RELATED APPLICATION DATA

This application is a National Stage Application under 35 U.S.C. 371 of PCT application number PCT/JP2018/036859 designating the United States and filed Oct. 2, 2018; which claims the benefit of JP application number 2017-206157 and filed Oct. 25, 2017 each of which are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

The present disclosure relates to a threaded connection for steel pipe, and more particularly to a threaded connection for connecting steel pipes with large diameters and large wall thicknesses.

BACKGROUND ART

Steel pipes called oil-well pipes are used, for example, for prospecting or producing oil or natural gas in oil wells or natural-gas wells (hereinafter collectively referred to as “oil wells” or the like), developing non-conventional resources such as oil sand or shale gas, retrieving or storing carbon dioxide (Carbon dioxide Capture and Storage (CCS)), geothermal power generation, or in hot springs. A threaded connection is used to connect steel pipes.

Such threaded connections for steel pipes are generally categorized as coupling type and integral type. A coupling-type connection connects a pair of pipes, one of which is a steel pipe and the other one is a short pipe called coupling. In this case, a male thread is provided on the outer periphery of each of the ends of the steel pipe, while a female thread is provided on the inner periphery of each of the ends of the coupling. Then, a male thread of the steel pipe is screwed into a female thread of the coupling such that they are made up and connected. An integral-type connection connects a pair of pipes that are both steel pipes, and does not use a separate coupling. In this case, a male thread is provided on the outer periphery of one end of each steel pipe, while a female thread is provided on the inner periphery of the other end. Then, the male thread of one steel pipe is screwed into the female thread of the other steel pipe such that they are made up and connected.

A connection portion of a pipe end on which a male thread is provided includes an element to be inserted into a female thread, and thus is usually referred to as “pin”. A connection portion of a pipe end on which a female thread is provided includes an element for receiving a male thread, and thus is referred to as “box”. A pin and a box constitute ends of pipes and are thus tubular in shape.

An oil well is drilled along while its side wall is reinforced by oil-well pipes to prevent the side wall from collapsing during digging, which results in multiple oil-well pipes arranged in one another. In recent years, both on-land and offshore wells have become deeper and deeper; in such environments, to connect oil-well pipes, threaded connections in which the inner and outer diameters of the connection portions are generally equal to the inner and outer diameters of the steel pipes are often used in order to improve efficiency in developing oil wells. The use of such threaded connections minimizes the annular gaps between the multiple oil-well pipes arranged in one another, which improves efficiency in developing a deep oil well without significantly increasing the diameter of the well. A threaded connection is required to have good sealability against pressure fluid from the inside (hereinafter also referred to as “internal pressure”) and pressure fluid from the outside (hereinafter also referred to as “external pressure”) under the above-described restrictions on the inner and outer diameters. Generally, the sealability against the internal pressure is called “internal-pressure sealability”, while the sealability against the external pressure is called “external-pressure sealability”.

Known threaded connections that ensure sealability include those having a seal that uses metal-to-metal contact (hereinafter referred to as “metal seal”). A metal seal is an arrangement in which the diameter of a pin sealing surface is slightly larger than the diameter of a box sealing surface (the difference between these diameters will be referred to as “amount of interference”) and, when the threaded connection is made up and the sealing surfaces are fitted onto each other, the amount of interference causes the diameter of the pin sealing surface to decrease and the diameter of the box sealing surface to increase and each of the sealing surfaces tries to recover their original diameters and thus generates elastic recovery forces, which produce contact pressures on the sealing surfaces to cause them to adhere to each other along the entire periphery, thereby providing sealability.

JP 2006-526747 A (Patent Document 1) discloses a technique used for a threaded connection for steel pipe including a pin and a box, where a nose is provided on the pin to improve the external-pressure sealability while maintaining its internal-pressure sealability.

JP 2015-132285 A (Patent Document 2) discloses a technique used for a threaded connection for steel pipe including a pin and a box, where galling is to be prevented while ensuring internal-pressure and external-pressure sealability.

Japanese Patent No. 4535064 (Patent Document 3) teaches that a threaded connection including one metal seal and one nose has a higher external-pressure sealability than a threaded connection without a nose.

The following prior art documents are incorporated herein by reference.

[Patent Document 1] JP 2006-526747 A

[Patent Document 2] JP 2015-132285 A

[Patent Document 3] Japanese Patent No. 4535064

SUMMARY OF THE INVENTION

An object of the present disclosure is to provide a threaded connection that improves its internal-pressure sealability while maintaining the high external-pressure sealability of a threaded connection having a nose used in a steel pipe with large diameter and large wall thickness.

A threaded connection for steel pipe according to the present disclosure is a threaded connection for connecting steel pipe bodies each having an outer diameter equal to or larger than 7 inches and a wall thickness equal to or larger than 0.7 inches. The threaded connection includes a tubular pin and a tubular box. The box is made up on the pin as the pin is inserted therein. The pin includes a nose, a pin shoulder surface, a pin taper surface, a pin round surface, a male thread, and a pin sealing surface. The nose is provided on a tip portion of the pin and has an outer diameter smaller than an inner diameter of a portion of the box that faces tip portion of the pin when the connection has been made up. The pin shoulder surface is provided on a tip of the nose. The pin taper surface is provided on an outer periphery of the nose. The pin round surface is provided on an edge between the pin shoulder surface and the pin taper surface. The male thread is provided on an outer periphery of the pin. The pin sealing surface is provided on the outer periphery of the pin and located between the nose and the male thread. The box includes a box shoulder surface, a female thread, and a box sealing surface. The box shoulder surface faces the pin shoulder surface and is in contact with the pin shoulder surface when the connection has been made up. The female thread corresponds to the male thread and provided on an inner periphery of the box. The box sealing surface faces the pin sealing surface and is in contact with the pin sealing surface upon make-up.

Expression (1) provided below is satisfied and tB/tL>1.4, where δ [mm] is the clearance between the outer surface of the pin and the inner surface of the box upon make-up as measured at the borderline between the pin taper surface and the pin round surface, D [mm] is the outer diameter of the steel pipe body, t [mm] is the wall thickness of the steel pipe body, L [mm] is the distance between a pivot point, which is a position, in the region of contact between a pin sealing surface and the box sealing surface, which is closest to the tip of the pin, and the tip of the pin upon make-up as measured along the pipe-axis direction, tL[mm] is the wall thickness of the pin as measured at the pivot point, and tB[mm] is the wall thickness of the box as measured at the pivot point.
0.30+min(antL+bnD,n=1,2,3)>δ>min(antL+bnD,n=1,2,3)   (1).

Here, min is a function that takes the minimum of the three factors in its parentheses,

a1=(L/t)×0.0115×(tL/t)−1.68778−0.00247×(tL/t)−2.02052;

b1=(L/t)×0.0076×(tL/t)−0.65672−0.00120×(tL/t)−1.06817;

a2=(L/t)×0.00725×(tL/t)−1.67341−0.00157×(tL/t)−1.93212;

b2=0;

a3=−(L/t)×0.00256×(tL/t)−2.40620+0.00372×(tL/t)−2.19899; and

b3=(L/t)×0.01986×(tL/t)−0.88615−0.01048×(tL/t)−1.00532.

