Threaded joints are key components of core drilling tools. Currently, core drilling tools generally adopt the thread structure designed by the API Spec 7-1 standard. However, fractures easily occur in this thread structure due to high stress concentrations, resulting in downhole accidents. In this paper, according to the needs of large-diameter core drilling, a core barrel joint was designed with an outer diameter of Φ135 mm and a trapezoidal thread profile. Subsequently, a three-dimensional simulation model of the joint was established. The influence of the external load, connection state and thread structure on the stress distribution in the joint was analyzed through simulations, from which the optimal thread structure was determined. Finally, a connection test was carried out on the threaded joint. The stress distribution in the joint thread was indirectly studied by analyzing gas leaks (i.e., the sealing effect) under axial tension. According to the test data and the simulation results, the final joint thread structure was optimized, which lays a good foundation for the design of a core barrel.

In the past decades, horizontal directional drilling (HDD) has been successfully used to install various pipelines in different strata. However, construction accidents such as drill‐burying and drill‐sticking occur occasionally when pipelines installed by HDD method in an unstable stratum such as sand cobble stratum. Recently, HDD with dual drill pipes was used to install pipelines in unstable stratum, and the effect is significant. The law of cuttings migration for HDD with dual drill pipes is still unclear. Therefore, it is necessary to study the law of cuttings migration in reverse circulation with dual drill pipes. This study performs numerical simulations and experimental research on the cuttings‐carrying process in reverse circulation directional drilling with dual drill pipes. Based on the assumption of dual concentric pipes, simulations of fluid‐solid two‐phase flows are conducted in different flow channels between the inner and outer drill pipes. An experimental cuttings‐carrying model is then established. By combining the results of the numerical simulations and experimental investigation, the hydraulic parameters of the dual drill pipe system are optimized, and the rationalities of the drill tool design and the grading selection are validated. The results of this study provide a reference cuttings‐carrying model during reverse circulation HDD with dual drill pipes.

In horizontal directional drilling (HDD) construction, the rig pulling load is the load directly applied to the rig, which is an important basis for selecting power and checking capability of drilling equipment. Existing models cannot directly calculate the rig pulling load during the pull-back process. In this study, a model that takes into account the influence of the drill string and the soil pressure was developed to predict rig pulling load. The accuracy of the model is verified by comparison with existing models and field tests. The results show that pullback capability of rig should be determined by the maximum rig load instead of the maximum pull head load. For lighter high-density polyethylene (HDPE) pipes, using the mud with good lubrication function is the most effective to reduce the maximum rig load, however, for heavier steel pipes, the most effective way to reduce the maximum rig load is choosing pipe delivery method with less resistance. The field test proves that the model can predict the rig load accurately when installing pipes in water-rich unstable strata.