Maxi-HDD Pull Loads for Nonlevel Grade for Polyethylene Pipe

Abstract

ASTM F1962, “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 pipe primarily as a function of the drilled path and buoyant weight of the pipe in the drilled hole. The original equations initially were developed assuming a level grade or that the horizontal directional drilling (HDD) entry and exit elevations were the same. In practice, however, it is not uncommon that there would be a finite grade or difference in elevation between the entry and exit points of the drilling operation. It is, therefore, of interest to extend the equations and methodology of ASTM F1962 to address the installation of polyethylene pipe for the case of a nonlevel grade. The primary source of drag corresponds to the frictional drag associated with the buoyant weight of the polyethylene pipe, which buoyancy also tends to pull the pipe upwards, opposite gravity, along sloped portions of the bore path. The effects on the pull load depend on the degree to which the drilled hole is free to drain slurry from the lowest access (entry or exit) point during the pullback operation, and the possible deployment of antibuoyancy measures (i.e., water ballast added to pipe interior). This paper addresses the installation of the pipe in the absence of antibuoyancy measures (i.e., no ballast). A subsequent paper will address the case with added ballast. This paper provides a better understanding of the factors influencing the pull loads in the maxi-HDD installation of pipelines.