Assessing Friction Coefficient in HDD Using Analytical Models

Abstract

In horizontal directional drilling (HDD), accurate determination of the pullback force for pipe installation (pullback) during the design phase is critical to the success of the project. For the calculation of pullback force, a friction coefficient of 0.3 is generally suggested for the lubricated borehole in the design. In this paper, friction coefficients are determined from data collected during running in hole (RIH), i.e., moving the drill assembly toward the cutting face without drilling, for the drilling of the pilot hole and reaming stage. This gives a friction coefficient that is calculated for hole conditions similar to those in the pullback process. The friction coefficient is back-calculated based on the equilibrium of thrust force (μF) and torque (μT), using three models. The main difference among the three models is whether or not the model incorporates the effect of annular pressure at the drill bit and viscosity of drilling fluid in the calculation. Results indicate the friction coefficient obtained based on the equilibrium of thrust force is larger than that for equilibrium of torque. The range of μF is between 0.10 and 0.40 and that of μT is between 0.05 and 0.2. This paper also compares three different models based on the calculated friction coefficients to identify the effect of annular pressure and viscosity on the calculation. The results indicate that the difference in terms of the calculated friction coefficient between the models is less than 10%. This paper provides an overall idea of the range of the friction coefficient for a clean hole in HDD based on data collected during an HDD project.