Forecasting the Life of Rock-Bit Journal Bearings

Abstract

Summary This paper describes an analytical procedure for forecasting the life expectancy of rock-bit journal bearings, Actual performance data and reliability analyses are used to establish empirical relationships and a graphic procedure that relates the risk of bearing failure to operating parameters and drilling cost. Introduction Although most journal-bearing rock bits are retired with effective bearings, the risk of bearing failure continues to be a major concern because it can cause cutter loss and result in an expensive and time-consuming fishing job. As a consequence, many bits are retired with useful lives remaining, and operating practices are often tempered to yield increased bearing reliability at the expense of penetration rate. These circumstances led to the development of a technique for estimating the risk of bearing failure through analysis of actual bit performance in commercial drilling applications. Causes of Rock-Bit Journal-Bearing Failure and Measures of Bearing Life The effectiveness of a rock-bit bearing is controlled by the seal, the lubrication system, and the bearing itself; failure of any of these constitutes a bearing failure. Lubrication system failures are very rare; usually the bearing or the seal fails. For approximately 20 years, the O-ring has been the most popular seal. As Fig. 1 illustrates, it is squeezed between the stationary journal and rotating cutter (cone). This arrangement caused continuous wear at one or both of the contact surfaces, and this wear eventually allows drilling fluid to enter the bearing. Such wear is always present in a used bit, but significant bearing wear is rarely observed in the absence of seal leakage. Thus, when a bearing failure is experienced in an O-ring sealed bit, seal failure is normally considered to be the cause. Fig. 2 shows an alternative to the O-ring, the metal-face seal. This design provides a dynamic seal between the contacting faces of two metal rings supported and held in contact by compressed O-ring energizers. With this arrangement, the O-rings act as static seals and do not experience continuous rubbing. In addition, the high wear resistance and low frictional drag afforded by the metal rings permit these seals to operate effectively over a range of surface speeds and ambient temperatures much broader than the range of the O-ring. As a result, bearing surface wear is currently the most common cause of bearing failure with the metal seal. Sliding-contact wear is usually proportional to the product of contact pressure and distance traveled. Thus, seal life should be a function of pressure between sealing surfaces, bearing size, and bit revolutions. Similarly, bearing life should be a function of weight on bit (WOB), bearing size, and bit revolutions.