Perhaps the most commonly overlooked part of any Horizontal Directional Drill (HDD) design is the product pipe pullback support. Although there is commonly significant engineering evaluation put into the development of the below-ground geometry (such as Annular Pressure / Hydrofracture and Inadvertent Return risk calculations, design radius and minimum allowable radius calculations, pull-force evaluations, buoyancy control calculations, etc.), in general much less thought is put towards how the product pipe is handled above ground, before it enters the designed bore-path. However, as will be discussed here, the design of this portion of the HDD process can be just as important (or more so) than the downhole design in ensuring a safe and successful installation.
Fifteen years ago, pullback (or lift-plan) designs were almost non-existent. It was up to the general pipeline contractor to provide equipment to lift the pipe such that it entered the borehole at the specified angle. As long as the sidebooms could lift the pipe to align with the hole, there was not much additional analysis or thought put into the process.
With the increasing capabilities of HDD equipment and planning, larger and longer pipe crossings started to become more common, and with some large pipe and challenging geometry in HDD designs, site-specific lift-plans started to become a project requirement. Although not always the case, within the United States lift plans are predominantly prepared by the general pipeline contractor or their subcontractors who are responsible for the lifting of the pipe, well after the design of the HDD drillpath itself has been finalized. HDD designers may draft up plans to show required pullback alignment for temporary workspace consideration, but in many cases, the design of the pullback support does not go beyond this preliminary stage during the detailed HDD design. For a multitude of reasons that are discussed below, this really should not be the case if the ultimate goal of the design is to ensure safe, efficient, and cost-effective HDD installations.
Justin Taylor, PE, CCI & Associates Inc.
Ashkan Faghih, M.Sc., P.Eng., CCI Inc.