Application of Torque-Position Assembly Technology for API Connections
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 134563, ’Continuing Application of Torque-Position Assembly Technology for API Connections,’ by J.P. Powers, SPE, ExxonMobil De velopment Company, and M.S. Chelf, SPE, ExxonMobil Upstream Research Company, originally prepared for the 2010 SPE Annual Technical Conference and Exhibition, Florence, Italy, 19-22 September. The paper has not been peer reviewed.

American Petroleum Institute (API) threaded casing and tubing connections are expected to maintain structural integrity and sealability performance throughout the life of the well. Current industry standards specify thread dimensions and tolerances, but the critical makeup operation is specified with either torque only or stand-off (position) control. One operator developed, and for the last 20 years has been using, a quality-control process for makeup that combines torque monitoring and a measurement of final assembly position. This assembly method, called “torque position,” has provided gas-tight strings for critical well applications worldwide.    

API Thread Standards

Threads intended for downhole application in well casing and tubing service are specified in API Specification 5B. Specification 5B includes dimensions and tolerances of the threads and thread forms as well as gauging and inspection guidance. Several thread profiles are specified in the standard; however, the most common examples are buttress (BTC) and eight-round. BTC features a more rectangular thread form that is tapered, while the eight-round thread features a 60º flank angle with rounded roots and crests, and it also is tapered. Millions of feet of all these connections have been run in all types of wells around the world since API first standardized them in 1939.

The BTC and eight-round thread-form specifications have not changed much over the decades, but machine-tool capability and computer numeric-controlled (CNC) lathes have made these threads more precise. Gauging practice defined in API Specifications 5B and Recommended Practice 5B1 requires various measurements such as pitch diameter, taper, and thread length. API thread forms rely on radial interference between the mating pin and coupling threads to provide sealability performance. The taper creates the radial interference during makeup by the relative axial travel of the pin into the coupling. The size of the two parts and how quickly interference develops during the makeup are critical contributors to performance. The critical pitch-diameter dimension presents a challenge because it is intended to be measured at a specified axial distance from the pin nose or coupling face, on a theoretical pitch cone in approximately the middle of the height of the tapered, helical thread at that distance from the nose or face. The API gauging practice addresses this challenge by requiring the installation of a hardened and ground ring onto the pin and plug into the coupling, with a measurement of standoff when the ring or plug binds in the newly threaded part. Standard 5B defines standoff in units of turns, and standoff serves as the only estimate of pitch diameter. When the tolerances on the pin and coupling are combined, the total tolerance ranges from one turn for the BTC to four turns for the eight-round thread form. Converted to a diameter, this tolerance on pitch diameter is relatively large, considering the advances in tools and hardware over the years.