Directional drilling is a method of installing pipes, ducts and services with minimum excavation. In the broad sphere of ‘trenchless technology’, directional drilling falls between impact moling, which is generally used for small diameters and short lengths, and microtunnelling, which focuses mainly on longer drives in diameters from about 500mm upwards.
For many years directional drilling has been a major growth area in the trenchless technology industry. It is now a common procedure in the gas, water, electricity and telecommunications sectors, and recent improvements in accuracy have allowed directional drilling to be used for installing sewers and other gravity pipelines.
The terms ‘guided boring’ and ‘directional boring’ are often used. At one time, guided boring referred mainly to smaller installations, while directional drilling was reserved for larger rigs and applications, but recently the terms have become largely interchangeable. To distinguish it from vertical well drilling, the process is commonly known as horizontal directional drilling or HDD.
Among its advantages, HDD is steerable, unlike most impact moling systems, and it is invariably cheaper than microtunnelling. The drill path may be straight or gradually curved, and the direction of the drilling head can be adjusted during the bore to steer around obstacles or under highways, rivers or railways. Drilling can be carried out between pre-excavated launch and reception pits, or from the surface by setting the machine to drill into the ground at a shallow angle.
Installation of the product pipe or duct is at least a two-stage operation. A pilot hole is first drilled along the required path, and the bore is then back-reamed to a larger diameter to accommodate the product pipe. During this second “pull-back” stage, the pipe is attached to the reamer by means of a swivel connector, and is pulled into the enlarged bore as the drill string is withdrawn. In difficult ground conditions, or where the bore enlargement is considerable, there may be intermediate reaming stages during which the bore diameter is increased progressively, the product pipe being pulled in during the final stage.
Apart from the obvious environmental benefits of trenchless installation, the relative cost of HDD has fallen below that of trenching for many applications, even ignoring the costs of traffic disruption and delay that can be incurred by trenching.
Methods
Most guided boring machines use a fluid-assisted drill head which is pushed through the ground on the end of a string of drill pipes. The head is usually angled so that constant rotation of the drill string produces a straight bore, whereas keeping the head in one position causes the line to deviate. A sonde or beacon may be built into the head or fixed close to it, and signals it emits are picked up and traced by a receiver on the surface. This allows the direction, depth and other parameters to be monitored. Hard-wire guidance systems may also be used, with the cable running through the drill string, particularly where the bore path cannot readily be traced on the surface, or where the depth of the bore is too great for accurate location by radio-frequency methods.
The drilling fluid may be simply water, but for longer or larger installations a properly formulated bentonite/water or polymer/water mix is normally used. The drilling fluid (or “mud”) carries the debris in suspension and can be filtered through a recirculation system. On completion of the pilot bore, the thixotropic mud stabilises the hole ready for back-reaming. The fluid also helps to cool the drilling head and the tracking sonde.
Most smaller machines rely mainly on axial thrust for the pilot bore, whereas larger rigs do most of the work by the rotation of the drill string. Mud motors, powered by the drilling fluid, can be used to drive rock cutting heads, and this technique may be used with some of the more powerful rigs.
Some systems are designed for dry operation without the use of water or drilling fluids. These are simpler to operate, create less mess and do not require as much on-site equipment, but there are limitations to the sizes that can be installed and the ground conditions that the machines can cope with.
Percussive action may be used to complement axial force and rotation. This can be achieved either with a percussive hammer at the bore-head, or by generating the percussion at the machine on the surface and transmitting it along the drill string. Percussion allows improved penetration and directional control in stony soils or weak rock, but is not intended for drilling through solid rock or large masses or hard material such as concrete.
Surface-launched directional drilling machines are often track-mounted and can be moved into position under their own power. Although they do not require starter or reception pits to install the new pipe, excavations are nevertheless required to make the connections at each end. Assuming that these connecting pipes are at some depth below ground, the first few metres of new pipe may be wasted in drilling down to the required depth.
Pit-launched machines require an excavation at each end of the bore, but may be operated in restricted spaces. Some of the more compact machines can work from an excavation just slightly larger than that needed to make the joint after installation. The length of individual sections of drill pipe is restricted by the dimensions of the excavation, and this may influence the speed of installation and the cost of the drill pipe.
Pit-launched rigs use the faces of the excavation to provide reaction to the thrust and pullback forces. Surface-launched rigs have some form of stake-down system to anchor them to the ground. On the more sophisticated machines, the stake-down system may be hydraulically powered.
An automatic drill pipe loading system may be incorporated, in which the lengths of drill pipe are contained in a ‘carousel’ and are automatically added to or removed from the drill string as boring or back-reaming progresses. There may also be an automatic vice arrangement which screws the drill pipes together or unscrews them during back-reaming. Automatic pipe handling speeds up installation, improves safety and reduces manpower requirements.
The capabilities of directional drilling machines depend greatly on ground conditions. In general, homogeneous clays are the most favourable soils, while sand can present problems, especially if it is below the water table or is not self-supporting. Gravel can be penetrated at the expense of accelerated wear to the bore-head. Standard machines without percussive action or mud motors are generally unsuitable for penetrating rock or hard inclusions which can stop the bore-head or throw it off line.
Fluid-assisted boring
Some surface-launched machines, especially the small and medium sized rigs, are self-contained, having on-board mixing tanks and pumps for the drilling fluid, together with associated power supplies, valves and control systems. Alternatively, separate mixing and pumping units may be mounted on a truck or in a container. The fluid is pumped through the hollow drill string to the bore-head, and returns through the space between the drill string and the walls of the bore. The fluid, together with the excavated material mixed with it, can be pumped into a filtration unit for separation and recycling.
The drilling fluid may lubricate the cutting head and reduce wear, soften the ground, convey excavated material in suspension back to the launch pit, stabilise the bore prior to backreaming, lubricate the product pipe during backreaming and insertion, and power mud motors for drilling through hard ground.
Dry boring
Dry boring machines tend to be more compact and simpler than fluid-assisted rigs, and both surface-launched and pit-launched rigs are available.
Instead of relying entirely on thrust and rotation generated at the rig, dry boring machines use a high-frequency pneumatic hammer at the bore head to penetrate and compact the ground for the pilot bore. In this respect, the concept is not unlike an impact mole on the end of hollow drill pipes which also act as the pneumatic feed. As with fluid-assisted systems, the chisel head in front of the hammer is angled, allowing the bore to be steered by stopping the rotation at a particular orientation.
For small diameter pipe, duct or cable installation, a cone-shaped reamer with tungsten-carbide cutting teeth may be connected directly to the drill rods. The expander is fitted with air jets, fed through the drill string, and a high velocity air flow helps to clean out the bore during backreaming. The expander is rotated and pulled back to enlarge the bore, with the pipe attached to the rear using a swivel connector and some form of towing head.
For larger diameters, a pneumatically powered reaming hammer is used, again with the pipe string attached to the rear of the device by means of a swivel. The percussive effect of the reaming hammer, rather than the pull-back force of the machine, is the main agent in expanding the bore, and no rotation is required during backreaming.
Tracking and guidance systems
Most guided boring techniques, other than some short-distance pit-launched applications, rely on accurate bore location and guidance systems. The most common types, known as ‘walk-over’ systems, use a sonde or beacon contained in a housing behind the bore-head. This emits a radio signal which is picked up by a receiver on the surface. In addition to giving the position and depth of the bore-head below ground, the data transmitted will often include the inclination of the drill bit, the orientation of the head, beacon battery status and beacon temperature. It is common for this information to be relayed to a satellite receiver at the drilling machine.
Where there is no access to the surface directly above the bore-head, tracking can be achieved either by a ‘hard wire’ system, or by a beacon containing an on-board electronic compass.
To avoid subjecting electronics to severe dynamic loading, a location and guidance system based on magnetometry is used with dry guided boring machines which employ percussive hammer action. Permanent magnets are housed in a section of the pilot hammer, and a magnetic field is created as the hammer rotates. The strength and fluctuation of this field is detected by magnetometers on the surface, and a computerised processing unit translates this data to give location, depth and roll angle of the bore-head.
The choice of backreaming tools and accessories is very wide. Most reamers are bullet-shaped with an arrangement of tungsten carbide teeth and fluid jets. The rear of the reamer has a coupling to which a towing head can be attached for pulling in the product pipe. Special designs are available for difficult ground conditions, including hole-openers for reaming in rock.
Various types of towing heads are available, including pressure-tight heads and versions aimed specifically at directional drilling. One function of these is to prevent the ingress of drilling fluid or debris into the product pipe, which may be important for potable water pipes.
Swivel connectors are an essential component during the backreaming and pipe-pulling operation, and models are available with capacities from less than five tonnes to more than 200 tonnes.
Some contractors use ‘breakaway connectors’ to protect the product pipe. The connectors have a series of pins designed to break under a predetermined load, and are set according to the permissible tensile load on the product pipe.
Other ancillary equipment may include butt-fusion machines for jointing polyethylene pipe, pipe support rollers and cable pullers.
Conclusion
Horizontal directional drilling is a low-disruption technique for new pipeline installations. It has a long track record and is more economical that most other steerable techniques such as microtunnelling. Recent improvements in guidance technology enable HDD to be used to install gravity pipelines such as sewers.
