Horrizontal Directional Drilling
Directional boring, commonly called horizontal directional drilling or HDD, is a steerable trenchless method of installing underground pipes, conduits and cables in a shallow arc along a prescribed bore path by using a surface launched drilling rig, with minimal impact on the surrounding area. Directional boring is used when trenching or excavating is not practical. Directional boring minimizes environmental disruption. It is suitable for a variety of soil conditions and jobs including road, landscape and river crossings. Installation lengths up to 6,500’ (2,000m) have been completed, and diameters up to 56" (1,200mm) have been installed in shorter runs.
Pipes can be made of materials such as PVC, polyethylene, Ductile iron, and steel if the pipes can be pulled through the drilled hole.The drilling begins with the set-up of the equipment needed to complete the bore. The first step is to view the geological surroundings and see if boring is an option. Boring is not suggested if there are voids in the rock or incomplete layers of rock. The best material is solid rock or sedimentary material. Soils with cobble stone is not recommended. There are different type of heads used in the pilot hole process and they depend on the geological material.
The equipment used in a directional drill operation depends on the size of the pipe, length of the run, and surrounding locations. For the large bores, a 100,000 pound pulling power drill is used with a reclaimer, excavator, and multiple pumps and hoses to move the fluid. The drilling steel is a 3-in. diameter pipe with male and female threads. The head of the operation comes in multiple designs and depends on the rock or soil being penetrated. The drilling head has multiple water ports to allow removal of material. A talon bit involves the diamond tipped cutters. These allow for steering and cutting the material. Another head is a mud-motor which is used in rocky landscapes.
Directional boring is used for installing infrastructure such as telecommunications 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. 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 however necessitates that the operator must have the complete information about the existing utilities so that he can plan his whole alignment to avoid damaging those utilities. Since uncontrolled drilling can lead to such damages different agencies/government authorities owning the urban 'right-of-way' or the utilities have formed their rules for safe work execution. For standardization of the techniques different trenchless technology promoting organizations have developed guidelines for this technique.
The beginning of the process starts with receiving hole and entrance pits. These pits will allow the drilling fluid to be collected and reclaimed to keep the cost down and prevent excessive waste. The first stage drills a pilot hole on the designed path and the second stage 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. The driller increases the diameter according to the material being cut and creates for optimal production. The third stage places the product or casing pipe in the enlarged hole by way of the drill steel and 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 Locating and guidance.
Location and guidance of the drilling is a very 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 two types of locating equipment for locating the bore head: the ‘walk-over’ locating system or a 'wire-line' locating system. In both of the systems a sonde, or transmitter, behind the bore head registers angle, rotation, direction and temperature data. This information is encoded into an electro-magnetic 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 decoded and steering directions are relayed to the bore machine operator. In a wireline system, this information is transmitted through the cable fitted within the drill string. Both systems have their own merits and depending upon the site requirements a particular system is chosen.