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.