HUGHES CHRISTENSEN
The ability to drill cement and associated downhole float equipment with eccentric tools has become increasingly important in deepwater Gulf of Mexico drilling. The extra trip required by conventional eccentric tools can add significant cost to a well. Hughes Christensen recently introduced a modified steerable-ream-while-drilling system called DOSRWD that has demonstrated the ability to drill out all types of float equipment. The DOSRWD features a field exchangeable pilot bit so the tool can use standard tricone or polycrystalline diamond compact (PDC) bits to optimize performance in a particular formation (Figure 1).
DOSRWD development
The SRWD framework includes several features that improve the tool's durability and function when compared to a conventional bicenter bit. These include hole-opening blades and a pilot stabilization pad (PSP). The hole-opening blade reduces loading on the first blade when transitioning from pass-through to drill size. Similar to a low-friction pad on an anti-whirl PDC bit, the PSP pad stabilizes the cutting structure in the pilot hole.
Until recently, it was not possible to drill out cement plugs or rotate inside casing without damaging the SRWD tool or the inside of the casing. Problems encountered while rotating inside the casing include impact damage to PDC cutters close to the pass-through circle as well as scarring and gouging of the casing. If a PDC pilot bit is used for the drill-out, the cutters in the center of the bit rotate backward while drilling the float equipment. But with new-generation cutters, drillers can drill cement and float equipment without damaging the cutter, enabling the DOSRWD system to use a pilot bit optimized for drilling the formation rather than for drilling cement.
Full-scale testing
While developing the drill-out concept, a sequence of full-scale laboratory tests was performed at Hughes Christensen's drilling simulator and the Baker Hughes Experimental Test Area (BETA) near Tulsa, Okla. The tests were performed with 97/8in. by 83/8in. and 12¼in. by 105/8in. SRWD bodies.
These tests enabled development of the DOSRWD product from conception to commercialization in less than 6 months. Testing revealed it was critical to implement a pass-through bearing surface that allows rotation in the casing and keeps the cutters from scraping the casing, which would cause damage to both.
The pass-through bearing surface consists of nonaggressive tungsten carbide ovoids and inserts placed on the blades that make contact with the pass-through circle. These ovoids reduce the side-cutting aggressiveness of the gauge cutters and help protect the casing. The pass-through bearing also limits lateral motion and absorbs impact that could damage the cutters.
Field testing
More than 30 field runs documented the cost benefits of eliminating the drill-out run. Pioneer Drilling of Corpus Christi, Texas, was the first customer to pioneer the new drill-out SRWD. While drilling a well in South Texas, a 97/8in. DOSRWD tool allowed Pioneer to drill to total depth, eliminating the need for a liner. This dramatically cut costs normally associated with the hole section and enabled Pioneer to finish drilling the well 4 days ahead of schedule.
According to Hughes Christensen, the company's ability to test prototypes at its laboratory and BETA test site enhanced the ability to deliver a field-worthy product the first time out.
Hardfacing
Hughes Christensen's metallurgical engineers have employed a technique that allows for a super-thick application of hardfacing on steel-tooth bits that significantly retards tooth wear while sustaining high rates of penetration. The new XLX steel-tooth package with Endura II hardfacing provides added durability and protection for the XLX-1 and MX-1 steel tooth bits (Figure 2). The XLX package extends tooth life, eliminates rounded gauge, reduces fluid erosion of the spearpoint and provides improved shirttail protection in abrasive formations. This technology is especially suited to areas where high penetration rates in soft, abrasive applications are required.
The Endura II hardfacing material has a spherical cast carbide geometry that provides increased strength and durability for teeth and gauge areas while reducing matrix erosion. Wear tests have shown Endura II to be superior to prior macrocrystalline applications.
With the XLX package, a 50% increase in tooth crest hardfacing thickness retards tooth wear while sustaining high rates of penetration. A wear-specific formulation of Endura II, containing an even greater percentage of spherical cast carbide than that used on the teeth, is welded to the full length of the gauge to improve gauge holding and increase bearing life. In addition, extra hardfacing is applied to the spearpoint on the No. 1 cone, covering the teeth, areas between the teeth and the cone shell area under the spearpoint to provide greater fluid erosion and wear resistance.
In short, it's not just which hardfacing material is used, it's where it is used that is important.
REED-HYCALOG
Cutter technology is an area of focus for Reed-Hycalog, a division of Schlumberger, because of the performance gains that already have been realized in the field. The PDC testing laboratory in Stonehouse, England, evaluates PDC cutters supplied by manufacturers from around the world.
Cutter manufacturing
PDC cutters are manufactured in a specialized high-pressure, high-temperature press. Micron size, monocrystalline diamond crystals are placed in a metal can, together with a cobalt-cemented tungsten carbide substrate, and subjected to pressures of 1 million psi and temperatures of 2,551°F (1,400°C). Under these conditions in the presence of a catalyst, the individual micron diamonds fuse together to form the many-crystalled or polycrystalline diamond layer. The essential cobalt catalyst for the process may be drawn from the substrate or premixed with the micron diamond crystals before pressing. Cobalt also is key in attaching the diamond to the substrate.
Internal stress
Diamond and cobalt-cemented tungsten carbide have different mechanical and physical properties, especially with the coefficient of thermal expansion. On reduction from sintering conditions, the diamond in a cutter tends to be larger than the tungsten carbide. However, since the two materials are intimately attached at the interface, this is not possible; at the interface they must be the same size. Consequently, the diamond at the interface is in compression while the tungsten carbide is in tension; therefore the cutter is internally stressed.
In a planar interface cutter, this stress is dissipated over a narrow band across the interface. In a nonplanar interface (NPI) cutter, the stress is dissipated across a wider band of texture of the interface, thereby reducing the peak stresses and enabling higher service loads to be applied to the cutter before failure occurs. This manifests itself as a higher impact resistance.
Finite element analysis clearly shows the internal stress at the diamond/carbide interface of cutters. It also identifies two high-stress rings, one on the perimeter of the diamond just above the diamond/carbide interface and the other in the face of the diamond close to the edge. Further analysis shows these stresses increase as the diamond layer thickness increases. However, the level of stress in these high-stress rings is reduced in cutters with correctly designed rim geometries. Thus, an increased diamond layer thickness is achieved without increasing internal stress.
Performance
PDC cutter performance is dependent on abrasion resistance, impact resistance and diamond volume. Abrasion resistance, an inherent property of the diamond layer, is not dependent on the interface and rim geometry. Impact resistance is influenced by the rim geometry due to the internal stress regime and by the NPI geometry. In the absence of impact damage, how far a cutter will drill is dependent on the abrasion resistance and the volume of diamond at the cutting tip.
Reed-Hycalog examined the statistics of NPI cutters, particularly those with thick rims. The evaluations showed an incidence of dome-type spall failures. In these failures, part or most of the diamond layer was lost around the rim while a dome is retained in the center. Terry Matthias, PDC research manager, said this is a result of a complex force interaction between the cutter and pocket, which gets worse as the diamond becomes thicker and under load impact.
Matthias' group developed an Iris cutter with a partial tungsten carbide rim that supports the 19mm Iris cutter. This integral rim of tungsten carbide acts as a cushion in the base of the cutter pocket in the bit body. Since the tungsten carbide rim only goes partially around, the diamond layer is exposed on one side, which becomes the cutting edge. The tungsten carbide rim supports and cushions the diamond layer from destructive forces around the pocket, especially under impact loads. In addition, it provides increased area for braze attachment of the cutter to the bit body.
Cutter field test
Reed-Hycalog first field-tested the experimental Iris NPI cutters with a tungsten carbide rim on an offshore well in the Gulf of Mexico. The test was made with a 12¼in. DS40HF+GNU in water-based mud on a rotary assembly. The bit drilled 6,365ft (1,941m) at an average of 41.2 ft/hr and was dull graded 0:1:ER:N/T:X:In:WT. The 19mm test cutters were in excellent condition with only minor wear flats. In contrast, the conventional planar gauge trimmers had suffered chipping and minor delaminating indicative of impact damage. The gauge pads showed signs of abrasive wear.
The performance of the rim cutters is enhanced not only by improvements in geometry, but also by the use of better, more abrasion-resistant diamond table material. In addition, the improvements made in impact toughness contribute to better abrasive wear characteristics because subtle microchipping of the diamond table is reduced.
SMITH BITS
Due to advancements in PDC bit designs and engineering materials, engineers have pushed PDC bits into drilling applications previously reserved only for rock bits or diamond-impregnated bits. PDC bits are being used to drill rock with twice the compressive strength of the rock drilled in the early 1980s (Figure 3). This practice has put a lot of pressure on the lowly API bit pin. A pin washout may result in hours of searching for the leak after a pressure drop is detected. A pin twist-off is one of the most difficult and expensive problems that can occur in a drilling operation. Such failures can result in costly time spent attempting to fish for the bit head or sidetracking around it.
Most bit pin failures are fatigue-related. During drilling, bit pins are under a combination of axial, bending and torsional loads resulting from weight on bit, hole angle changes and driving torque. Most fatigue failures occur at a pin's last engaged thread, about 0.75in. to 1in. from the pin shoulder.
Although the API Recommended Practices Manual governs most of the design and material property requirements for bit connections, it does not clearly recommend what makeup torque should be used when bits experience tough drilling conditions. It indicates the minimum recommended makeup torque for connecting drill pipe, drill collars and PDC bits (Figure 4), but only suggests that higher makeup torque should be used for challenging formations. Although the minimum makeup torque from API Recommended Practice 7G has been widely adopted, the effect of makeup torque on bit pin reliability has remained a question, especially since PDC bits are being selected to drill directional wells and hard formations.
To meet the demands of these applications, Smith Bits has developed materials and processes to provide pin strength and toughness levels that exceed API requirements for tool joints. The company also conducted full-size rotating fatigue tests to study other methods of improving the fatigue life of the bit pins. The study evaluated the use of the API stress relief groove (SRG), as well as increased make-up torque, on the rotating fatigue life of full-size 4½in. API regular bit pins. The use of higher makeup torque was made possible by the high yield strength and toughness of the new bit pins.
Experimental results
Two groups of samples were tested: 4½in. API pins with and without SRG. Pin components were welded to an extension bar to provide a large moment arm. The assembly was made up into a sub with a box connection and chucked in the fatigue test machine as in a lathe. Bending stress was induced into the connection by applying a bending load at the opposite end of the extension bar. The entire assembly was rotated at 168 rpm, which induced cyclic stresses in the connection (Figure 4). Results are listed in Table 1.
As anticipated, the pins with an SRG fared better than those without an SRG. When makeup torque was set at 17,750 ft-lb (the API minimum), SRGs increased the number of cycles to failure by 260%. Performance was improved 300% when makeup torque was set at 21,000 ft-lb. However, the largest improvement came from the tests run at 25,000 ft-lb.
The two pins with the highest makeup torque during fatigue testing were inspected afterward. No indications of thread stretching or distortion were seen. One of the pins having the highest cycles (1,108,910) that did not fail also underwent magnetic particle inspection, and no cracks were found on the pin.
As seen in Figures 5 and 6, the pin without an SRG failed within 1in. of the shoulder, while the pin with an SRG failed at the root of the SRG near the shoulder. Finite element analysis has revealed the last engaged thread takes about 30% of the load. The SRG feature removed this hazardous stress concentration, thus improving the pin performance.
The fatigue tests showed the SRG and higher makeup torque dramatically increased pin fatigue life when the connection was subjected to severe cyclic bending loads. When bit pins with material properties such as those on drill pipe or drill collar tool joints are provided, makeup torque can be increased above the API minimum values. This will enable PDC bits to handle the higher stresses experienced in some field applications and eliminate premature fatigue failures.
Hundreds of bit runs in the field have validated the beneficial effects of the SRG and higher makeup torque. Since incorporating these enhancements into production bits, not a single pin failure has been reported.

SECURITY DBS
There is no question that improvements in materials and greater understanding of design characteristics have resulted in better designs for roller cone bits. Improved bearing precision, increased bearing-load capacity and longer-lasting seals have significantly extended bit life. Advances in diamond technology and better wear properties of tungsten carbide have extended roller cone bit application to harder, more difficult lithologies. In addition, new software developments allow bit designers to correlate formation lithology to bit design for optimum bit selection.
However, to realize the full potential of such new and varied technologies, something is needed to bring it all together. "It requires a system approach," said Mark Stringer, technology champion with Security DBS, "the ultimate goal being to continually improve responsiveness to customer needs."
Stringer explained Security DBS re-engineered the company to enhance the advantages of new technologies for specific bit solutions. "X Series is the result of a total re-evaluation of our roller cone product line," he said.
The new series is the culmination of the company's Cascade business process, which puts experienced design personnel alongside the customer, equips them with GeoMechanics, the most sophisticated drilling analysis software available, and facilitates rapid response to their requests through streamlined manufacture and delivery. The result has been substantial improvement, including longer bit life as a function of design integrity. For example, in East Texas applications where seal effectiveness rates historically have been in the 70% range, performance has improved to about 90%, Stringer said.
Flexibility is the key
Security DBS is able to deliver custom-designed roller cone bits, mixing standard with diamond-enhanced cutting structures to match particular drilling conditions. By organizing design functions at the customer interface and delivering prototype bits quickly, Security DBS expands the choices for roller cone bits with flexibility that allows mixed formats and various combinations within a single bit.
Re-engineering the division for Design at the Customer Interface (DatCI), the company has placed trained bit designers closer to customers, eliminating the centralized design function that impeded responsiveness. "Bit design, inventory management and area strategy fall to a locally experienced, extensively trained application design engineer (ADE)," Stringer said. "This approach has cut as much as 75% from the design-delivery cycle for new bits by shifting key responsibilities to the field."
Priorities set in the field
For the customer, the ADE provides a valuable resource to produce solutions more efficiently because he or she knows the customer's specific performance needs, rig capabilities and lithological problems, and is able to respond with direct solutions more quickly. "The design engineers provide a responsive resource for customers, serving as partners in the development process. In turn, the customer has the opportunity to offer more input and become more involved in creating technology that serves him most effectively," Stringer said. "Bit selection has always been more of an art than a science, with optimization based on trial and error and personal experience. Even among the best drilling engineers familiar with an area, opinions will differ on bit selection. Using GeoMechanics provides the most accurate insight into lithology, porosity and rock strength, allowing for more accurate bit selection, and significantly speeding up the learning curve."
Because customers were able to involve Security DBS earlier in the planning process, they could see their bit design developments on a computer screen before the bits were built. For example, in an East Texas field, the software tools enabled the operator and vendor to gain a better understanding of the hard, abrasive formations in the area and what the existing equipment capabilities were. This, in turn, helped determine what was needed from a materials and design standpoint.
Custom designs
From bearing systems to hardfacing, the new X Series bits incorporate advanced innovations and improvements in application-specific designs. Instead of building technology upon an existing system, Security DBS designed a new bearing system from scratch, taking advantage of today's materials and metallurgy and manufacturing know-how. "The entire system was revamped, with the result being a simpler and more robust bearing system," Stringer said.
The bearing system is a three-part configuration composed of the bearing, seal and lubrication and pressure compensation system. The new bearing is larger and has significantly more contact surface for higher load capacity. Its thicker thrust flange provides an additional radial load surface and better cone retention. In addition, for larger sizes, new floating bushings reduce heat generation and resist galling. Stringer said the company has altered the manufacturing process to achieve better surface finish and dimensional consistency, which is especially important in protecting the seal. The new seal material provides enhanced resistance to temperature and abrasive wear. In addition, larger cross-sectional seals improve sealing and last longer.
In the lubrication system, the grease volume has been more than doubled. "It's now a simple cartridge system, installed in one piece during manufacturing," Stringer said. A new pressure-relief valve is also much simpler and more reliable for controlling internal pressure.
Working with suppliers of tungsten carbide inserts and hardfacing, Security DBS also has developed more wear-resistant materials for its cutting structures, especially for the critical gauge area. In addition, X Series premium steel-tooth bits feature the company's patented Claw tooth combination of innovative internal tooth design and Diamond Tech2000 hardfacing. The premium tooth design offers faster rate of penetration and extended cutting structure life, even replacing insert bits in several areas.
Field results prove out
Performance improvements with introduction of the 77/8in. XS73 in the East Texas field bear out the advantages of the system approach. Employing DatCI allowed direct, local interaction with customers and development of more accurate and timely response to their needs. Resulting design improvements have yielded significant savings from fewer rig-days per well, rigs freed for other opportunities, and more efficient, safer drilling.
"Re-engineering our manufacturing plant in Dallas added a further dimension to the process," Stringer said. "By totally re-evaluating manufacturing processes and equipment, we were able to significantly reduce the time required to build bits and, in the process, improve the quality and consistency." By using a rapid prototyping concept, the company reduced the size of the bit production line almost 80%, replacing older, less efficient equipment with modern precision machines. "This reduced lead times and allowed us to test, evaluate and modify in a much shorter period of time," he said.
The new Cascade process goes beyond placing design engineers in the field, Stringer said. "Security DBS is rebuilding our entire roller cone product line with the purpose of becoming more responsive to customer needs. That means providing leading technology and leading innovations in all aspects of bit design."