Mechanical Model for Determining the Crossover Point of Slurry Flow in Pull-Back of Horizontal Directional Drilling Installations

At present, various predicting models are developed to predict pull-back loads for HDD projects and results at the end-point of pull-back derived from them fit quite well with the results obtained in field tests. However, the maximum loads that usually occur in the process of pull-back can not be predicted by any of them because of slurry drag effects not considered adequately. There is a crossover point in the process of pull-back where slurry reverses its flow direction while the maximum loads usually occur, since drag effects take a large proportion in total pulling force for small and medium scale HDD installations. The paper presents a model to calculate pressure drop of slurry in concentric annulus assuming laminar flow of power-law fluid. Crossover point is located by comparing pressure drops of two kinds of annulus (pipe in borehole and drill string in borehole). Pressure drops are calculated based on equation prescribed by the Society of Petroleum Engineers (SPE) as well. The result calculated from the model agrees with HDD field test data more accurately compared with the SPE equation. It is observed that the rheology parameters under low shear rates have a significant impact on pressure drop calculation.