Pipe joints or couplings – With casing – lining or protector
Reissue Patent
1999-04-27
2001-05-08
Nicholson, Eric K. (Department: 3627)
Pipe joints or couplings
With casing, lining or protector
C175S325100, C148S519000
Reissue Patent
active
RE037167
ABSTRACT:
This invention relates to drill pipe generally and, in particular, to drill pipe used in drilling deep wells, such as wells over 10,000 ft. deep.
Oil and gas producers are having to drill deeper and deeper wells as they strive to maintain or increase their reserves of oil and gas. Wells 10,000 to 15,000 ft. deep have been common for many years. Today, wells 28,000 to 30,000 ft. deep are becoming more commonplace.
Drill bits on the end of a drill string drill the wells. Drill bits have a finite life and have to be replaced periodically. This means that the entire string of pipe must be pulled from the well to allow a new bit to be installed on the lower end of the string after which the drill string is run back into the hole. This operation is referred to as a “round trip” or “trip” for short. During a trip the pipe will usually be pulled in stands of multiple joints. Each stand is unscrewed from the pipe string and set back in the derrick until the stand is subsequently reconnected into the string as the pipe is run back into the hole. After each stand is pulled from the well, the portion of the pipe string that remains in the hole is supported in the rotary table by slips as the stand is disconnected and set back in the derrick. The same situation exists when each stand is being reconnected into the string. The slips have inserts with teeth that are forced against the wall of the pipe by the wedging action between the slips and the slip bowl. To support the pipe, the teeth will have to cut notches in the wall of the joint of pipe in the slips. The pipe is not only subjected to notching by the slips during a trip but also whenever a joint of pipe is added to the string during drilling operations. This operation begins with the weight of the pipe string being supported by slips that engage the top joint in the string just below the upper tool joint while the kelly or power swivel is disconnected from the top joint. Another joint is then connected to the top joint and the pipe string is lowered until the new joint can be supported by the slips. The kelly or power swivel is then reconnected and drilling resumes.
In addition to the problem of the slip marks or notches, the slip area of a joint of drill pipe is subjected to increasing compressive hoop stress when supporting a string of drill pipe in the rotary table due to the increasing length and weight of drill strings as wells are being drilled to greater depths.
Thus, the slip area of the pipe, i.e., the area of the pipe engaged by the slips, usually an area about 24 to 36 inches long the top of which is about 28 to 32 inches below the upper tool joint, is repeatedly subjected to notching by the slip teeth. These notches or slip marks accumulate over time and eventually require the pipe to be downgraded because of reduced wall thickness or retired because of cracks in the slip area. Slip marks can also result in a premature failure of the drill pipe. At a time when oil and gas prices are low, preventing drill pipe failures may make the difference in showing a profit or a loss in drilling a new well. The most common cause of drill pipe failure is fatigue.
It is well known that steel fails under repeated loading and unloading, or under reversal of stress, at stresses smaller than the ultimate strength of the steel under static loads. The magnitude of the stress required to produce failure decreases as the number of cycles of stress increase. This phenomenon of the decreased resistance of steel to repeated stresses is called “fatigue”. Drill pipe that is rotated while bent is subjected to a reversal of stress every 180° of rotation. As long as the stress is uniform in the pipe and below the endurance limit of the steel, the pipe will last forever. theoretically. If, however, there is a stress concentration produced by a change in the cross-section or by a local defect such as a notch cut in the wall of the pipe by slip teeth, a fatigue crack can appear. Once formed, the crack spreads due to the stress concentrations at its ends. This spreading progresses under the action of the alternating stresses until the cross-section becomes so reduced in area that the remaining portion fractures suddenly under the load.
New upset designs and new tool joint designs have improved the fatigue life of drill pipe, but we still have slip damage, which prevents the full benefit of these innovations from being achieved. It so happens that slip damage is located in or near the high stress area of the upset fadeout on the box end or the pin end if the pipe is run with the pin up. As discussed above, slips are designed to bite into the pipe and hold it from sliding down the hole, while a connection is being made up. Slip damage can be more severe if the pipe is allowed to turn in the slips or if the slips and slip bowl are not properly maintained. Slip cuts cause stress risers which in turn generate cracks and fatigue failures.
U.S. Pat. No. 3,080,179 that issued Mar. 5, 1963 to C. F. Huntsinger proposed a drill pipe construction that would include a thick-walled “protector tube” in the slip area of the drill pipe to solve the slip damage problem. Specifically, Huntsinger proposed:
“an elongate steel protector tube extending downwardly from said upper tool joint and secured to the upper end of said main portion, said main portion having a much lesser wall thickness throughout substantially its entire length than said protector tube and being made of a steel having substantially greater hardness and unit tensile and torsional strength than the steel of said protector tube, . . . said protector tube being disposed in said drill pipe section at a location for engagement by supporting slips at the top of the well bore, the cross-sectional area of said protector tube being such that the total tensile and torsional strength of said protector tube is no less than the total tensile and torsional strength of said main portion, whereby said protector tube has less notch sensitivity and greater resistance to inward crushing than said main portion.”
Huntsinger obtains “less notch sensitivity” in the protector tube by reducing the hardness of the metal in the protector tube below that of the main portion of the drill pipe. At line 62, col. 6, Huntsinger states with reference to FIGS.
2
and
5
:
“The fact that the protector tube portion
20
a is not as hard as the main portion
18
a of the drill pipe section renders it less susceptible to notching, minimizing, if not fully eliminating, fatigue failures.” (emphasis added)
In fact, Huntsinger emphasized many times in his patent that the primary improvement of his patent came from the reduction of hardness and the subsequent decrease in the notch sensitivity of the material in the protector tube. Huntsinger recommends a protector tube made from grade E tubing with a chemistry which is equivalent to AISI 1040 carbon steel. This is a high carbon, normalized material that is relatively soft. Its micro-structure has large grains, which result in the metal having low impact strength (low toughness). Drill pipe slips would cut deeply into this material greatly reducing the wall thickness of a protector tube made of this weak material and cause it to fail in fatigue in a short time.
It is therefore an object and feature of this invention to provide a protector tube for drill pipe that is made of steel having high strength and high hardness with a small, close knit grain size (called martensite). This hard material (30-38 HRc) reduces the penetration of the slips and thereby increases the wall thickness under the shallow notches, made by the slips, when compared to the soft material recommended by Huntsinger. This increased area would result in less bending stress per unit area. The improved microstructure of the martensitic material will also be resistant to crack initiation and its high toughness will be more resistant to crack propagation. Fatigue testing of full scale test specimens, with the protector tube of this invention, showed that with slip damage, it will last more than 600% longer than standard drill pipe with
Browning Bushman
Grant Prideco, Inc.
Nicholson Eric K.
Torres Carlos A.
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