WorldWide Drilling Resource

7 JANUARY 2022 WorldWide Drilling Resource® Dual Rotary Drilling ~ A Heavy Hitter Compiled by the Editorial Staff of WorldWide Drilling Resource® The right drilling equipment is important for any project, but because dual rotary rigs are large and expensive, many contractors don’t own any. Instead, site goals may be achieved using other tools, even if it costs customers more time and money. But there are distinct advantages to using a dual rotary rig and/or dual rotary technology. Dual rotary technology, the actual method of drilling, is ideal for difficult formations with unconsolidated overburden, such as boulders, cobbles, gravel, and sand, because the borehole is cased while drilling. Dual rotary rigs are extremely powerful with two rotary drive mechanisms, an upper rotary head and a hightorque lower drive unit. The top and lower drives feed independently, allowing the bit position to vary relative to the bottom of the casing; this capability provides the ability to case off upper unstable formations and continue with open hole drilling techniques. The top drive rotary head simultaneously handles a drill string equipped with a down-the-hole hammer, drag bit, or rolling cone rock bit to drill inside or ahead through the casing. When the casing is advanced ahead of the bit or air hammer, cross contamination is minimized, and borehole stability problems associated with artesian conditions are eliminated. The independent lower rotary drive is used to advance steel casing through the unconsolidated overburden, and is equally effective at extracting casing and exposing well screens. Due to a rugged carbide-studded casing shoe welded to the end of the first joint of casing, combined with independent drives for the drill head and casing, dual rotary can drill the straightest holes through the overburden. Through rotation and pull-down pressure applied to the casing, using the lower drive assembly, the steel casing can be seated into bedrock without disturbing or contaminating the surrounding formation. Then the drill string can advance to create a deep rock socket. In most cases, the drill bit is advanced flush with, or slightly ahead of, the casing for the best penetration rates. Telescoping casings can be implemented to case off upper zones of contamination or to stabilize previously drilled formations. Drill cuttings are typically evacuated with air and discharged to a containment vessel. Increased depth capabilities are made possible by reducing sidewall friction on the casing and drill strings. Dual rotary rigs can also be configured to use bentonite-based drilling fluids, foam, or flooded reverse circulation drilling. Their versatile platform allows the operator to easily swap out drilling methods, so if one technology hits a formation it cannot overcome, there is no need to bring in another drill rig. Instead, the tooling for a more appropriate drilling method can be added to keep going. Once the casing has been drilled to the required depth or set into solid rock, the rig can continue to drill open hole in the same manner as a conventional top drive air or mud rotary drill without tripping out the drill string to change the bit. For projects requiring deep drilling, the dual rotary technology can provide large diameter boreholes which can set casings up to 40 inches in diameter. This makes drilling deeper a possibility because the initial wide diameter provides room to telescope. With projects where using mud is not preferable, such as environmental sampling, dual rotary can succeed without risking sample contamination from any sort of circulation fluid. Dual rotary allows telescoping to go pretty deep, and if you switch to reverse circulation, you can achieve even greater depth. If a project involves gravel or basalt in the formation, there is the potential for lost circulation while drilling. Dual rotary can provide both casing and sealing before moving forward with drilling. When a project contains multiple formations, a dual rotary rig can allow the option of using a variety of drilling methodologies as conditions require. Granted, dual rotary is not the fastest method of advancing casing. The cost per foot for dual rotary is typically higher due to the cost of steel casing, welding, and installation time required to obtain the desired depth. However, it is a timesaver on tough formations because it may be the only method to make its way through. You won’t have to bring out another rig to overcome refusal or unstable formations. The predictability of penetration in known formations translates into accurate project cost estimates. What used to take days or even weeks to get the casing installed and hole completed can now be performed in one easy operation, and in a fraction of the time, making dual rotary one of the most efficient and cost effective methods for drilling holes in difficult formations. Photo courtesy of Foremost. C&G Irrigation by: Rain Bird Academy Training February 7-11 ~ Wichita, KS February 7-11 ~ Bozeman, MT February 21-25 ~ New Orleans, LA February 21-25 ~ Nashville, TN February 22-24 ~ Grand Rapids, MI February 28-March 2 ~ Nashville, TN phone: 800-498-1942 E-mail: Safety by: South Carolina Rural Water Assn Trenching, Shoring & Confined Space Competency February 1 ~ Kershaw, SC phone: 803-667-9699 More education opportunities during events can be found by clicking here online at: Education Connection