Failure analysis and solution studies on drill pipe thread gluing at the exit side of horizontal directional drilling

With its advantages of a shorter construction period, fewer external constraints, and higher construction quality, horizontal directional drilling (HDD) technology has become widespread throughout the world's trenchless industries. However, thread gluing accidents have often occurred at the exit side during construction. Not only has this caused an excessive number of drill pipe failures, but it has also significantly extended the construction time, thus greatly limiting the development of HDD technology. In order to reveal the failure causes, the authors of this study researched the construction conditions of the Lanzhou-Zhengzhou-Changsha third crossing of the Yangtze River. Then, material tensile property tests and make-up and break-out tests of the same batch of drill pipe were performed to observe the material properties and structural parameters. A 3D drill pipe thread finite element model (FEM) was established based on principles of virtual work, nonlinear contact theory, and the elastic-plastic yield criterion, which may be loaded by various combining loads, such as make-up torque and bending moment, which cannot be loaded on a 2D model. Analysis of the construction situations with this FEM showed that insufficient make-up torque was the main predisposition factor and bending moment generated by the drill pipes hanging were the immediate causes of the failure accidents. Some improvement measures have been proposed according to the failure causes, and successfully applied in engineering. A beveled shoulder thread (BST) was proposed, which is superior to the API thread by having a higher bending strength, larger flexural rigidity and a stronger seal performance withstand bending load, and is thus suitable for HDD. The work presented in this paper is a reliable guideline for reducing thread gluing accidents and thus reducing construction time in HDD projects.