1. Field of the Invention
The invention relates generally to rolling cutter earth boring drill bits used for the exploration and retrieval of petroleum and other minerals from the earth. In particular, the invention is a new form of bearing failure indicator for a rolling cutter earth boring drill bit.
2. Description of the Related Art
It is commonplace during drilling into the Earth for minerals such as oil and natural gas to drill boreholes thousands of feet deep. A rolling cutter drill bit used to drill these wells is so remote from the surface that even high quality instrumentation located near the bit while drilling is not able to accurately indicate the impending failure of the bearing in the drill bit. Oftentimes the drill bit fails suddenly and causes the drilling operation to be halted while the xe2x80x9cjunkxe2x80x9d left behind by the failed bit is removed from the bottom of the borehole. The time lost in recovering this junk may cost the drilling company many thousands of dollars, and unless all the junk left by the failed bit is recovered, the replacement drill bit may also fail prematurely from damage by the junk.
Detecting impending bit failure is particularly difficult with rolling cutter type drill bits that utilize sealed and lubricated friction bearing systems. The reason for this is that when these bits first experience bearing failure, only relatively minor changes in drilling torque and drilling rate of penetration occur. Since these changes are usually within the range of normal torque and ROP variations, the bearing failure is usually not detected at the surface.
Often, the only indication of failure is the sudden decrease in drilling rate of penetration that occurs when bearing failure is total and junk is left in the hole. Although instrumentation packages built into measuring while drilling tools may at times be able to accurately detect impending bit failure, they are not able to detect it reliably. Furthermore, these MWD packages often add considerable expense to the drilling operation, and are therefore used sparingly. Consequently, it is highly desirable that any bearing failure mechanism be made into the drill bit.
Bearing failure indicator schemes are disclosed in numerous different rolling cutter drill bit designs, including U.S. Pat. Nos. 3,058,532, 3,011,566, 3,062,302, 3,363,702, 3,678,883, 3,853,184, 4,346,591, 4,436,164, 4,548,280, 4,655,300, 4,785,894, 4,785,895 and 5,183,123, all incorporated by reference herein for all they disclose. The complexity of these designs, and/or their tendency to falsely indicate a bearing failure have limited their utility. In fact, these designs have had only limited commercial success.
A new type of bearing failure indicator for sealed and lubricated rolling cutter drill bits is disclosed. The rolling cutter drill bit comprises a bit body adapted for rotation about a longitudinal axis, a plurality of extending legs, and a cantilevered bearing spindle formed on each leg. A plurality of rolling cone cutters are rotatably mounted upon the bearing spindles with the cone apices adjacent to the longitudinal axis of the bit. A plurality of cutting inserts are secured in the rolling cone cutters, and arranged in a plurality of rows. At least two of the rolling cone cutters are intermeshing cutters, arranged such that they have intermeshing rows of cutting inserts. At least one of the intermeshing cutters has at least two rows of cutting inserts arranged as two inner rows. A groove is formed intermediate the two inner rows. Within the groove is a row containing a plurality of generally flat top bearing inserts.
In the normal operation of the drill bit, the rows of generally flat top bearing inserts do not contribute to the drilling action of the drill bit. However, when a bearing assembly fails in operation, the generally flat top bearing inserts engage the intermeshing rows of inserts in the adjacent intermeshing cutter.
This engagement causes a sudden, relatively large increase in the drilling torque of the drill bit. This torque increase is readily discernable at the drill rig by the drilling crew, providing a reliable indication of a failed bearing. The disclosed arrangement provides an extremely reliable means of indicating a failed bearing in a rolling cutter drill bit.