Stress distribution of oil tubing thread connection during make and break process

https://doi.org/10.1016/j.engfailanal.2003.10.007Get rights and content

Abstract

The operations of make-up and break-out in oilfields have a strong impact on the service life of oil tubing thread connections. A series of full-scale tests were performed on samples of J5573.03×5.51 mm API nonupset tubing thread connection. Forty foil gages were placed on the inner surface of the pin thread and the outer surface of the box thread along the axial and circumferential direction. Using XYQ3E type oil tubing hydraulic tongs that are used in oilfields and the DH5937 type dynamic strain testing system, the axial strain, hoop strain and shearing strain of the tubing connection were tested dynamically under the action of different make-up and break-out torque and velocity, and then the failure surfaces of the tubing thread were analyzed by SEM. Major factors, which affect thread gluing and the galling position of the tubing connection, were studied and their effects were summarized through analyzing the experimental data and results. The dynamic distribution of stress field in the location where the thread connection exists are ascertained during the whole process of make-up and break-out.

Introduction

The amount of oil tubing is only lower than that of casing among the oil country tubular goods (OCTG), and it plays an important role in oil development industry. The tubing thread connections need to be made up and broken out quite a few times when the oil tubing is used in oilfields. However, in recent years, thread gluing and thread off failure happens more and more frequently, which brings great economic loss to oil and gas fields [1]. So it is very necessary and significant to research the failure problem of oil tubing.

According to the failure cases from oilfields, it is known that thread gluing and thread off failure happen on many kinds of tube including nonupset tubing, upset tubing, low grade, high grade, coupling copperizing, coupling phosphonation, and so on. Moreover, the failure position always focuses on the make-up end of the tubing. Sometimes the failure cases also happen even though the tubing quality completely meet API standard [1]. So the failure must be related to the operations of make-up and break-out. The magnitude, distribution and other mechanical properties of the stress field between pin and box are very important parameters to analyze the causes of tubing thread gluing. The load, the stress and strain state of the casing have been researched in detail at home and abroad [2], [3], however, the theory and experimental analysis of oil tubing is reported rarely. Though the casing and tubing both have taper screw threads, their stress and strain state are very different because they have unlike load state, geometry dimensions and other parameters [4], [5].

Therefore, the general objective of this research is to investigate in depth the static and dynamic stress field distribution of the J5573.03×5.51 mm API nonupset tubing thread connection under different load conditions via experimental tests. These load conditions include make-up, break-out and tensile load. Material property tests were performed for the samples and thread measurement were conducted before and after the full-scale tests. The experimental data was analyzed, and the failure thread surfaces were also analyzed by scanning electron microscope (SEM).

Section snippets

Mechanical properties of grade J55 tubing material

The mechanical properties of tubing material are an important input to investigating the failure problem of tubing thread connections. The experimental samples were manufactured from the pipe body of J55 tubing made at home and abroad. Tensile experiments were performed on the material test system. The typical curve of stress and strain for the J55 under tensile load is shown in Fig. 1. It shows the following characteristics:

  • 1.

    All sorts of mechanical properties of the domestic and foreign tubing

Experimental equipment

The tubing hydraulic tongs. The experiment adopted the XYQ3E type hydraulic tong that is manufactured by Yancheng Teda Petroleum Machinery Limited Company and is applied widely in oil and gas fields. The hydraulic tong is shown in Fig. 2. Its torque capacity is 3 KN-m; its specified speed is 28 rpm; its rated operating pressure is 10 MPa; and its rated oil supply is 901 pm.

Strain gauge. The DH5937 type dynamic stain gauge was applied in this experiment. Its sampling frequency is 100 Hz, the

Analysis of the experimental results

The tubing thread connection is made up and broken out four times during the whole experimental process. The thread surfaces have no failure after the thread connection is broken out two times. When the thread connection is broken out at the third time, there are some burrs and scratches on the surface of the first thread from the head of the pin, furthermore, the first and the 13th thread from the head of the box are polished on the whole circumference. After the thread connection is broken

Conclusions

  • (1)

    The main factor for tubing thread gluing failure is the uneven distribution of contact stress on the pin and box and the rigidity of the thread surface. So the key measure to improve the anti-galling performance of the tubing thread is by improving the rigidity of the thread surface and decreasing the local stress concentration.

  • (2)

    The make-up and break-out torque and velocity impact on the stress field distribution of the tubing thread connection during the makeup and breakout processes. It is an

Acknowledgements

The authors thank Baoshan Iron and Steel Limited Company and Drilling and Technology Research Institute of ShengLi Oilfield for their support to this research. The authors also thank Professor Lin Yuanhua and Dr. Wang Li for their many helpful comments and suggestions in doing experiment.

References (6)

  • Yi Zhang et al.

    Influence of using hydraulic tongs on thread gluing of tubing

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  • A.P. Assanelli et al.

    Numerical/experimental analysis of an API8-round connection

    J. Energy Resources Technol.

    (1997)
  • Li Wang et al.

    Experimental research of stress field distribution on API round thread casing connection

    J. Univ. Sci. Technol. Beijing

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