Prediction of rotational torque and design of reaming program using horizontal directional drilling in rock strata

► The data of projects before are analyzed to help build models. ► An improved means of estimating the torque is provided. ► A useful means is offered for engineers to select appropriate size reamers. Horizontal directional drilling (HDD) has been widely used to efficiently and cost-effectively install pipelines in a variety of conditions. In particular, during application of HDD in the recent West–East Natural Gas Transmission Project in China, rocky conditions were encountered. In such difficult situations, it is necessary to predict the torque required for performing the reaming operation, as well as to select the appropriate type and size of reamers. However, there is presently not an appropriate and convenient method to accomplish these tasks, especially for large pipelines, such as 1219 mm (48 in.) diameter, which typically require a borehole of approximately 1600 mm (64 in.). The process of expanding the initial borehole to the ultimate required diameter in rocky strata is often a complicated procedure involving several passes of intermediate size reamers. Although available information for drilling techniques does provide some means of predicting the torque, it is not sufficient for meeting the present needs, including providing guidelines for determining the corresponding reaming program. Therefore, the results of recent research, which addresses rock crushing energy, have been incorporated to develop an improved prediction method. The results reflect rock characteristics collected at different job sites, which may be applied to new projects containing similar rock strata. The improved methodology is a valuable aide for engineers in their selection of an appropriate back-reaming program.