Horizontal directional drilling (HDD) is a commonly used trenchless installation method. This article discusses the rig components, the machine mechanism, typical maintenance and advantages of HDD application.
Drill Rig Components
An HDD rig is comprised of several components. A main component is called a drill rig. The rig includes storage where drill rods are prepared. An operator will link drill rods together to form a drill string for advancing the drill bit. Behind the drill bit, there is a sonde housing that exerts a signal. The signal will be picked up by another technician equipped with a transmitter receiver to trace the location. The data is processed to inform the operator on the rig who navigates and directs the drilling.
A tank and pump are also utilized to supply drilling fluid to the rig. This is to ensure that drilling fluid is continuously circulated throughout the drilling process. (Read also: An In-Depth Look at the Role of Drilling Fluid Systems in Trenchless Construction.)
HDD Installation Mechanism
HDD is accomplished in three steps: developing the pilot hole, enlarging the pilot hole, and hole conditioning. Throughout the process, hydraulic fluids are injected into the hole and drill bits to assist in breaking soft soil, cool the drill bits, and lubricate the bore. In rock formations, stronger drill bit material, such as tungsten carbide alloys or Polycrystalline Diamond (PCD), are also required in addition to hydraulics force.
HDD installation is initiated with pilot hole drill. The pilot hole is drilled from the entry point to the exit point following a profile and alignment. Drill bits are used to initially create a smaller hole from the entry site to the exit site. The profile and alignment should be designed and programmed by the installer. During the pilot hole drilling, a directional guidance system is used to navigate the pilot hole along its pre-designed profile. The drilling tools and rig equipment selected for each job is largely determined based on the results of the geotechnical investigation, the size of the crossing (length and diameter), as well as the installer’s judgment.
The entry angle of the drill string is typically 8° to 16° degrees. The drill string is advanced by a combination of rotation torque and thrust supplied by the rig. This allows the operator to change the direction and properly navigate the pilot hole along its pre-designed profile. Most drill bits have a slant-face and can be designed to navigate through different types of soil, from clays and sands to rock. (Read also: The Right Drill Bit for Soft, Medium & Hard Ground Conditions.)
A walkover tracking system is used to monitor the location of the bore. The system is comprised of a transmitter and receiver. The transmitter emits a continuous magnetic signal, which is picked up by a portable handheld receiver. On-board controls allow the operator to monitor the orientation of the bit and the change in the general direction of the bore. To move in a straight line, the rig operator both rotates and pushes the drill string. To change direction, the operator stops rotating the drill string and pushes the string. The path will change in the direction that the bit’s slant-face is pointing.
At the exit site, the drill bits are switched to a larger reamer to enlarge the hole according to the required pipeline diameter by pulling pack the pilot bore towards to entry site. In applications for small diameter crossings, the pipeline can be directly installed in the pilot hole. However, in most projects, pilot hole needs to be enlarged. The enlargement is known as “pre-reaming”. Pre-reaming is required to provide a bore diameter large enough so that the proposed pipeline can be installed in the drilled crossing. Pre-reaming may include one or more stages based on the final desired diameter and soil conditions.
Pipe installation commences after the pre-reaming is completed to the required diameter and the hole is cleaned adequately. The pipeline is preassembled in a single string and placed on rollers. This setup leads to a process called pullback. For pullback, a reamer is connected to the drill pipe. Behind the reamer, a swivel is connected allowing the drill pipe to rotate the reamer without transferring torque to the pipe. The drill pipe is then pulled back towards the entry point until the pipeline is fully installed. The finished diameter is verified using swab tool.
Maintenance
Methods for drilling rig maintenance can vary depending on the use. This article presents a select few standard practices for a drill rig maintenance: (Read also: Proper Maintenance for Drill Rig Equipment)
- Daily checks.
- Lubrication maintenance.
- End-of-working-day clean up.
Daily Service Checks
Every rig comes with its own service manual. The manual shows recommendations for operators for servicing hydraulic pumps, fluids, oils, coolant and other components. The manual also includes daily maintenance checks. These simple processes when followed closed will reveal any issues left over from the previous usage. Early detection and maintenance will help keep a machine running properly. Spending 5-to-10 minutes a day conducting service checks will not only save time, money and costly damages from any overlooked maintenance problems, but also keep machines operating at their peak level on the jobsite.
Maintaining Lubrication
Maintaining proper lubrication and rotation of the drill rig is essential. It will not only help lengthen the life of your equipment but lead to its overall success. Proper lubrication of a drill pipe is important as well. Every manufacturer has its own standard of proper lubrication, which is designed to optimize thread compression and tolerance. Maintaining automated greaser machines for proper hydraulic pressure helps contractors achieve proper torque, reducing the chances of the pipe unthreading and breaking downhole.
End-of-Day Maintenance
At the end of every work day, it is recommended that each machine is cleaned to eliminate any blockage. Manufacturers often design machines with a high-pressure pump that gets plugged into the circuit system within the machine, creating an efficient way to perform an end-of-day wash down. During the clean up, contractors can also look for any hydraulics leak. Identifying these problems ahead of time eliminates the chance of costly future issues. Maintenance tasks may seem tedious but are important to ensure the overall success on any project. Through these checks and routine cares, contractors can keep their machines in the best shape to operate which saves overall time and money.
Benefits of HDD
HDD is the preferred trenchless approach especially for large river crossings. Advantages of HDD include quicker completion time where existing site condition is congested or has space constraints. HDD has less surface disturbance, cost savings and is less invasive compared to open-cut installation. HDD can generally be completed with a smaller crew which means less labor costs and more efficient use of project budget.