DUAL-ROTARY DRILLING
"National Driller"
February 1, 2010
Getting the job done in unstable overburden formations
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 Conditions
The 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.
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.
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.”
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