Horizontal Directional Drilling Machines (HDD) Information

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Horizontal directional drilling machines (HDD) are steerable, trenchless drilling rigs for the installation of pipes, conduits, and cables underground in a shallow arc.  They are commonly used for installations below waterways and existing roadways to avoid disturbing the ground above. Horizontal drilling machines are typically heavy-duty vehicles fitted with track drives, operator controls, and drilling machinery. Materials used for pipes include PVC, polyethylene, polypropylene, Ductile iron, and steel. 


Directional drilling machines are used to install both small and large diameter lines (3” to 48”). Installation is typically a two-step process. The first step is to use a drill head to drill a one to five inch pilot hole at an angle, using the ground as reference.  The second step involves replacing the drill head with a reaming device such as a fluid-assisted drill, a high pressure fluid jetting system or a dry bore system to enlarge the hole to the desired size.  As the reamer is pulled back through the pilot hole it enlarges the hole and pulls the pipe or conduit back with it. For larger pipes an additional step of reaming will be required. The final size of the drilled hole is 50% larger than the pipe diameter to allow for drilling fluids to drain and to accommodate the bend radius of the pipe. Directional drilling machines come in a range of sizes, with the largest machines capable of drilling to depths of 200 feet and lengths of 6000 feet.


Drillhead Guiding Systems


Guiding the drill head is an important part of the drilling operation. There are three types of guiding systems for locating and steering the drill head.


Walk-over locating system -- A sonde or transmitter located behind the drill head registers angle,

rotation, direction, and temperature data. This information is encoded into an electro-magnetic signal and transmitted to the surface. A handheld receiver is manually positioned over the transmitter where the signal is decoded and steering directions are given to the machine operator.


Wire-line locating system -- With a Magnetic Guidance System (MGS) the tool reads inclination and azimuth. The guidance system also has a secondary means of location verification utilizing wire grids laid on the ground surface. This information is transmitted through a wire-line fitted within the drill string. At the drilling machine, a navigator performs calculations to confirm the direction and location of the drill head.


Gyro-based locating system -- The Gyro-based locating system is considered one of the most accurate systems available. These locating systems are free of some of the limitations inherent in other systems. Limitations include vulnerability to magnetic disturbance, inability to access areas above a drill path, and a large tolerance of trajectory. Horizontal directional drilling machines are divided into three categories large-diameter HDD (maxi-HDD), medium-diameter HDD (midi-HDD), and small-diameter HDD (mini-HDD). There is no significant difference in the operation of these systems. The categories represent the drive length, depth, torque and pullback capabilities of each system.