Horizontal Directional Drilling and It's Application

HORIZONTAL DIRECTIONAL DRILLING (HDD) is a construction procedure that has been employed internationally for some 25 years. It describes a specific form of trenchless technology that, by definition, enables a variety of underground ducts and pipes to be installed with minimum disruption to the ground surface. The technique combines the steering technology developed in the oil industry with boring methods employed for conventionalhorizontal trenchless crossings. It requires little, if any, excavation to install pipes and conduits in a range of lengths, sizes and depths along pre-assigned vertical and horizontal alignments.

The technique has gained worldwide acceptance and is considered an established method for installing pipes and ducts ranging in sizes from 50 mm to 1 500 mm beneath large obstructions such as rivers, wetlands, highways, airfields and buildings.

In 2001 it was estimated that there were over 10 000 HDD rigs of various sizes worldwide, yet in South Africa there are probably fewer than ten, suggesting that local application of the system has been limited. No doubt there are many reasons for this, not least of which being the high cost of importing the machinery, tooling and consumables, compounded by the reluctance of clients and consulting engineers to accept an apparently new and untested technology as a viable alternative to conventional construction methods.

APPLICATIONS

As with all trenchless methods, HDD has the advantage of minimising the disruption caused by conventional open cut and cover excavations for service installations, and it improves safety as it eliminates the need to support the sidewalls of deep trenches. HDD has the added advantage of enabling crossings to be constructed beneath large surface obstructions along preset horizontal and vertical alignments where direct, indirect and social costs would otherwise make such an alignment unfeasible if conventional construction techniques were to be employed.

Environmental applications are wide and varied, including limiting surface disruption in sensitive areas such as wetlands. Others include the installation of well screens along horizontal curves within or near a contaminated zone, thereby optimising the efficacy of contaminated water recovery by enabling substantial lengths of screen to be placed in the areas most needed. In addition, HDD offers the ability to enhance bio-remediation of pollutants by the injection of appropriate liquids or gasses. Post-installation of leachate drains in tailings and landfill sites is another application of the technique in the environmental field, as is the sealing off of contaminated areas by horizontal injection of polymers or grouts beneath polluted underground plumes.

HDD can be employed to provide horizontal water extraction wells beneath or within rivers or lakes where, owing to set-up costs over water, conventional vertical boreholes may prove to be too costly.

Another possible application, which requires further study and research, is the use of horizontal guided drilling for site investigations.

THE TECHNIQUE

HDD essentially involves three processes:

1. The first stage entails installing a pilot hole drilled from the surface at a pre-determined angle and along a prescribed path. The drill path comprises straight inclined and straight horizontal sections connected to large radii curves. The latter are dictated by the size of the drill string, pipe material and pipe diameter and are formed by a small kink, or bend, in the drill string just behind the drill head, termed a 'bent sub'. Steering is controlled by rotating the drill head to the desired direction and then pushing the drill string forward until the required direction is obtained, after which drilling is continued along the realigned path. The size of the pilot hole is dictated by, among others, soil conditions, the size of the drilling machine and the drilling method. The pilot hole is supported by slurry, consisting of a mixture of water and bentonite, and sometimes polymers.
2. The second stage enlarges the pilot hole by reaming to achieve the required size within which the product pipe or duct is to be installed. Typically, the final hole should be between 1,3 and 1,5 times the outside diameter of the product pipe, and for large diameter holes a number of reaming operations may be required. The process is normally carried out by attaching the reamer to the drill string on the side opposite to that of the drill rig, from where it is rotated and pulled back into the pilot hole, which is enlarged by cutters attached to the reamer. Lengths of drill pipe are added to the reamer as it is pulled back to the rig. Slurry is again used to support the hole. Depending on ground conditions, forward reaming can also be carried out. In this process the reamer is thrust forward by the drill rig and guided by the pre-drilled pilot hole.
3. The final process entails pulling the product pipe into the pre-drilled hole. The pipe, usually steel or HDPE, is prefabricated to the required length on the side opposite to the drill rig, from where it is connected to the drill string via a pulling head onto which a swivel is attached. The swivel prevents rotation of the pipe as it is pulled by the drilling rig into the hole until the full length of pipe is in place.

EQUIPMENT

The equipment required to carry out HDD work includes the drilling rig, tracking system, mixing and recycling equipment, tooling, including rods, bits and reamers, and consumables such as bentonite and polymers.

Drilling rigs are classified into three groups termed mini, midi and maxi rigs. The type of rig selected depends on the length of pipe to be installed, the hole size, thrust and pullback capacity, torque and depth to be drilled. Table 1 illustrates the industry classification of HDD systems developed in 1994. Since then substantially more powerful rigs have been manufactured, enabling larger, deeper and longer crossings to be achieved, and the classification presented here is probably due for updating.

Mini rigs are typically used for short distances arid small-diameter pipes and conduits associated with municipal service reticulation lines. Maxi rigs, on the other hand, are used for longer distances and large diameter pipes, such as for bulk water and sewer, and beneath sizeable crossings. Intermediate distances and pipe diameters require a midi rig, the capabilities of which fall between those of the mini and maxi. Table 2 outlines typical applications for the various rigs classes.

In South Africa there are estimated to be seven mini and three midi rigs, most of which are located in Gauteng. No maxi rigs are available locally, although they can be purchased from overseas suppliers and manufactures.

Tracking systems are required to ensure that the position of the drill head is correctly aligned vertically and horizontally. These include the walkover and wireline steering systems, which provide information on the magnetic azimuth for horizontal control and inclination for vertical control.

The walkover system employs a transmitter, or 'sonde', located near the drill head. When positioned directly above it, a receiver at ground surface locates the position and depth of the drill head. The wireline system consists of a magnetic sensor placed in a non-magnetic bottom hole assembly, which is connected to a computer at the surface by wires which pass through the inside of the drill pipe. Magnetic readouts are interpreted by the computer, providing information on the alignment and depth of the bore.

In South Africa the walkover system is the cheaper and more popular, but it does have its limitations, some of which are discussed below.

Tooling includes drill bits, rods and reamers, the correct choice of which is vital to the successful completion of an HDD project.

Drill bit selection depends on the subsoil conditions and geology and range from duck bill and jetting or shovel bits for soft soils, through to tricone roller bits for stiff and dense clays and sands, and downhole hammer and Tricone bits with mud motor for very dense soil and rock conditions.

Drill rods are typically 127 mm (D150) to 171 mm (D300) OD hollow steel pipes in lengths of 9,1 m. Rod selection is related to the rig specifications, crossing length and product pipe diameter.

Reamers come in a range of sizes and shapes, each selected to suit the soil conditions encountered. They include spiral or fluted reamers, which are the most versatile since they can be used in sands, clays, gravels, cobbles and soft rock. Others include wing or open reamers for stiff clays, barrel reamers for loose sands and mixed soils, blade reamers for soft and firm clays, and rock reamers for rock formations.

Slurry mixing and recycling consists of a chamber for mixing the bentonite, water and additives and a collection pit where the returned mixture with cuttings from the hole are passed through a system of sieves and hydro-cyclones to separate the cuttings, which are disposed of, from the bentonite mixture, which is re-used.

Drilling fluid is one of the most important components of the entire HDD operation since it fulfils a variety of functions, including holding the cuttings in suspension and transporting them to surface, stabilising the hole, lubricating the drill bit and rods, cooling the tooling and preventing loss of slurry through micro-fissures and voids. Polymers and additives are sometimes mixed with the bentonite slurry. These are expensive but they do aid in improving lubrication, controlling fluid loss, enhancing suspension of the cuttings, and controlling viscosity.

SOME HDD PROBLEMS

Proper investigation, planning and the correct selection of equipment and tooling, and the use of experienced operators can overcome many of the problems associated with HDD projects.

Geotechnical conditions underlying the route are probably the most important factors to be considered in any HDD project, as they govern the selection of suitable tooling and provide information for the correct slurry design. Variations in subsoil conditions, both laterally and with depth, the presence of obstructions, the depth to a perched or regional water table and the presence of shallow rock are just a few of the many potential difficulties that may be identified by undertaking a geotechnical investigation before embarking on any HDD project. Potential problems can then be anticipated and addressed at the planning stage before undertaking the work.

Steering problems normally arise when soft clay or loose sand overlies very dense soil, such as pedocretes, or rock, more so at shallow entry angles. Under these conditions the drill head may deflect off the harder underlying material, leading to deviations from the desired route. Pushing the drill rod without rotation may assist in this regard. A similar steering difficulty may be encountered when the drill deflects off boulders occurring within the formation. Very soft low strength clays may also present steering difficulties, as they are unable to offer any significant shear resistance to the steering tool when a change in trajectory is required.

Tracking or locating systems, particularly the walkover system, present problems where metallic objects or magnetic and electrical interferences are present. Such sources include microwave towers, traffic signal loops, power lines and electrified security fences. The walkover system requires the receiver to be located above the transmitter at the drill head, and access to certain areas such as over rivers and wetlands or beneath buildings becomes problematic. The walkover system also has depth limitations. Although more expensive, the wireline tracking system is one method of overcoming some of these difficulties.

Borehole stability is governed by the material through which the hole is advanced and collapse of the hole may lead to numerous difficulties. These include high pullback forces, high torque and surface settlement, which occurs if the hole is large or near to the ground surface. While slurry often addresses potential borehole collapse, excessive use may cause swelling of some active desiccated clays owing to water absorption by the clay. This causes the borehole to slowly close with time, resulting in a hole size smaller than the product pipe. While potentially preventing hole collapse, thicker, and hence more viscous, slurry requires that more torque be applied by the rig.

Slurry loss is not uncommon, particularly in sands and at shallow depths where seepage occurs through permeable material. Hydraulic fracture in fissured clays and jointed weathered rock is another source of fluid loss. Increasing the fluid density and control of drilling pressure can partially assist in reducing these losses.

Drill rod failure occurs when excessive torque and pullback forces are applied to the string by the rig, mainly during reaming. Tensile and bending failure may occur during pullback along sharp curves which induce high stresses in the rods at the bends. Steep entry angles require high pullback forces. The grinding action of abrasive silica rich materials such as quartzite may wear the rod where it rests on the rock, reducing its diameter and hence strength.

LIMITATIONS

HDD is not the panacea for all installation projects and it does have its limitations.

For example, machine size limits the length and alignment for a given pipe diameter. Although sizeable projects can be undertaken with smaller rigs, the risks are increased, particularly during reaming and pipe installation. Careful planning, detailed investigation and considerable operator skill are required to undertake an HDD project employing a machine with lower thrust, pullback and torque than that ideally required.

Space limitations may in certain situations obviate the use of HDD, especially in developed urban areas. For a given depth, entry angle, pipe size and material type, the drilling rig must be set back to a predetermined entry point. If obstructions such as buildings and roads prevent this, then HDD is not feasible at the location intended. Similar requirements are necessary at the exit point. Space is also required for the rig, slurry mixing and recycling plant, disposal and for storage of the tooling and consumables.

Pipe type and diameter place limitations on the radii within which the pipe can be installed. Sharp curves with small radii place undue stresses on the drill rod during pullback, and both tensile and bending stresses on the pipe itself. Large pipes require large bend radii with concomitant large set-back distances requiring additional space, which, if at a premium, makes the technique unviable.

Buried obstructions such as boulders can seriously effect steering, as discussed above. Fill areas containing large blocks of reinforced concrete rubble are even more problematic. Not only does this affect steering, but the steel interferes with the tracking system and severely wears the drill bit.

PROJECTS

A few projects that were undertaken recently are described below, together with the problems encountered and the methods employed to overcome them.

• A 200 m long crossing beneath a river underlain by alluvial deposits comprising cobbles and sand was undertaken in the Eastern Cape. A 315 mm HDPE pipe was installed after pilot boring with a Trihawk and roller-cone drill bit and staged reaming with hole openers. Problems were encountered in penetrating the dense tightly packed cobbles, which may have been overcome had a deeper bore been designed and a mudmotor employed, as originally proposed. Owing to limited funds, a shallower depth was selected and an alternative drilling technique to that ideally required was used.
• A 284 m long crossing beneath a river underlain by silt and silty sand was undertaken in Mozambique. A 200 mm steel pipeline placed within a 315 mm HDPE sleeve was installed after the pilot bore had been enlarged to accommodate the sleeve.
• Two 200 mm diameter HDPE pipe ducts for electric cables were installed beneath a railway line, a road and a stormwater channel, over a distance of some 150 m and within residual granite comprising uniform clayey sand of stiff consistency. The crossings were located near to a substation and beneath a railway line where high electromagnetic fields were generated, leading to problems with the walkover tracking system. This was overcome by utilising a dual frequency sonde.
• Numerous HDPE sewer pipes, ranging in size from 200 to 315 mm, were installed over a total distance of some 1 200 m to depths of the order of 4 to 5,5 m in a busy urban area. Variable ground conditions were encountered, ranging from transported silty sands through to clayey silt, residual andésite, residual shale gravel and dense sand, and residual sandstone. Tracking was carried out with Digitrak locating systems and a Magenta 0,1 % drilling sonde, which enabled drilling to the tight tolerances required for the sewer lines.
• A number of crossings for a gas line were installed in soils ranging from fill, alluvial gravels and cobbles, expansive clay and residual sandstone, to quartzite and dolerite. Pipes consisted of bitumen-coated steel of 204 mm OD. Tooling included a standard jetted system, mud motor, directional air hammer and hole openers. all bores were designed and logged and the final 'as-built' bore installation with details and drawings provided.

The main problem was the variable geotechnical conditions at each of the crossings. No information was provided prior to establishing on site and ground penetrating radar was used in an attempt to estimate the in-situ density of the soils, and by inference consistency, and to locate services.

The bitumen-coated cover to the pipe also created some difficulties, particularly in areas underlain by weathered rock, as the gravel-sized particles tended to adhere to the coating, necessitating large pullback forces to overcome the increased frictional resistance. Care had to be taken to ensure that all gravel and sand suspended in the bentonite was removed before installation of the pipe was attempted.

In some locations the underlying active clays were dry and stiff. When slurry was introduced, they expanded, resulting in some difficulties in installation. Slightly larger holes were reamed where these conditions occurred. Loss of slurry through fissures, which are common in these clays, particularly in the upper horizon, also occurred when crossings were too close to the ground surface.

• A 450 mm diameter 74 m long HDPE sewer pipe was installed in dense ferricrete in a township. Very few problems were encountered on this contract. Laying out the product pipe before installation did interfere with access to driveways and some roads, but the local residents were informed of this beforehand and were very accommodating.
• A 137 m long crossing beneath a road and located in rock was undertaken employing a 25 ton midi rig and adirectional air percussion hammer. Because of space constraints and other restrictions the rig was set up on a platform some 4 m above the road level. Complex curves were required and continual directional changes had to be made throughout the bore, together with allowances for bend radii of the drill rod, sleeve pipe and the product line. Rock hole openers were used to enlarge the bore diameter to the required size before it was finally cleaned and the sleeve pipe installed.
• In Saudi Arabia, a maxi rig was used to install two crossings, one a 406 mm diameter 1 000 m long steel pipeline located beneath a 167 m high sand dune. No problems of any significance were encountered except that of designing a facility capable of pumping and recycling 2 000 l/min of slurry down the hole during the reaming stages. The second installation consisted of a 610 m long 762 mm diameter steel pipeline placed beneath a highway interchange. Sandy clay and sandstone were encountered in the bore. The only available water in the area was saline, which required a special blend of drilling fluids. Of the few problems encountered, one was that the viscosity of the drilling fluids increased from 40 to 120 seconds owing to the sandy nature of the material. An additive and water were used to constantly adjust the fluid mixtures. Buoyancy of the pipe was calculated and adjusted throughout the pulling process. The entire installation was completed in 17 days.

CONCLUSIONS

HDD is a useful and internationally accepted technique for the installation of pipes and ducts covering a range of sizes and lengths in areas of surface and buried obstructions. It has applications in many fields, including civil infrastructure, water abstraction and environmental monitoring and remediation.

The method requires fairly sophisticated equipment and tooling and the experience of skilled operators. While it has its advantages, there are limitations for which the technique is suitable and it can be fraught with problems if undertaken by inexperienced contractors or with inadequate equipment.

Since it was first implemented, a great deal of research has been undertaken and significant developments have been made in the machinery, tooling, tracking equipment and slurry design, and HDD has now reached a level of advancement that makes it a viable, competitive and cost-effective alternative to conventional subsurface installation methods. Projects can now be undertaken which, for social, environmental or financial reasons, would not otherwise have been possible.

For various reasons the method has yet to receive sufficient recognition and application in South Africa, which has placed limitations on the number of contractors carrying out HDD work, and on the number and size ofdrilling rigs and tooling available.

Sumber : Harrison, Brian; Wood, André. "Horizontal Directional Drilling and It's Application". 29 Januari 2014. http://search.proquest.com/docview/221094812?accountid=31562