Roller cone core bit

The present invention relates to an improved roller cone core bit comprising a plurality of cone cutters facing inboard and a single cone cutter facing outboard. The single outboard facing cone cutter provides additional cutting elements for cutting a core. The single outboard facing cone cutter also creates an unbalanced loading condition on the core bit causing the bit to drill smoother than conventional reversed cone core bits.

BACKGROUND OF THE INVENTION 
This invention relates to an improved roller cone core bit used to cut a 
core of a subterranian formation. More particularly, the improved roller 
cone core drill bit includes a plurality of cone cutters facing inboard 
and a single cone cutter facing outboard. 
Conventional roller cone core bits include a plurality of inboard facing 
cone cutters. The cone cutters are frusto-conical in shape with rows of 
cutting elements, such as milled steel teeth or wear resistant tungsten 
carbide inserts. During drilling operations, the cone cutters cut a 
cylindrical core in addition to cutting the bottom and outer diameter of 
the borehole. With insert roller cone core bits, the nose inserts, located 
about the apex of the cone cutter, cut the cylindrical core. There are 
fewer inserts in the nose area than on the remainder of the core cutter. 
Thus, relatively few inserts actually cut the core. The remaining inserts 
cut the bottom of the borehole. When the bit is rotated, the nose inserts 
move slowly relative to the formation in comparison with the outer row of 
inserts that cut the gage diameter of the borehole. The slower moving nose 
inserts tend to drag across the formation which leads to accelerated wear 
on the inserts, especially in abrasive formations. Increased wear on the 
nose inserts leads to nose area failure which is one of the most common 
modes of failure in rotary cone core bits. 
To overcome the problem of nose area failures, practitioners in the coring 
industry increased the number of inserts cutting the core by reversing a 
plurality of cone cutters to face outboard. The outboard facing cone 
cutters, or reversed cones, had the rows with the most inserts cutting the 
core. The prior art reversed cone core bits were symmetrically balanced 
and did not impart a continuous lateral outward load on the borehole while 
coring. Conventional reverse cone core bits were symmetrically arranged 
with, for example, four cone cutters facing inboard and two cone cutters 
opposite each other facing outboard; with three cone cutters facing 
inboard alternating with three cone cutters facing outboard; or with two 
cone cutters facing inboard alternating with two cone cutters facing 
outboard. Due to the symmetrical arrangement, there was negligible 
continuous lateral loading against the borehole while coring. 
Conventional reversed cone core bits reduced the number of nose area 
failures by providing more inserts for cutting the core. However, 
reversing 2 of 4 cones or even 2 of 6 cones places a higher percentage of 
the available inserts than is necessary to the inner area of the borehole. 
While this improves the life of the bit adjacent the core, it diminishes 
the life of the bit at the outer areas by removing too many inserts from 
the zone where the largest volume of rock is being removed. Accordingly, 
conventional reversed cone cutters have an inadequate percentage of 
inserts, or cutting elements, to cut the remainder of the borehole. The 
useful life of the conventional reversed cone core bits suffered because 
of the insufficient number of inserts for cutting the remainder of the 
borehole. 
The present invention overcomes the problems associated with conventional 
roller cone core bits and conventional reversed cone core bits. The 
present invention includes a plurality of inboard facing cone cutters and 
a single outboard facing cone cutter. The single outboard facing cone 
cutter provides additional inserts for cutting the core, thereby reducing 
the potential for nose area failure of the core bit. At the same time, the 
present invention leaves more inserts for cutting the remainder of the 
hole than conventional reversed cone core bits. Unlike conventional 
reversed cone core bits, the bit of the present invention is not starved 
for cutters for cutting the remainder of the borehole. 
In addition to the above mentioned benefits, the present invention creates 
an unbalanced load on the bit, with the net result being a continuous 
lateral load imparted on the borehole by the bit. This unbalanced loading 
is created by the asymmetrical arrangement of the plurality of the inboard 
facing cones and the single outboard facing cone. As a result of the 
unbalanced load, one side of the bit will always be forced against the 
borehole wall causing the bit to run smoother than conventional reversed 
cone core bits. The unbalanced loading reduces the natural whirling action 
of the bit causing it to drill much smoother. A smoother drilling core bit 
reduces the likelihood of breaking and jamming the core in the core barrel 
and thus increases the likelihood of recovering a longer core section. The 
reduction of the whirling action of the bit is also beneficial because the 
whirling action tends to reduce the diameter of the recovered core. Thus, 
the roller cone core bit of the present invention also increases an 
operator's ability to recover a full gage core. Reduction in whirling also 
contributes to more efficient cutting action and longer bit life. The 
energy which would normally fuel the whirling action is now available for 
drilling. Whiffing causes lateral shock loading on rock bit bearings and 
stresses the seals. It also causes shock loading on the inserts which can 
result in early failure due to breakage. The lateral motion caused by 
whirling also causes more wear on the inserts. 
SUMMARY OF INVENTION 
The present invention relates to an improved roller cone core bit. More 
particularly, the improved roller cone core bit includes a plurality of 
cone cutters facing inboard and a single cone cutter facing outboard. The 
outboard cone cutter provides additional cutting elements for cutting the 
core. The single outboard cone cutter also creates unbalanced loading on 
the cone cutters which causes the core bit to drill smoother than 
conventional reversed cone core bits. The smoother drilling roller cone 
core bit of the present invention increases the likelihood that a longer 
continuous core can be cut and recovered.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, core bit 1 has an internally threaded box 2 on its 
upper end for securing the core bit to the core barrel and drill string. 
Core bit 1 has a plurality of journal segment arms 5 mounted on its 
lowermost end. A frusto-conical cone cutter is rotatably mounted and 
secured on a journal which extends downward from the bottom of each 
journal segment arm 5. The frusto-conical cone cutters drill a doughnut 
shaped hole in a subterranean formation 4, leaving a cylindrical plug or 
core 10 in the center. As drilling progresses, core 10 rises inside a 
hollow tube or inner core barrel (not shown) above the core bit 1 where it 
is captured and subsequently retrieved at the surface. Core catcher 12 
helps guide the core into the hollow tube or inner core barrel above the 
bit while being configured to prevent the downward movement of the 
captured core so that the core may be recovered at the surface. The 
various configurations of core catcher 12, along with its use with the 
inner core barrel, are well known in the coring industry. 
The upper end of the journal segment arms 5 are securely affixed by welding 
or other appropriate means to journal segment pocket 14 at the lower end 
of a cylindrical bit body 3. Core bit 1, as shown in FIG. 1, includes a 
plurality of cone cutters that are rotatably mounted on journals 20 with 
sliding beating surfaces. This is meant to include journal bearings either 
with or without journal bushings. Journal bearings carry the load on 
surfaces which slide relatively to each other. Although not illustrated, 
the present invention may also be utilized with roller cone cutters 
mounted on sealed or unsealed roller bearings. As shown in FIG. 2, core 
bit 1 includes a plurality of inboard facing cone cutters 8 and a single 
outboard facing cone cutter 9. The cone cutters have rows of tungsten 
carbide inserts for cutting the borehole and core. The tungsten carbide 
inserts are press fitted into circumferential rows of receiving apertures 
in the cone surface. The number of inserts on each row will depend on the 
circumference of the row. In general, more inserts can be used on a given 
row as the circumference of the row increases. While the preferred 
embodiment shown in the accompanying drawings illustrate wear resistant 
inserts for the cutting elements of the bit, the present invention may 
also be used with roller cone cutters having rows of milled steel teeth. 
Each journal segment arm 5 includes a passageway 22 for inserting retaining 
balls about journal 20. Passageway 22 also serves as a channel for 
supplying lubricant to the bearing surfaces. A lubricant reservoir 25 is 
also provided in each journal segment arm 5 for maintaining a sufficient 
quantity of lubricant for the bearing surfaces. Details of reservoir 
systems are commonly understood in the art and are not illustrated in this 
sketch. A cone/journal seal 30 protects the bearing structure from drill 
cuttings and other foreign debris. The cone/journal seal 30 may be an 
elastomeric packing ring, such as an o-ring seal, or other suitable 
structure. 
The embodiments shown in FIGS. 1 and 3 include three inner rows of active 
inserts, collectively illustrated as 15, on outboard facing cone 9 for 
cutting core 10. The innermost row 35, referred to as the core heel row, 
includes the core heel inserts that cut core 10 to gage diameter. Inboard 
facing cones 8 have nose area inserts 36 that also cut core 10 to gage 
diameter. However, since more cutters can be arranged on core heel row 35 
than on the nose area of the inboard facing cones, core bit 1 is less 
likely to fail from nose area failure than conventional roller cone core 
bits. By way of example, cone 9 may include approximately 70 active 
inserts in rows 15 for cutting a core, while the inboard facing cones may 
have only 5 or 6 nose area inserts for cutting the core. There are enough 
active inserts on the reversed cone to continue cutting the core even 
after the nose area inserts on the inboard facing cones are worn away. The 
number of inserts described in the above example is for illustrative 
purposes only and is not meant to limit the present invention in any way. 
The primary purpose of reversed cone 9 is to provide additional cutters for 
cutting core 10. Cone 9, as illustrated in FIG. 3, also includes 
additional rows of smaller inactive inserts 40. These inactive inserts 
serve primarily to protect the integrity of the apex of cone 9 from wear. 
Inserts 40, as illustrated, are not designed to cut virgin formation and 
hence are referred to as inactive inserts. Other embodiments of the 
invention, however, may have active cutters for cutting virgin formation 
on all rows of cone 9. 
Superimposed on an inboard facing cone 8 in FIG. 1 are the wear resistant 
inserts for the other inboard facing cones. The superimposed inserts on 
cone 8 in FIG. 1 illustrates the number of rows of inserts on the inboard 
facing cones that cut the bottom of the borehole. As shown in FIG. 1, 
there is a sufficient number of inserts on the inboard facing cones to 
adequately cut the remainder of the borehole. Unlike conventional reversed 
cone core bits, the remainder of the borehole is not starved for cutters. 
Accordingly, the core bit of the present invention will last longer than 
conventional reversed cone core bits. 
The single outboard facing cone cutter 9 causes an unbalanced loading 
condition on the core bit 1. As a result, a continuous lateral load is 
imparted on one side of the borehole by the bit. The unbalanced loading 
causes the core bit to drill smoother than prior art roller cone core 
bits. Conventional roller cone core bits tend to whirl about the 
longitudinal axis of the borehole. This whirling action is detrimental for 
several reasons. The whirling action tends to reduce the diameter of the 
core. The whirling action also increases the likelihood of the core 
breaking and jamming inside the core barrel, thus reducing the amount of 
recoverable core. The unbalanced loading created by the single reversed 
cone of the present invention reduces the whirling action thus increasing 
the likelihood of recovering longer cores of gage diameter. 
Due to the side loading of the bit, core bit 1 requires stabilization 
within the borehole. Preferably, core bit 1 includes stabilizer blades 
that are spirally oriented about the longitudinal axis 40 of bit body 3. 
Preferably, the spirally oriented stabilizer blades have up to 360.degree. 
of wall contact with a full gage borehole. Spirally oriented stabilizer 
blades for roller cone core bits are disclosed in U.S. Patent application 
Ser. No. 08/007,257, incorporated herein by reference. FIG. 3 illustrates 
the core bit of the present invention with stabilizer blades 42 spirally 
oriented about the longitudinal axis of bit body 3. The stabilizer blades 
42 are fixedly attached to the circumference of bit body 3 and extend 
substantially from the top of core bit 1 to the top of the journal segment 
arms 5. The spiral stabilizer blades 42 extend radially from the core bit. 
Stabilizer blades 42 as shown in FIG. 3, radially extend to substantially 
the gage diameter of the core bit. Between the stabilizer blades are junk 
slots 44 which provide passageways for cuttings removal by the drilling 
fluid. Alternatively, a stabilizer may be mounted on the outer core barrel 
(not shown) near the core bit. 
It will be understood by those skilled in the art that some variations and 
modifications can be made without departing from the spirit and scope of 
the invention as defined herein and in the appended claims. 
By way of example, a variation of this bit is also being used to open a 
small cored hole to a larger diameter borehole. This bit requires a large 
hole in its center through which the smaller coring bit and coring string 
can pass. Thus, instead of being connected to a conventional core barrel 
assembly, this embodiment has a sufficiently large center hole extending 
longitudinally through the bit body for passage of a separate coring 
assembly through the center of the bit. Once the smaller coring assembly 
has cut a core and been retrieved, the bit can be rotated as a hole opener 
to open the cored hole to a larger diameter hole. By way of example, a 12 
inch diameter bit having a plurality of inboard facing cone cutters and a 
single outboard facing cone cutter is used as a hole opener for opening a 
4 inch cored hole to a substantially 12 inch diameter borehole. The 
reversed cone provides smoother operation during normal drilling and 
provides additional cutting inserts in instances when the cored hole 
deviates from the path of the 12 inch bit.