Horizontal Directional Drilling (HDD), is a minimal impact trenchless method of installing underground pipe, conduit, or cables in a relatively shallow arc or radius along a prescribed underground bore path by using a surface-launched drilling rig. With respect to the pipeline/utility industry, the term “Directional Boring” or “Bore” is generally reserved for mini/small sized drilling rigs, small diameter bores, and crossing lengths in terms of hundreds of feet. Generally, the term Horizontal Directional Drilling (HDD) is intended to describe large/maxi sized drilling rigs, large diameter bores, and crossing lengths in terms of thousand of feet. Although directional boring and HDD are similar in some respects to directional drilling associated with the oil industry, an equal comparison cannot be drawn between the procedures as they serve two entirely different functions. Directional Boring/HDD offers significant advantages over traditional cut and cover pipeline/utility installations and are routinely used when trenching or excavating is not practical. Directional Boring/HDD can be utilized with various pipe materials such as PVC, polyethylene, polypropylene, ductile iron, and steel as long as the pipe is sized appropriately to withstand installation stresses imparted during pullback.
Directional Boring/HDD is generally accomplished in three principle phases. First, a small diameter pilot hole is drilled along a directional path from one surface point to another. Next, the bore created during pilot hole drilling is enlarged to a diameter that will facilitate installation of a pipeline. Lastly, the pipeline is pulled into the enlarge hole, thus creating a continuous segment of pipe underground and exposed only at the two initial end points.
Directional Boring/HDD is suitable for a variety of soil conditions and obstacles including road, landscape, wetland/marshes, and river crossings. Problematic soil conditions most often encountered in evaluating the feasibility of an HDD installation is large grain content in the form of gravel, cobbles, and boulders. Other subsurface conditions which can impact the feasibility of an HDD installation include excessive rock strength and abrasivity, poor rock quality, and solution cavities in karst formations.
The equipment used in horizontal directional drilling depends on the outer diameter of the pipe, length of the run, ground conditions and the surroundings above ground.
For larger bores, directional drills equipped with as much as 600 tonnes 1 320 000 lb (or more) of thrust/pullback (Vermeer D1320x900) is used in conjunction with a mud reclaimer, excavator, and multiple pumps and hoses to supply the drilling fluid to the drillstem. Directional drilling stems are made from heat-treated high-carbon steel in diameters of 8 – 15 cm. Drill stem sections are manufactured in 3.0, 4.6, and 9.1 meter lengths with male and female threading on opposite ends. It is common for a directional drill to carry as much as 305 m of drilling stems. Drilling heads are available for different types of rock or soil. The drilling head has multiple water ports to allow removal of material. A talon bit head uses carbide-tipped blades to cut through material while steered by the operator. Another type of head is a mud-motor that is used in rocky landscapes. Supporting equipment is often required, and may include a drilling fluid (mud) recycling system, shale shaker, mud cleaner, centrifugal pump, and mud tanks.
Directional boring is used for installing infrastructure such as telecom and power cable conduits, water lines, sewer lines, gas lines, oil lines, product pipelines, and environmental remediation casings. It is used for crossing waterways, roadways, shore approaches, congested areas, environmentally sensitive areas, and areas where other methods are costlier or not possible. It is used instead of other techniques to provide less traffic disruption, lower cost, deeper and/or longer installation, no access pit, shorter completion times, directional capabilities, and environmental safety.
The technique has extensive use in urban areas for developing subsurface utilities as it helps in avoiding extensive open cut trenches. The use requires that the operator have complete information about existing utilities so that they can plan the alignment to avoid damaging those utilities. Since uncontrolled drilling can lead to damage, different agencies/government authorities owning the urban right-of-way or the utilities have rules for safe work execution. For standardization of the techniques, different trenchless technology promoting organizations have developed guidelines for this technique.
The process starts with the receiving hole and entrance pits. These pits will allow the drilling fluid to be collected and reclaimed to reduce costs and prevent waste. The first stage drills a pilot hole on the designed path, and the second stage (reaming) enlarges the hole by passing a larger cutting tool known as the back reamer. The reamer’s diameter depends on the size of the pipe to be pulled back through the bore hole. The driller increases the diameter according to the outer diameter or the conduit and to achieve optimal production. The third stage places the product or casing pipe in the enlarged hole by way of the drill stem; it is pulled behind the reamer to allow centering of the pipe in the newly reamed path.
Horizontal directional drilling is done with the help of a viscous fluid known as drilling fluid. It is a mixture of water and, usually, bentonite or polymer continuously pumped to the cutting head or drill bit to facilitate the removal of cuttings, stabilize the bore hole, cool the cutting head, and lubricate the passage of the product pipe. The drilling fluid is sent into a machine called a reclaimer which removes the drill cuttings and maintains the proper viscosity of the fluid. Drilling fluid holds the cuttings in suspension to prevent them from clogging the bore. A clogged bore creates back pressure on the cutting head, slowing production.
Location and guidance of the drilling head is an important part of the drilling operation, as the drilling head is under the ground while drilling and, in most cases, not visible from the ground surface. Uncontrolled or unguided drilling can lead to substantial destruction, which can be eliminated by properly locating and guiding the drill head.
There are three types of locating equipment for locating the bore head: the walk-over locating system, the wire-line locating system and the gyro guided drilling, where a full inertial navigation system is located close to the drill head.
♦ Walk-over locating system — A sonde, or transmitter, behind the bore head registers angle, rotation, direction, and temperature data. This information is encoded into an electromagnetic signal and transmitted through the ground to the surface in a walk-over system. At the surface a receiver (usually a hand-held locator) is manually positioned over the sonde, the signal is decoded and steering directions are relayed to the bore machine operator.
♦ Wire-line locating system — The wire-line system is a magnetic guidance system. With a magnetic guidance system (MGS), the tool reads inclination and azimuth. The MGS, also has a secondary means of location verification utilizing wire grids laid on the ground surface. It is the only system that has the capability of verifying the location. This information is transmitted through the wire-line fitted within the drill string. At the surface, the navigator in the drill cab performs the necessary calculations to confirm the parameters have been met. The MGS, even without the use of the wire grid, has been accurate to over 2 km with an accuracy of 2% at depth. The operator of the MGS communicates with the driller and guides him towards a predetermined engineered drill path.
♦ Gyro-based locating system — The gyro based system is fully autonomous and therefore one of the most accurate systems where sufficient diameter (200 mm) is available and where long distances (up to 2 km) have to be traversed with minimal deviation (less than 1 m position error). Currently the actual depth is not verifiable without the use of surface coils, a near surface transponder or sonde used in walkover systems.
All three systems have their own merits, and a particular system is chosen depending upon the site requirements.