The Effects of Pipe Stiffness on HDD Pull Loads

ASTM F 1962, Standard Guide for Use of Maxi-Horizontal Directional Drilling for Placement of Polyethylene Pipe or Conduit Under Obstacles, Including River Crossings, provides a procedure for estimating pull loads and stresses on polyethylene (PE) pipe as primarily a function of the drilled path and buoyant weight of the pipe in the borehole. The ASTM theoretical model does not consider the impact of pipe bending stiffness on maxi-horizontal directional drilling (maxi-HDD) installations, since it assumes that such effects may be ignored for flexible PE pipe material. In reality, the magnitude of any additional corresponding installation forces depends on the bending stiffness of the pipe, as well as the borehole characteristics. It is therefore of interest to gain an improved understanding of the possible impact of the greater stiffness of other pipe materials. The present investigation provides an analytical means to conservatively estimate the reaction forces due to bending and the associated frictional drag, allowing the original methodology for estimating pull forces to be extended to potentially cover a broader range of pipe materials. Based on some practical examples, consistent with reasonable design and installation practices, the magnitude of the additional drag forces does not appear to be significant for many maxi-HDD applications.