Owners and operators of utility systems appreciate the benefits of trenchless construction with horizontal directional drilling equipment.
Directional drilling has become the method of choice for an increasing number of project segments, and project planners are specifying its use for a greater variety of applications in more types of soil conditions than ever before. They want to drill farther and install larger diameter materials, and they are sending drillers to job sites with subsurface formations that often push the limits of equipment capabilities.
Drilling in rock remains a challenge for both operators of HDD equipment and for companies who design and build drill rigs and downhole tools. Manufacturers continue to search for new ways to make compact HDD rigs more productive in rocky conditions while keeping machine and tool costs compatible with the fees project owners are willing to pay.
Experienced drillers agree that actually drilling through rock isn't the primary problem they face - current technology can cut through most of the formations encountered at the relatively shallow depths where directional equipment operates.
Effectively steering in rock remains the issue - the slant-face bits used for drilling in dirt simply won't work in solid rock, and they are not as efficient in formations containing loose gravel or cobble.
Several manufacturers have developed mechanical rock tools that allow rigs in the 15,000- to 35,000-pound pullback range to work through many types of difficult conditions, including softer rock. However, the mud motor, used with HDD equipment capable of generating the fluid volumes necessary to operate it, still is the only practical method to work in hard, solid rock.
"There presently is no practical alternative to the mud motor for hard-rock drilling, and I don't see anything replacing the mud motor in the immediate future," says John Shea, director of drilling operations for C&S Network Construction, a MasTec North America company.
In addition, says Shea, mud motors now are being developed specifically for the HDD market, so there is a larger selection of motors available.
"We drill a lot of rock," says James Ditto, project manager/superintendent for C & B Associates, Mineral Wells, TX. "Intermediate steps have been tried, but if you are in hard rock and need control, basically you have to use a mud motor."
"Hard rock" and "rocky conditions" are imprecise terms, and they often mean different things to different drillers. Rock drilling can mean working in soil containing broken pieces of rock, gravel, cobble, boulders or it may entail drilling through solid rock. Loose gravel and solid granite both are "rocky," but obviously there is a big difference in how a driller will approach these very different conditions.
There are many types of solid rock with varying degrees of hardness. In HDD construction, rock hardness usually is defined by how many pounds of force per square inch is required to crush a sample: 2,500 psi is "soft" everything above 10,000 psi is "hard."
Small machines using slant-nose bits often can complete installations in gravel and cobble and in soft rock formations such as shale and siltstone with hardness ranging between 2,500 psi and 5,000 psi.
Medium rock- between 5,000 and 10,000 psi - includes sandstone, limestone, and marble. Some drillers use conventional drill rigs equipped with various mechanical downhole rock tools or rock systems, while others believe mud motors are faster and operate more smoothly and with less wear on equipment.
Hard formations include quartz, basalt, and rhyolite, and they are mud motor projects. The hardest formations, found near the surface where directional drilling installations take place, rarely exceed 40,000 psi.
"Drillers would love to have one tool that works in all soil conditions," says Jay Miller of Inrock, Houston. "Unfortunately, it is unlikely that such a tool can ever be developed - there will always be a need for different types of tools for different conditions."
Choosing the correct equipment that can most cost-effectively handle the work is a challenge all drillers face and one that changes with shifting demands of the market.
Mud motor technology came from the oil field and innovative directional drillers were quick to recognize the potential for expanding the capabilities of early HDD equipment.
The downhole mud motor is powered by drilling fluid channeled through the sections of pipe making up the drill string. The mud motor converts the flow to rotary speed and torque, the keys to effective mud motor operation.
"Drilling mechanics is directly related to the torque output of the motor," says Steve Cornwall, vice president of sales and marketing for Sharewell, Houston. "The motor must produce enough torque to overcome the compressive strength of rock and allow the tri-cone bit to penetrate the rock."
A mud motor achieves steering in rock with a bent sub - a slight (1 1/2 to 3 percent) bend in the motor's housing. The motor, powered by drilling fluid, can be rotated independently of drill string pipe. To go straight, the drill stem is rotated as the drill bit, powered by the mud motor, cuts into rock.
To change direction, the drill operator stops rotation of the drill string, positions the bent sub in the direction of the desired change, and applies thrust, causing the bore path to change in the direction of the bend. Because of the small angle of bend, directional changes are gradual. Information needed to guide the bore path is provided by conventional walk-over electronic trackers or wireline guidance systems.
Mud motor companies recognized the potential of their products for directional drilling sooner than many drill rig manufacturers, and limited fluid capabilities of early drill rigs couldn't accommodate flow rates necessary to operate mud motors. To use a mud motor, drill owners had to upgrade fluid systems and bring in auxiliary fluid units.
HDD mud motor applications are more effective today because mud motor manufacturers have developed products that meet the specific needs of directional drilling, and - just as important - drill rig makers are producing equipment compatible with mud motor requirements.
"All major rig manufacturers now recognize the importance of having enough mud pump flow to allow drillers to use mud motors," says Cornwall. "Many smaller rigs (20,000 to 40,000 pounds pullback) now are plumbed to handle higher flow rates, allowing the driller to use a supplemental pump to make rock bores with a mud motor. Mud cleaning and recirculation systems also have evolved to provide complete rock-drilling packages."
Development of low-flow mud motors has expanded drilling capabilities of smaller rigs, but the productivity of such equipment packages declines as rock hardness increases.
Impact cutting tools
Mud motors aren't the answer to every rock drilling application.
While many job conditions are too difficult for compact rigs using conventional downhole tools, drillers can't justify the higher cost of larger machines and mud motors, and in many cases space limitations prevent bringing in larger rigs or separate fluid systems.
Drillers want mechanical downhole tools that will effective work in conditions that fall in the broadcategory between dirt and hard rock, and numerous new mechanical cutting bits, backreamers, and hole openers have been developed to expand the capabilities of small-to-medium-range directional machines. In addition, there are jetting tools and mechanical rock systems available: one manufacturer has marketed a dual-pipe mechanical rock option on a 22,500-pound machine for several years, and percussive tools to hammer into hard rock have been also been introduced.
Denis Fox, senior vice president of sales and marketing for American Augers, believes percussive hammer downhole tools will come to be widely accepted for directional drilling in rock formations.
"Pilot hole drilling with air and water percussive equipment, has been used in the past year in the United States and Europe," says Fox.
"Downhole tool manufacturers are being asked to design multi-purpose bits that will cut the many types of formations that are encountered during a long bore," Fox continues. "Cutting consolidated rocks can be accomplished: however, drilling in sand, heavy clays and cobble continues to be challenging along bore paths that lead in and out of solid rock formations."
To change the direction of a pilot bore in rock, mechanical bits cut a hole for the tool to follow.
"In most rock, the driller cuts a relief, then backs off and drills into the relief," says Wayne Lanier, director of sales and marketing for Railhead Underground Products. "To steer to 12 o'clock, the operator pushes in at 10 o'clock and carves to 2 o'clock. Do this a few times to cut a relief. Then, when you drill into the relief, the bit naturally takes the path of least resistance."
Enlarging pilot holes
Whether the pilot hole is made with a mud motor or mechanical bit, backreaming in rock requires special tools.
Various types of cutters are available for different types of rock, and matching the tool to the type of formation is important. Backreaming in rock places added stress on equipment, and using the wrong tool compounds the problem.
"A driller would never consider using the same type bit for all formations," says Inrock's Miller. "It is equally important to use the correct hole opener for enlarging pilot bores."
Of the many types of backreamers and hole openers available, drillers develop preferences for designs they find most effective and many - especially those with oil field experience - design and make their own downhole tools.
No simple answers
At best, horizontal directional drilling is a complex, often difficult discipline, and working in rock increases the potential for complications.
What type of rock is going to be encountered, and how hard is it? Is a mud motor necessary, or will mechanical tools work? If a mud motor is used, are drill rig and mud motor correctly matched? What fluid additives will be most effective? Is the drill rig's fluid system adequate, or is an auxiliary system needed? What type bits and backreamers will work best?
"As a downhole tool manufacturer, the biggest battles we encounter is misinformation and the lack of any information at all about soil conditions," says Cornwall.
With the pressure to maximize production and not waste time, who has time to take samples and conduct rock hardness tests?
"But with the necessary information," continues Cornwall, "trained operators and crews can properly plan a bore and will successfully complete the job more often than not. Then they can review that bore and make logical changes to increase production and lower operating costs on future projects."
"As the HDD drill rig industry continues to grow," concludes Cornwall, "and prices paid to contractors drop, manufacturers must develop more economical ways to help drillers work in rock. Higher production at lower cost is the key to success."
RELATED ARTICLE: What Manufacturers Say About Rock
Denis Fox, senior vice president for sales and marketing: "Mud motors are tools of choice for pilot hole drilling in rock and cobble, and equipment with sufficient rotary torque, speed and mud flow capabilities are proven to be most efficient for drilling the pilot hole and backreaming in rock formations. Our self-contained machines are designed with large diameter rotary gearbox spindles, mud swivels and tool joints that incorporate large-diameter watercourses for increased mud volumes with little downhole operating pressure losses. Downhole tool manufacturers are being asked to design multi-purpose tools that will cut through the many types of formations encountered in a long bore. American Augers is focusing on the backream process and is working with Numa to redesign this company's percussive downhole hammer for the HDD marketplace. In the cowing weeks, we will be testing the backream hammer prior to introducing the dual-fluid backreamer to the market."
Mike Dvorak, downhole tool product manager: "HDD operators are looking for efficient, tough downhole tools for drilling in rock, and manufacturers face the challenge of making tools and equipment that makes efficient use of downhole horsepower for effective production, but which is also economical. For long bores in hard rock, mud motors play a big role. To accommodate a mud motor, drill units must provide adequate mud flow and drill pipe that can accommodate fluid volumes necessary to turn the motor. Today's improved downhole tools allow drill units to pretty well handle most hard soils, and some new products are working well in softer rock formations. Currently available tools probably drill in chunk rock best, and there is the opportunity to develop improved tools for glacier till and cobble. Our RS 8/60 dual-pipe rock system is a proven, economical machine for many rock formations. Our Rhino and Glacier bits and jetting assemblies are suited for rock drilling and the new Three-Wing and Kodiak Cobble backreamers perform well in many types of rock."
Jay Miller, sales and marketing manager: "Mud motors are still the most cost effective means to drill pilot holes in rock. Rock drilling performance with mud motors is more determined by fluid volume than by rig size, and today's HDD rig packages are able to drill rock more effectively because of improvements in mud pumping capacity and cleaning. We often see rigs as small as 24,000 pounds using auxiliary fluid pumps to run larger size mud motors. Inrock offers Black Max mud motors and a variety of drill bits and rock reaming tools with various type tooth structures for use in both soft and hard rock formations. We also sell and rent auxiliary mud pumps and recyclers."
Railhead Underground Products
Wayne Lanier, director of sales and marketing: "Mud motors will always have their place, based on the hardness of rock to be drilled. But our Incredibit tool is used routinely to drill through tough conditions, including solid rock. It is available in four sizes to fit equipment from 7,000 to 80,000 pounds of pullback. It's sonde housing is designed to stand up to tough conditions. Steering is accomplished by cutting a relief in the rock and then drilling into the relief. In April, Railhead will introduce a new rock backreamer that has been tested in really tough conditions in various parts of the country."
Steve Cornwall, vice president of sales and marketing: "Evolution of today's HDD rigs and downhole tooling for rock has been dramatic. All major drill manufacturers now recognize the importance of having high enough mud pump flow rates to enable the use of mud motors, and the development of low-flow mud motors has expanded the capabilities of smaller drilling machines, The biggest challenge today for HDD manufacturers is providing enough horsepower to optimize all of the drill units major functions at the same time. Sharewell sells and rents a full line of mud motors in outside diameters from 2 7/8 to 8 inches for use with all types of bits. Sharewell mud motors are designed to be shorter in length than other tools on the market. Lo-Torque hole openers for soft and hard rock are available in sizes from 6 to 68 inches. Jumbo Lo-Torque hole openers are designed for large-diameter holes in all rock conditions."
Mark Van Houwelingen, trenchless product market analyst: "Mud motors are the chosen tools for hard formations over longer distances. But new methods have promise for shorter bores. Impact cutting systems can be effective in certain conditions. Our patent-pending Rockfire is an air-powered impact cutting system for rock which uses foam and polymer to remove cuttings from the hole. The TriHawk drill head is designed for softer rock and other adverse conditions. The Navtec computer-aided steering system has been effective with conventional drag-cutting techniques. The use of over-center bits and the availability of different sizes of bits allow operators to adjust steering capabilities in particular ground conditions.
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Mud motors 158 HDD rock tools 159