Oil country tubular goods such as tubing and casing used for digging of oil wells for crude oil or gas oil are typically connected to each other using tubular threaded joints. In the past, the depth of oil wells was at most 2,000-3,000 meters but it sometimes reaches 8,000-10,000 meters in recent deep wells such as in offshore oil fields. The length of oil country tubular goods is typically 10 some meters, and the periphery of tubing through which fluid such as crude oil flows is surrounded by a plurality of casings. Therefore, the number of oil country tubular goods which are connected by threaded joints reaches a huge number.
In their environment of use, tubular threaded joints for oil country tubular goods are subjected to loads in the form of axial tensile forces caused by the mass of oil country tubular goods and the joints themselves, to compound pressures such as internal and external pressures, and to geothermal heat. Therefore, it is necessary that the threaded joints maintain gas tightness without being damaged even in such a severe environment.
A typical tubular threaded joint used for connecting oil country tubular goods (also referred to as a special threaded joint) has a pin-box structure. A pin, which is a joint component having male threads, is typically formed on both ends of an oil country tubular good, and a box, which is a mating joint component, having female threads which threadingly engage with the male threads, is typically formed on the inner surface of both sides of a coupling, which is a separate member. As shown in FIG. 1, a seal portion is provided on the outer peripheral surface in the vicinity of the end surface on the side closer to the end of the pin than the male threads and on the inner peripheral surface of the base portion of the female threads of the box, and a shoulder portion (also referred to as a torque shoulder) is provided on the end surface at the end of the pin and on the corresponding rearmost portion of the box. The seal portions and the shoulder portions of the pin and the box constitute unthreaded metal contact portions of the tubular threaded joint, and the unthreaded metal contact portions and the threaded portions of the pin and the box constitute the contact surfaces of the tubular threaded joint. Below-identified Patent Document 1 shows an example of this type of special threaded joint.
When performing makeup of such a tubular threaded joint, one end of an oil country tubular good (a pin) is inserted into a coupling (a box), and the male threads and the female threads are tightened until the shoulder portions of the pin and the box contact each other and interfere under a suitable torque. As a result, the seal portions of the pin and the box intimately contact each other and form a metal-to-metal seal, whereby gas tightness of the threaded joint is guaranteed.
Due to various problems when lowering tubing or casing into an oil well, a tubular joint which was previously made up is sometimes broken out, the joint is raised from the oil well, it is remade up, and then it is again lowered into the well. API (American Petroleum Institute) requires galling resistance such that unrepairable severe seizing referred to as galling does not take place and gas tightness is maintained even when makeup and breakout are carried out 10 times for a joint for tubing and 3 times for a joint for casing.
In order to increase galling resistance and gas tightness, a viscous liquid lubricant (a lubricating grease) containing heavy metal powder and referred to as compound grease has been previously applied to the contact surfaces of a threaded joint each time makeup has been carried out. Such compound grease is prescribed by API BUL 5A2.
With the object of increasing the retention of compound grease and improving its sliding properties, it has been proposed to subject the contact surfaces of a threaded joint to various types of surface treatment such as nitriding treatment, various types of plating such as zinc plating or composite plating, and phosphate chemical conversion treatment to form one or more layers on the contact surfaces. However, as described below, the use of compound grease raises the concern of an adverse effect on the environment and humans.
Compound grease contains a large amount of heavy metal powders such as zinc, lead, and copper powders. At the time of makeup of a threaded joint, the applied grease is washed off or squeezed out to the exterior surface, and there is the possibility of an adverse effect on the environment and especially on sea life particularly due to harmful heavy metals such as lead. In addition, the process of applying compound grease worsens the operating environment and operating efficiency and may cause harm to humans.
As a result of the enactment in 1998 of the OSPAR Convention (Oslo-Paris Convention) with the object of preventing maritime pollution in the northeast Atlantic, in recent years, strict environmental restrictions are being enacted on a global scale, and in some regions, the use of compound grease is already regulated. Accordingly, in order to avoid an adverse effect on the environment and humans in the process of digging gas wells and oil wells, a demand has developed for threaded joints which can exhibit excellent galling resistance without using compound grease.
As a threaded joint which can be used for connecting oil country tubular goods without application of compound grease, the present applicant proposed in below-identified Patent Document 2 a threaded joint for steel pipes having a viscous liquid or semisolid lubricating coating formed thereon, and in below-identified Patent Document 3, they proposed a threaded joint for steel pipes having a solid lubricating coating formed thereon.
Below-identified Patent Document 4 discloses forming a high-friction lubricating layer on the entirety of the contact surface of a pin or a box and forming a low-friction lubricating layer on specified portions of the contact surface of a pin or a box (when the low friction layer and the high friction layer are disposed on each other, the low-friction layer is the upper layer). The specified portions on which the low-friction lubricating layer is formed are specifically a metal-to-metal seal portion and thread crests and roots, and it is described in that document that only the high-friction lubricating layer preferably remains on a shoulder portion and thread load flanks. However, it is very difficult to form the low-friction lubricating layer on only the thread crests, roots, and stab flanks of the threaded portion without forming it on the thread load flanks