Joints and connections – Interfitted members – Radially interposed shim or bushing
Reexamination Certificate
2000-06-19
2001-07-24
Browne, Lynne B. (Department: 3629)
Joints and connections
Interfitted members
Radially interposed shim or bushing
C285S123500
Reexamination Certificate
active
06264395
ABSTRACT:
FIELD OF THE INVENTION
This invention generally pertains to apparatus for holding pipe or other tubular goods in a vertical position and, more particularly, to such apparatus which is useful in oilfield operations for drilling, setting casing or placing or removing any tubular goods from a wellbore. Even more particularly, the purpose of this invention is to improve the strength of commercially available drill pipe slip assemblies and to develop a method to manufacture new drill pipe slip assemblies with improved strength.
In the drilling or workover of oil and gas wells, it is necessary to thread together numerous links of tubular goods, or pipe. These could form either a drill string which rotates a bit at the bottom thereof, or a pipe conduit such as production tubing or well casing which is placed and cemented in the wellbore to prevent its walls from collapsing. In the drilling operation, at least some of the weight of the pipe string extending into the wellbore is supported by a traveling block and tackle arrangement from a derrick which extends upwardly from the floor of the drilling rig.
When it is necessary to add or remove additional pipe to or from the top end of the drill string, the rotary motion of the drill string is stopped and it is suspended at the floor of the drilling rig while an additional pipe section is threadedly connected to the uppermost pipe section in the drill string. Alternatively, it may be unthreaded and removed from the uppermost pipe section in the drill string. In these instances, the drill string is typically suspended by a slip assembly which is mounted in the floor of the drilling rig and through which the drill string extends downwardly Into the wellbore. Referring to
FIG. 1
, a prior art slip assembly comprises a slip bowl
56
which is typically installed in a table bushing
57
and which has a tapered inner surface having a cylindrical hole through which the pipe
60
at the upper end of the drill string extends. The slip assembly usually also includes a plurality of slip segments
74
, typically three, having external tapered surface
74
(
a
), which conform to the shape of the inner surface of slip bowl
56
as shown in FIG.
1
. Each such slip segment has a plurality of dies, together forming an internal cylindrical surface within the assembly. Thus, each slip segment includes gripping elements directed toward the pipe to be contained within the slip assembly. When the pipe is lowered within the interior of the slip assembly, a camming action between the slip segments of the assembly, and their respective dies, forces the slip segments, and their respective dies inwardly into the pipe, thus gripping it and suspending it from the slip assembly.
When drill pipe is so suspended, an additional joint of pipe may be threadably engaged with the uppermost pipe section on the drill string. The slip segments are then removed from the slip bowl so that the dies are not in engaging contact with the pipe, and rotary motion is imparted to the drill string to continue drilling.
Also during the drilling operation it may be necessary to remove the drill string to change the bit, to add casing to a portion of the well, or for other reasons. While removing the drill string, rotary motion is stopped and the drill string is suspended in the slip assembly. Thereafter, an elevator which is suspended from the traveling block, in the block and tackle arrangement mentioned previously, is used to grip the pipe just above the slip assembly and the slip segment dies of the slip assembly are disengaged. The traveling block is then raised, the slip segments are reinstalled and the stand pipe extending above the drilling rig floor may be unthreaded and removed. Thereafter, the elevator grasps the pipe extending from the slip assembly, the slip segments are again released from contact, and the traveling block again raised. This process may be repeated until the drill string is entirely removed from the wellbore.
Drill pipe slip assemblies are designed to allow supporting of an oil well drill string at virtually any location along the length of the drill string. In this way, the drill pipe and suspended weight can be repeatedly moved up or down and secured structurally to the drill floor as needed during drilling operations. The slip assemblies are typically composed of a “bowl” which is located in the rotary table that includes a tapered bore. The tapered bore is such that the bowl is smaller in diameter at the bottom than the top. Within the tapered bore, a plurality of (typically three) long circumferential gripping assembly segments are located that are formed with an outer taper that matches the tapered bore of the bowl. These slip segments are interconnected by hinges so that the segments maintain a consistent axial relation to one another and may be simply opened and lifted away from the pipe by rig workers when not needed.
The slip segments with gripping assemblies, when installed in the slip bowl, form a cylindrical hole in the center that is roughly the same size as the drill pipe body which is manually lowered into the annular area between the bowl and the drill string when it is desired to suspend the drill string. The assembly naturally grips onto the pipe as it is wedged in the annular taper angle formed between the bowl and the slip segments.
Within each circumferential slip segment, multiple hardened “dies” are located for contact with the drill pipe surface. In one known example, there are three axial rows of six dies for a total of 18 hardened dies secured within each slip segment. These hardened dies typically include “tooth” profiles on the pipe interface surface that enhance the gripping capability of the dies on the pipe by actually penetrating the pipe surface slightly. The hardened dies are necessary because the contact stresses with the pipe can be quite high and the dies are subject to considerable wear.
As the oil industry seeks to drill in ever-deeper offshore waters, the length and weight of the longest drill strings in service have increased accordingly as well as the weight of the suspended loads such as casing strings and liners. As a result of the high repeated loads experienced in many of the deep well applications, bothersome cracking has been noted in the slip segments in the critical “nose” areas that support the loads from the dies. If these cracks are allowed to grow to the point of complete failure to support the dies, the result could be the loss of the drill string downhole as well as loss of the suspended load. This could result in huge remedial costs, or complete loss of the well.
Drilling supervisors choose to replace the slip assemblies at the first sign of cracking, usually in the nose area, to prevent the worst failure scenario from occurring. This is expensive and time consuming.
The problem we have found is in the conventional method used to secure the dies with the three slip segments. The conventional practice for securing the dies is to machine axial “dovetail” shaped grooves in the slip segments. The hardened dies are formed with a mating profile to the dovetail grooves so that the dies may be simply inserted into the dovetail grooves and stacked on top of one another. In a typical slip segment, there are three internal longitudinal dovetail grooves each containing six “stacked” dies. A segmented die retainer ring is bolted above the top die in each groove so as to contain the dies from upward movement and release from their respective grooves.
This arrangement allows the dies to be quickly changed, a welcome convenience feature. However, this arrangement also relies on the load from each die to be supported by the die immediately below it such that, within each axial row, the load accumulates such that the supporting slip segment material below the lowest die (critical nose region) carries the load from he entire set of dies in each axial row.
Another problem with this construction is that the dies have some “slack” or free movement axially in the dovetail grooves and the friction resulting between individual
Allamon Jerry P.
Miller Jack E.
Allamon Jerry P.
Browne Lynne B.
Eriksen Clarence E.
Stafford McGlinchey
Walsh John B.
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