Couplings for standard A.P.I. tubings and casings and methods of assembling the same

A method of providing couplings for A.P.I. standard tubing or casing lengths each having first and second threaded ends in which the threads are in accordance with predetermined A.P.I. tolerances including pitch, taper and thread profile, the couplings incorporating collars having internally threaded first and second ends defining thread bearing regions meeting in a central region wherein the crests of the threads in the central region are truncated to flattened surfaces defining a central interior bearing region of a given inner diameter while the full effective thread bearing regions are retained, the dimensions of the collar length and the internal threads being within predetermined tolerances selected to be more restrictive than A.P.I. tolerance limits for acceptable collars, comprising the steps of:

• prescreening the A.P.I. tubing or casing to reject those in which thread taper and dimension are greater than predetermined limits even though still within A.P.I. tolerance limits;
• partially assembling tubing or casing lengths together with individual collars, by first threading a first end of each tubing or casing into the first end of a precision collar to a selected number of turns of engagement past the hand tight plane;
• and
• then assembling a string from the partially assembled elements and collars by threading a second end of a different tubing or casing into the second end of a precision collar previously attached at its first end to a different tubing or casing length into a predetermined relation to the inserted end of the first tubing or casing and repeating the sequence with further tubing or casing lengths and collars, whereby a string of coupled tubing or casing is assembled having enhanced physical and sealing properties relative to a standard A.P.I. coupling string because thread bearing engagement and sealing throughout the string exceeds that which can be derived when using only the variations which are allowed by A.P.I. tolerances.

A method of upgrading the applicable usage of A.P.I. standard threaded tubings and casings for a given application meeting specific mechanical or corrosion resistance conditions that are more than those specified for the same A.P.I. standard tubings and casings including the steps of:

• forming a collar having a collar length and with A.P.I. standard compatible internal threads extending from mill and field ends of said collar toward a central thread region, with said collar length and said internal threads being within tolerances selected to be more precise than A.P.I. standard tolerances;
• providing said central thread region of said collar with flattened thread crests defining a given diameter central bearing area having a constant maximum diameter at the flattened crests;
• prescreening threaded ends of A.P.I. standard tubings and casing and selecting only those tubings and casings having a thread taper and dimension within specific limits that are tighter than standard A.P.I. tolerance;
• partially assembling each of said selected tubings and casings with one of said collars by threading a first end of each said selected tubing or casing into a mill end of one of said collars to a selected number of turns;
• optionally installing one of a number of different types of interior rings into said central bearing area of each said collar with an outer diameter of said selectively installed ring being in close engagement with said flattened thread crests in the central bearing area such that said selectively installed interior ring will be engageable with said first end of said tubing or casing;
• and
• assembling a plurality of said selected tubings or casings and collars by threading said second ends of said selected tubings or casings into said collars previously partially assembled with said first ends of said tubings or casings to form a string of said assembled tubings or casings and collars with said string having physical and sealing properties better than those provided by a standard A.P.I. coupling string.