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.
