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Horizontal Directional Drilling

Overview
HDD is one of the most versatile trenchless technologies used for the installation of many product lines including service connections, pipes and cables under highways and watercourses etc. In the simplest of terms, an HDD rig will 'drill' a pilot tube from which the 'hole in the ground' can be enlarged if required to permit the installation of a larger sized pipe or cable. Generally, HDD is best suited for installing pressure pipes and ducts/conduits where precise grades are not required.
In HDD machine technology, rapid developments are quite common and within the last few years the performance of HDD machines and accessories has increased enormously. With numerous additional functions, automated steps, improved mixing units, advanced power transmissions, optimised tooling, etc. a bore performance can now be achieved for which a machine with twice the capacity would have been required three or four years ago.


Application
Generally HDD methods are suitable for the installation of gas, water and sewage pressure pipes, cables, siphons etc. and may occasionally be used for gravity sewers but is very dependent on whether an even, constant gradient can be achieved which in turn is dependent on ground conditions.

Technology Description
The application of the technology for utility service projects has been taken from the oil and gas industry which developed the method to drill to/from offshore installations. For utility applications, 3 basic rig types have been developed: Mini, Midi & Maxi while the Mega rig is also available for some major project needs – see table below.
The process of drilling using HDD technology is usually a relatively simple method for small diameter product pipe, say covering a short distance but much more complicated when the final product is large and the distances are long.

Bore rig (type) Max. pulling force (kN) Max. torque (kNm) Weight (tonnes)
Mini 150 7 to 13 <7
Midi >150 to 400 15 to 30 7 to 25
Maxi >400 to 2,500 30 to 100 25 to 60
Mega >2,500 >100 >60

Classification of bore rigs in accordance with the DCA Technical Guidelines

An HDD set up comprises of: -
• directional drill rig sized for the job at hand;
• drill rods linked together to form a drill string for advancing the drill bit and for pulling back reamers and product pipes or cables;
• transmitter/receiver for tracking and recording the location of the drill and product;
• tank for mixing and holding drilling fluid/mud;
• pump for circulating the drilling fluid/mud;
• drilling bits, reamers, swivels and pulling heads.
It is usual to operate the HDD rig from an excavated launch site where the rig is set-up with an entry angle of 8° to 16° and positioned to drill a pilot bore along a planned path to an exit or reception pit where either the product pipe, reamer (to enlarge the drill hole) or product pipe reamer is attached and pulled back through the bore hole – see HDD diagram. When drilling fluids/muds are used, the launch pit is deepened to capture the returning, contaminated fluids - a similar pit is used at the planned exit point. The drill string comprises of a series of drill rods and advanced by a combination of rotation and thrust supplied by the HDD rig. There are a number of bit types to enable the drill head to navigate through different types of soil, from sands, clays and rock. Most drill bits have a slant-face, the orientation of which determines the direction that the bit will advance. To move in a straight line, the rig operator both rotates and pushes the drill string and to change direction, the rotation is stopped and the drill string just pushed. The path of the drill head and that of the drill string will change in the direction that the bit's slant-face is pointing. On-board controls allow the operator to monitor the orientation of the bit and the change in general direction of the bore.
Once the pilot bore reaches the reception pit, the reaming and installation the final product pipe phase begins. The hole is reamed in one or more passes to the required diameter which is usually about 1½x the size of the product pipe - reamers are designed to operate best in certain types of soil and the larger the final product pipe size required, the more passes will be required to open up a hole capable of receiving the product pipe along the full drill length. The product pipe is attached to the drill string with a pulling head and swivel and pulled back to the launch pit.
Tracking and guiding the drill head with a transmitter/receiver system is usually managed by one of two standard methods: a walk-over tracking system or an in-line wire. Such systems are described in 'Location, Detection & Inspection' part of the UKSTT web site. The information received allows the operator to track location along the planned bore and to make changes as necessary. The in-line wire method is usually used for large scale projects where tracking across water for instance is required.
When using the Mini Rig, the need to use a drilling fluid/mud is not usually necessary particularly for service pipe and cable installation apart perhaps in areas of poor ground conditions. For smaller drilling operations, the returned material can be removed by vacuum technology for disposal. However for economical and efficiency reasons, for larger projects it is normal practice to recycle the drilling fluid using a combination of screens, centrifugal pumps and hydro cyclones to remove the cuttings from the fluid. The drilling fluids or muds which are pumped down the drill line through the hollow drill rods and holes in the drill bit, are key to keeping the transmitter electronics cool, stabilizing the hole, particularly in poor ground conditions and removing the returned fluid from the bore hole. Drilling fluid/mud technology is an 'art' in itself and the mixtures are carefully designed and prepared to address the solid conditions that are anticipated along the planned path The quality of the fluid can be monitored throughout the drilling process.
Depending on the machine components, the pit bore rig can be equipped with a hammer for difficult soils which allows un-steered bores to be driven in light to medium heavy rock – this set up is driven by a combined hydraulic/pneumatic unit.

Mini Bore Rigs
Mini bore rigs are mostly used in inner-city areas for the installation of PE pipes or cables. The bore rigs develop maximum pulling forces of approximately 150kN, maximum torques of 10 to 15kNm and weigh up to 7 tonnes. Many of these mini rigs are operated on (rubber) caterpillar tracks.
The mini rig machines can be started out of a small pit or shaft and mainly install property service connections for gas, water, telecommunications and electricity but they may also be used to lay drains for connecting buildings to sewer pipes if the gradient and ground conditions allow. Road crossings and smaller under-river crossings are also possible. Depending on the machine performance, the maximising upsizing diameter is 250mm. Advantages: -
Whenever longer or more complicated connections to buildings are to be made
Whenever a controlled bore is required for safety reasons
Whenever a quick, clean, precise and economic solution is required
Whenever a non- steerable soil displacement hammer cannot be used

Midi Bore Rigs
Midi Bore rigs are often used in inner city areas, for smaller under river crossings or special tasks such as environmental technology. They develop maximum pulling forces of 150 to 400kN, torques of 15 to 30kNm and weigh from 5 to 25 tonnes. As a rule these rigs are built on caterpillar tracks and are therefore self- propelled and have all-terrain access.
Maxi Bore Rigs
Maxi bore rigs are used for extensive bore lengths and diameters. They can often be found on pipeline system routes where they are used to produce under-river, under-railway or large under-road crossings. Maximum pulling forces of these bore rigs are between 400 and 2500kN, torques vary between 30 and 100kNm and weigh from 25 to 60 tonnes.

Mega Bore Rigs
Mega bore rigs are designed for extreme bore lengths and borehole diameters, for example their use on pipeline routes in Eastern Europe and Asia. Maximum pulling forces of these bore rigs are more than 2500kN, torques are above 100kNm and the machines weigh over 60 tonnes.

Horizontal Directional Drilling
Horizontal Directional Drilling

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