Academic Journal of Science and Technology 
ISSN: 2771-3032 | Vol. 14, No. 3, 2025 

 

9 

Double	Shoulder	Thread	Mechanics	Study	and	Life	
Analysis	of	Drilling	Tools	
Junming Zhang1, 2, 3, Liangliang Dong1, 2, 3 

1School of Mechatronic Engineering, Southwest Petroleum University, Chengdu 610500, PR China 
2State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, PR China 
3Oil and gas equipment technology, Sharing and Service Platform of Sichuan Province, China 

 

Abstract: Under the trend of the continuous in-depth development of global oil and gas exploration and development, the 
torque in deep drilling is excessively high, and drill string failure accidents occur frequently. The threaded connection part of the 
drill string is the main weak link of the drill string, and improving its torsional resistance performance is the main way to reduce 
the risk of drill string fracture and failure. Based on the mechanical analysis of the torque load of the double-shoulder drill string 
thread in this paper, a double-shoulder drill string thread model was established. By comparing it with the stress collected from 
experiments, the rationality of the simulation model was verified. The sensitivity analysis of the torsional resistance performance 
of the double-shoulder joint thread under torque load was studied, and its torsional resistance performance is better than that of 
the API thread. The sensitivity analysis of the fatigue life of the double-shoulder joint thread under torque load was carried out, 
and its torsional resistance performance is significantly improved compared with that of the API thread. The research results 
show that the torsional resistance performance of the double-shoulder joint is increased by 20.8% compared with that of the API 
joint, and the fatigue life is increased by 147%. 

Keywords: Drill string thread; Double shoulder; Mechanical analysis; Fatigue life. 
 

1. Introduction 
Under the trend of the continuous in-depth development of 

global oil and gas exploration and development, deep well 
and ultra-deep well operations have become increasingly 
common. As the core component of drilling operations, the 
performance of the drill string directly affects the progress 
and benefits of the operations. However, the problem of drill 
string failure has always been a difficult issue to solve, 
plaguing major oil fields[1-2]. The fracture diagram of the 
drill string is shown in Fig.1 According to incomplete 
statistics, drill string failure accidents occur frequently in oil 
fields across the country every year, reaching hundreds of 
cases[3]. In the oil fields in western China, the cost of a single 
drilling operation can reach tens of millions or even hundreds 
of millions of yuan. Once the wellbore is scrapped due to drill 
string failure, the resulting economic losses are 
astronomical[4-5]. 

 

 
Figure 1. Failure diagram for drilling tool joints 

 
According to statistical analysis, the threaded connection 

part of the drill string is the main weak link of the drill string, 
and most failure events originate from this part[6]. Excessive 
torque load is the main factor leading to the failure of the joint 
thread. With the increase in the operation depth, the double-
shoulder drill string thread has been widely used in the 
industry because it can effectively improve the torsional 
strength of the thread and balance the load distribution[7]. 
However, it cannot be ignored that there are still many 
shortcomings in the current research on the double-shoulder 
drill string thread[8]. On the one hand, the mechanical 
analysis is difficult to accurately simulate the complex and 
changeable load conditions downhole. For example, the 
combined effects of extreme loads such as bending moment, 
torque, erosion, friction and vibration are often simplified or 
ignored. On the other hand, the accuracy of the life calculation 
method is not good enough, and it cannot provide a reliable 
reference for the service life in actual operations[9-10]. 

In this paper, through the analysis of the mechanical 
analysis of the double-shoulder drill string thread under actual 
working conditions, the mechanical performance under 
torque load is deeply explored. At the same time, combined 
with the failure cases in actual production, the root causes of 
the failure are analyzed, aiming to lay a solid theoretical 
foundation for the optimized design and reasonable use of the 
double-shoulder drill string thread. 

2. Finite Element Analysis of Drilling 
Tool Threads 

2.1. Equation for threaded connection of 
drilling tools 

The drilling tool threaded connection is preloaded by 
suitable upper buckle torque to ensure the sealing 
performance of the joint threads, enhance the resistance of the 
joint to the tensile load effect, and prevent the threads from 
unbuckling under the tensile load, so determining the suitable 



 

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upper buckle torque is one of the keys to the design of the 
drilling tool joints. The torque balance equation (1) obtained 
from the deformation coordination formula shows that when 
the drilling tool male buckle is subjected to torque, the 
external torque 𝑇 is equal to the sum of the frictional torque 
𝑇  on the thread engaging surface and the frictional torque 
𝑇  on the contact surface of the table shoulder, and the sum 
of axial force 𝐹  on the thread engaging surface is also equal 
to the combined force 𝐹  applied on the table shoulder[11]. 
Fig.2 shows the mechanical characteristics of a variable tooth 
height double shoulder drilling tool thread under torque 

loading. 
𝐹 𝐹 ，𝑇 𝑇 𝑇  (1) 

𝐹 𝐹 𝐹  (2) 

𝑇 𝑇  (3) 

 
Where 𝑛—The total number of thread engagement turns; 

𝑖 —Number of threaded turns; 𝐹  —Axial forces on the 
shoulder surface; 𝑇 —Torque generated by the shoulder. 

 

 
Figure 1. Analysis of torque forces on double shoulder drilling tool joints 

. 
The figure indicates that the plane is facing inward, 

indicates that the plane is facing outward, and is a fixed 
boundary condition. 

It can be seen that the shoulder bears part of the torque for 
the thread, and the double shoulder structure can increase the 
contact area of the shoulder, improve the friction torque 𝑇  
and axial force 𝐹   on the shoulder surface, and reduce the 
friction torque 𝑇   and axial force 𝐹   between the thread 
engaging surfaces. 

2.2. Three-dimensional finite element modeling 
of drilling tool threads 

In order to better simulate the contact of the threads of the 
joint of the drilling tool, the 3D model of the external threads 
of the pipe body and the internal threads of the coupling is 
established with the help of 3D drawing software. After the 
modeling of the external and internal threads is completed, 
the fit is set in the 3D drawing software for assembly, and the 

assembly is imported into the finite element software for 
meshing.  

(1) API threads 
Taking the 7-inch API drilling tool joint thread as an object, 

NC50 thread specification is used as the connection thread of 
the tool, and the tooth type is selected as V-0.038R. 3D 
drawing tools are used to construct a 3D model of the 
conventional buckle, which is then imported into the finite 
element simulation software for mesh delineation. In order to 
ensure the accuracy of calculation, hexahedral mesh is 
selected as the mesh type, and the cell type is set as C3D8R. 
In the thread contact area and its periphery, high-density fine 
mesh division is implemented; while in the part far away from 
the thread contact, sparse mesh division is used to improve 
the calculation efficiency, in which the number of cells for 
external thread division is 114894, and the number of nodes 
is 149180, and that of the internal thread is 123,496, 160 
nodes and 123496. 123496, the number of nodes is 160933, 
as shown in Fig.3. 

 

 

 
(a) external thread 

 
(b) female thread 

Figure 3. API thread 3D mesh model 
 

(2) Double shoulder joint threads 
The double shoulder structure adds a sub-shoulder on the 

basis of the conventional API thread, which increases the 
contact area of the shoulder surface and the friction torque of 



 

11 

the shoulder surface, thus reducing the load on the threaded 
part and making the load distribution uniform at the beginning 
and end of the thread. The three-dimensional model is shown 
in Figure 34. The mesh division is consistent with the above, 

in which the number of cells for external thread division is 
116154 and the number of nodes is 150650, while the number 
of cells for internal thread is 121156 and the number of nodes 
is 158515, as shown in Fig.4. 

 

 
Figure 4. Three-dimensional mesh model of a double-shouldered structure 

 

2.3. Simulation Model Accuracy Verification 
In order to verify the accuracy of the finite element 

calculation results, this paper refers to the literature[12] 
experimental testing of Ø139.7mm × 7.72mm long round 
threaded casing fitting, experiments were used in this casing 
fitting with different compression loads, in different number 
of upper buckles and under different tensile loads to carry out 
research experiments, for the theoretical analysis of casing 
fittings threads and numerical analysis to provide 
experimental results of the support of the data. Under the 
experimental conditions of axial tension of 1200.96kN, the 
data of stress change on the outer surface of the casing were 
collected, and the stresses of 10 data points on the outer 

surface of the casing were taken in the simulation results and 
compared with the collected data of the corresponding points 
on the outer surface of the experimental casing, as shown in 
Fig.5. 

As can be seen from Fig.5, the calculation results obtained 
by the two methods show a basic consistent trend, with a 
maximum error of 8.72%. It proves the accuracy of the finite 
element simulation model of the drilling tool threads 
constructed. The model can be used to gain a deeper 
understanding of the stress distribution of the threaded casing 
joint of the drill pipe and provide an important theoretical 
basis for optimizing the design of the main parameters of its 
threads. 

 

 
Figure 5. Comparison of experimental and simulation results 

 

3. Analysis of Thread Torque Carrying 
Performance of Double Shoulder 
Joints 

The stress characteristics of the two couplings under 
torsional loading are simulated and analyzed based on the 
three-dimensional finite element method to obtain the stress 

distribution pattern and then evaluate their torsional strength. 
The boundary conditions are fixed at one end of the joint 
threads, and the other end is applied with torsional load at the 
coupling point, which is loaded slowly by using a smooth 
loading method. The torsional performance of the two 
structures is compared by calculating the stresses under 
torsional loading. The results of the API thread calculations 
are as follows:  

 



 

12 

 
Figure 6. Torque Load - API Threaded Joints Thread Stress Clouds 

 
Fig.6 shows the stress cloud diagram of API threads under 

torsional loads of 40kN·m, 50kN·m and 60kN·m. As the 
torque decreases, the thread stress decreases, and the stress 
exceeds the yield limit (930MPa) under the 60kN·m torque. 
It can be seen from the cloud diagram that the maximum stress 
of API joint threads under torsional load is concentrated in the 
first turn of threads on the shoulder side of the main platform, 
and the friction moment generated on the shoulder side of the 
main platform resists part of the torque load, and the first turn 

of the threaded teeth is significantly larger than the rest of the 
threaded teeth, and the friction moment generated on the 
threaded surfaces resists the rest of the torque load, so that the 
distribution of the stress on threads shows that the maximum 
stress of the first turn of the threads is decreasing in turn 
afterward. The stress distribution is extremely uneven, which 
can easily cause the stress concentration in the first turn of the 
thread, resulting in fatigue damage.  

 

 
Figure 7. Torque Load-Double Shoulder Threaded Joints Thread Stress Clouds 

 
Fig.7 shows the stress cloud diagram of double shoulder 

threads under torsional loads of 40kN·m, 50kN·m and 
60kN·m. As the torque decreases, the thread stress decreases, 
and the stress exceeds the yield limit (930MPa) under the 
60kN·m torque. Under the action of torsional load, the stress 
part of the thread of the double shoulder joint is at the main 
shoulder and the secondary shoulder, at this time, the main 
shoulder surface and the secondary shoulder surface are in 
contact with the friction moment to share part of the load, and 
the main stress part of the thread is located in a circle of the 
thread close to the main and the secondary shoulders, and the 
distribution of the thread stress shows that the maximum 

stress at the head and the tail is decreasing towards the middle 
in order of the distribution. The double shoulder structure 
resists the torsional load through the friction moment 
generated between the main shoulder surface, the secondary 
shoulder surface and the thread engaging surface. Compared 
with API joint threads, the double shoulder structure can 
reduce the load of the threaded part, reduce the load bearing 
pressure of the first loop of threads, and increase the load 
bearing of the last loop of threads, so as to make the 
distribution of the stress more uniform and to improve the 
performance of torsional resistance of the threads.  

 



 

13 

 

  
Figure 8. Comparison of torsional properties of 

male threads 
Figure 9. Comparison of torsional properties of 

female threads 
 

From Fig.8 and Fig.9 shows the torque corresponding to 
the maximum stress value of the female and male threads, it 
can be seen that: the maximum stress value of the two 
structures and the magnitude of the torque load is basically a 
linear change in the relationship between the maximum stress 
value and the magnitude of the torque load, in the same torque 
load, the double shoulder structure of the peak stress is lower 
than that of the API structure, and as the torque increases, the 
torsional performance of the double shoulder structure 
improves even more. According to the maximum stress as the 
strength evaluation criterion, the comparison of the results 
shows that the torsional performance of the double-
shouldered structure is about 20.8% higher than that of the 
API structure under 60kN·m torque. 

4. Thread Fatigue Life Analysis of 
Double Shoulder Joints 

4.1. Fatigue damage and fatigue life of drilling 
tool joints 

Fatigue damage and fatigue life are key concepts in 
assessing the performance of materials and structures under 
cyclic loading. Fatigue damage refers to the phenomenon of 
progressive damage accumulation until rupture that occurs 
when a material is subjected to alternating stresses below its 
static load limit strength, or even below its yield strength, for 
a long period of time. It is significantly different from static 
load damage, characterized by low stress, suddenness and 

sensitivity to material microscopic defects. According to the 
number of cycles and stress level, fatigue damage is divided 
into high-cycle fatigue and low-cycle fatigue, the former 
cycle number is greater than 104 times less than 107 times, 
elastic strain dominated; the latter cycle number is around 104 
times, plastic deformation plays a major role[13]. 

Fatigue life, on the other hand, is the number of cycles or 
time that a material or structure undergoes from the beginning 
of cyclic loading until fatigue damage occurs. There are 
various models for calculating fatigue life, such as the two-
stage model that divides it into crack formation and expansion 
stages; the three-stage model that further subdivides the small 
crack stage; and the multi-stage model that refines the small 
cracks by microscopic, physical, and structural levels. 
Accurate understanding of the fatigue damage mechanism 
and fatigue life calculation method is crucial for predicting 
the reliability and service life of double-shouldered drilling 
tool threads under complex downhole cyclic loading 
conditions[14]. 

4.2. Calculation of drilling tool thread fatigue 
life 

This paper mainly analyzes the research drilling tool joint 
threads under high torque load, based on the finite element 
life analysis software, simulation and analysis under torque 
load, to get the life of drilling tool threads and evaluate its 
fatigue life. The calculation results are as follows: 

 

 
(a) 60kN·m 

 
(b) 50kN·m 

 
(c) 40kN·m 

Figure 10. API Fatigue Life Cloud at Thread Torque 



 

14 

 
Fig.10 shows the cloud diagram of the number of fatigue 

cycles under 40kN·m, 50kN·m, and 60kN·m torsional loads 
for API. Although the maximum stress of the female buckle 
is greater than the maximum stress of the male buckle, the 
three results are all male buckle fatigue life is less than the 
fatigue life of the female buckle, indicating that the high stress 

level of the male buckle has a wider range, resulting in a lower 
number of fatigue cycles of the male buckle, and they all 
appeared in the first ring of the male buckle, which is in line 
with the engineering of the first ring of the male buckle is 
prone to the occurrence of fatigue fracture phenomenon. 

 

 
Figure 11. Fatigue Life Curves at API Thread Torque 

 
Fig.11 shows the fatigue life of API joints versus torque 

load. As the torque increases, the number of fatigue cycles of 
male and female buckles of API structure decreases, and 
under the torque of 60kN·m, the number of fatigue cycles 

decreases to the order of 104. The number of fatigue cycles of 
the male buckle are smaller than the female buckle, indicating 
that the male buckle is more prone to fatigue failure, need to 
improve the fatigue performance of API joints. 

 

 
(a) 60kN·m 

 
(b) 50kN·m 

 
(c) 40kN·m 

Figure 12. Fatigue life cloud under double shoulder thread torque 
 

Fig.12 shows the cloud diagram of the number of fatigue 
cycles of double shoulder structure threads under 40kN·m, 
50kN·m and 60kN·m torsional loads. Although the maximum 
stress of the female buckle is greater than the maximum stress 
of the male buckle, the three results are all male buckle fatigue 
life is less than the fatigue life of the female buckle, indicating 

that the high stress level of the male buckle has a wider range, 
resulting in a lower number of fatigue cycles of the male 
buckle, and they all appeared in the first ring of the male 
buckle, which is in line with the phenomenon of fatigue 
fracture that is prone to occur in the first ring of the male 
buckle in the engineering position. 

 



 

15 

 
Figure 13. Fatigue life curve under double shoulder thread torque 

 
Fig.13 shows the fatigue life of double shoulder joints in 

relation to the torque load curve. With the increase of torque, 
double shoulder joint male buckle female buckle fatigue cycle 
number decreases from high week fatigue 106 magnitude 
down to 104. male buckle fatigue cycle number are smaller 
than the female buckle, indicating that the male buckle is 
more prone to fatigue failure, but the overall number of 
fatigue cycles is less than the API structure, has a better 
fatigue resistance. 

5. Conclusions 
In this paper, for deep and ultra-deep oil and gas drilling, 

the problem of excessive torque and frequent failure of the 
drill column, the thread structure of double shoulder drilling 
tool with better anti-torsion performance is mechanically 
analyzed and life calculation, and the following conclusions 
are obtained: 

(1) Analyze the mechanical analysis of joint thread torque 
load, establish a finite element model for calculating the 
thread strength of the three-dimensional structure of double-
shoulder thread, and compare the calculated outer wall node 
stress with that obtained from the experimental results, which 
verifies the reasonableness of the simulation model. 

(2) Comparative analysis of API and double-shoulder 
structure through torque load calculation, API stress is mainly 
concentrated in the main shoulder side, and then decreases 
sequentially, while the stress of double-shoulder structure 
becomes the distribution state of threads with two large ends 
and a small middle, and the thread stress is more balanced, 
and the torsional performance of the double-shoulder 
structure is improved by 20.8% compared with that of API 
threads. 

(3) Through the fatigue life calculation, API and double-
shoulder structure are compared and analyzed, the fatigue 
failure location of API structure is mainly located in the first 
turn of the thread, while the fatigue failure location of double-
shoulder structure is at the two ends of the thread, and the 
double-shoulder structure shows a better fatigue resistance, 
and the fatigue life is improved by about 147% compared with 
API thread. 

Acknowledgement 
This research was supported in part by the Key Core 

Technology Research and Development Project of China 
National Petroleum Corporation, “Research and 
Development of High - Power Downhole Electric Heater” 
(Project No. 2024ZG23). 

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