Threaded joint for pipes

#This invention relates to a threaded joint for pipes which has a metal-to-metal seal portion and which is for use with oil well pipes used in the development, production, and transport of petroleum and natural gas, for example.

#The conditions of use of joints for connecting oil well pipes used in the development and production of petroleum and natural gas are becoming more severe as oil wells and gas wells become deeper, and a higher level of performance is required thereof with respect to strength and airtightness (also referred to below as sealing properties).

#In such wells, with threaded tubular joints such as buttress joints or round thread joints which have conventionally been used in which joining is performed solely by threaded engagement of API standard threads, it is not possible to provide either strength or sealing properties adequately.

#FIG. 1 is a schematic assembly view of a threaded joint for pipes having a metal-to-metal seal portion.

#FIG. 2 is an enlarged view showing the vicinity of the metal-to-metal seal portion of the threaded joint.

#FIG. 3 is a figure showing the average pressure ratio Pm/Py and the results of a seal test for threaded joints for pipes having a surface roughness Ry on the order of 25 μm.

#The present inventors investigated the relationship of the surface roughness of the sealing surface, the pressure, and the width in the axial direction of the annular contact portion which receives the pressure to sealing properties for a metal-to-metal seal portion of a threaded joint by the following method.

#A steel pipe having a nominal outer diameter of 88.9 mm and a nominal wall thickness of 6.45 mm and made of low carbon steel for oil well pipes (L-80 of API standards) was prepared as a test pipe. As shown in FIGS. 1 and 2, a male thread 1b and an unthreaded sealing surface 1a for providing plane-to-plane contact (a conical unthreaded sealing surface like that proposed in U.S. Pat. No. 5,137,310) were formed by machining on the outer peripheral surface of both ends of the test pipe (at its tip of both ends). The test pipe had a shoulder portion 1c at each end.

#A coupling 2 made of the same material as described above for connection to the test pipe 1 had a female thread 2b for threaded engagement with the male thread 1b of the test pipe and an unthreaded sealing surface 2a for abutting against the sealing surface 1a of the test pipe formed on its inner peripheral surface at both of its ends. It had an inner shoulder portion 2c for abutting against the shoulder portion 1c of the steel pipe.