Horizontal Directional Drilling
Horizontal Directional Drilling (HDD) is a steerable trenchless technology process for the installation of utility pipelines and conduits along a desired profile using a surface-launched drill rig. It is most often used in the water industry for the purpose of bypassing natural obstacles such as rivers and lakes, with pipelines drilled underneath the water body.
Man-made structures such as roads and buildings are also traversed using HDD. The technology originated from the oil fields in the 1970s and evolved through a combination of technologies used in the utility and water well industries. Since then, HDD has been widely used in the municipal pipeline installation industry by specialty contractors.
Typical HDD Operations – Horizontal Directional Drilling is typically accomplished in three phases. The first phase consists of drilling a small-diameter pilot bore along a designed directional path; the equipment and method has a unique capability to track the location of the cutter head and steer it during the drilling process.
The second phase consists of enlarging the pilot bore to a diameter suitable for installation of the product pipe through the use of a reamer. Depending on the diameter of the product pipe, multiple enlargements may be required. To avoid surface heave and to reduce the chances of damaging adjacent utilities, the reaming or enlargement process is achieved by pulling only, and not pushing.
During back reaming, drill mud is simultaneously pumped into the bore to both lubricate and also to provide structural stability to the borehole. Finally, in the third phase, the product pipe is pulled into the hole to complete the crossing.
An Engineered Pipe Material Solution for Exact Project Needs – Welded steel pipe is often specified for the HDD of raw and potable water lines when crossings are long; thermoplastic pipes such as HDPE, used in directional drilling are limited in how far they can be pulled due to their viscoelastic physical/mechanical properties. Their availability to exacting project specifications is limited; off-the-shelf type products are not suited for customized manufacture.
Steel pipes on the other hand can be ▪fully engineered for the required application, and▪the distances to which they can be pulled is limited only by the limitations of the installation drill rig. ▪Wall thicknesses, ▪internal diameters, ▪joint types and ▪lining and coatings are all custom designed to an Owner and Engineer’s specifications.
Allowable Bend Radii of Steel Pipe – In accordance with the American Society of Civil Engineers (ASCE) Manual of Practice, MOP 108, Pipeline Design for Installation by Horizontal Directional Drilling (ASCE 2014), the allowable bend radius for steel pipe is typically specified as 1200 times the nominal diameter of the pipe in inches, which is then typically converted into feet. So for a 36-inch ID pipe, the allowable bend radius would be 3600-ft.
The 1200-times-nominal-diameter (in inches) relationship has been developed through experience with constructability in the HDD industry, primarily in oil and gas applications, as opposed to being based on pipe stress limitations. However, the maximum allowable tensile stress imposed on a steel pull section during installation is typically limited to 90% of the pipe material’s minimum yield strength.
Bending stresses imposed on a steel pull-section during installation are also limited per conditions discussed in MOP 108. Typically, the minimum radius derived using a stress-limiting criterion is substantially less than 1200 times the nominal diameter. For this reason, bending stress limitations rarely govern geometric drilled path design but are applied, along with other stress limiting criteria, in determining the minimum allowable radius of curvature.
Spiral welded AWWA C200 steel water pipe is available for the most challenging HDD needs of Owners and Engineers. ▪Wall thicknesses can be specified to an accuracy of one thousandth of an inch. ▪Joints can be single or double lap-welded, or butt welded. ▪Typical coatings and lining specified for HDD applications includes flexible bonded polyurethane per AWWA C222.
In late 2014, the Cities of Lake Oswego and Tigard, OR, as part of the Water Partnership Project, successfully directionally drilled 3600-ft of 36-inch ID polyurethane lined and coated AWWA C200 steel water pipe under the Willamette River through extremely challenging geotechnical conditions. Delivery of the steel pipe on barges began in October 2014.
Northwest Pipe Company also offers the Permalok® Type HDD joint for small diameter applications, typically no larger than 12-inch, though it was not used on this project. This eliminates the need to weld long strings of steel pipe together and can be performed in a “cartridge method.” The modified joint profile incorporates an additional locking key on the male portion of the joint to greatly increase its tensile pull strength. A longer joint body allows more flexibility in bending, and with the spacing between interlocking ribs being greater, joint make up is faster and requires less force to engage.