Threaded joint for oil well tubing

A threaded joint for an oil well tubing comprising:

• a pin having a nose portion on a distal end of a male threaded portion;
• and
• a box having a female threaded portion which engages with the male threaded portion of the pin by fitting engagement and portions opposedly facing the nose portion, wherein
• a shoulder portion positioned on a distal end of the nose portion of the pin is brought into contact with a shoulder portion of the box which opposedly faces the shoulder portion of the pin in an axial direction,
• an outer surface of the nose portion of the pin is formed of a projecting curved surface, and an inner surface of the box which opposedly faces the outer surface of the nose portion is formed of a tapered surface having an inclination angle θ with respect to the axial direction,
• both the outer surface of the nose portion of the pin and the inner surface of the box are configured to be brought into metal-to-metal contact with each other in a radial direction at the time of making up the joint thus forming a structure which seals a fluid,
• a seal point is defined as a position in the axial direction where an overlapping margin of seal portions of both the pin and the box in the radial direction becomes maximum in a state where cross sectional views in the axial direction of both the pin and the box are made to overlap with each other such that the shoulder portions mate with each other, and
• following parameters including sizes of the threaded joint at the seal point and the shoulder portions satisfy following formulae 1, 2 and Ls/Ln is set to 0.2 to 0.6:
• (Ds12−Ds02)/(D12−D02)≥0.30 (formula 1)
• tan θ>(ΔD+δ)/{2(Lt−Ls)} (formula 2)
• where
• Ds1:
• diameter (inch) expressing position of upper end of a shoulder contact surface;
• Ds0:
• diameter (inch) expressing position of lower end of the shoulder contact surface;
• D1:
• outer diameter (inch) of a pin non-formed portion;
• D0:
• inner diameter (inch) of the pin non-formed portion;
• θ:
• taper angle (°) of the seal portion of the box;
• ΔD:
• difference (ΔD=Dn−Dsp) (inch) between outer diameter Dn (inch) of connecting portion of the nose portion with the threaded portion of the pin and outer diameter Dsp (inch) at the position of the seal point;
• δ:
• seal interference amount (overlapping margin of the seal portions per diameter at the position of the seal point) (inch);
• Lt:
• index (inch) of finish position of a seal taper portion of the box (expressed by length from a distal end of the pin in axial direction);
• Ls:
• index (inch) of a seal point position (which is expressed by length from the distal end of the pin in axial direction);
• and
• Ln:
• length (inch) of the nose portion.