Drill Rods
The rods that carry out the bore come in different diameters (2 cm – 20 cm) and lengths (3 m – 9 m) depending on the size of the rig. A rod is essentially a hollow pipe with tapered threads at each end so that rods can be joined together. The drilling fluid is pumped into the bore through this hollow core.
Rods are designed to resist tension, compression and torsion. To create an elliptical profile underground, HDD rods need high elastic deformation and low plastic deformation: however much a rod bends under an applied force, it should return to its original shape when the force is removed.
Material and manufacture
Rods are made of special alloys, most commonly carbon (C) and titanium (Ti) alloys. Because the tapered threaded ends wear much faster than the body, these parts are made of CrMo (chromium-molybdenum) steel.
The two parts are then joined by friction welding (rotated at very high speed on the same axis and forged together).
Use on site
Drill rods can be fed by the rig's automatic magazine, or from outside by crane or by hand. Rod boxes are generally carried on the rig up to medium-class machines; above 50 tonnes the rods become heavier and larger, so they are fed to and collected from the rig one by one from outside.
Longer rods mean fewer joints and save time on long crossings. Medium rigs can, however, use rods of different lengths; the choice belongs to the supervisor.
Rod selection and maintenance
Drill rods are selected together with the rig class: rod diameter and thread type must match the rig's rotary head, and rod length must suit the loader and the site. The most important technical value of a rod set is its allowable bend radius; the bore profile cannot be designed tighter than this radius. Depth, entry and exit angles and obstacle crossings in an HDD project are always drawn to the rod's bending limit.
Rods are the most heavily worn component in horizontal directional drilling. The tapered threads wear with every make-up and break-out; dry threads, incorrect torque and missing lubrication lead to thread galling and breakage. A broken rod means not only lost equipment but, because of the piece left in the pilot hole, usually re-drilling the route from scratch. Tool-joint compound is therefore applied regularly, make-up torque is limited to the rig manufacturer's value, rods are inspected visually and, where necessary, ultrasonically on every project, and bent or thread-damaged rods are removed from the set.
Deltek keeps a spare rod set for every rig class in its Istanbul and Izmir fleet; on long crossings the longest possible rod length is chosen to reduce the number of joints. For the tools fitted to the end of the rods, see the steering head and reamer pages.
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