Directional drilling is a trenchless method of installing pipelines underground. Depending on the project, HDD directional boring can also be used to install cables or conduits.
The method usually consists of three steps. First, the drill string creates a pilot bore to determine the path in which the pipe will be installed. The pilot hole is then enlarged via backreaming. This process creates the space which will accommodate the product pipe. Finally, the prefabricated pipe is attached to the drill string and is pulled into place.
The drill path can be entirely straight or gradually curved. All directional drilling equipment use drilling fluid which provides lubrication to the bore and the drill string itself.
Horizontal directional drilling or HDD has significant environmental advantages over traditional installation methods. Directional boring is often used when conventional trenching or excavation is not suitable or if the ground surface cannot be disturbed during installation. HDD is a great underground drilling option if you have to install pipes across an area with rivers, railways, roads or natural reserves.
To help you better understand horizontal directional drilling, here is the general rundown of the entire process.
Assessing the area and preparing entry/exit sites
Before a pilot hole is drilled, the contractors must assess the area and see if there are any existing utilities that could affect the drilling process.
Entry and exit sites at both ends of the predetermined path will also need to be excavated beforehand. The size of these sites will depend on the measurements of the pipe to be installed.
The entry pit is where the drilling begins. Naturally, the drilling rig will be set up at this site. On the other hand, the exit pit is where the reamers are attached to the drill string. As a result, the reaming tools and the final product pipe are usually set up at the receiving site. These entry and exit sites will also collar the pilot bore and allow contractors to collect drill cuttings during the process.
Step 1: Pilot hole
First, a pilot hole will be drilled through the ground to create a small path between the entry and exit points. This path acts as the predetermined route for the product pipes.
During this step, the direction of the drill can be adjusted at any time, allowing contractors to avoid existing utilities or any other obstacles. Using a tracking beacon, the contractors will be able to know the depth and position of the drill head. Directional control is possible due to an asymmetrical leading edge as well as a non-rotating drill string. The asymmetrical leading edge creates a steering bias while the non-rotating drill string allows the head to stay at a specific position if necessary.
As it creates the pilot hole, drill fluid is continuously expelled by the string. As the fluid makes way back to the entry pit, it mixes with the debris created by the drill head. This process allows the contractors to collect the drill cuttings at the entry site by filtering it from the returning fluid. Drill fluid also lubricates the string, cools the cutting tools and stabilises the path.
When the drill string reaches the exit pit, the pilot hole step is complete.
Step 2: Pre-reaming
Once the predetermined route has been drilled, the pilot hole will have to be enlarged to accommodate the pipe. This is done through backreaming. Depending on the size of the pipe, the contractors might have to undertake multiple reaming passes to safely enlarge the bore.
At the exit pit, the contractors will attach a reamer to the drill string. Backreaming tools usually involve large circular cutters and drilling fluid jets. In most cases, these cutters will have to be custom-made to ensure that the size of the hole is suitable for the product pipe. Once the tools are safely attached, the drill string is then pulled back towards the entry pit, enlarging the pilot hole. As with the previous step, drill fluid is continuously pumped at the head for lubrication and stabilization purposes.
It’s worth noting that there are different ways that the reaming process can be completed.
Pre-reaming refers to the process of enlarging the pilot bore before installing the pipe. Other methods involve simultaneously reaming the bore and installing the product pipe. This is when the pipe is added immediately behind the reamers as it makes its way back to the entry pit. This method ensures that the product is properly placed and aligned in the reamed path
Regardless of the method, mud mixture is used to fill the space between the reamed space and the product pipe.
Alternatively, it’s also possible to attach the reamers at the entry pit and thrust the drill string through the pilot bore and away from the drilling rig.
Step 3: Pipe installation
If the product pipe wasn’t installed in the previous step, the third stage involves pulling the pipe through the reamed path via the drill string.
At the exit site, sections of the product pipe are prepared. These sections are welded together and then water tested. After determining that the product is working properly, the pipe will then be attached to the drill string.
Once the drill string is connected to the pipes via a swivel, the pullback commences and the pipe is dragged towards the entry pit. The swivel is used to minimise the torsion that will be transmitted to the product pipe during the process. For support, roller stands and flotation ditches will be installed to prevent major damages to the pipeline.
There’s a lot to consider when it comes to drilling projects. Product size and soil conditions must be taken into account before choosing an appropriate drilling process.
The HDD method comes with a lot of conveniences. However, just like with any method, it’s suitability will depend on the nature of the task. Consider the process we’ve outlined above and think about how it suits your particular project.