With trenchless pipeline construction techniques such as horizontal directional drilling and microtunneling, new pipelines can be built with minimal disruption to the environment. This is because these methods allow for water, gas, power sewer or telecommunication cables installation without having to dig trenches in order to install utilities underground.
In comparison to open-cut methods, both approaches have gained appeal due to their lower carbon footprint and less disruptive nature. However, there are a few key distinctions between these systems that might aid contractors in selecting the best method for a given project.
Launch and Retrieval
HDD is a method used to install underground facilities in areas where traditional tunneling is prohibitively expensive. This trenchless installation process can be launched from the surface and requires no digging or excavation, making it an ideal option for those concerned with environmental impacts of traditional methods as well as costs.
It’s a surface-to-surface approach that doesn’t necessitate the excavation of a launch/entry shaft and a reception/exit shaft. However, adequate space for storage of pipelines and horizontal directional drilling equipment must be given on both the drill rig side and the exit side.
Pipelines have been installed successfully through long crossings, environmentally sensitive areas, and demarcated lands using this method. It’s widely used to lay pipelines beneath palustrine wetlands, creeks, streams, airports, and other structures.
Microtunneling, on the other hand, is a trenchless installation method that uses a laser-guided steerable remote control and is launched from a pit. The excavation face is supported indefinitely using this procedure. A jacking system and thrust wall are installed in an entrance or launch pit. The product pipe and microtunnel-boring machine (MTBM will be collected in the area where an exit hole will be dug. To handle the slurry, a facility for slurry separation is provided at the surface.
Pipelines that traverse infrastructure like highways, railway lines, and significant rivers are typically installed using this method. When compared to other trenchless approaches, MTBMs use high-end guided systems with live monitoring for real-time corrective capability.
Type Of Soil, Size, And Precision
The drill bit used in HDD, which can be suspended indefinitely within the drilling fluid and removed after a project is complete. The best type of soil for this technique are non-cohesive sand, clayey soils or silt as they cause less wear on the rig’s machinery. It also has an extended installation length up to 1800 meters deep with depths of 15 meters below surface level possible when drilled into alkaline earth materials such as limestone or dolomite found near water sources where contamination by groundwater is a potential issue.
This method can be used to install pipes with diameters ranging from 50 to 1200 mm. HDD has a +/- 100 mm precision. HDD can induce ground movements such as heaving or collapse, as well as the loss of drilling fluid, when utilized in shallow ground cover.
Microtunneling is most suited to soft formations with high water tables, although it can also be used to bore through sand, soil, clay, and even hard rock. Installation lengths of up to 225 meters are possible, with varied depths. This technology can install pipes with sizes ranging from 250 to 3000 mm. When big diameter pipes are involved, the accuracy gained can be as excellent as +/- 10 mm.
Most Suitable For
To ensure precise installation, HDD rigs require a variety of systems. This approach cannot be used to install gravity pipelines since it is launched from the surface and arcs down into the ground before emerging on the other end. Thus, it’s ideal for setting up water lines, pressure lines, cables, pump sewer systems, gas pipes, and conduits. Also, pipeline installation utilizing horizontal directional drilling equipment is less expensive than microtunneling.
For microtunneling, it’s best for installing gravity sewers and other pipelines that require exact grades. Other pipes, such as water, gas, and cable lines, are also installed using this method. Even though microtunneling is slightly more expensive than HDD, the accuracy, dependability, and lower post-installation maintenance costs of pipelines make it a preferable alternative in most circumstances.
The Drilling Process
Horizontal directional drilling has two stages. The first stage entails using a steerable guided drill to drill a pilot hole along a predetermined path. Along the intended pipe alignment centerline, drill a pilot hole with about 1 to 5 inches diameter. The second stage starts once the drill string gets to the exit point.
Attaching a reamer of the same size as the product pipe to the drill string’s end is the second stage. It may be necessary to make numerous passes to attain the desired bore diameter.
The bore diameter is approximately 50% bigger than the pipe diameter to allow for easy pipe string pull-through. The reamer is attached to the product pipe, which is then drawn through the borehole. To achieve the highest degree of accuracy, electronic monitoring of the drill route is carried out throughout the process. A slurry system aids in the circulation of drilling fluid and the transportation of drill cuttings to the surface.
For microtunneling, the jacking frame is installed when a launch shaft is excavated to the depth of pipe installation. The MTBM’s rotating cutter head excavates the ground as it travels. The MTBM’s laser guidance system constantly sends grade and position to the operator. The product pipe is pushed into the borehole made by the MTBM using the thrust wall at the back of the launch shaft.
Cuttings and drilling fluid are transported to the surface for settlement and recirculation using a closed-loop slurry system. Unexpected soil conditions like high water table or groundwater may require corrective procedures such as freezing or dewatering, as It necessitates stable ground. A variety of cutter heads are available to cut through various soil types, as well as rocks if necessary.
Takeaway
While the timeframe and initial costs of an HDD installation are frequently lower than those of microtunnelling, the long-term picture is somewhat different. The costs of microtunnelling may be outweighed by the risks of greater maintenance costs and increased pipe failure when utilizing HDD.
It all boils down to budget and risk tolerance: if you’re on a tight budget and don’t care about accuracy or long-term expenditures, HDD might be the way to go. However, if the pipeline must be constructed precisely or long-term expenses must be considered, microtunnelling is the ideal alternative.
