The physical size of the equipment and the scope of horizontal directional drilling projects may vary significantly.

The physical size of the equipment and the scope of horizontal directional drilling (HDD) projects may vary significantly. Small HDD projects and operations differ substantially from larger operations, while the general practices are similar.

The capab ility of a rig to perform a given installation is a function of the ground conditions, experience of the crew and the resources (i.e. support equipment, supplies, downhole tooling) available. Subsurface conditions strongly influence the selection of downhole tools, drilling fluid additives, drilling techniques and schedule.

The small rig category is defined as rigs with less than 40,000 pounds. thrust/pullback, less than 4,000 foot-pounds. torque, and with drilling fluid pumping systems of less than 75 gpm. Small rigs often are mounted on a trailer, truck or a self-propelled track undercarriage. The latter are self-contained units with engine, hydraulic power and drilling fluid pump on-board. Others that are auxiliary-powered and have off-board drilling fluid pumping systems are referred to as "umbilical units." Systems belo w 20,000 pounds thrust/pullback primarily are used for installation of utility cable and small diameter pipes in congested urban areas. The higher end of this category is suitable for drilling gravel, cobble or other formations where borehole stability is difficult to maintain - so long as bore lengths and installed pipe diameter are not excessive. Specialty drilling systems and downhole tools are now available for drilling through rock of medium hardness (5,000- to 10,000-psi unconfined compressive strength), as well as cobbles.

The medium rig category is defined as rigs with 40,000 pounds to 100,000 pounds thrust/pullback, 4,000 foot-pounds to 20,000 foot-pounds torque, and with drilling fluid pumping systems of 50-200 gpm. Many are self-contained units with engine, hydraulic po wer, and drilling fluid pumps on-board. Medium rigs are used to install conduits and pipelines up to 16 inches in diameter. Installation distance may range up to 2000 feet. They are particularly suitable for the installation of municipal pipelines, as the y are sufficiently compact to be used in urban areas, while at the same time they have the capacity to install relatively large diameter water, gas and sewer force mains beneath highways, rivers or other obstacles. Bores can be installed in soft to hard soils and soft rock formations. Use of a downhole mud motor and specialty backreamers allows successful drilling through hard rock.

Large rigs typically involve a large operation with multiple trailer-mounted support equipment and substantial mobilization and demobilization periods (i.e. one to two weeks). High operating costs and large required work areas make their use cost-prohibit ive in the urban utility installation market. They are employed primarily in the large diameter pipeline industry or where exceptionally long bores are required. These large units may be used in the installation of large diameter products (16 in. to 48 in.) or exceptionally long boreholes (up to 6,500 ft.). The large rig category is defined as rigs with greater than 100,000 pounds thrust/pullback, more than 20,000 foot-pounds torque, and with drilling fluid pumping systems greater than 200 gpm.

Thi s article is provided through the courtesy of the HDD Consortium. It is excerpted from the Consortium's "HDD Good Installation Guidelines." The guidelines are available in print form or on CD-Rom from the Consortium members: Directional Crossing Contract ors Association (972-386-9545), Distribution Contractors Association (972-680-0261), Equipment Manufacturers Institute (312-321-1470), National Utility Contractors Association (703-358-9300), North American Society for Trenchless Technology (703-351-5252), and Power & Communications Contractors Association (703-212-7734).