Oil well pipes such as tubing or casings are used for drilling an oil well to mine crude oil or gas oil. The oil well pipes are connected (fastened) to each other generally using a tubular threaded joint. The depth of an oil well according to the related art was 2000 m to 3000 m. The depth of a deep oil well such as an undersea oil well in recent years may reach 8000 m to 10,000 m.
On the tubular threaded joint for the oil well pipes, a load called an axial tensile force caused by the masses of the oil well pipes and the joint themselves under the use environment, a complex pressure such as internal and external surface pressures, and geothermal heat are exerted. Therefore, for the tubular threaded joint for the oil well pipes, maintaining gastightness is required without breakage under such severe environments.
A typical tubular threaded joint (called special threaded joint) used for tightening the oil well pipes has a pin-box structure. The pin-box structure is constituted by a member called a pin which is formed at both end portions of the oil well pipes, and a member called a box which is formed on the inner surfaces of both sides of a threaded joint component (coupling). The pin has male threads. The box has female threads. Seal portions are respectively formed at the outer peripheral portion near the end surface on the tip end side of the pin from the male threads and at the inner peripheral surface of the base portion of the female threads of the box. Shoulder portions (also called torque shoulders) are respectively formed at the end surface of the tip end of the pin and at the corresponding innermost portion of the box.
One end (pin) of the oil well pipe is inserted into the threaded joint component (box) and the male threads and the female threads are fastened until the shoulder portions of the pin and the box abut on each other and interfere with each other at an optimum torque. Accordingly, the seal portions of the pin and the box come into close contact with each other and form a metal-to-metal seal, thereby ensuring gastightness of the threaded joint. The seal portions and the shoulder portions form unthreaded metal contact portions of the tubular threaded joint. The unthreaded metal contact portions and the threaded portions (the male threads and the female threads) become surfaces (called contact surfaces of the tubular threaded joint) that come into contact with each other during fastening. An example of the special threaded joint is described in Patent Document 1 and Patent Document 2.
During an operation of lowering the oil well pipe to an oil well, due to various problems, the oil well pipe is temporarily pulled up from the oil well, and a threaded joint which is fastened once is loosened to release the fastening of the threaded joint. There is a case that the oil well pipe in which the fastening of the threaded joint is loosened is lowered after re-fastening the joint. API (The American Petroleum Institute) requires seizure resistance in such a sense that even when fastening (make-up) of a threaded joint and loosening (break-out) thereof are performed a plurality of times, seizure called galling does not occur and gastightness is maintained. For example, ten successful times are required for a tubing joint, and three successful times are required for a casing joint.
At the time of fastening, in order to achieve enhancement in seizure resistance and gastightness, a viscous liquid lubricant (grease lubricant), that is called a “compound grease”, containing a large amount of heavy metal powder is applied to the contact surfaces of the threaded joint. Such a compound grease is specified in the Standard API BUL 5A2.
For the purpose of enhancing the maintenance of the compound grease and improving lubricity, surface treatments for a single layer or two or more layers, such as a nitriding treatment, various plating processes including zinc-based plating and dispersion plating, and a phosphate chemical conversion treatment are performed on the contact surfaces of the threaded joint. However, there is a problem with the use of the compound grease in that there is concern about a harmful effect on the environment or human bodies as described below.
The compound grease contains a large amount of heavy metal powder such as zinc, lead, and copper. Particularly, there is a possibility that harmful heavy metals such as lead that is contained in the compound grease may have a harmful effect on the environment, marine organisms, and the like. Further, an operational environment is worsened by an operation of applying the compound grease, and there is a concern about a harmful effect on human bodies.
In recent years, upon the opportunity that OSPAR Convention (the Oslo and Paris Conventions, OSPAR) regarding the prevention of ocean pollution of the northeast Atlantic Ocean has taken effect since 1998, environment restrictions have strictly proceeded on a global scale. Accordingly, the use of the compound grease is also restricted in some regions. Therefore, in the operation of drilling a gas well or oil well, in order to avoid the harmful effect on the environment or human bodies, a threaded joint capable of exhibiting excellent seizure resistance without the use of the compound grease has been required.
As a threaded joint which can be used for fastening oil well pipes without the compound grease being applied, in Patent Document 3, a threaded joint for an oil well in which a resin coating containing lubricating powder is formed on the contact surfaces thereof is suggested. In addition, the applicant suggested a tubular threaded joint in which a solid coating is formed on each of a pin and a box in Patent Document 4.