Design and mechanical properties analysis of drill pipe's joint thread with unequal taper under complex loads

Abstract

The design of drill pipe joint thread with unequal taper is proposed to investigate the fracture failure of the API NC38 used in the drill pipe joint of the SU36-8-4H2 well. The effect of changes in thread taper on the stress distribution and mechanical properties of drill pipe joints is analyzed and compared with the API standard thread to determine the optimal thread structure with unequal taper. The results reveal highly concentrated stress at the last engaged thread root of API NC38 single-shoulder thread (SUT) may cause early yield failure of the joint threads. Adjusting the unequal taper of the pin thread mitigates uneven stress distribution in NC38 single and double-shoulder threads and enhances connection strength, particularly for SUT-II and DUT-I. However, altering the box thread's unequal taper modifies the stress concentration slightly in NC38 single and double-shoulder threads. This offers limited tensile and compressive strength improvement. The maximum Mises stress value of SUT-II is reduced by 56.69% compared to SUT. The maximum Mises stress value of DUT-I is reduced by 34.87% compared to NC38 double-shoulder joint thread (DUT). This design approach can guide the optimization of other API threads and enhance joint strength for non-API and specialized taper threads.

Keywords: Complex load; Drill pipe joint thread; Mechanical behavior; Unequal taper.

Fig. 1

Pin and box thread model parameters. (a) Single-shoulder box thread, (b) Single-shoulder pin thread, (c) Single-shoulder thread type, (d) Double-shoulder box thread, (e) Double-shoulder pin thread, (f) Double-shoulder thread type.

Fig. 2

Force characteristics of drill pipe’s joint under make-up torques. (a) Single shoulder, (b) Double shoulder. Note: ⊕ represents the force acting vertically inward on the paper; ⊙ represents the force acting vertically outward on the paper; △ represents the built-in boundary; P is the total stress, MPa; F_P is the force on the main shoulder, N; T_T is the main shoulder torques, N·m; F_s is the force on the sub-bench shoulder, N; F_n is the force on each thread tooth of the pin, (n = 1, 2, 3, …), N; is the force on each thread tooth of the box.

Fig. 3

Computational modeling of drill pipe joint threads. (a) Single-shoulder drill pipe joint thread, (b) Double-shoulder drill pipe joint thread, (c) Mesh delineation of single-shoulder drill pipe joint threads, (d) Mesh delineation of double-shoulder drill pipe joint threads, (e) Contact setting of single-shoulder drill pipe joint threads, (f) Contact setting of double-shoulder drill pipe joint threads, (g) Restraint settings for single-shoulder drill pipe joint threads, (h) Restraint settings for double-shoulder drill pipe joint threads.

Fig. 4

Relationship between the number of threaded mesh elements and the maximum Mises stress in drill pipe joints.

Fig. 5

Comparison between finite element model and theoretical model calculation results.

Fig. 6

Mises stress distribution color map of a single shoulder with unequal taper under make-up torques. (a) Mises stress distribution of SUT, (b) Mises stress distribution of SUT-II.

Fig. 7

Mises stress of single-shoulder drill pipe’s joint thread with unequal taper under make-up torques. (a) Mises stress on single-shoulder pin thread, (b) Mises stress on single-shoulder box thread.

Fig. 8

Stress color map of drill pipe joint thread under axial tension of 1250 kN. (a) Mises stress distribution of SUT, (b) Mises stress distribution of SUT-II.

Fig. 9

Maximum stress on single-shoulder under different axial tension.

Fig. 10

Mises stress of drill pipe joint thread under single-shoulder unequal taper tension. (a) Mises stress on single-shoulder pin thread, (b) Mises stress on single-shoulder box thread.

Fig. 11

Stress color map of drill pipe’s joint thread under axial pressure of 1250 kN. (a) Mises stress distribution of SUT, (b) Mises stress distribution of SUT-II.

Fig. 12

Maximum stress on single-shoulder under different axial pressure.

Fig. 13

Stress color map of drill pipe joint thread under internal and external pressure differences of 20 MPa. (a) Mises stress distribution of SUT, (b) Mises stress distribution of SUT-II.

Fig. 14

Maximum stress on single-shoulder under different internal and external pressure differences.

Fig. 15

Mises stress distribution color map of double-shoulder unequal taper under make-up torques. (a) Mises stress distribution of DUT, (b) Mises stress distribution of DUT-I.

Fig. 16

Mises stress of double-shoulder drill pipe joint thread with unequal taper under make-up torques. (a) Mises stress of double-shoulder pin thread, (b) Mises stress of double-shoulder box thread.

Fig. 17

Stress color map of drill pipe joint thread under axial tension of 1250 kN. (a) Mises stress distribution of DUT, (b) Mises stress distribution of DUT-I.

Fig. 18

Mises stress of drill pipe joint thread under double-shoulder unequal taper tension.

Fig. 19

Mises stress of the effective engagement thread tooth of the drill pipe joint under the tension of box thread with unequal taper. (a) Mises stress of double-shoulder pin thread, (b) Mises stress of double-shoulder box thread.

Fig. 20

Stress color map of drill pipe joint thread under axial pressure of 1250 kN. (a) Mises stress distribution of DUT, (b) Mises stress distribution of DUT-I.

Fig. 21

Maximum stress of double-shoulder under different axial pressure.

Fig. 22

Stress color map of drill pipe joint thread under an internal and external pressure difference of 20 MPa. (a) Mises stress distribution of DUT, (b) Mises stress distribution of DUT-I.

Fig. 23

Maximum stress of double-shoulder under different internal and external pressure differences.