Threaded connection for oil well

A threaded connection for connecting pipes for an oil well, comprising:

• two pins each provided on an end of a corresponding one of the pipes and in continuity with a body of the corresponding pipe having a wall thickness not smaller than 12 mm;
• and
• a tubular coupling including two boxes and a recess portion, the boxes being provided on both ends of the coupling, each of the boxes being capable of being made up on the associated pin, the recess portion being located between the two boxes,
• each of the pins including:
• a nose forming a tip portion of the pin, an outer peripheral surface of the nose being not in contact with the box made up on the pin;
• a pin shoulder surface being formed at a tip surface of the nose;
• a male thread provided on an outer peripheral surface of the pin and located closer to the body of the corresponding pipe than the nose, the male thread being a tapered thread;
• and
• a pin sealing surface provided on the outer peripheral surface of the pin and located between the nose and the male thread,
• each of the boxes including:
• a female thread provided on an inner peripheral surface of the coupling to correspond to the male thread, the female thread being a tapered thread;
• a box shoulder surface located closer to a middle of the coupling as determined along a pipe-axis direction than the female thread, the box shoulder surface being in contact with the pin shoulder surface when the connection has been made up to form a shoulder;
• and
• a box sealing surface provided on the inner peripheral surface of the coupling and located between the female thread and the box shoulder surface, the box sealing surface being in contact with the pin sealing surface when the connection has been made up to form a seal,
• wherein the recess portion has a length not smaller than 10 mm as measured in the pipe-axis direction,
• the connection has a connection efficiency smaller than 1, the connection efficiency being calculated by A1/A2, where A1 is the area of a transverse section of the coupling as measured at the ends, adjacent to the seal, of the male thread and the female thread engaging each other when the connection has been made up, and A2 is the area of a transverse section of the body, and,
• when the wall thickness of the body is expressed as tpipe, the wall thickness of the recess portion and the length of the recess portion as measured in the pipe-axis direction are expressed as t1 and 2×L1, respectively, and the length of each of the boxes as measured in the pipe-axis direction is expressed as L2, and when the following expression (1) defines T and the following expression (2) defines L, T and L satisfy the following expression (3):
• T
• =
• t
• 1
• t
• pipe
• (
• 1
• )
• L
• =
• L
• 1
• L
• 1
• +
• L
• 2
• (
• 2
• )
• T
• ·
• L
• 1
• /
• 15
• >
• 1.4
• .
• (
• 3
• )