Lubricating coating composition suitable for tubular threaded joints

This invention relates to a lubricating coating composition suitable for lubricating coating treatment of tubular threaded joints which are used to connect oil country tubular goods (abbreviated as OCTG) to each other.

A tubular threaded joint having a lubricating coating formed by treatment with a composition according to the present invention can be used to connect OCTG without application of a lubricating grease which contains a large amount of heavy metals and hence raises the a concern of causing environmental pollution, even if the joint is of the type having an unthreaded metal-to-metal contact portion which provides the joint with improved sealability but which makes the joint susceptible to galling.

OCTG are tubing and casing which are used to excavate oil wells. They are normally assembled on site by connecting steel tubes having a length on the order of ten some meters to each other using tubular threaded joints. Conventionally, the depth of oil wells has been 2,000-3,000 meters, but in recent deep sea oil fields, it may reach 8,000-10,000 meters.

In its environment of use, a threaded joint for connecting OCTG is subjected not only to a load in the form of an axial tensile force caused by the weight of OCTG and joints connected thereto but also to a combined internal and external pressure and subterranean heat. Therefore, it must maintain an airtight connection without breakage even in such a severe environment.

During the operation of lowering tubing or casing into an oil well, a tubular threaded joint which has once been tightened is sometimes loosened and then retightened. API (the American Petroleum Institute) requires a tubular threaded joint for OCTG to have sufficient galling resistance to make it possible to carry out tightening (makeup) and loosening (breakout) ten times for a joint for tubing and three times for a joint for casing without the occurrence of galling (unrepairable severe seizure) while maintaining airtightness.

A tubular threaded joint having good sealability when used to connect OCTG is of the pin-box structure capable of forming a metal-to-metal contact seal. With a typical tubular threaded joint of this type, a pin is formed on the outer surface of each end portion of a steel tube and includes a threaded portion with a male (external) thread and an unthreaded metal-to-metal contact portion, while a box is formed on the inner surface of a coupling, which is a separate connecting member, and includes a threaded portion with a female (internal) thread and an unthreaded metal-to-metal contact portion. The tubular threaded joint is tightened by inserting the pin into the box and tightening the male and female threads until the unthreaded metal-to-metal contact portions of the pin and the box tightly contact each other to form a metal-to-metal contact seal. Prior to tightening of the joint, a lubricating grease called compound grease is usually applied to the surfaces of the threaded portions and the unthreaded metal-to-metal contact portions which are the engaging portions of the joint when it is tightened, in order to provide these portions with improved galling resistance and airtightness. The engaging portions of a tubular threaded joint may be pretreated so as to have an increased surface roughness by surface treatment such as phosphating in order to increase the retention of a compound grease.

However, a compound grease contains a large amount of powder of heavy metals such as lead, zinc, and copper in order to provide the compound grease with sufficient lubricity and corrosion resistance. Therefore, the applied grease causes environmental pollution if it is washed off or squeezed out to its surroundings. In addition, the process of applying a compound grease worsens the work environment and decreases the efficiency of the operation of assembling OCTG. Accordingly, there has been a demand for a tubular threaded joint which fulfills its function sufficiently without application of a compound grease.

With the aim of providing a tubular threaded joint which can be used without application of a compound grease, a lubricating coating composition for forming a lubricating coating on the engaging portions of a tubular threaded joint and a lubricating coating or layer formed from such a composition are proposed in the following Japanese patent documents:

Patent Document 1: JP 2002-173692 A1

Patent Document 2: JP 2004-53013 A1

Patent Document 3: JP 2004-507698 A1.

Since the OSPAR (Oslo-Paris) Convention pertaining to preventing maritime pollution in the Northeast Atlantic came into effect in 1998, strict environmental regulations have been increasing on a global scale. In the excavation of gas wells and oil wells on offshore rigs, in order to minimize the discharge of causative agents of marine pollution, there is a recent trend that any substance which is used on a rig and which is capable of being discharged to the environment requires an environmental impact assessment thereof. If the substance does not meet the standards in that country or region, use of the substance is prohibited.

The properties to be assessed in this environmental impact assessment are prescribed in the OSPAR Convention as the HOCNF (Harmonized Offshore Chemical Notification Format). Among these properties, biodegradability (abbreviated as BOD) is particularly important.

For a tubular threaded joint, an environmental impact assessment of a compound grease is of course required, since its use unavoidably involves application and washing operations on a rig. Lubricating coatings and coating compositions for forming such a coating as proposed in the above-listed patent documents should also be subjected to such an assessment since there is a possibility of such a coating being discharged to its surroundings during washing of a joint.

However, although the lubricity and anticorrosive properties of lubricating coatings and coating compositions which have been proposed in the prior art are taken into consideration, their compositions are not designed with consideration of biodegradability, which is now an important item requiring assessment, and they have not been assessed with respect to their environmental impact. Therefore, their use has become difficult under recent conditions in which environmental regulations are becoming increasingly strict.

It is an object of the present invention to provide a tubular threaded joint suitable for use in connecting oil country tubular goods (OCTG) which eliminates the above-described problems of the prior art.

A more specific object of the present invention is to provide a tubular threaded joint which can be used to connect OCTG without application of a lubricating grease such as a compound grease and without problems related to lubricity, which is prevented from rusting and exhibits improved galling resistance and airtightness, and which can be used even in countries or regions having strict environmental regulations.

Another object of the present invention is to provide a lubricating coating composition for use in the manufacture of such a tubular threaded joint.

In order to achieve the above objects, the present inventors carried out investigations on various lubricating coating compositions suitable for a tubular threaded joint with respect to biodegradability, lubricity, and anticorrosive properties.

(A) Biodegradability (BOD)