The rapid growth of cities needs more and more underground infrastructures such as electrical and communication networks, water and sewer piping system and other small utility tunnels. In metropolitan cities, the jamming of road traffic is a severe problem for the society. The traffic problem get increased when the infrastructure is constructed using the traditional methods of open trench construction, because, it causes significantly disruption of traffic.
In addition, unsafe trenches and risky conditions can cause danger to pedestrians and workers. Besides, the traditional methods always disturb the environment, such as, site clean up difficulty and soil movement problem. In recent years, making horizontal bores under the ground has become the preferred construction method for installation of the new underground infrastructure.
The most of the above problems can be eliminated by using trenchless technology which can be performed at the same or even lower cost than the conventional open trench methods. Being a trenchless method for installation, underground horizontal boring is particularly suited for locations that are inaccessible by conventional open cut techniques. This may include situations where the proposed installation lies in an environmentally sensitive location, crossing of buildings, roads, highways and railway lines. The low environmental and social impact of this technology has made the underground boring extremely viable and desirable by various departments and municipalities where there is a high investment in surface infrastructure and the commutation of traffic makes open cut alternatives inconvenient.
The trenchless methods allow installation of most underground infrastructure systems with minimum surface disruption. With the view of demand for trenchless construction practices, the horizontal directional drilling, microtunneling, impact moling & pipe jacking industry has grown substantially over the past few years.
Horizontal Directional Drilling (HDD) technology is evolved from the merging of oil field and water well drilling technologies. The main advantage of this technology over conventional services laying techniques is that it is a trenchless method that requires a minimum amount of excavation to complete an installation. This technique allows flexibility in installation of the pipes etc. as the path can be drilled straight or curved to avoid surface and subsurface obstacles.
The directional drilling industry has grown very fast and is being utilized by several users. It was first used in the oil and gas industries and then expanded to include utility installations, environmental remediation and the installation of sewers etc. The installation of pipe utilizing directional drilling is a multi stage technique completed in two phase operations which includes the drilling of a pilot hole and its subsequent reaming to install the product pipe. Installation of pipe is conducted from the surface and commences with the drilling of a pilot hole along the path of installation. The pilot hole is launched from the surface at a certain angle and then gradually becomes horizontal when the required depth is reached. The hole can be steered and tracked from the surface using a walk over or wire line locator system to direct the hole to the exit location. Once the drill string reaches the surface at the exit location, a reamer is attached to the drill string and pulled back to the entry point. This process enlarges the hole for the installation of the product line. To achieve the required size of hole, the several reaming operations can be performed, Figure 1. Generally, all reams prior to the actual product installation are referred to as pre-reams and the final ream to which the product pipe is attached is referred to as the back reaming.
The product line is installed once the borehole is enlarged to a diameter that is generally 1.5 times of the outside diameter of product pipe or conduit. This is necessary to allow for an annular void for return of drilling fluids & spoils and to allow for bend radius of the product line. There is a variety of different reamers for each type of soil. A blade reamer is used for soft soils, a barrel reamer for mixed soils and a rock reamer with tungsten carbide inserts is used for rock formations.
The wet drilling process requires large volume of drilling fluid which provides various functions like hydraulic cutting with jet, lubrication of cutting head, transportation of cuttings out of the bore, stabilization of the bore hole against collapse etc. The slurries most commonly used as drilling fluid are bentonite based.
The advantage of using this method is its low operational cost, effectiveness in congested urban areas for crossing of roadways, Low restoration cost as it requires very small entrance pit.
The possible application of horizontal directional drilling can be in soil vapour extraction, ground water extraction and infiltration wells.
Guidance and Control system :-
The actual path of the pilot hole is monitored during drilling by taking periodic readings of the inclination and azimuth of the leading edge. Readings are taken with an instrument, commonly referred to as a probe, inserted in a drill collar as close as possible to the drill bit. Transmission of downhole probe survey readings to the surface is generally accomplished through a wire running inside the drill string. These readings, in conjunction with measurements of the distance drilled since the last survey, are used to calculate the horizontal and vertical coordinates along the pilot hole relative to the initial entry point on the surface. Azimuth readings are taken from the earth's magnetic field and are subject to interference from downhole tools, drill pipe, and magnetic fields created by adjacent structures. Therefore, the probe must be inserted in a non magnetic collar and positioned in the string so that it is adequately isolated from downhole tools and drill pipe. The combination of bit, mud motor (if used), subs, survey probe, and non magnetic collars is referred to as the Bottom Hole Assembly or BHA.
The pilot hole path may also be tracked using a surface monitoring system. Surface monitoring systems determine the location of the probe downhole by taking measurements from a grid or point on the surface. An example of this is the TruTracker System. This system uses a surface coil of known location to induce a magnetic field. The probe senses its location relative to this induced magnetic field and communicates this information to the surface.
The drilling process for HDD can be described as follows:
- Excavate and prepare the driving pit and receiving pit.
- Set up the control unit and any other auxiliary equipment beside the driving pit.
- Set up the HDD rig at driving pit.
- Position the HDD rig in front of the driving pit with proper inclination .
- Set up and calibrate navigation (down hole) system.
- Set up the hoses ( water , bentonite, hydraulic ) with the HDD rig unit.
- Pilot Hole Directional Drilling :Pilot hole directional control is achieved by combination of using simultaneous rotational and thrust movement of cutting head , with steering option by cutting bit bend.
- drilling progress by hydraulic cutting with a jet nozzle
- The actual path of the pilot hole is monitored during drilling by taking periodic readings
- The pilot hole path may also be tracked using a surface monitoring system. Surface monitoring systems determine the location of the probe downhole by taking measurements from a grid or point on the surface
11.Reaming : Enlarging the pilot hole is accomplished using either prereaming passes prior to pipe installation or simultaneously during pipe installation. Reaming tools typically consist of a circular array of cutters and drilling fluid jets
12.Prereaming :a prereaming pass, reamers attached to the drill string at the exit point are rotated and drawn to the drilling rig thus enlarging the pilot hole. Drill pipe is added behind the reamers as they progress toward the drill rig. This insures that a string of pipe is always maintained in the drilled hole
13..Pullback : Pipe installation is accomplished by attaching the prefabricated pipeline pull section behind a reaming assembly at the exit point and pulling the reaming assembly and pull section back to the drilling rig. This is undertaken after completion of prereaming or, for smaller diameter lines in soft soils, directly after completion of the pilot hole. A swivel is utilized to connect the pull section to the leading reaming assembly to minimize torsion transmitted to the pipe. The pull section is supported using some combination of roller stands, pipe handling equipment, or a flotation ditch to minimize tension and prevent damage to the pipe.
14..Buoyancy Control : Uplift forces resulting from the buoyancy of larger diameter (pipe 30 inches or over in diameter ) lines can be very substantial. High pulling forces may be required to overcome drag resulting from buoyancy uplift.
- The most common method of controlling buoyancy is to fill the pipe with water as it enters the hole. This requires an internal fill line to discharge water at the leading edge of the pull section (after the breakover point). An air line may also be required to break the vacuum which may form at the leading edge as the pull section is pulled up to the rig. The amount of water placed in the pipe is controlled to provide the most advantageous distribution of buoyant forces.
Diameter range :-
100 mm to 1500 mm
Depth of installation :-
A minimum of 1.65 m of cover or a depth-of-cover to diameter ratio of 2 is usually recommended for MT to avoid heave or settlement of the surface
Drive length :-
Drive length more than 300 m. in a single pass has been achieved by Atoz Infracon Pvt. Ltd. Recent past in NH-2,Budwan , West Bengal, India. With state of art technology drive length of 1 km and above is also possible.
Type of pipe :-
The most common types of pipe used with HDD are steel, HDPE pipe in India so far .
Required working space :-
Adequate working space needs to be provided at the drive shaft to accommodate the required equipment and materials for the HDD operation. The space requirement is determined by the drive Pit size, Pipe diameter etc.
Soil condition :-
The most favorable ground condition for HDD is wet clay type soil . However, a wide selection of HDD cutter heads is available that provide the capability to handle a range of soil conditions, including sand, boulders , morrum and rock.
An HDD crew of Six to eight can obtain a production rate of 30 m to 300 m per shift, however productivity largely depends on many other factors like utility to be installed , soil condition, water availability, congestion etc .
Atoz Infracon is considered to be pioneer in India in HDD Technology its expertise is hired across the country by governments and as well as private companies to implement Trenchless HDD Projects.