Jon Schroder Maurizio Di Pasquale Baker Hughes Inc. The Woodlands, Tex.
Alun Richards Baker Hughes Inc. Dhahran, Saudi Arabia Slim Bearings
Jesse Yorty Timken Co. Canton, Ohio
A new tapered bearing assembly, developed specifically for roller-cone rock bits, significantly increased bit life for a Middle Eastern operator. This technology can help oil and gas customers avert premature bit failures, minimizing nonproductive time, and decreasing costs. Longer-life expectancy of the tapered bearing and seals package improves the attractiveness of roller-cone bits in applications where fixed-cutter polycrystalline diamond compact (PDC) bits traditionally have been selected.
Engineers used analytical modeling and simulations to build 28-in. tungsten carbide insert (TCI) bits, which were tested in a laboratory and then sent for a Middle Eastern exploration run in which the bearing assembly's first iteration increased reliability and endurance.
The run had 70% more bit revolutions and 58% more footage than the field average. Bit life traditionally constrains roller-cone technology. Traditional journal bearings and cylindrical-roller bearings with ball-lock cone retention have inherent play, reducing bearing and seal life.
Shorter bit life increases drilling costs by requiring more trips and more drill bits. Recent innovations effectively packaged application-specific, tapered-bearing technology into roller-cone assemblies (Fig. 1).
A preloaded bearing package eliminated axial and radial play, which also stabilized the sealing interface between the head and cone, promoting extended seal life. Analytical modeling helped designers define and simulate various loading conditions downhole, including reaming, up-drill, directional drilling, and high-weight.
Proprietary bearing-analysis software enabled development of bearings that perform effectively across various loading conditions. Roller-cone rock bit bearings originally were designed for vertical shallow wells, which differ greatly from today's very deep wells and horizontal wells.
Opposed-taper roller bearings eliminate play and stabilize cones, but need to be configured into roller-cone design. Many roller cones use inserts in drilled holes having sufficient depth to retain them.
Shell thickness between a hole's bottom and the internal bearing assembly must be kept above a minimum to ensure sufficient strength to withstand downhole drilling conditions. The cone assembly also must be attached to the leg with enough strength to endure downhole drilling.
Attachment occurs via blind-hole assembly. The bearing assembly must be precisely preloaded to allow for optimum bearing life and seal performance. Lab tests validated the opposed-tapered roller bearings (TRB) as did successful field trials.
Roller-cone bits still play a role in drilling, but bearing technology has not kept pace with the hybrid technology that has merged PDC bits and roller-cone bits. TRB application increased both capacity and life of rolling-cone bits.
Anti-friction bearings can meet numerous needs. Various anti-friction bearing types exist, requiring compromises during bit design and selection decisions.
Roller-bearing or ball-bearing types can satisfy boundary (or fixed) conditions in widely varying applications.
Customized bearings improve performance in specific operating conditions. The bearing raceway and roller profiles can be designed to control maximum stress levels, boosting durability and enhancing performance in demanding applications. Optimization normally targets bearing profile, surface finish, material, coating, and precision setting.
Key boundary conditions to be considered include:
• External loads; e.g., radial, thrust, moment, shock, and combination loads.
• Operating temperature range, such as extreme limits and thermal cycling.
Early cylindrical-roller bearings (CRB) featured two roller-guide flanges on the outer raceway and none on the inner raceway. Most current CRB deploy crowned rollers to avoid premature spalling from roller-edge loading. Crowning addresses loading concentration from inner-race misalignment caused by combined bearing and system deflections.
In rock bits, the assembly attaches via a central beam between the cone assembly and leg section. The beam attaches to the leg (or static side of the bit assembly), providing a structure to the bearing and supporting bend loading. The leg's bearing section needs to be made of low-carbon materials that can be welded. Since the beam is independent of the head, high-strength alloys establish a strong overall leg-section bearing pin.
Bearing lubrication also must be addressed. While the central beam is keyed to the leg section to prevent rotation, targeted grease channels can be integrated to the beam's shaft in a non-loaded direction. From precise flats on either side of the shaft of the central beam, lubricant flows through holes feeding the opposed tapered bearings. This assembly diverts the lubrication channels away from the bearing axis' center, typically the assembly's highest-stressed area.
The cone-bearing assembly uses one axial contact point and precise shimming controls tolerances. A preload setting of the opposed tapered-bearing configuration increases system stiffness and reduces deflection.
A permanent shim, under the head of the central beam, establishes preload settings in the bearing assembly. Axial loads stem from two sources:
• Taper in the bearings that split any loading into axial and radial load components.
• Inward loading on the cone assembly, particularly prevalent during directional drilling or reaming.
Applying calculated torque settings beyond severe-loading conditions to a nut on the end of the central beam prevents elongation of the beam during operation.
The overall size and cutting structure of the drill bit is as important as the bearing assembly. The varying size of the cones and the natural taper of the cutting structure decrease the radial space available toward the end of the cones.
The package needs adequate clearance for cutting-structure elements while maintaining load capacity. Simulations showed that available space played a primary role in developing a nominal effective bearing spread through taper angles and bearing profiles.
In blind-hole application of bearings, a threaded ring is used to retain the bearing assembly in the cones. The ring threads down until it contacts the outer race of the main bearing, fixing its position. The bearing assembly is installed with no gaps in the system, to prevent movement. The threaded ring seals the cone assembly to the leg section.
A static o-ring seals the ring and the cone together. A dynamic seal between the ring and the leg enables a smaller-diameter seal, linearly reducing the seal-sliding speed which reduces overall wear and extends seal life. Lower speeds reduce seal-face temperatures and slow degradation.
Bearing fundamentals, tribology, and advanced modeling are a few aspects of total system analysis when maximizing bearing-fatigue life (or service life) under high-loading, high misalignment, thin lubrication film, debris, or other prevalent drilling conditions. Bearing evaluation involves a detailed review of boundary conditions and performance expectations.
Designers determined bearing loads by evaluating bit loading from various new-well profiles and historical drilling data (Fig.2).
Varying cutter profile designs, bit designs, and applied weights on bit prompted bit-by-bit development of the expected duty cycle. Consideration also was taken for boundary conditions.
Although current roller-cone bits use CRB and ball arrangements, preloaded tapered-roller arrangements increase bit performance. A preloaded tapered-roller bearing offers clearance advantages over other bearing types, especially when the bearing encounters system misalignment.
Improving load distribution between the two bearing positions increased power density. Heavy loading with repeated stops and starts of the roller-cone bits creates misalignment. Traditional CRB and ball lock roller-cone design and the new tapered bearing series were evaluated in larger bit sizes (Fig. 3)
Optimizing roller-cone geometry allows handling of the most severe-loading conditions, which result in off-set loading and uneven load distribution on the bearings. The roller-cone load location and load angle induce an over-turning moment force on the bearings. Such load scenarios often reduce seal performance and bearing reliability. Heavy loading with repeated stops and starts of the roller-cone bits creates misalignment of the roller-cone and bearing axis.
A proprietary bearing-analysis program accounted for multiple loading conditions, misalignment, lubrication effects, and determined fully adjusted bearing-performance results. Simulations considered nominal load directions, heavy outward loads, and severe inward loads from reaming or directional drilling. Analysis prompted changes that increased calculated bearing-fatigue life and improved potential seal performance.
A tapered-roller bearing has as much as six times more radial stiffness as a comparably sized angular contact ball bearing and twice as much radial stiffness as a comparably sized cylindrical-roller bearing for a zero-clearance condition. Increased stiffness allows for only two tapered-roller bearings and increases seal-face stability, leading to a more evenly distributed load and extended seal life.
Estimated loads and physical space limitations allow optimizing of bearing geometric-spread. The inherent taper moves an effective bearing center-location from on center, for indirect mount like a CRB, to further outside. The angularity of the bearing, or "K factor," is a function of the half-included cup angle and is the ratio of basic dynamic radial-load rating (C90) to basic dynamic axial-load rating (Ca90) in a single row-bearing.
Too much cup angle can result in reduced radial load capacity. Selection of a nominal tapered-roller bearing cup angle improved tilting stiffness or resistance to overturning loads, helping stabilize the cone assembly during drilling.
Bearing preload minimized axial movement of the roller cone under operating load. Axial preload values were reviewed to maintain a minimum load zone in the setup-bearing row. Bit designers refined the setting and balance life, contact stress and distribution, heat generation, and other bearing performance indicators.
Excessive axial preload can lead to unwanted heat generation, lubrication problems, premature bearing damage, and reduced bearing-fatigue life. Excessive axial play results in fewer rolling elements carrying the load, increasing individual roller load and reducing bearing life. Optimizing the bearing setting improves bearing life and load sharing between bearing rows.
The enhanced internal geometry enables reduction of geometric stress concentrations at the extreme edges of the roller-raceway contacts. Specific component profiles were designed for a uniform stress distribution under normal and severe loading conditions. Standard and inward loading conditions of traditional CRB configuration vs. TRB were studied with displacement amplitude reduced by 50 times. Plots showed loading distribution on standard ERB rollers (Fig. 4).
It is difficult to control load distribution across the CRB rows. Raceway and roller diameter tolerances result in a minimum mounted-radial internal clearance that is non-adjustable and required for assembly of the roller cone onto the leg. The peak load is shared in TRB arrangement, stresses are reduced, and load is more evenly shared by the system's rollers.
High overturning moment loads, deflection, or misalignment resulted in less than optimal roller-raceway contact stress distribution using a standard bearing geometry. An ideal pressure distribution involves an optimized roller profile with loading relatively centered between the two bearing positions.
When studying contact stresses at the most loaded roller during off-set loading, high overturning loads, or misaligned conditions, non-optimized profiles resulted in uneven pressure across the roller-raceway contact. Geometric modifications improved the center and edge stress values along with the pressure slope across the roller length.
Testing the first bit assembly with the tapered-roller bearing configuration in a lab under controlled conditions yielded initial proof of concept. Water was used and the test conducted only under atmospheric pressures, which don't impact bearing capability. The bit was a 28-in. TCI-roller cone that drilled into two different limestones. It was run twice for just more than 36-in. of effective drilling per run. Rotating speed was brought to 120 rpm and held at 90,000 lbf weight on bit (WOB). Cone-rotating torque was smooth and cones spun freely with no difference in rotating torque. This bit was sent for field testing.
The bearing was tested in a Middle Eastern application. The bearing package along with a metal face-seal system was installed on the 28-in. tungsten carbide insert bit IADC 445, as classified by the International Association of Drilling Contractors.
Section length was typically 2,500-3,500 ft, and IADC 435 or 445 roller cones on performance motors delivered optimum results. Bearing and seal life constitute a limiting factor that resulted in more runs on rotary bottomhole assembly and multiple trips. Roller-cone bits were pulled out of hole at a maximum of 1.3 million revolutions (MRevs) and an average of 0.91 Mrevs. The tapered-bearing bit extended the maximum to more than 1.7 MRevs.
The subject bit was run with a performance motor assembly (12¾-in. OD, 5/6 lobe, 0.13 rev/gal) that drilled through a section of 10,000-20,000 psi unconfined compression-strength rock consisting of anhydrites, limestone, and dolomitic limestone with minor shale layers.
Limiting penetration rate to 30 ft/hr prevented circulation loss. Compared with average parameters in offset runs, the bearing and seals experienced the same WOB while rpm was 100% higher. Results showed 31% higher revolutions compared with previous best performance.
In another test run in the Middle East, the same bit design with a rotary assembly drilled comparable lithologies within 10,000-20,000 psi unconfined compressive strength. Crews drilled 3,555 ft, reporting the bit accumulated 216 hr and 1.6 MRevs with 60,000-80,000 lb-ft WOB and speeds of 70-100 rpm. The run included backreaming twice per stand. The bit came to the surface because of a suspected drill-string washout and the dull grading satisfied customer requirements (Figs. 5-6).
Any type of reaming typically causes premature bearing and seal failures, but this was not the case for the tapered bearing assembly. The bearing assembly maintained preload with a visually polished surface on raceways and rollers. Inward loading caused more nose-bearing wear than main-bearing wear.
The cone profile traces on the main bearing showed very good conformity and very little wear across the raceway surface. No spalling resulted and bearing integrity remained uncompromised.
The use of tapered-roller bearings reduces and potentially eliminates axial play and angular misalignment while providing superior reliability, including but not limited to high-speed motor bottomhole assembly (>350 rpm and up to 500 rpm).
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The authors acknowledge A al Darwaish and M. Mohamed, both of Baker Hughes Inc., for their efforts in developing the technical paper on which this article was based.
Deep Groove Bearing The authors Jon Schroder (Jon.Schroder@bakerhughes.com) is Baker Hughes product development engineer for drill bits. He supports the development of new drill bits technologies and solutions worldwide. Schroder has been with Baker Hughes since 2010. He previously worked on mechanical development of industrial gas compression engines for Cummins. He earned a BS in mechanical engineering (2008) from Texas Tech University. Maurizio Di Pasquale (Maurizio.DiPasquale@bakerhughes.com) is Baker Hughes product manager for tricone bits. He supports the introduction and commercialization of new drill bits technologies and solutions worldwide. Di Pasquale has been with Baker Hughes since 2007, working in Europe and North Africa drill bits operations. He earned an MS in automation engineering (2003) from the University of Bologna in Italy and is a licensed professional engineer. Alun Richards (Alun.Richards@bakerhughes.com) is Baker Hughes applications engineering manager for drilling systems and drill bits. He has worked in various engineering roles in the UK, Middle East, and Asia during his 17 years in the upstream oil and gas industry. He earned a BS in geology (1994) and an MS in geotechnical engineering (1997), both from the University of Wales, Cardiff. Jesse Yorty (Jesse.Yorty@timken.com) is a principal application engineer for Timken Co. where he has worked since 2005 in various roles. He has extensive experience with the practical application and performance analysis of rolling element bearings. Yorty's main responsibility is technical application support for a broad industrial customer base, including heavy industries and power transmission. He earned a BS in mechanical engineering (2005) from the Worcester Polytechnic Institute.