Advances in midsize HDD technology help underground contractors conquer rock
Rock drilling isn’t the beast it used to be. Technologies developed and refined over the last two decades – specifically in midsize horizontal directional drilling (HDD) systems – are allowing utility contractors and others in the underground construction industry to more easily and costeffectively bore through even the hardest rock.
Still, conquering the strongest sub-terrain isn’t a simple matter. HDD in rock requires planning, the right equipment and the right people in place.
These factors are crucial because rock structure and hardness can vary significantly. The compressive strength of the rock can be deceptive upon first assessment – rock that appears to be soft may in fact be hard, and vice versa. Unlike vertical drillers who generally reach consistent formation after the first 500 to 1,000 feet of earth, horizontal drillers can encounter a variety of formations – from rock to cobble to dirt – along their path.
Innovations in HDD, including dual-pipe technology and downhole air hammers, have armed contractors with the highly productive tools needed to satisfy customers who want buried utilities – regardless of the underground terrain.
Municipalities and other customers are increasingly demanding that electrical wires, telecommunications lines and other utilities go underground for aesthetic and more practical reasons. For example, overhead lines pose a danger during inclement weather. Using new trenchless technologies, underground construction companies are capitalizing on these demands.
Compact HDD units were developed in the early 1990s to install utility pipe and cable, and advancements in the technology eventually provided tooling that could penetrate rock. However, steering through rock with a first-generation slant-nose bit could be slow and tedious.
Many in the industry decided the only way to drill horizontally through rock was with a mud motor – a system with a downhole motor driven by drilling fluid. However, these large units didn’t fit the needs of many utility projects because they were too large and costly to operate, partly due to the large amount of mud required.
Since those early days, slant-nose bits have been modified to improve their ability to carve through various types of soil and rock. Today, modified rock bits are widely available in various configurations for small to large drill units.
Meanwhile, the underground equipment industry has made leaps forward in engineering midsize HDD systems that can tackle rock. A major breakthrough was the introduction of a mechanical dual-pipe system. The system has been refined and is now an effective option for drilling in hard rock with midsize HDD equipment.
Today, that dual-pipe system provides a variety of options for directional drilling in rock. The dual-pipe system allows midsize HDD models with 30,000 to 100,000 pounds of pullback to effectively drill in rock that is beyond the capability of conventional machines with comparable pullback ratings.
One of the primary advantages of an HDD machine equipped with the dual-pipe system is the capability to accurately steer while drilling through rock. The mechanical motor system has an inner rod that drives the rock bit and an outer pipe to provide steering. Unlike similarly-sized mudmotor systems, the tracking electronics of dual-pipe systems are at the drill head, not 10 or 15 feet behind at the pipe connection.
The mechanical system delivers maximum power to the drill bit and enables simultaneous steering through rock and rocky soils while using no more drilling fluid than conventional equipment.
Another key advantage is the system’s all-terrain capability: If it encounters loose cobble or other mixed soils while drilling rock, the machine can bore through them. Loose cobble is one of the most troublesome soils in HDD because directional changes are extremely difficult. Dual-pipe systems often are the most effective choice for making an installation in mixed soil conditions and rock.
Downhole air hammers
For directional drilling in hard, solid rock, air hammers can be more effective than rotary methods in high compressive strength rock formations. Volumes of compressed air power the hammer at pressures up to 350 psi. Downhole air hammers can be used on single drill-stem equipment or dual-pipe models. While air hammers can be utilized in lower compressive formations, their production levels are more effective in harder rock applications (25,000 psi and up).
The effectiveness of air hammers can be limited, especially during long-distance drilling. That’s because high-volume air pressure is necessary to clear cuttings from the borehole – the longer the bore, the lower the velocity to transfer those spoils. This can reduce productivity.
Steering in layered underground formations can be difficult with a single-pipe air hammer using a bent sub and moderately slanted bit. Recent advances in dual-pipe rock drilling systems have been equipped to operate air hammers. They have also proven to be very effective with the bent sub on the outer pipe for steering and the inner pipe providing continual rotation of the air hammer.
Advances in HDD mud motors – such as newer low-flow units – have made them more efficient as well. Mud motors are routinely used with large drill units operated by contractors who specialize in installing complex bores for large-diameter products, such as pipelines.
Preparation is key
The choice of drilling equipment depends largely on what the contractor will encounter beneath the soil. Sometimes they already know the underground conditions because of their experience drilling in that territory.
But the unknown can come with serious consequences when drilling in rock. The results can range from damage to high value equipment to expensive project delay penalties.
To gain the necessary underground knowledge, some contractors will simply take a backhoe and dig until they hit rock, so they visually access the local formations.
However, the most accurate way to know what lies ahead is to perform a geological survey. Mitigating this risk will increase your productivity and profits. Using information from a geological survey, contractors can better determine what tools will be needed to complete the job most effectively.
Drilling mud is another major consideration when drilling in rock. The right quality and quantity helps to clean the bore, keep conditions cool and carry the cuttings out of the hole.
Drilling in rock is not like boring through soil and clay – you can’t push the material away from the bore and compact into surrounding formation. Small pieces of cut rock must come out of the hole to prevent those chunks from acting like a vice and locking down the drill pipe.
So you must have the ability to lift the loose chunks and the velocity to push them out of the hole. A rule of thumb when drilling rock is to use a bentonite base mix for viscosity. The clay platelets in the bentonite provide the support to lift small pieces of rock and carry them out of the bore hole. And depending on the formation, polymers can help with friction downhole.
The success of any drilling project will rely on the ability of the operator. That’s especially true when drilling through rock, where often “the tortoise beats the hare.”
Rock drilling requires operators who understand the intricacies of selecting the appropriate bit, mud type and penetration rate for specific underground formations.
Operators also need enough experience to have a feel for the drill and know the right way to move the rock and surrounding soils. When it comes to directional drilling in rock, patience is a virtue for operators.