Drill pipe joints fatigue analysis and optimization under special working conditions

Recently, the number of ultra-deep and extended reach wells has continuously increased. The failure of the drill pipe is prominent when drilling. The thickened area of the drill pipe body and the position of the drill pipe joint often appear fatigued, especially the internal thickening zone for the nonlinear structure of the cast component. Fatigue failure occurred at the drill pipe joint due to non-linear contact between the male and female snaps. The fatigue life S-N curves show that the overall life of drilling with high-resistance torsion drill pipe is increased by 136% compared with API drill pipe, though the maximum stress value was reduced by 8.4%. The high-bend drill pipe has an overall life improvement of 174% over the API drill pipe. High flexural drill pipe reduces stress by 10% compared to the maximum value. At the same time, the drill pipe’s fatigue period and safety factor can be improved by adjusting the drilling speed and annulus pressure properly.


Introduction
During the drilling operation, the drill pipe environment is complicated, and failure accidents occur, which brings great financial losses to the oilfield development.The main part of the failure of the traditional API drill pipe is the joint connection thread.This is due to the geometry of the thread and the stress concentration effect.Under the compound alternating stress, the bottom of the thread is prone to fatigue crack, which is expanded until fracture [1,2] under cyclic stress and corrosion.
Given the above problems, the NC50 double shoulder drill pipe joint is an example to analyse the problems of insufficient torque resistance and serious stress concentration of API standard drill pipe joints, especially in the fields of extended reach wells and ultra-deep wells.The shoulder drill pipe plays an increasingly important role.

Mechanical model
By studying the stress condition of the drill pipe under different working conditions, it is helpful to understand the stress distribution of the drill pipe under special working conditions and provide a theoretical basis for designing a new structure drill pipe (Fig. 1).By simulating the stress distribution of the drill pipe under different working conditions, it can be seen that the stress concentration of the drill pipe in the ultra-deep well is mainly reflected in the joint.The maximum failure point of the Mises stress is the first thread of the male buckle and the large displacement of the drill pipe.It is embodied in the male buckle drill pipe, and its maximum failure stress is still the first thread of the male buckle.Therefore, it is very important to study the drill pipe joint.

Stress analysis of pulled, twisted, and bent threaded joints
Under ideal conditions, the axial direction of the drill string is subjected to torsional stress, tensile and compressive stress, longitudinal bending moment force, and eccentric wear of the drill pipe and the borehole wall.Under non-ideal conditions, in addition to the above forces, it will be subject to axial and longitudinal polarization [3,4], which will make the size and direction of the drill pipe and the borehole wall irregularly scarping, thus eliminating the effects of lateral polarization and eccentric wear (Fig. 2).When the drill pipe is rotated and penetrated, there is auto-transmission and revolution along the drill pipe.The revolution is broken up into two parts: the longitudinal polarization and the bending and twisting vibration caused by the polarization during slanting penetration.The size of the disturbance caused by the polarization is generally 15 mm, especially up to 30 mm.The bending and torsion lead to disturbance changes.By consulting the test data in laboratory experiments, the single root of the drill pipe does not exceed 550 mm.This simulation ignores the influence of polarization on the disturbance [5].
The physical model is a cantilever beam model in a longitudinally and transversely curved beam; one end is completely constrained, and the other gives a certain torque and provides axial and longitudinal bending moment forces.
Different from the axial force of the bending section, there is no compression bending and centrifugation in the lower part of the drill string, so the torque caused by the correlation coefficient of the inclination angle is different.

Static tension.
Taking a depth of 3000 m, the diameter of φ127 mm drill pipe, and the wall thickness of 9.19 mm as an example, the maximum tensile force is the weight of the drill string.Considering the lateral extrusion force of mud buoyancy and hydrostatic pressure, the axial tensile force at any section of the drill string is equal to the tensile force of the drill string in air multiplied by the buoyancy reduction factor of the mud.

Pulling force during drilling.
Since a part of the total weight of the drill string is applied to the drill bit for the weight-on-bit, the bottom drill string is compressed along the axis direction [6,7].For steel structural materials, the yield limit of the compression is about ten times the yield limit of the tension.Therefore, in the force analysis of the threaded joint, the influence of the alternating load on the thread of the drill pipe joint is considered when the drill pipe thread is pulled because the compressed condition is safe.

Torque calculation.
During the drilling process, the entire drill string is subjected to torque, so shear stress is generated on each cross-section of the drill string.The torque experienced by the drill string during normal drilling operations depends on the power delivered to the drill string by the turntable [8].

Bending calculation.
According to Baryshnikov et al. [9], Wilson et al. [10] and Wang [11], most of the drill failures are caused by fatigue, and the reverse movement of the drill string is the direct cause of the alternating stress.The bending stress of the drill string comes from the reverse movement of the drill string, and the other parts come from the forced vibration of the drill string under self-excited transverse vibration.

Drawing of joint thread grid
The paper takes the S135 grade API NC50 steel drill pipe joint as the object; the outer and inner diameters of threaded joints are 168.28mm and 69.85 mm.The thread type is V-0.040.The mesh drawing software is used to change the structure of the joint shoulder to maintain the same outer diameter.Other parameters are changed according to the calculation.The joint portion is divided into 428,728 units by the tetrahedral grids, wherein the number of internal thread units is 219,383, and the number of external threaded units is 209,345.To reduce the calculation time, the finite element analysis of the threaded part of the joint is only carried out for stretching, bending, or torsion.The finite element of the drill pipe joint thread is played, as shown in Fig. 3.

Simulation results of the joint structure while pulling and twisting
According to the test data and load conditions, the isotropic elastoplastic material is selected as the material of the drill joint model.The material used for the drill pipe joint is 37CrMnMoA, the elastic modulus is 2.12×10 5 MPa, the Poisson's ratio is 0.28, and the friction coefficient is 0.12 [12,13] between the contact faces.Simulating the thread force of different shoulder joints in the depth of a 7000-meter section, the maximum stress value of the high-torsion shoulder Mises is 802 MPa, and the maximum stress of the API drill pipe Mises is 875.85MPa.The stress distribution area of the first thread of the female buckle is less than that of the API, but the stress value is still large.So, reducing the maximum stress value of the male buckle and improving the safety factor of the drill pipe are important tasks for the maximum stress position of the high anti-torsion joint.At the first thread root of the male buckle, the maximum stress position of the API joint is the initial position of the male thread, which is related to the upper buckle torque of the shoulder (Fig. 4).
Using the Ncode module to analyze further the force of the drill pipe, the fatigue cycle value is simulated by the S-N stress fatigue cycle curve.Fig. 5 shows that the high torsion drill pipe is 136% higher than the API drill pipe life during drilling at a depth of 7000 m.The API fatigue cycle is 2.93×10 5, and the high torsion drill pipe fatigue cycle is 5.46×10 5 .The fatigue failure life of the high torsion drill pipe is 86% higher than the API drill pipe life.Some drilling parameters in the Tarim oilfield can be referred in [14][15][16][17].For the 6500~7000 m ultradeep well section, given a 120 kN weight on bit, the safety factor of the drilling operation alters the weight on bit and the rotating speed (Fig. 6).The speed of 75~100 r/min rotating speed can significantly increase the safety factor.When the speed is up to 85 r/min, the safety factor is the largest, and then the speed exceeds 110 r/min, the safety factor decreases.According to Yin et al. [18], Teng et al. [19] and Aslaksen et al. [20], the weight data on bit and drilling rotating speed at the section of 6550~7000 meters are brought into the simulation, and the reasonable safety factor is integrated by the interpolation method.The weight on the bit is 105~130 kN.The rotating speed is the safest in the 70 ~ 90 r/min.Fig. 7 and Fig. 8 show the API joint (right) and the high-resistance drill pipe (left) Mises stress diagram at a depth of 4,000 with borehole curvature of 10°/30 m, where the maximum stress of the API drill pipe is 749 MPa, high-strength drill pipe is 670 Mpa, and the maximum stress value is reduced by 10%.Since the maximum force surface is the bottom of the joint curved surface when the drill pipe is bent, the drill pipe will not generate excessive stress load for the upper part, and the sinusoidal load spectrum is used for analyzing the fatigue life, and its amplitude is between 0 and 1.

Conclusion
(1) The stress distribution simulated of the drill pipe under different working conditions provides an advisory opinion for the design of the joint shoulder.Also, it explains the reasonable failure of the drill pipe during the drilling process.
(2) When drilling ultra-deep wells, the life period of a high torsion-resistant drill pipe is 136% more than the API drill pipe life at the depth of a 7000-meter position.The life period affected by the lithology is changed slightly compared with the rotational speed and the weight-on-bit.
(3) When the deep kickoff point is selected, the slope level is reduced as much as possible, and the reasonable relationship between the drilling speed and the weight is adjusted to improve the life value of the drill pipe.

Figure 1 .
Figure 1.Stress distribution of high torsion (top) and high bending (down) drill pipe.

Figure 2 .
Figure 2. Top view of the drill pipe movement under ideal conditions.

Figure 3 .
Figure 3. Grid type of joint thread.

Figure 4 .
Figure 4. 7000 m well deep joint thread stress distribution (left high torsion right API).

Figure 5 .
Figure 5. Fatigue failure table of well depth and drill rod pulling and twisting.

Figure 6 .
Figure 6.Double impact of speed and weight on the safety factor.