Development and Application of a New Roller Cone Bit with Optimized Tooth Orientation
Abstract: 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 … Show more
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A roller cone bit with balanced cutting structure is developed in this paper. A balanced cutting structure refers to energy balancing and/or force balancing. This paper describes why traditional roller cone bits are unbalanced, how the balancing condition of an existing roller cone bit is evaluated by a computer drilling simulator, and how an unbalanced cutting structure can be balanced by a design optimization procedure. Finally, bit performance in the field is discussed and compared with offset bits. It is found that a force balanced roller cone bit has better durability and drills faster than the offset bit. Introduction The force balanced PDC bit has been widely used with success in the oil and gas drilling industry. A force balanced PDC bit refers to a bit in which the ratio of the imbalance force to applied weight on bit (WOB) is minimized. Field runs have proven that a force balanced PDC bit drills much smoother and has better durability than conventional designs. This is especially true in directional drilling where bit stability is a key issue. It is also well known that bit imbalance forces are major causes of bit vibration1,2,3. However, the application of the concept of "force balancing" to a roller cone bit was considered very difficult. The first difficulty comes from the limitation of the geometric space. Most soft to medium roller cone bits are designed so that the teeth on one cone are intermeshed with teeth on adjacent cones in order to get a self-cleaning effect. Any profile change on one cone will affect that on other cones. The tooth size and the distribution of rows on cones are also constrained by the requirement of cone/teeth clearance. The second difficulty is the evaluation of a balanced cutting structure. No analytical model was available until recently. Computer programs have been developed which predict and simulate the bottom hole patterns created by roller cone bits by combining the complex movement of the teeth with a model of formation failure4,5,6,7. In this paper, a computer drilling simulator is first developed. Using the results from the drilling simulator, a procedure for evaluating of balanced cutting structures is then developed. The unbalanced cutting structure can be balanced by this optimization procedure. Finally the field performance of the bit with balanced cutting structure is summarized. It is shown that this kind of bit can drill faster and smoother with longer bit life. Its enhanced stability is especially suitable for directional drilling. The Concept of a Balanced Roller Cone Bit A roller cone bit may have one, two, three or four cones. In this paper only three cone bits will be discussed, but the principle can be applied to other types of bits with two or four cones. The cutting structure of a roller cone bit refers to the cones and the rows of teeth on the cones. As the bit is rotated, the cones will rotate around their own axis and the teeth will crush the formation beneath them. Each cone removes part of the hole bottom. The formation removed by each cone may be different. As a result, the force acting on each cone may be different. This leads to the development of two concepts:Energy Balancing: when all three cones remove the same amount of rock during drilling, then the cutting structure is said to be energy balanced.Force Balancing: when all three cones/bearings are subject to the same forces, then the cutting structure is said to be force balanced.
A roller cone bit with balanced cutting structure is developed in this paper. A balanced cutting structure refers to energy balancing and/or force balancing. This paper describes why traditional roller cone bits are unbalanced, how the balancing condition of an existing roller cone bit is evaluated by a computer drilling simulator, and how an unbalanced cutting structure can be balanced by a design optimization procedure. Finally, bit performance in the field is discussed and compared with offset bits. It is found that a force balanced roller cone bit has better durability and drills faster than the offset bit. Introduction The force balanced PDC bit has been widely used with success in the oil and gas drilling industry. A force balanced PDC bit refers to a bit in which the ratio of the imbalance force to applied weight on bit (WOB) is minimized. Field runs have proven that a force balanced PDC bit drills much smoother and has better durability than conventional designs. This is especially true in directional drilling where bit stability is a key issue. It is also well known that bit imbalance forces are major causes of bit vibration1,2,3. However, the application of the concept of "force balancing" to a roller cone bit was considered very difficult. The first difficulty comes from the limitation of the geometric space. Most soft to medium roller cone bits are designed so that the teeth on one cone are intermeshed with teeth on adjacent cones in order to get a self-cleaning effect. Any profile change on one cone will affect that on other cones. The tooth size and the distribution of rows on cones are also constrained by the requirement of cone/teeth clearance. The second difficulty is the evaluation of a balanced cutting structure. No analytical model was available until recently. Computer programs have been developed which predict and simulate the bottom hole patterns created by roller cone bits by combining the complex movement of the teeth with a model of formation failure4,5,6,7. In this paper, a computer drilling simulator is first developed. Using the results from the drilling simulator, a procedure for evaluating of balanced cutting structures is then developed. The unbalanced cutting structure can be balanced by this optimization procedure. Finally the field performance of the bit with balanced cutting structure is summarized. It is shown that this kind of bit can drill faster and smoother with longer bit life. Its enhanced stability is especially suitable for directional drilling. The Concept of a Balanced Roller Cone Bit A roller cone bit may have one, two, three or four cones. In this paper only three cone bits will be discussed, but the principle can be applied to other types of bits with two or four cones. The cutting structure of a roller cone bit refers to the cones and the rows of teeth on the cones. As the bit is rotated, the cones will rotate around their own axis and the teeth will crush the formation beneath them. Each cone removes part of the hole bottom. The formation removed by each cone may be different. As a result, the force acting on each cone may be different. This leads to the development of two concepts:Energy Balancing: when all three cones remove the same amount of rock during drilling, then the cutting structure is said to be energy balanced.Force Balancing: when all three cones/bearings are subject to the same forces, then the cutting structure is said to be force balanced.
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.
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