What Does Horizontal Directional Drilling (HDD) Mean?
Horizontal directional drilling (HDD) is a trenchless construction method used to install pipes underground without disturbing the ground surface. The drill is launched from one end of the designed bore path and retrieved at the other end, and except for the launch and retrieving spaces above ground, the entire process takes place underground, out of sight.
HDD has become a choice alternative to conventional pipe-laying methods that require opening up the earth’s surface up to the depth of installation for the entire pipeline route.
With thanks to Hadlee & Brunton NZ
Benefits of HDD
HDD is a minimum impact method and especially useful in environmentally sensitive areas such as wetlands, estuaries, rivers, and lakes.
It is a quick and accurate method that enables the operators to accomplish creditable accuracy even when crossings are complicated.
Continuous monitoring and control during the operation allow operators to navigate in constrained spaces between existing utility lines.
Drilling can be carried out at greater depths to bypass existing utilities altogether and requires only one launching shaft at the entry point and one reception shaft at the exit point.
Horizontal Directional Drilling (HDD) is a construction technique whereby a tunnel is drilled under a waterway or other designated area, and a pipeline or other utility is pulled through the drilled underground tunnel.The underground tunnel follows an arc line from the entry point, down under the special crossing area, and then resurfaces on the opposite side. Using advanced technology and highly trained technicians, a drill head guides the drilling pipe electronically to ensure the angle, depth, and exit point adhere to carefully designed engineering plans. Throughout the drilling process, the tunnel is kept open and lubricated by circulating a watery mud-slurry mixture, typically composed of about 95 percent water and 5 percent bentonite clay – a natural, non-toxic substance. The drilling mud also helps coat the walls of the tunnel and remove drill cuttings. When the tunnel is 12 inches wider than the diameter of the pipe going through it, the welded pipeline segment can be pulled through the underground arched tunnel from one end to the other end. These pipeline segments are typically made of thicker steel and have a protective first layer of coating to prevent corrosion and a second abrasion-resistant coating to protect the pipe during the pull through the tunnel. The two ends of the pipeline segment can then be welded together with the rest of that pipeline system.
Why is HDD used?
Long-distance underground pipelines are built across open areas, such as rural farmlands and urban utility corridors. Typically, construction crews clear an approved route, dig a specially prepared trench, lower welded sections of pipeline into the trench, and carefully replace removed soil for the restoration process. Usually, this method is preferred for open area installation and can be completed in a way that minimizes environmental and stakeholder
impact, even in sensitive areas. For large rivers or certain sensitive crossings, directional drilling is an appropriate and preferred construction method, depending on
geologic conditions and other factors. HDD construction is considered suitable for large river crossings, extreme congestion (e.g. crossing rail yards), or other unique site-specific situations because it minimizes impacts on the area above the drill. Although land around the drill entry and exit locations is disturbed during HDD activities, it will be restored to its pre-drill state.
Directional drilling is a three stage process:
1. 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 magnetics 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 stem. 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.
The second step is to pre-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. Pre-reaming speeds will vary depending on existing soil conditions and the amount of cuttings that are removed from the hole.
Bentonite and other additives will be used to ensure a clean and stable hole. Bentonite is used to create a “cake layer” around the outside of the hole during pre-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.
3. PIPE PULLBACK
The final step is the pullback of the pipe within the pre-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.
In co-operation with our business partners Trident Group of Rotterdam-Netherlands, we will undertake the training and recuitment of personnel related to HDD, so if you are interested, please let us know by using the button below.