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. 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.
Horizontal directional drilling is a technology in pipe and utility installation that allows greater accuracy and flexibility in placement and ends the need for costly digging, large crew, road closures and other complications of traditional digging and pipe installation.
Horizontal directional drilling is a steerable guided method of installing underground utilities (Viz, OFC, Gas Pipe line, Water & Sewerage Pipe Line etc.) in a prescribed bore path using a surface launched HDD Machines.
The HHD is used when open trench or open excavation is not possible. The tools and technique used in the HDD method are high demanding technology in gas pipe line projects. It is now common the HDD machine usage for laying OFC, power cable, water and sewerage pipe line of telecommunication companies and city municipals respectively.
Directional control of the HDD drill path is limited. An operator can adjust the direction of the bit by stopping the rotational movement and then thrusting the bit forward. The slanted face of the drill bit causes it to adjust its angle into the ground. HDD rigs allow for a transmitter to be installed just behind the drill bit. A person holding a receiver above the ground over the drill bit’s location will be able to determine the position and direction of the drill. This information is used by the operator to make minor adjustments to compensate for any deviation.
Drilling mud is pumped down the drill string and into the drilling bit, where it exits through holes in the bit. The purpose of the drilling mud is to keep the transmitter cool, stabilize the walls of the tunnel and remove the spoil. As it washes back up the hole to the surface, it removes the spoil from drilling along with it. Drilling mud is recycled by using screens and hydro cyclones to remove the spoil. It is processed back into a tank on the surface before being pumped into the drill string again. This is called reclaiming or recycling the mud.
Installation of a pipeline string by HDD is generally accomplished in following six stages.
- Soil Investigation
- Drill path design
- Drilling the pilot hole
- Pre-reaming the horizontal boring or reaming
- Pull back operation
- Installation of conduits
Different types of drilling bits/drill heads (diamond bit/ tungsten bit) are used in the pilot-bore process. Selection of drill bit is depend on the type of soil strata. The purpose of the soil testing is not only to determine the feasibility of HDD crossing, but also to establish the most efficient way to accomplish it. Soil with cobble stones or rock having voids or incomplete layers of rock are not considered suitable for drilling. On the geo-tech information we can determine the best crossing route along with selection of drilling tools and execution methodology.
Following shall be investigated during the geo-tech survey:
- Soil identification to locate rock, rock inclusions. loose deposits, discontinuities etc.
- Soil strength and stability characteristics
- Groundwater level
Other relevant data may be obtained from existing records, eg. recent nearby bridge construction, other pipeline/cable crossing in the area.
Soil strata investigation is performed by analyzing the soil sample extracted from bore-holes drilled along the pipeline route called as bore log data. For long crossings, bore logs are typically taken at 200 m intervals. For short crossings i.e. crossings which are less than 300 m length, as few as three bore log may be sufficient. The borings should be near the drill-path to give accurate soil data, but sufficiently far from the borehole to avoid pressurized mud from following natural ground fissures and rupturing to the ground surface through the soil-test bore hole. A thumb rule is to take borings at least 10m to either side of bore path. Although these are good general rules, the number, depth and location of boreholes is best determined by engineer.
River crossings require additional information such as a study to identify river bed, river bed depth, stability and river width. Typically pipes are installed to a depth of at least 6m below the expected future river bottom. Soil boring for geotechnical investigation are generally conducted to 12m below river bottom.
Drill path design
One of the key considerations in the design of the drill-path is creating as large a radius of curvature as possible within the limits of the right-of-way. Small radius of curvature induces bending stresses and increases the pullback load due to the capstan effect. The capstan effect is the increase in frictional drag while pulling a pipe around a curve due to a component of the pulling force acting normal to the curvature. Higher tensile stresses reduce the pipe’s collapse resistance. Curvature requirements are dependent on site geometry (crossing length, required depth to provide safe cover, staging site location, etc.) But, the degree of curvature is limited by the bending radius of the drill rod and the minimum elastic bending radius of the pipe.
The designed drilling profile consists of a series of straight lines and curves. The straight lines are referred as tangents. The straight sections are those in which the drilling hole curvature is ideally zero. This implies that any pipe section can be considered as straight section if the curvature of that section is less than that necessary to make the pipe deviate beyond the walls of the hole, which is roughly 1.5 times larger in diameter than the pipe itself.
Drilling the Pilot Hole
The steering tool is placed within the Bore Hole Assembly (BHA). Generally, the BHA is made up of non-magnetic drill collars. The lead collar of the BHA is placed on the alignment of the particular crossing. After the alignment, the steering probe is energized with electrical current (wire-line steering) and a bearing for the drill path is established and logged into the surface computer. The drilling rig is set precisely on line with a transit. The non-magnetic lead collar (with steering probe) and the directional deviation tool are started exactly at the designated entry point. In most cases, one Non-Magnetic Drill Collar (NMDC) is used behind the BHA. A 10 m non-magnetic collar shall serve as a buffer between the steering probe (in the lead collar) and the steel drill pipe. Drill pipe is often highly magnetized due to the continual making up and breaking out the tool joint connections and can affect the tool parameters.
Pilot hole drilling typically is considered the most challenging and time consuming step. As each piece of drill pipe is advanced, the next drill pipe is fitted with a wire inside. This wire is attached to the corresponding wire of the drill pipe previously drilled. This internal wire is the vehicle used for the signal to be sent from the steering probe located in the BHA to the surface computer. This process is repeated until the bit is advanced along the predetermined path and comes out at predetermined exit location as per the designed drilling path.
Pre-Reaming the Horizontal Boring/Reaming Operation
Once the drilling bit exits out (punch out) of the pilot hole, the lead pieces/ drill pipes are unscrewed. The hole opener/ reamer is then attached to the leading pipe to start reaming operation. The reaming operation consists of using an appropriate tool to open the pilot hole to a slightly larger diameter than the carrier pipeline. The percentage oversize depends on many variables including soil types, soil stability, depth, drilling mud, borehole hydrostatic pressure, etc. Normal over-sizing may be from 1.4 to 1.5 times the diameter of the carrier pipe. While the over-sizing is necessary for insertion, it means that the inserted pipe will have to sustain vertical earth pressures without significant side support from the surrounding soil.
Good grade of bentonite is continuously pumped through the reamers to flush the cuttings and stabilize the hole. Similar procedure is repeated for all stages of reaming.
Swab Pass: While pulling the reamer back to the shore if the Driller feels that the hole is not conditioned or if there is a collapse of the hole, additional swab passes are made with the same size of the reamer.
The pipe shall be strung and welded, on the rollers, in the same line as the drilled hole from entry side to exit side. The welds of the pipe may be subject to visual inspection and/or non-destructive testing. After welding of the total pipe string, in a single segment length, it is pre-hydrostatically tested at a pressure of 1.25 times x design pressure of the pipeline. After successful completion of pre-hydrostatic testing, test header is removed and pull head is welded on the rig side of the pipe string. The near to hole section of the pipe string is lifted with the help of adequate lifting equipment to make a necessary over bend.
Pipe Pull Back
The pullback operation involves pulling the entire pipeline string in one segment (usually) back through the drilling mud along the reamed-hole pathway. The pulling equipment is attached to the leading end of the drill pipes string, and the prepared pipe string is fed gently into the bored hole. Proper pipe handling, cradling, bending minimization need to be followed. Axial tension force readings, constant insertion velocity, mud flow circulation/exit rates, and footage length installed should be recorded. The pullback speed ranges usually between 1 to 2 feet per minute.
Benefits of Directional Drilling
- Reduced soil disturbance.
- A single location area can be used to install different pipes.
- Reduces the fractures to existing rock formations.
- Reduces the contamination of groundwater pollution.
- Protects the ecosystem and adjacent areas.
- Reduces the excavation and shoring costs.
- It is a safer operation than open cut.
- Weather will not impact directly on the process.
- Limited traffic and landscape disruption. Ideal for sites sensitive to surface disruption such as heavy roadways, airport runways, golf courses, etc.
- Ability to drill beneath surface obstructions or ongoing site operations.