The directionally controlled horizontal drilling process was developed in the U.S., and has become commonly used for installing pipelines under natural or manmade obstacles, especially river crossings. This method has revolutionized complicated river crossings for pipelines which were initially done by conventional dredging methods or were rerouted through long distances and crossed over at a bridge location.

The method, an outgrowth of the oil well drilling technology, was reportedly first developed in the early 1970s by Titan Construction, of Sacramento, California. The first installation was accomplished in 1971 for Pacific Gas & Electric Co., and involved the installation of approximately 600 lf (180 m) of four-inch diameter steel pipe under the Pajaro River near Watsonville, California. Prior to 1979, the method was limited to the installation of short lengths. In 1979, the method was acquired by Reading & Bates Construction Company (Now Inarc Drilling, Inc.), Tulsa, Oklahoma. Since 1979, the method has progressed to state-of-the art where long lengths of crossing with a wide variety of pipe sizes can be accomplished.

From 1971 to 1979, only 36 crossings were made using this method, all of them in the United States. However, in the next seven years, over 175 crossings were made, with several accomplished in South America, Europe, Africa and Asia. Currently, there are several firms competing in the market, and the total number of crossings exceed well over 500.

For a time, horizontal directional drilling (HDD) was primarily used by the oil and gas industry on large-diameter, cross-country pipeline transmission lines. Increasingly, it is being approved and used for small-diameter gas distribution lines in urban and suburban areas, as well for municipal water and telecommunication cable crossings at airports, highways, and waterways.

HDD has become the preferred technique of many pipeline and utility companies by virtue of its lower costs and decreased surface disruption. Because it minimizes the negative impact of construction on the surrounding areas, HDD provides a method of installation that is unsurpassed in its ease and overall “friendly” nature. Pipeline companies have been increasingly turned to HDD over traditional, open-trench methods for underground construction projects.

Equipment and systems

Introduced in the 1970s, the first HDD rigs were considered difficult to operate. But improvements came quickly, and more manufacturers developed products as utility companies and contractors recognized the advantages offered by the technique.

A basic HDD system is highly user friendly, and includes a drill frame, power source, hydraulics, drilling fluid and guidance systems. No starting pits are needed; bores are launched from the surface and proceed downward at an angle until the necessary depth is reached. Then the path of the bore is leveled, and the bore head is steered to a designated exit point, where it is brought to the surface.

As the bore progresses, lengths of drill pipe are added. When the initial bore is complete, material to be installed is attached to the drill string, often with a backreamer, to enlarge the diameter of the borehole. The installation is made by pulling the product pipeline back through the borehole, with drill pipe sections being removed as the drillstring approaches the drill frame.

Drilling fluid is used in making the initial bore, and during pullback. For utility work, bore lengths generally range from 50 to 600 ft, but, with some systems, can extend as far as 1,800 ft.

HDD equipment has evolved dramatically over the past 10 years. The introduction of a reliable walk-over locating system spawned the initial growth of the mini- and midi-class drills that use high drilling fluid pressure (up to 5,000 psi) and low drilling fluid volume (2-10 gpm). This method of drilling is effective in soft to medium ground conditions.

Initially, manufacturers custom-built each drill, making improvements and design changes based on suggestions and feedback from their customers. The next hurdle to overcome for the mini- to midi-class drills was to be able to successfully bore in hard soil and solid rock conditions.

A boom in oilfield horizontal drilling forced the development of small-diameter downhole mud motors. The maxi-class HDD rig operators had been using large-diameter mud motors successfully for years. Industry demands led the manufacturers to develop the necessary design changes, to enable the use of small-diameter, lower-flow mud motors (minimum 30-40 gpm required). Having expanded the capacities of the mini- and midi-HDD rigs with higher flow mud pumps, the need to limit job setup space or rig footprint led to the concept of self-contained HDD Rigs.

The development of small HDD Rigs with the power sources, drilling mud fluid pump and hydraulic pumps all mounted on a mobile drill carriage then began, and manufacturers of self-contained HDD move rapidly forward. Currently, a contractor can obtain mini-HDD rigs fully self-contained, including an on-board drilling water supply to midi-HDD rigs supported only by an additional water/drilling mud source.

Along with the development of these higher flow output mini- and midi-class rigs, the need for a mud cleaning, mixing and recirculating system began to evolve. Minimizing drilling mud spillage at the bore entry site while lowering mud cost made the cleaning and mixing system a logical investment for many contractors.

At the same time HDD rigs were evolving, so were various downhole supporting tools. Drill pipe, once thought by many to be a low-cost, expendable item, is now usually the second-highest dollar amount on a purchase order for a new HDD rig. Early examples of pipe threads varied widely ­– left-hand, right-hand, even “rope” threads have been replaced with API-approved thread designs. Forged drill pipe has become standard on most midi-class rigs.

The evolution of walk-over locating systems has been almost as dramatic as that of the HDD rigs themselves. Early systems were limited to very shallow depths, and information update times were extremely slow by today’s standards. Now, not only do contractors have a choice of sondes to match their project scopes, but wireline steering tools have been simplified. Walk-over system status readouts that are broadcast back to the driller’s console enable the driller and locator to see the information simultaneously.

The variety of drill bits or heads and back reamers can be overwhelming to a contractor. Each style of bit or reamer has a designated purpose. Most contractors will find a bit/reamer design that consistently works in their typical soil conditions, and will employ that style almost exclusively.

Currently, the trend in the industry is contractor diversification. They want to have the capacity to not only be able to bore in the service lines from the right-of-way to the house, but also be able to handle longer, harder formation bores. Realistically, this type of project work requires more than one class of machine. The development of the mini-, self-contained drills has satisfied a majority of service-type work.

The midi-class of rigs has undergone big changes in recent years. Various manufacturers have developed midi-rigs that are relatively compact in size, yet powerful enough to tackle the occasional difficult bore.

Some of the more experienced HDD contractors have gained enough confidence to move into the maxi-class of rigs. This move, according to some contractors, can be as overwhelming as buying their first HDD rig. Larger projects require more planning, more time and more support. However – assuming that no unexpected disaster occurs – the financial reward is usually greater.

A growth industry

The use of HDD grew throughout the 1980s, but was still not as commonplace as it has become today. By the end of the decade, directional drilling was a technique to be tried when other construction methods could not be used.

But the technique saw tremendous growth in the 1990s, as the need for environmentally friendly installation methods grew. In 1996, about 30% of all underground work was completed with directional drilling equipment. That figure increased to close to 50% in 1999 and 2000, as demand from the telecommunications companies joined that of pipelines and utilities.

That rapid growth has slowed with the recent downturn in the telecommunications and internet-broadband industries. But demand for trenchless installation techniques from pipeline companies and utilities has remained strong, and is expected to increase in the future. This is due to advantages HDD offers in the environmental, technical, contractual and economic arenas.

Today, utilities and contractors often make HDD their first choice. In the early days, the only obvious drawback to choosing HDD was that the technology was so new, there was not a large pool of experienced personnel to draw from. This has changed as well. In 2004, there is a much larger amount of experience regarding HDD, on the part of operators, contractors, public officials and regulators.

In no small part, this is because contractors, equipment manufacturers, and service companies teamed together to develop educational HDD programs. These programs aid in the development of competent HDD employees. The initial learning curve of a new horizontal directional drill owner is dramatic in its rapid up-slope. However, after the initial knowledge is attained, moving to the next level can be difficult without support from the manufacturers, dealers and service companies. Recognizing this need, schools and one-on-one training courses are available from an increasing number of sources.

Utility construction, particularly in the gas and telecommunications industries, ensures a growing demand for the use of directional drilling techniques into the 21st century. The potential for work spans the spectrum, from major river crossings to utility installation in congested urban areas, to various types of environmental remediation.

Compact HDD systems can be used to install service lines to residences, without damaging private or public property, and other utility systems. These (usually smaller) rigs are able to bore beneath driveways and sidewalks, existing utility lines, and underground sprinkler systems. Larger directional drilling equipment can be used to go under parking lots, highways, freeways, and even rivers and lakes.

In addition, HDD offers unique solutions to environmental problems. For example, directional systems can install horizontal remediation wells to access contaminated soil and ground water in areas where other procedures are uneconomical. Some in the industry predict the environmental market for HDD will eventually be larger that the utility market.

Overall, HDD reduces restoration time and costs in both urban areas and residential neighborhoods. The public appreciates the reduced inconvenience made possible by the use of directional drilling equipment.

Applications

Companies usually consider the technical, contractual, and economic aspects of a project when determining whether to use HDD. In the case of a river crossing, the project is determined to be technically feasible if it can be installed using existing tools and techniques, regardless of uncertainties surrounding installation cost. A crossing is contractually feasible if the installation cost can be accurately estimated in advance, allowing contractors to submit lump-sum bids. HDD is economically feasible if installation cost is less than the cost of competing construction methods.

Mini-HDD is a subsurface-launched installation technique that typically uses either controlled-fluid cutting or fluid-assisted mechanical cutting. However, this method can be use air for cooling or dry boring assistance, or can be a dry process where neither liquid (e.g., bentonite, water) nor air is used. Therefore, the choice of fluid-assisted or dry cutting depends on the nature of work (normal utility application or environmental application), size of the utility line (diameter and length), subsurface conditions, and impact on the environment.

The feasibility for using the mini-HDD technique should be initially established on the basis of diameter length, and required/achievable degree accuracy in alignment and grade. Mini-HDD currently is not practical for installing pipe to the precise alignment and grade tolerances required for gravity sewer line. However, mini-HDD is suited for installing new utility networks for water, gas, electric or telecommunication lines in developed areas. It can also be used to install pressure sewer line. The diameter of the product pipe or utility line to be installed using this technique should be somewhere in the 2 to 10-in. range. The depth should be less than 30 ft.

The dry system is suitable only for small-diameter pipelines (typically less than 4 in.) and short (typically less than 150 ft) utility line installations. It is useful for drilling through soft soil or drilling under a sensitive, contaminated site where the probability of contamination movement to ground water or to the surface is high. It may also be applicable where hazardous or toxic wastes have a high probability of contaminating drilling fluids, which would then require proper disposal.

The main drawbacks of dry boring are the limitation of pipe size and overheating of the drill head as the bore diameter and the length of the bore increases. Also, it should be remembered that the dry boring system (with or without air) is usually less responsive to steering corrections than fluid-assisted mechanical cutting.

Fluid-assisted mechanical boring is appropriate for most utility applications, and the low drilling fluid volume used is unlikely to cause voids or settlement problems. Moreover, the fluid assisted mechanical cutting system is very useful for coarse, saturated, sandy soils where drilling fluid stabilizes the borehole.

An important distinction between fluid-assisted mechanical boring and water-jet boring is that the high fluid volumes and pressures used in water-jet boring often cause erosion of the soil adjacent to the borehole. Water-jet boring uses large volumes of water at pressures as high as 15,000 psi for cutting the soil, while mini-HDD uses water pressure less than 4,500 psi to assist in mechanical cutting by the drill head.

Another factor that should be considered for selecting the method of installation is the steerability of different techniques. Steerability is important for curved space alignment, or where the clearance between the existing utility lines is small. It is important to determine the minimum radius of curvature attainable with specified diameter drill pipe for each method. The minimum radius of curvature is typically given as 125 ft for 1.5-in. diameter pipe, although some manufacturers state that radii of 42.5 ft for 1.25-in. pipe and 30 ft for 1.0-in. pipe are not unusual. Drill rod life can be adversely affected by use of sharp bends.

The mini-HDD method is best suited for soils with some cohesion (i.e., clays). The method is also successfully used in sandy soils by adding bentonite to the drilling fluid, to ensure borehole stability. Hard soils, caliche, shale, limestone and other rocks typically reduce the drilling rate and increase, and increase drilling head wear. The mini-HDD method is not well suited for gravelly soils (greater than 25% gravel sizes).