1. Field of the Invention
The invention relates generally to seal assemblies for sealing between a rotating and a static member. In one aspect, and more particularly, the invention relates to seals for rolling cone bits as used to drill boreholes for the ultimate recovery of oil, gas or minerals. Still more particularly, the invention relates to elastomeric seals that seal and protect the bearing surfaces between the rolling cone cutters and the journal shafts on which they rotate.
2. Description of the Related Art
An earth-boring drill bit is typically mounted on the lower end of a drill string. With weight applied to the drill string, the drill string is rotated such that the bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.
A typical earth-boring bit includes one or more rotatable cone cutters. The cone cutters roll and slide upon the bottom of the borehole as the drillstring and bit are rotated, the cone cutters thereby engaging and disintegrating the formation material in their path. The rotatable cone cutters may be described as generally conical in shape and are therefore referred to as rolling cones.
Rolling cone bits typically include a bit body with a plurality of journal segment legs. The rolling cones are mounted on bearing pin shafts (also called journal shafts or pins) that extend downwardly and inwardly from the journal segment legs. As the bit is rotated, each cone cutter is caused to rotate on its respective journal shaft as the cone contacts the bottom of the borehole. The borehole is formed as the action of the cone cutters removes chips of formation material (“cuttings” or “drilled solids”) which are carried upward and out of the borehole by the flow of drilling fluid which is pumped downwardly through the drill pipe and out of the bit. Liquid drilling fluid is normally used for oil and gas well drilling, whereas compressed air is generally used as the drilling fluid in mining operations.
Seals are provided in glands formed between the rolling cones and their journal shafts to prevent lubricant from escaping from around the bearing surfaces and to prevent the cutting-laden, and thus abrasive, drilling fluid from entering between the cone and the shaft and damaging the bearing surfaces. When cuttings are conveyed into the seal gland, they tend to adhere to the gland and/or seal component surfaces, and may cause undesirable increased deflection and wear to the seal components. Moreover, the cuttings can accelerate abrasive wear of all seal components and of the bearing surfaces.
In oil and gas drilling, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed before reaching the targeted formation. This is the case because each time the drill bit wears out or fails as a borehole is being drilled, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section by section, in order to replace the bit. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. It is therefore advantageous to maximize the service life of a drill bit. Accordingly, it is always desirable to employ drill bits that will be durable enough to drill for a substantial period of time with acceptable rate of penetration (ROP).
The durability of a bit and the length of time that a drill bit may be employed before it must be changed depends upon numerous factors. Importantly, the seals must function for substantial periods under extremely harsh downhole conditions. The type and effectiveness of the seals greatly impact bit life and thus, are critical to the success of a particular bit design.
One cause of bit failure arises from the severe wear or damage that may occur to the bearings on which the cone cutters are mounted. These bearings can be friction bearings (also referred to as journal bearings) or roller type bearings, and are typically subjected to high drilling loads, high hydrostatic pressures, and high temperatures.
As previously mentioned, the bearing surfaces in typical bits are lubricated, and the lubricant is retained within the bit by the seals. The seal is typically in the form of a ring, and includes a dynamic seal surface, that is placed in rotating contact against a non-rotating surface, and a static seal surface that engages a surface that is stationary with respect to the seal ring. Although the bit will experience severe and changing loading, as well as a wide range of different temperature and pressure conditions, the dynamic and static seal surfaces must nevertheless remain sealingly engaged in order to prevent the lubricant from escaping and/or cuttings from entering the lubricated areas. These seals should perform these duties throughout the life of the bit's cutting structure.
In one typical arrangement, the seal includes a static seal surface adapted to form a static seal with the interior surface of the roller cone, and a dynamic seal surface adapted to form a dynamic seal with the journal shaft upon which the roller cone is rotatably mounted. The seal must endure a wide range of temperature and pressure conditions during the operation of the drill bit and still prevent lubricants from escaping and/or contaminants from entering the journal bearing. Elastomer seals known in the art are conventionally formed from a single type of rubber or elastomeric material, or may be made of two or more materials bonded together.
The rubber or elastomeric material selected to form the seal for the journal bearings has a particular hardness, modulus of elasticity, wear resistance, temperature stability, and coefficient of friction, among other properties. Additionally, the particular geometric configuration of the seal (along with the dimensions of the seal gland) produces a selected amount of seal deflection that defines the contact pressure and seal footprint applied by the dynamic and static seal surfaces against respective journal bearing and roller cone surfaces.
The wear, temperature, and contact pressures encountered at the dynamic seal surface are different than those encountered at the static seal surface. Therefore, the type of elastomeric material and the geometry that is selected to form each seal surface is aimed at satisfying the particular operating conditions experienced by the different dynamic and static seal surfaces.
In certain prior art bits, the elastomeric seal rings are generally adapted to form static seals on outer surfaces and dynamic seals on inner surfaces thereof In such bits, the OD seal surface is arranged to form a static seal with an adjacent and concentric surface of a seal gland (where the seal gland is formed on an internal surface of a roller cone). During operation, localized temperature increases, caused by inadequate lubrication or abrasive penetration, may result in the ID seal surface becoming static by sticking to the journal shaft, and the OD seal surface then becoming dynamic. When rotation occurs at the OD seal surface, which is usually formed from a relatively soft elastomer and has a relatively poor wear resistance, the OD seal surface experiences severe wear, and the seal may fail after a short time.
The service life of bits equipped with such elastomeric seals is generally limited by the ability of the seal material to withstand the different temperature and pressure conditions at each dynamic and static seal surface. Where such seal components experience damage, the lubricant is able to escape, and cutting-laden drilling fluid is allowed to enter the seal gland causing still further deterioration and damage to the seal components. Eventually, enough cuttings may pass into the journal gap and/or enough lubricant may be lost from the bearing area such that rotation of the cone cutter is impeded and drilling dynamics are changed, eventually requiring the bit to be removed from the borehole. Accordingly, protecting the integrity of the seal is of utmost importance.
Additionally, to provide the appropriate sealing pressure and contact footprint, it is imperative that the seal and the seal gland be precisely manufactured. For example, if the gland is too large or the seal too small, the appropriate squeeze on the seal will not be provided and, in turn, the desired seal footprint and sealing pressure on the journal surface will be lacking. In such instances, the seal will not perform its intended function and the bit may prematurely fail. Likewise, if the seal is too large or the gland too small, an excessive sealing pressure and footprint may result, causing excessive heating and thermal failure of the seal. Once again, this can lead to bit failure. Accordingly, for these reasons, the seals must be precisely molded and the seal glands precisely machined to create the desired contact pressure and footprint on the journal shaft.
The requirement for the elastomeric seal to provide the precise contact pressure and footprint against the adjacent sealing surfaces creates difficulties for bit manufacturers. For example, an optimal seal design for a particular application may indicate that an elastomeric seal with a non-conventional or complex geometric profile be employed in the bit. This, in turn, may require a difficult-to-machine seal gland be formed in order to retain the non-conventional seal. In this instance, manufacturing the bit could be extremely expensive or even cost prohibitive, requiring that a compromise be made by the bit designer in which the bit design would surrender certain features desirable for good seal performance in order to ease manufacturing difficulties. As another example, for a given size of bit, the different rock formations and depth of borehole in which the bits are used may dictate different sealing pressures and footprints for these bits. Even for the same size bit, the manufacturer may be required to machine many types of seal glands for the same size of seals and bits, resulting in an increase in manufacturing cost. Still further, bit manufacturers make and inventory a wide variety of bit designs and, for each such design, there may be a relatively large number of sizes of such bits. In turn, the differing bit sizes require the manufacturer to make and inventory a relatively large number of cone cutters and seals. Depending upon the application and the particular design, the manufacturer may be required to manufacture a large number of O-ring seals and corresponding seal glands to meet its various requirements. In turn, this leads to the manufacturer being required to make and inventory a large number of seals of substantially similar construction and materials, but of a myriad of cross-sectional areas. Assembly of such bits must be carefully accomplished to be sure of identifying correctly the precise seal that is required for a particular bit that is being manufactured, and ensuring that the appropriate seal is installed in the seal gland. The manufacturing is further complicated and made more expensive by the requirement that this large number of differently-sized seal rings be molded and, once completed, retained in inventory. Similarly, a variety of different milling programs or procedures must be maintained in order to properly machine the correct seal gland for the particular cone being manufactured.
It is therefore desirable that a new, long lasting and effective seal assembly be devised that maintains the appropriate contact pressure and footprint, to provide the appropriate seal on the static and dynamic sealing surfaces. In addition, it would be desirable that the seal assembly allow drill bit manufacturers to manufacture a wide range of bit sizes while minimizing or reducing the number of different sized seals and seal glands that must be made and kept in inventory.