1. Field of the Invention
This invention relates generally to oil and gas well tubing and more particularly has to do with the construction of high pressure tubular joints operable when made up and run into a well to withstand extreme fluid pressures and for sealing off thereof to prevent escape of high pressure fluid or gas through the tubular assembly at the joints.
2. Description of the Prior Art
The search for oil and gas reserves has brought about the exploration of ever deeper formations. These deeper formations require longer strings of production pipe, casing, liners and drill pipe used in the exploration and production of oil and gas. Such tubular goods are made of steel material. Such wells may be subject to extremely high pressures from formation zones. The increased length of tubular strings imposes the upper portion of the string to very high tensile loads and where high pressure exists from deeper formations, may also expose the upper portion of the string to high internal pressures where there is little or no offsetting external pressure on the tubing. With standard joints there are limitations to the depths to which a string of tubing can be run.
There have been prior art attempts to provide metal-to-metal seals which can withstand the extreme pressures that the tubular joint is required to withhold.
U.S. Pat. No. 2,893,759 issued in the name of Thomas L. Blose, one of the inventors of the present invention, is entitled "Conically Tapered Screw-Type Casing Joint With Metal-To-Metal Seal". The patent discloses a metal-to-metal seal where a different sealing taper is provided on the box and pin sealing surfaces to generate an area of radial deformation of the sealing surfaces upon make-up of the connection. The sealing surface of the pin is provided with a higher taper angle than that of the box sealing surface, so that upon joint make-up a line contact is initially established between the sealing surfaces. Upon further make-up, at least one of the members is deformed radially to generate a surface contact between the sealing surfaces. Since the sealing of the pin has a higher taper or angle of incline with respect to the coaxial axis of the pin and box, the trailing edge of the pin sealing surface is the first to contact the annular sealing surface of the box on initial make-up.
The conical and crown-type metal-to-metal sealing geometries in use today for high pressure joints for the oil and gas industry, typically have pin and box members machined to the same nominal angle of engagement as measured from the tubular axis. This angle is typically between two degrees (2.degree.) and fifteen degrees (15.degree.), but may be of any angle. A typical conical seal of any given width would have both pin and box seals machined to a nominal fourteen degrees (14.degree.) for their entire widths. A crown-type seal of fourteen degrees (14.degree.) is disclosed in U.S. Pat. No. 2,992,019 issued in the name of MacArthur where a crown seal is provided on the pin member and is machined at the same angle as the sealing surface on the box member.
It has been found that for parallel or near parallel mating surfaces such as sealing surfaces of conical or crown-type metal-to-metal seals of the same or substantially the same angles, the bearing loads shift from the leading edge of the seal on initial contact to the trailing edge upon final make-up. Depending on the amount of radial interference induced, the leading seal edge can be depleted of all bearing load as the load is transferred further and further back toward the trailing seal edge with the increase in metal-to-metal interference. At very high levels of interference the trailing edge of the seal can receive bearing loads sufficient to cause not only unnecessarily high stress levels of interference in the seal area, but even yielding of the material in a seal surface. This yielding or plastic metal surface distortion, sometimes also referred to as galling, can lead to the metal-to-metal seal losing its pressure containment ability. For this reason, the amount of interference between sealing members has heretofore been limited to avoid attainment of the galling threshold. This, however, has tended to lower the maximum pressure containment limit of typical metal-to-metal seals, particularly on thick wall, high yield strength tubular goods.
The metal-to-metal seal described in U.S Pat. No. 2,893,759 mentioned above provides that the angle of incline of the sealing surface on the pin member be greater than the angle of incline of the sealing surface on the box member. This arrangement necessarily causes the trailing edge of the pin sealing surface to first come in contact with the sealing surface on the box member and upon further make-up, at least one of the members is deformed radially whereby the bearing load on the final seal is greatly distorted at a particular point, typically the trailing edge of the seal.
It is therefore an object of this invention to provide a metal-to-metal seal in a tubular joint that has a substantially uniform loading distribution across the axial extent of the metal-to-metal contact between the male and female members.
It is a further object of this invention to provide a casing joint with a pin and box member where both internal and external seals are provided, wherein the bearing load distribution across each seal is approximately uniform.
It is a further object of the invention to provide a high strength leak resistant sealing surface in a tubular joint which is better adapted to withstand the high pressures and tensile loads encountered in the exploration and production of deep oil and gas well formations.