The effects of downhole vibrations on polycrystalline diamond compact (PDC) bit performance have been well discussed in recent years. However, most research on the downhole dynamics of roller cone bits has focused on axial vibration and its effects on bit performance. The questions which remain to be answered are: What kind of vibrations does a roller cone bit experience, and what are the effects of these vibrations on roller cone bit performance? To investigate these questions, a series of field tests was conducted in a full scale drilling research rig, with downhole vibration data recorded by an advanced instrumented sub. Soft to medium hard formations were drilled using four 81/2-in. roller cone bits (IADC Code 517. Three kinds of harmful vibrations were observed, and their effects on bit performance in terms of rate of penetration (ROP), insert breakage, and bearing/seal and bit life are discussed.

Summary The effects of downhole vibrations on the performance of polycrystalline diamond compact (PDC) bits have been well discussed in recent years. However, most studies on the downhole dynamics of roller-cone bits have focused on axial vibration and its effects. The questions that remain to be answered are what kinds of vibrations a roller-cone bit experiences and what the effects of these vibrations are on roller-cone bit performance. To investigate these questions, a series of laboratory and field tests was conducted. Downhole vibration data were recorded by an advanced instrumented sub. Soft- to medium-hard formations were drilled with four 8 1/2-in. roller-cone bits (IADC 517). Three kinds of harmful vibrations were observed, and their effects on bit performance in terms of rate of penetration, insert breakage, and bearing/seal life are discussed. Introduction Based on their directions, downhole drilling vibrations may be divided into three types - axial, torsional, and lateral. Furthermore, two specific types of vibrations are often observed during drilling - stick-slip and whirl. Stick-slip vibration is a specific kind of torsional vibration. By definition, bit stick-slip vibration involves periodic fluctuation in bit rotational speed, ranging from near zero to a high value, often more than twice the rotational speed measured at the surface. By contrast, other kinds of torsional vibrations occur in which bit rotational speed may be near-constant. Similarly, bit whirl (backward or forward) is a specific kind of lateral vibration. Similar to the case of stick-slip vibration, bit whirl is not the only form of bit lateral vibration. That is, a bit may vibrate laterally but may not necessarily be whirling. Only when the bit's geometric center moves around the hole's center line (either forward or backward) is the bit truly whirling. In addition, in most cases, bit whirl is coupled with other forms of lateral vibration. Therefore, the trajectory of the bit center is usually not a circle. For PDC bits, it has been found that these types of vibrations - stick-slip and whirl - have a significant negative effect on several aspects of bit performance, including rate of penetration (ROP) and bit life. It has been also found that the cutting structure plays a major role in the onset of backward whirl. The antiwhirl and track-lock design concepts of PDC bits1,2 are two examples of how the onset of bit whirl can be successfully discouraged through the cutting structure's design. To date, most research in the field of roller-cone bit downhole dynamics has focused on axial vibrations, or "bit bounce."3,4 The stick-slip and backward-whirl vibrations of roller-cone bits were observed only in recent years and mainly in laboratory tests.5,6 Issues like the effect of stick-slip and whirl vibration on bit performance, the influence of the bit cutting structure on bit vibration, and the effectiveness of downhole tools to suppress vibrations are still unclear and need to be studied further. Test Conditions To investigate how roller-cone bits vibrate during drilling and what the effects are on bit performance, a series of laboratory and field tests was designed. The field tests were conducted in a full scale research test rig at the Advanced Drilling Intl. Facility located in Montrose, Scotland. Four different 8 1/2-in. roller-cone bits were each tested with 12 different combinations of weight on bit (WOB) and rotational speed. A comparison of the four bits is shown in Table 1. In the field tests, drilling with rotary assemblies began at a depth of 2,000 ft. The formation is homogeneous consolidated sandstone made of volcanic detritus minerals. This is a soft to medium formation. The inclination angle of the well was approximately 20°. Surface data were monitored with conventional techniques. Downhole vibrations were recorded by an advanced, instrumented dynamic sub that uses accelerometers, strain gauges, and contact sensors to measure axial, torsional, and lateral vibrations, as well as instantaneous bit rotational speed, bit center trajectory, and actual WOB and torque on bit (TOB). A detailed description of the instrumentation may be found in Ref. 7. Stick-Slip Vibration of Roller-Cone Bits In Fig. 1, the downhole instantaneous rotational speed of Bit A is charted vs. time for a portion of its run and demonstrates a typical case of stick-slip vibration. Bit A is a conventional design. The surface rotational speed of 60 rpm is approximately equal to the mean value of the downhole rotational speed (63.9 rpm), confirming the reliability of the measurement. The maximum speed during stick-slip is 146 rpm, more than twice the indicated surface speed. The minimum speed is zero. Fig. 2 shows the spectrum of bit rotational speed. The stick-slip frequency is approximately 0.7 Hz. This frequency is lower than but close to the first torsional natural frequency of the drillstring calculated with a dynamic stiffness method.8 The numerical model shows that the stick-slip frequency is independent of the bit rotational speed.9 For a PDC bit run at the same rig, it was found that the frequency of stick-slip, once initiated, was also approximately 0.7 Hz.7 These facts indicate that, once initiated, stick-slip frequency is dependent on the bottomhole assembly (BHA) rather than bit type or bit-formation interaction. However, bit-formation interaction may play some role in the initiation of bit stick-slip vibration. It is interesting to note that even when the effective duration of bit rotation is reduced to approximately 70% of the total drilling time, the penetration rate of the bit in stick-slip is not reduced. In other words, the stick-slip vibration does not adversely affect the ROP. This phenomenon was also observed in laboratory and field tests for PDC bits.5,10 An analysis of WOB and TOB during bit stick-slip is shown in Figs. 3 and 4. There is considerably less variation in both WOB and TOB during the "stick" period than during the "slip" period. This is because during the "stick" period, the bit stops drilling while the WOB and TOB are still applied. During the "slip" period, the bit rotational speed changes from 0 to 146 rpm, WOB increases from 8,000 to approximately 21,000 lb, and TOB changes from 200 to 800 lb-ft. Such significant fluctuations of both WOB and TOB may be the major cause of insert breakage and may be partly responsible for premature bearing/seal failure.

A significant research and development effort plus extensive laboratory andfield tests have resulted in a new generation of roller cone bits, namely anenergy balanced roller cone bit. An energy balanced roller cone bitincorporates three patented features:balanced cutting structure,optimized tooth orientation, andoptimized anti-tracking mechanism. This paper details the principles of these three features and their application torock bit design. Field performance of energy balanced bits is evaluated by rateof penetration, footage drilled, durability of cutting structure, andreliability of bearing/seal system. Significant performance improvement hasbeen observed in the field. Introduction Optimum combination of drilling efficiency (the rate of penetration) anddurability (the bit life) for specific formations is always an objective in thedesign of a roller cone bit. Durability of a roller cone bit is determined bythe shorter of cutting structure durability and seal/bearing durability. Thispaper focuses on the design of bit cutting structures in order to improvedrilling efficiency and durability and to extend bearing/seal life. However, the design of a seal/bearing system is not the topic of this paper. Drilling efficiency is directly associated with the cutting structure. Fromthe aspect of bit design, a roller cone bit may be designed to drill very fastfor a given formation by increasing the tooth extension, reducing the number ofteeth, increasing the cone offset, etc. However, such a design may lead to veryearly damage of the cutting structure such as insert breakage and insert loss.From the aspect of bit application, a roller cone bit may drill fast byapplying high weight on the bit and/or by applying high rotational speed. Butif the energy level applied to bit is too high, not only cutting structures, but also bearings and arms may fail unexpectedly. Usually it is very hard to design a cutting structure of a roller cone bitthat has high drilling efficiency and high durability simultaneously. Themodifications of roller cone bit design are based on years of experience inevaluating bit run records and dull bit conditions. The best solution so far isto develop a compromise between efficiency and durability. Since drill bits arerun under inhospitable conditions, it is usually difficult to determine whatcaused a bit failure. It is also difficult to evaluate a bit's performance inthe field because drilling conditions vary from well to well. Anotherdifficulty is to identify design weaknesses based on performance of individualbits. Obviously, cutting structure design plays a key role in roller cone bitperformance. There must be an optimized cutting structure that not onlyincreases rate of penetration but also increases cutting life and extendsseal/bearing life. In this paper, the principles of three patented technologies, namely,balanced cutting structure,optimized tooth orientation, andoptimizedanti-tracking mechanism, are detailed. The effects of these technologies on bitdrilling efficiency and bit durability as well as on seal/bearing life arediscussed. It is well known that seal/bearing durability can be described byreliability curves expressed as a function of bit life1. In order tosystematically evaluate the performance of energy balanced bits, the durabilityof cutting structures is also obtained by using information from bit dullconditions. Thus, the reliability of a roller cone bit is appropriatelyestablished by the combined reliabilities of both the cutting structure and theseal/bearing system. Energy Balanced Roller Cone Bit Balanced Cutting Structure Roller cone bits may have one, two, three, or four cones. However, in thispaper the discussion will be focused only on roller cone bits with three cones.Each cone removes a part of formation and takes a part of the weight on a bit.If all three cones remove the same volume of formation during drilling, thenthe bit is volume balanced. When each of the three cones is subject to the sameforces, then the bit is force balanced. Fig. 1 depicts three major forces acting on each cone duringdrilling. If a roller cone bit is both volume and force balanced, then the bitis called an energy balanced bit.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractBased on computer drilling simulation results, a new, unique roller cone bit has been developed 1 . All the teeth on this new bit are oriented optimally such that the leading face of the elongated crest of a tooth is perpendicular to its trajectory during interaction with a formation. This paper describes the computer model, the calculation procedure for determining the trajectories of the teeth, and the method for designing the new bit. Laboratory and field tests have shown that the new bits drill up to 25% faster and longer than the same type of bits with conventional tooth design.