Not applicable.
1. Field of the Invention
The invention relates generally to the field of roller cone drill bits used to drill wellbores through earth formations. More particularly, the invention relates to systems and methods for determining possible failure of a bearing on a roller cone drill bit.
2. Background Art
Roller cone drill bits known in the art include one or more roller cones having cutting elements thereon rotatably mounted on a bearing journal pin. The bearing journal pin forms part of a bit body coupled to a drill string. A typical roller cone drill bit is shown in U.S. Pat. No. 5,189,932 issued to Palmo et al, for example.
Typically, the bearing surfaces of the journal pin are lubricated to reduce wear by rotation of the cone thereon by a lubricant that is stored in a reservoir formed in the bit body. FIG. 1 of the Palmo et al. ""932 patent shows a cross section of one of the bearings and roller cones, including the lubricant reservoir. The lubricant reservoir is pressure compensated by a piston or the like so that the lubricant in the reservoir is maintained at substantially the same fluid pressure as the ambient pressure of a drilling fluid (xe2x80x9cdrilling mudxe2x80x9d) outside the bit body in the wellbore. Equalizing lubricant and wellbore fluid pressures reduces the tendency of the drilling fluid to bypass a seal disposed between the roller cone and the bearing journal pin. As is known in the art, entry of wellbore fluid into the lubricant reservoir or the bearing area of the journal pin can contaminate the lubricant to a degree as to make it useless as such. When the lubricant no longer can function, bearing failure typically follows rapidly afterward. Bearing failure is undesirable not merely because the bit will have to be replaced prematurely, but also because upon bearing failure, there is a substantial risk of failure of a cone locking mechanism. If the cone locking mechanism fails, the cone may fall off the bit, making for an expensive and time consuming operation to xe2x80x9cfishxe2x80x9d (find and remove) the cone from the wellbore. Accordingly, it is desirable to reduce the possibility of premature bearing failure and attendant cone lock failure.
One system known in the art for indicating possible loss of lubrication function or expected bearing failure in a roller cone bit is described, for example, in U.S. Pat. No. 5,813,480 issued to Zaleski et al. The system disclosed in the Zaleski et al. ""480 patent includes a number of different types of sensors disposed at selected positions in and on the bit body, and a signal processing and recording system adapted to record measurements made by the various sensors, and/or transmit the measurements to a telemetry unit for ultimate transmission to the earth""s surface for detection and interpretation. Sensor types disclosed in the ""480 patent include various forms of temperature sensor and pressure sensor.
It is also desirable to have a sensor in a drill bit which can indicate changes in the character of the lubricant which may result in loss of lubrication function. It is also desirable to have a system in a drill bit which can maintain a selected positive differential pressure in a lubricant reservoir to reduce the likelihood of lubricant contamination by the drilling fluid.
One aspect of the invention is a roller cone drill bit which includes at least one roller cone rotatably mounted to a bit body. The bit body includes therein a lubricant reservoir adapted to supply lubricant to bearings on which the roller cone rotates about the bit body. The bit includes a sensor adapted to detect drilling fluid contamination of the lubricant. The bit includes a processor/transmitter operatively coupled to the sensor and adapted to communicate signals corresponding to detected contamination. In one embodiment, the transmitter is adapted to communicate signals to a measurement-while-drilling collar.
One particular embodiment includes a first pressure sensor adapted to measure fluid pressure in the reservoir, a second pressure sensor adapted to measure fluid pressure in a wellbore outside the bit, and a pressure compensator adapted to control the fluid pressure inside the reservoir. The pressure sensors and the pressure compensator are operatively coupled to the processor/transmitter. The processor/transmitter is adapted to operate the pressure compensator to maintain a selected pressure differential between the reservoir and the wellbore.
Another aspect of the invention is a roller cone drill bit including at least one roller cone rotatably mounted to a bit body. The bit body includes therein a lubricant reservoir adapted to supply lubricant to bearings on which the roller cone rotates about the bit body. The bit includes a processor, a first pressure sensor adapted to measure fluid pressure in the reservoir, a second pressure sensor adapted to measure fluid pressure in a wellbore outside the bit, and a pressure compensator adapted to control the fluid pressure inside the reservoir. The pressure sensors and the pressure compensator are operatively coupled to the processor, and the processor is adapted to operate the pressure compensator to maintain a selected pressure differential between the reservoir and the wellbore.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.