Horizontal Directional Drilling (HDD)

Horizontal directional drilling (HDD), also known as directional boring, is a trench-less construction technique and method used for underground drilling. HDD provides higher flexibility and accuracy for installing, repairing, and maintaining underground pipelines, utility cables, and service conduits.

Such technique involves the use of horizontal directional drilling (HDD) machinery, and associated drilling and steering systems, to accurately drill along the chosen bore path, back ream, and pullback the required pipe No matter the soil condition, whether hard or loose, this sustainable technique is perfect in crossing natural or artificial obstacles, from rivers and valleys to highways and railways alike. Additionally, HDD is ideal for replacing burst pipes, intakes and outfalls, installing gravity sewer pipes, and mining exploration to name a few. The use of horizontal directional drilling (HDD) techniques and methods carry numerous advantages. For instance, it is cheaper than trench-based installation methods, as it does not require large digging crews or costly repairs to structures damaged during the process. Moreover, avoid the inconveniences of road closures and obstruction of railway lines. Additionally, it is environmentally friendly, as drilling occurs under planted areas rather than though them. Horizontal directional drilling (HDD) is a three-stage process: pilot hole, reaming, and pipe pullback.

Horizontal Directional Drilling (HDD) Processes

Pilot Hole

The first stage consists of drilling a small diameter pilot hole. Drilling fluid is pumped through the drill pipe to the drill bit where high-pressure jets and the bit will grind the soils ahead of the drill stem. The drilling fluid will also carry the cuttings back to the entrance pit at the drill rig. Tracking of the pilot hole can be done in several ways depending on the size and complexity of the shot.

Smaller shots are done using a walkover guidance system, whereas the larger more complex shots have a wire line magnetic type system. With both methods there is a transmitter or steering tool located near the drill head which sends a signal to the location engineer giving the exact coordinates of the drill head. Readings are constantly taken which check the depth, alignment, and percent slope of the drill head.

Corrections will be made by the operator and locator to keep the pilot hole along the predetermined bore path. The speed of the pilot hole will vary upon existing soil conditions and the amount of steering which is required. Upon reaching the exit point, the beacon housing and bit is detached and replaced with a reamer.

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The second step is to ream the pilot hole and enlarge it to a size sufficient to safely install the product lines. A reamer is pulled back and rotated while pumping drilling fluid to cut and remove solids to enlarge the hole. Reaming speeds will vary depending on existing soil conditions and the amount of cutting that is removed from the hole

Bentonite and other polymers will be used to ensure a clean and stable hole. Bentonite is used to create a ‘filter cake layer’ around the outside of the hole during reaming. This will help with the stability of the bore hole and with fluid loss or infiltration. Additives such as polymers are used to help break up the clay soils. A more evenly mixed drilling fluid will prevent any blockages inside of the bore hole.

Pipe Pullback

The final step is the pullback and install the pipe within the reamed hole. The drill rod and reamer will be attached to a swivel, which is utilized between the product line and the reamer to prevent any torsional stress from the rotating drill string being transferred to the product pipe. As the product pipe is pulled into the drill hole, drilling fluid is pumped downhole to provide lubrication to the product pipe.


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