Pipe joints or couplings – Particular interface – Tapered
Reexamination Certificate
2002-09-25
2004-06-08
Nicholson, Eric K. (Department: 3679)
Pipe joints or couplings
Particular interface
Tapered
C285S390000, C285S055000, C285S094000
Reexamination Certificate
active
06746057
ABSTRACT:
TECHNICAL FIELD
This invention relates to a threaded joint for steel pipes and particularly oil well pipes which has improved galling resistance, rust-preventing properties and gas tightness, and which can be used without the need to apply a liquid lubricating grease containing a heavy metal powder such as a compound grease.
BACKGROUND ART
Oil well pipes such as tubing and casing used in the drilling of oil wells are typically connected to each other by threaded joints. In the past, the depth of oil wells was typically 2000 to 3000 meters, but more recently, the depth of oil wells may reach 8,000 to 10,000 meters in offshore oil fields and other deep wells. A threaded joint for oil well pipes is subjected to various forces in its environment of use, including loads such as axial tensile forces caused by the weight of the oil well pipe and the threaded joint, the combination of internal and external pressures, and geothermal heat which increases as the depth increases. Therefore, a threaded joint needs to be able to maintain gas tightness without breakage in such environments.
During the process of lowering tubing or casing into a well, a joint which is once fastened for connection often needs to be loosened and then re-fastened. API (American Petroleum Institute) requires that there be no occurrence of galling, which is unrecoverable seizing, and that gas-tightness be maintained even if fastening (make-up) and loosening (break-out) are repeated ten times for a tubing joint and three times for a casing joint.
Oil well pipes are generally connected using a threaded joint with a pin-box structure having a metal to metal contact portion forming a good seal. In this threaded joint, typically, a pin is formed on the outer surface of each end of an oil well pipe, the pin having an externally threaded portion and a metal contact portion without threads. Such a metal contact portion is hereunder referred to as an “unthreaded metal contact portion”. A mating box is formed on the inner surface of a separate, sleeve-shaped coupling member, and it has an internally threaded portion and an unthreaded metal contact portion so as to mate with the corresponding threaded and unthreaded portions of the pin. When the pin is screwed into the box for fastening, the unthreaded metal contact portions of the pin and the box are made to tightly contact each other to form a metal seal.
In order to guarantee adequate sealing properties by the metal seal of a threaded joint in the environment of an oil well pipe, an extremely high surface pressure which may exceed the yield point of the material for the threaded joint must be applied to the unthreaded metal contact portions during fastening. This high pressure makes it easy for galling to take place. Therefore, prior to fastening, a lubricating grease called a compound grease, which is a viscous, flowable liquid, is applied to the metal contact portions and the threads in order to impart an increased galling resistance and gas tightness. Since the compound grease has good rust-preventing properties, it is frequently applied to a threaded joint before shipping the joint so as to prevent the joint from rusting before use in an oil well.
It has been proposed in the prior art to subject a threaded joint to surface treatment in order to improve the retention of a compound grease and impart decreased friction.
For example, Japanese Unexamined Patent Publication No. 61-79797 (1986) discloses a threaded joint having a threaded portion plated with Zn or Sn and a metal seal (unthreaded metal contact portion) plated with gold or platinum. Japanese Examined Patent Publication No. 3-78517 (1991) discloses a pipe joint having a coating of a synthetic resin which contains 20-90% of molybdenum disulfide having a particle diameter of 10 micrometers or less dispersed therein. Japanese Unexamined Patent Publication No. 8-103724 (1996) discloses a method for the surface treatment of a steel pipe joint comprising forming a lower manganese phosphate chemical conversion coating and an upper molybdenum disulfide-containing resin coating. Japanese Unexamined Patent Publication No. 8-105582 (1996) discloses a method for the surface treatment of a pipe joint comprising forming a first nitride layer, a second iron or iron alloy plated layer, and a third, uppermost molybdenum disulfide-containing resin coating.
Japanese Unexamined Patent Publication No. 62-258283 (1987) discloses a joint for oil well pipes which has a porous plated coating of zinc or a zinc alloy at least on the surface of its metal seal portion, the coating being formed by blast plating which is performed by blasting with particles having a core of iron or an iron alloy coated with a zinc or zinc alloy layer. It is described therein that the plated coating does not suffer from delayed fracture caused by hydrogen embrittlement and has good retention of a lubricant due to its porous nature, resulting in the advantage that gas tightness and galling resistance can be attained even with oil well pipes of a high-strength, high alloy steel. Thus, the coating formed by blast plating is contemplated using a compound grease as a lubricant.
All the threaded joints disclosed in the above-described patent publications are supposed to use with a compound grease to assure sufficient resistance to galling. However, the use of a compound grease involves problems with respect to the environment and gas tightness at high temperature as described below.
A compound grease contains a large amount of a powder of a heavy metal such as lead, zinc, or copper. Since a part of a compound grease applied to a threaded joint is discharged into the surrounding environment by washing off or by running out during fastening, the use of a compound grease raises a concern of causing environmental pollution. In addition, the application of compound grease worsens the work environment.
Recently, high-temperature oil wells having a temperature as high as 250-300° C. due to an increased depth, as well as steam-injected oil wells into which steam at a high temperature close to the critical temperature (e.g., around 350° C.) is injected in order to improve oil recovery have increased. Threaded joints for oil wells which are used in such a high-temperature environment are required to have a good heat resistance, as evidenced by maintaining gas tightness in a test where a joint which has been fastened is kept at a temperature above 350° C. for a certain period and then subjected to loosening and re-fastening. With a compound grease, the lubricity may decrease at such a high temperature due to evaporation of one or more constituents of the grease, thereby leading to a failure to attain a desired gas tightness when the loosened joint is re-fastened in the test.
In order to eliminate the need to apply a compound grease, a threaded joint having a solid lubricating coating has been proposed. For example, a threaded joint having a solid lubricating coating which comprises a resin such as an epoxy resin having molybdenum disulfide as a solid lubricant dispersed therein is disclosed in Japanese Unexamined Patent Publications Nos. 8-233163 (1996), 8-233164 (1996), and 9-72467 (1997).
However, such a solid lubricating coating is inferior to a compound grease with respect to ability to protect a threaded joint from rusting under field conditions from shipment of the threaded joint to the use thereof in an oil well. Therefore, the threaded joint is apt to rust and thus form blisters or cause debonding in the solid lubricating coating, which in turn makes fastening of the joint unstable, leading to occurrence of galling or a decrease in gas tightness.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a threaded joint for steel pipes having improved galling resistance, gas tightness, and rust-preventing properties, which can assure gas tightness in its metal seal portion after being subjected to fastening and loosening repeatedly without application of a heavy metal powder-containing liquid lubricating grease such as a compound grease.
Another ob
Anraku Toshiro
Goto Kunio
Nicholson Eric K.
Sumitomo Metal Industries Ltd.
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