of turbine shaft connection thread under composite loads of compressive torsion,tension torsion and bending torsion based on the elastoplastic constitutive relation of the connection thread and finite element control equation.The results showed that the stress on external thread of turbine shaft would increase with the increase of weight on bit (WOB) under the condition of constant torque.The stress distribution on thread teeth was uneven,with large stress fluctuation on whole ring thread teeth.When the turbine shaft is subjected to a constant tension,the stress on the first tooth of external thread shoulder increases with the increase of torque,and there is a risk of fatigue failure.When in the condition of the torque is 25 kN·m,the wellbore curvature of wellbore is 10 and 20(°)/100m,the maximum stress of external thread is 700.0 MPa,and the connection thread of turbine shaft is relatively safe.However,when the wellbore curvature is 30°/100m,the internal/external threads of the thread shoulder will have the maximum stress of 903.6 MPa,which is close to the yield limit of materials.Studies indicated that the connection thread of turbine shaft easily produced stress concentration on the first tooth of external thread under bending and torsional loads,resulting in failure of the thread.Therefore,where the well section exhibits a high degree of curvature,turbine drilling tools should be avoided.