The distinguishing feature of the dual-rotary (DR) drill rig is a lower rotary drive, which is used to independently advance casing up to 40 inches, depending on the drill model. The lower drive transmits pulldown, pullback and rotational forces to the casing. A carbide-studded shoe, welded to the bottom casing joint, cuts through the overburden.
The rotary top drive has its own feed system, and raises and lowers independently of the lower drive. The rotary top drive handles the inner drill string, which can be tooled with a downhole hammer, drag bit or rolling-cone bit. Cuttings typically are removed using air from either the on-board compressor or an auxiliary compressor. Because the top drive and lower drive operate independently of one another, the drill bit can be positioned ahead or behind the casing shoe. For example, the casing can be advanced ahead of the drill bit, minimizing aquifer cross-contamination and loss circulation, and eliminating borehole stability problems associated with artesian conditions. In typical drilling operations, the drill bit is advanced slightly ahead of the casing for fastest penetration rates. It is worth noting that once the casing is chucked in the lower drive, it can be rotated either clockwise or counter-clockwise.
Once the casing has been set to the required depth, the DR drill can continue to drill open hole in the same manner as a conventional top-drive air drill without tripping out to change the bit.
All dual-rotary drill models share a number of unique design features:
Hydraulic feed system– Both the top- and lower-drive units are raised and lowered via directly connected hydraulic cylinders. Benefits of this design include a high pullback to weight ratio, zero load on the mast’s crown, and the elimination of cables, chains, sheaves and sprockets in the feed system for reduced maintenance.
Breakout using lower drive – In addition to providing the feed and rotation needed to advance casing, the powerful lower drive is used to make and break tool joints, hammers and bits.
Tilt-out top drive – The top drive has a hydraulic tilt-out feature that enables safe and efficient loading and unloading of drill pipe and casing. Some models also are available with an optional pipe tub and single rod loader.
Cuttings discharge swivel – All cuttings that rise up the annulus between the drill pipe and casing are diverted through a discharge swivel that attaches to the top of the casing. An integral bearing, protected by hard metal seals, provides support between the rotating casing and stationary discharge elbow. A cone seal prevents cuttings from blowing by as the drill pipe rotates. Cuttings are directed by a flexible hose to a convenient dumping point or sampling cyclone.
Casing shoe– A carbide-studded shoe, welded onto the casing, cuts through boulders and hard formations as the casing rotates. This shoe also enables the dual-rotary drill to seat casing into bedrock. It is important to note that casing shoes used with the DR have the same inside diameter as the casing I.D. Consequently, there is no reduction in the borehole diameter when switching to open-hole drilling. The outside diameter of the casing shoe is fractionally larger than the casing O.D. for reduced friction on the outer wall of the casing. Casing shoes are available in light duty, standard duty and heavy duty, and are distinguished by the spacing of the carbide buttons.
Casing jaws – The lower rotary drive engages the casing via a set of three power-activated casing jaws. Once the casing is locked in the jaws, it can be rotated clockwise or counter-clockwise, while simultaneously applying pulldown or pullback. The hardened steel jaws are available for all common casing sizes, and feature replaceable pipe tong inserts. The lower drive design enables the jaws to be changed out quickly in the field.
Overburden ConditionsThe dual-rotary method provides several advantages that deliver economic benefits to drilling contractors. On any DR rig, the maximum diameter of the casing is equal to the spindle diameter of the lower drive. On some projects, drillers have used adapters in the lower drive to set surface casing that is larger than the lower drive normally would allow.
The table (on next page) is based on operator experience, drilling in a variety of formations in different parts of the world. It highlights the capacity of the rig to advance casing in difficult formations to significant depths. These figures are guidelines only. Obviously, local drilling conditions will dictate the relationship between casing diameter and depth.
Boulders – The carbide-studded casing shoe enables the casing to be rotated through boulders without the need for under-reaming or blasting. As casing deflection is minimized, the hole remains plumb.
Heaving formations– Where heaving ground conditions are experienced, the bit can be retracted into the casing, creating a “plug” in the casing, which allows drilling to proceed under controlled conditions in most situations.
Installing and welding casing– The operation of installing and welding lengths of casing together is simplified with the dual-rotary rig. The tilting top drive allows drill pipe and casing to be added and secured to the discharge swivel with the operator standing safely at ground level. Once secure, the top drive is raised, and the drill pipe and casing are returned to the vertical position. The top drive is rotated to thread the drill pipe into the downhole string, and then the holdback wrench is removed. Next, the top drive is lowered until the two casings are lined up, and then they are temporarily tack-welded in place. Now, with the downhole casing held firmly in the lower drive, the main welding operation begins. As welding progresses, the welder can periodically rotate the lower drive enough to keep his work in front of him. With the DR method, the welder never has to climb in behind the table to complete the weld.
Setting screens– The lower drive is equally effective at extracting casing, simplifying the process of setting and exposing screens.
Sampling– The DR drilling method, employed in conjunction with a cyclone collection system, ensures an accurate sample of the formation being drilled. When the casing is advanced simultaneously with the drill bit, the cuttings that return to the cyclone do not mix with material from the borehole wall, and are representative of the substance at the bit face. Furthermore, in lieu of welding, threaded casing can be used with the DR rig for monitoring and sampling applications.
Angle drilling – An optional angle package enables drilling and/or casing at angles up to 45 degrees. Even shallower angles have been drilled in some special applications, such as the installation of a utility shaft under a highway at 21 degrees off horizontal.
Hole straightness – A key benefit of casing rotation is a straight hole, even when drilling in cobbles and boulders. A straight hole helps to minimize sidewall friction, reduce stress on casing joints and welds, and enable greater casing depths.
General versatility – In most situations, all that is required when drilling with a dual-rotary rig is a conventional drill string, casing and a casing shoe. However, a DR rig is versatile such that it can use a variety of common tools available on the market for specialized jobs or unique situations. Reverse-circulation systems are used in both airlift and reverse water-flooded applications. Mud drilling also is possible, as is under-reaming.
Dual-rotary ApplicationsEssentially developed as a water well rig, the DR drill has evolved to a point where it is routinely used with great success in a variety of applications. In addition to applications that involve setting or extracting casing, the DR rig can be used as a conventional top-drive drill with the added convenience of a tilting top drive.
Water Well DrillingOne dual-rotary operator was observed to drill a 6-inch cased hole to a depth of 580 feet in sand and gravel, with ample torque and pullback available to advance casing even deeper. DR models are available with an optional pipe tub and single rod loader for open-hole applications. DR rigs also are used for drilling domestic wells with operators reporting depths exceeding 1,300 feet.
Dual-rotary rigs have been used to drill large-diameter industrial wells to varying depths. For contractors using the DR method, the predictability of penetration rates in known formations translates into accurate project cost estimates, which provide a distinct advantage when responding to competitive bid invitations.
Some municipalities are specifying the dual-rotary method for drilling their wells to eliminate mud contamination. Mud disposal costs are increasing, and some landfills no longer accept used mud.
As the lower rotary drive is equally effective at retracting casing, the DR rig is highly suitable for well abandonment and casing recovery projects. Furthermore, the lower drive and casing can be used to over-drill in order to rescue lost tools.
Construction DrillingTruck-, track-, trailer- or crane-mounting options make the DR flexible for numerous job sites. The DR’s angle-drilling capability often is a crucial advantage in construction applications. Plus, the DR’s ability to control cuttings ensures a clean drill site.
Independent rotary drives produce straight holes, which are essential for hydraulic elevator shaft drilling.
Dual-rotary rigs excel in heaving, water-bearing formations where the potential for hole collapse exists. Angle-drilling capability allows for effective drilling of batters. The DR also is able to drill through industrial debris, such as steel and wood, commonly found on dock construction and rehabilitation projects.
The DR rig can penetrate where pile hammers and augers fail, which is especially important when bridge supports must be drilled beyond the overburden and seated into the bedrock.
The DR process allows you to drill through the large boulders and industrial debris often found on land reclamation projects. The technology allows a hole to be drilled and cased through these difficult formations without significant bending or deflection.
DR rigs offer the dual-purpose ability to dewater and set dam foundation anchors. For dam rehabilitation projects, DR is quite suitable for drilling grout injection holes. In circumstances like this, where the structure already is cracked or fractured, the casing is advanced ahead of the bit to minimize the chance of blowout. Also, the rotation of the casing minimizes the potential for vibration damage to the already fragile structure.
The DR rig does not have the positioning flexibility of a dedicated tie-back rig; however, for larger-diameter, high-risk tie-back holes, DR is a good option. For these jobs, casing can be set to guide the placement of the steel wire. Then, just as easily, the rig can extract the casing while grout is injected into the hole.
The DR rig can seat casing into bedrock without fracturing the surrounding formation. The independent top drive allows the drill string to advance ahead of the casing to create a deep rock socket.
DR rigs offer the accuracy to re-drill existing pilings, combined with ability to set sockets deep into bedrock. For example, a dual-rotary rig was used successfully for a seismic retrofitting project on the Benecia-Martinez Bridge in the San Francisco Bay. The project design required drilling 17-inch-diameter casing through existing steel-reinforced foundation shafts.
Drilling accuracy, the ability to advance casing without percussion, and control of cuttings discharge make the DR drill and excellent choice for drilling in populated urban areas or near sensitive or precarious structures.
Mining and ExplorationDR rigs provide drilling accuracy. This, combined with angle drilling capability, makes them well suited to drilling mine utility shafts or locating and backfilling abandoned sections of underground mines.
For placer drilling, the casing is advanced ahead of the drill bit to preserve the accuracy of cuttings sample.
Dewatering wells can be drilled through loose overburden material, including blasted rock and industrial debris.
Environmental ApplicationsWhether the goal is to contain environmentally hazardous cuttings, obtain an accurate profile of the formation or just keep a clean job site, the DR method can do the job.
As in placer drilling, casing can be advanced ahead of the drill string so cuttings do not mix with material from the borehole wall as they exit the hole. Cuttings remain representative of the material at the bit face. In recent years, some U.S. government authorities have specified the DR method for a growing number of projects, because it is possible to drill and case without introducing water or mud into the formation. On critical environmental sites, a DR rig can install casing, and retrieve cuttings into containers without releasing water or unfiltered air into the atmosphere. The rig also can be fitted with reverse-circulation hammers and tooling, which can be beneficial when drilling in contaminated soils, or when sampling in open-hole applications.
The ability to case a hole with minimum cross-contamination makes the dual-rotary method well suited to environmentally sensitive drilling projects. On some projects, it has been recommended as the only acceptable method.
DR effectively uses casing to seal off the strata above the recharge reservoir. This prevents undesirable material or substances from bleeding further down into the formation and causing cross-contamination. And, because drilling is accomplished without the use of drilling mud, there is no danger of plugging or contaminating the underground reservoir.
Other ApplicationsDR rigs offer an advantage when anodes must be buried in overburden, or when casing is being set to house the sacrificial anode.
For special limited applications, hollow-stem augers can be fitted to the lower rotary drive using an optional adapter.
The torque of the lower drive can be added – via kelly bar – to the torque of the top drive to enable large-diameter, open-hole drilling. This is useful where the required borehole diameter exceeds the I.D. capacity of the lower drive spindle, or when the torque requirements exceed the output of the top drive. Cuttings typically are evacuated using reverse flooding in this application.
The dual-rotary method is being used in oilfields to drill rat holes, mouse holes and conductor casing for oil and gas wells. In oilfield applications, a 20-inch conductor casing is externally cemented into a 24-inch borehole to depths of 300 feet or more. In some cases, the DR method is also being used to pre-set Range II surface casing.
In SummarySince its introduction, the dual-rotary drilling method has gained world-wide acceptance as a cost-effective drilling tech-nique. It has proven itself to be a viable alternative to more conventional methods when drilling in unstable overburden. Its flexibility and versatility continue to deliver for drilling contractors, and has contributed significantly to advancing the technology of overburden drilling.
This article is provided through the courtesy of Foremost Industries LP; it is excerpted from the company’s publication, “Benefits of Dual Rotary Drilling in Unstable Overburden Formations.”