Coupled connection for tight clearance casing strings in oil and gas wells

A casing string for use with a predetermined annular clearance inside a surrounding wall, the casing string comprising:

• tubulars each having a pipe body and pin ends, the pipe body having a first outer diameter and having a bore with a first inner diameter, the pipe body having a first critical cross-section defined by the first outer diameter and the first inner diameter, a first strength of the pipe body being defined by the first critical cross-section, the pipe body having a pipe tension resistance and having a pipe pressure resistance defined by the first strength, the first inner diameter being a first constraint dictated by the predetermined annular clearance, the first outer diameter being a second constraint dictated by the predetermined annular clearance, each of the pin ends having a turn-down recessed to a second outer diameter smaller than the first outer diameter, the turn-down having a second critical cross-section defined by the second outer diameter and the first inner diameter, a second strength of the turn-down being defined by the second critical cross-section, each of the pin ends tapering inward from the second outer diameter at the turn-down to a third outer diameter at a pin nose of the pin end, the pin end having a third critical cross-section defined by the third outer diameter and the first inner diameter, a third strength of the pin end being defined by the third critical cross-section, each of the pin ends having a pin thread formed externally on a portion thereof;
• and
• couplings each having a cylindrical body, the cylindrical body having opposing ends and having a through-bore, the cylindrical body having a fourth outer diameter, the through-bore having a second inner diameter, the fourth outer diameter being greater than the first outer diameter of the pipe body and being a third constraint dictated by the predetermined annular clearance to the surrounding wall, the through-bore having first and second portions of box thread formed internally thereon, the second inner diameter at the first portion tapering outward from a center of the through-bore to one of the opposing ends, the second inner diameter at the second portion tapering outward from the center of the through-bore to the other of the opposing ends, the couplings each having at least one fourth critical cross-section defined by the fourth outer diameter and the second inner diameter, a fourth strength of the coupling being defined by the at least one fourth critical cross-section,
• the pin threads of opposing ones of the pin ends being configured to thread respectively in a connection to the first and second portions of the box thread of the couplings,
• the pin noses of the opposing ones of the pin ends being configured to meet in the connection in the center of the couplings and are configured to engage with a contact pressure against one another,
• wherein (a) at least the second critical cross-section of the turn-down is selected within the first, second, and third constraints to increase the at least one fourth critical cross-section of the coupling relative to the first critical cross-section of the pipe body, and (b) at least the third strength of the pin end defined by the third critical cross-section of the pin end is iteratively selected relative to the fourth strength of the coupling defined by the at least one fourth critical cross-section of the coupling within the first, second, and third constraints to achieve:
• a connection tension resistance at a first targeted level that is at least 70 percent of the pipe tension resistance of the pipe body, and a connection pressure resistance for the connection at a second targeted level to that is at least 70 percent of the pipe pressure resistance of the pipe body.

A method of manufacturing a casing assembly with tubulars and a coupling for use with a predetermined annular clearance inside a surrounding wall, the method comprising:

• selecting a first outer diameter and a weight of a pipe body for the tubulars, the pipe body having a first critical cross-section defined by the first outer diameter and a first inner diameter, a first strength of the pipe body being defined by the first critical cross-section, the pipe body having a pipe tension resistance and a pipe pressure resistance defined by the first strength;
• selecting a fourth outer diameter of the coupling to fit within the predetermined annular clearance, the coupling having at least one fourth critical cross-section defined by the fourth outer diameter and a second inner diameter;
• and
• configuring a connection at box ends of the coupling to pin ends of the tubulars by:
• (i) forming a turn-down in the first outer diameter of the pipe body at each of the pin ends of the tubulars, the turn-down recessed to a second outer diameter smaller than the first outer diameter, the turn-down having a second critical cross-section defined by the second outer diameter and the first inner diameter, a second strength of the turn-down being defined by the second critical cross-section;
• (ii) configuring pin thread of opposing ones of the pin ends to thread respectively to box thread in the box ends of the coupling, the pin end having a third critical cross-section defined by a third outer diameter and the first inner diameter, a third strength of the pin end being defined by the third critical cross-section;
• (iii) configuring nose bearing area of pin noses on the opposing ones of the pin ends to contact with one another in a center of a through-bore of the coupling;
• and
• (iv) selecting at least the second critical cross-section of the turn-down to increase the at least one fourth critical cross-section of the coupling relative to the first critical cross-section of the pipe body and iteratively selecting at least the third strength of the pin end defined by the third critical cross-section of the pin end relative to a fourth strength of the coupling defined by the at least one fourth critical cross-section of the coupling to achieve:
• a connection tension resistance for the connection at a first targeted level to that is at least 70 percent of the pipe tension resistance of the pipe body, and a connection pressure resistance for the connection at a second targeted level that is at least 70 percent of the pipe pressure resistance of the pipe body.