A horizontal directional drill (HDD) is a trenchless construction machine that drills a steerable borehole along a shallow curved path under the ground and then pulls a pipe or cable duct back through it. The rig sits at the surface on one side of an obstacle, such as a road, river, railway or building, and the pipe comes out on the other side without an open trench along the route.
HDD is used to install water and gas mains, sewers under pressure, telecommunications ducts, fibre optic cables, power cables and oil and gas pipelines. Machines range from compact units that fit on a small trailer and install fibre ducts under driveways, to very large rigs with pullback forces of hundreds of tonnes that cross major rivers with large diameter steel pipelines.
Brief History of Horizontal Directional Drills
Directional drilling techniques were first developed in the oil industry to deviate vertical wells. The idea of drilling a shallow curved bore under an obstacle came from the utility and pipeline sector. In the early 1970s, Martin Cherrington, a contractor in California, carried out what is widely regarded as the first HDD river crossing, and the method was then developed for pipeline crossings of increasing length and diameter.
Through the 1980s, better downhole steering tools, mud motors and drilling fluids made longer crossings possible, and walkover tracking systems that locate a transmitter in the drill head from the surface made small bores practical. Compact self-contained rigs spread rapidly in the 1990s with the growth of telecommunications and cable television networks, and HDD became a standard method for utility installation. Today the technique is used worldwide, from short urban service connections to crossings several kilometres long.
How a Horizontal Directional Drill Works
An HDD installation normally has three stages: pilot hole, reaming and pullback.
- Rig and carrier. The drill is usually mounted on a crawler or trailer carrier. A rack or thrust frame, set at an entry angle, carries a carriage with a rotary drive that pushes, pulls and rotates the drill pipe.
- Pilot bore. A steerable drill head with an angled face or a bent sub is pushed and rotated along the planned path. To steer, the operator stops rotation and pushes the angled face in the desired direction.
- Tracking. A walkover locator above ground reads a signal from a transmitter behind the drill head, giving depth, pitch and roll. Longer bores use wireline or gyroscopic steering tools that report the position through the drill string.
- Drilling fluid. A mixing system and pump send fluid, usually bentonite and water, through the drill pipe. It cools the head, carries cuttings, stabilises the bore and lubricates the pipe.
- Reaming. Once the pilot bore reaches the exit point, a reamer enlarges the hole in one or more passes to a diameter larger than the product pipe.
- Pullback. The product pipe is attached behind the reamer through a swivel and pulled back into the bore as the drill string is withdrawn.
- Pipe handling. Rod loaders on smaller rigs, and pipe handling cranes or loaders on larger rigs, add and remove drill pipes.
Types of Horizontal Directional Drills
Mini and Compact HDD Rigs
Small rigs with pullback of up to about 10 t, used for fibre, telecom, power and water service connections in streets and gardens. Many are highly automated and have onboard rod loaders.
Midi HDD Rigs
Medium rigs used for distribution pipelines, larger ducts and river or road crossings of moderate length.
Maxi HDD Rigs
Large rigs, often built as modular spreads with separate power units, mud systems and pipe handling, used for long crossings of large diameter pipelines under rivers, estuaries and infrastructure.
Rock Drilling Rigs
Rigs fitted with dual-rod systems, mud motors or air hammers to drill through rock. Dual-rod rigs have an inner rod that turns the bit while the outer rod steers.
Related Trenchless Methods
Pipe ramming, auger boring, pipe bursting and microtunnelling are other trenchless methods. HDD is chosen for its steerability and speed, while microtunnelling is preferred for gravity sewers that need precise grades.
Size Classes
| Class | Typical pullback force | Typical use |
|---|---|---|
| Mini and compact | up to about 10 t | Fibre, telecom and service connections |
| Midi | about 10 to 45 t | Distribution mains, road and small river crossings |
| Large | about 45 to 100 t | Larger pipes and longer crossings |
| Maxi | over about 100 t | Long river crossings, transmission pipelines |
Figures are indicative. Classifications differ between manufacturers and countries.
Main Manufacturers
- Vermeer, a major US maker of HDD rigs, locators and fluid systems.
- Ditch Witch and American Augers, both part of Toro.
- Herrenknecht, with large HDD rigs alongside its tunnelling equipment.
- Prime Drilling, a German maker of large HDD rigs.
- TRACTO, a German specialist in trenchless equipment.
- XCMG and other Chinese manufacturers with wide HDD ranges.
- Digital Control Incorporated (DCI) and Subsite Electronics for tracking systems.
Main Uses of Horizontal Directional Drills
- Fibre optic and telecommunications duct installation.
- Water mains, gas distribution and pressure sewer pipelines.
- Electrical cables, including grid connections for wind and solar farms.
- Oil, gas and water transmission pipeline crossings.
- Crossings under roads, railways, rivers, airports and protected areas.
- Landfall connections for offshore cables and pipelines.
Planning an HDD Crossing
- Investigate the ground. Soil and rock type, groundwater and obstructions decide the tooling, fluid and whether a bore is feasible.
- Locate existing services. Utility records, ground penetrating radar and exposure by vacuum excavation reduce the risk of strikes.
- Design the profile. Entry and exit angles, depth of cover and bend radius must suit the pipe material and the rig.
- Manage fluids. Plan for mixing, recycling and disposal of drilling fluid, and for the risk of fluid escaping to the surface.
- Prepare the pipe. The product pipe must be fused or welded, tested and laid out in a straight line before pullback, which needs space on the exit side.
HDD or Open Trench?
HDD is not always the cheapest method for a single pipe in open country, where a trencher or excavator can often install it faster. Its advantages appear where the surface must not be disturbed: under roads with heavy traffic, railways, rivers, wetlands, protected habitats, historic areas and private gardens. Reinstatement costs, traffic management and disruption to residents can make HDD more economical overall even on short urban runs. The main limits are ground conditions, such as loose gravel and cobbles, the need for enough space for the rig at the entry point and the pipe string at the exit, and the accuracy required for gravity pipelines.
Safety
The most serious HDD hazard is striking an existing service, especially a live power cable or gas main. Many rigs have electrical strike detection systems, and crews use grounding mats and protective equipment. Rotating drill pipe and the moving carriage can cause entanglement and crushing injuries, so guards, interlocks and a clear two-way communication system between the operator and the tracking crew are important. Other hazards include high-pressure fluid lines, heavy pipe handling and traffic when working on roads. Inadvertent returns of drilling fluid to the surface or into watercourses must be monitored and contained.
Trends and Technology
HDD rigs are becoming more automated, with automatic rod loading, assisted drilling modes and recorded bore logs. Tracking systems now work in areas with electrical interference and can steer the rig through the locator. Gyroscopic steering tools improve accuracy on long crossings where surface access is limited. Demand for fibre broadband and grid connections for renewable energy is driving growth in both compact and large rigs, and battery electric compact rigs have been introduced for urban work.
Fluid management is another area of development, with more efficient recycling systems that clean and reuse drilling fluid on site and reduce the volume that must be transported and disposed of.
Key Terms
- Pilot bore: the first small borehole drilled along the planned path.
- Reamer: a tool pulled back through the bore to enlarge it.
- Pullback: the stage in which the product pipe is pulled into the bore; also the rig’s maximum pulling force.
- Walkover locator: a handheld receiver that reads the drill head position from the surface.
- Frac-out: an unintended release of drilling fluid to the surface.
- Dual-rod: a drill string with inner and outer rods for drilling in rock.
Frequently Asked Questions
How long can an HDD bore be? Compact rigs usually drill up to a few hundred metres. Large rigs have completed crossings of several kilometres, sometimes by drilling from both ends and meeting in the middle.
What pipe materials can be installed by HDD? Polyethylene, steel and ductile iron with restrained joints are the most common, along with bundles of cable ducts.
Is HDD suitable for gravity sewers? Not usually. HDD steering is not precise enough for many gravity sewers, which need constant grades. Microtunnelling or pipe jacking is generally preferred.
Why is drilling fluid so important? It keeps the bore open, removes cuttings, cools the tools and reduces friction during pullback. Poor fluid management is a common cause of failed bores.
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