New Technology for Improved Tubular Connection Performance
Authors: Weiner P.D. , Sewell F.D. 
Published: December 2007

The industry trend toward deeper, higher pressure wells demands development and use of new technology to meet the increasingly severe tubular goods requirements. Tubing and casing strings for deep wells are actually 2- to 4-mile long pressure vessels held together by threaded connections which must sustain stresses due to high internal and external pressures and to the pine string weight, in addition to being leak proof at gas pressures sometimes exceeding 10,000 psi. Successful performance of tubular connections requires

- an understanding of the connection vealing mechanism and make-up requirements to obtain the seal,

- proper evaluation of stresses induced in connections during make-up and while in service and

- a simple field means to insure that connections are properly made up.

Developments to meet these requirements include a unique device which permits simple field control of connection make-up variables. Effective field application of the new technology has resulted in large cost savings by minimizing connection leakage and pipe string failures.

Introduction

A space-age challenge confronting the industry is the construction of long pressure vessels which, in addition to sustaining high stresses due to tensile, burst and collapse loadings, must be leak proof at high gas pressures. Failure to meet this challenge may result in expensive workovers for tubing and casing string repair or replacement or, in some instances, total loss of the well. A recent API survey of industry pipe string failures indicates that 86 percent of the casing failures and 55 percent of the tubing failures occurred in the connection; operators lost an average of $100,000 per failure. The evolution of these long, high-strength tubing and casing strings from the rather common pipe strings for shallow, low-pressure wells has possibly masked the critical nature of assembling these pressure vessels upon which the economic success of the well may depend. Improper connection make-up can nullify precautions to insure satisfactory design, manufacture, transportation, inspection, handling and running of high-strength tubing and casing strings. These other facets of tubular goods performance are also critical in long, high-strength tubing and casing strings; however, their consideration is outside the scope of this article. Most deep well operators have observed with dismay that some connections leak while others in the same string made up with the same torque hold high gas pressures. This observation suggests that torque alone is an inadequate measure of connection make-up. All well designed connections have mechanisms which form seals when specific connection make-up requirements are met; leakage of connections is normally caused by failure to fulfill the connection make-up requirements. Stresses induced in connections during connection make-up or while in service must also be understood and properly controlled to avoid connection failure. Control of these rather complex variables must occur routinely in the field to insure installation of a leak-free pipe string which will remain intact until pulled from the well. A technical program directed at improving tubular goods performance included development of a unique device to permit simple field control of connection make-up variables. Field applications of the control device have been highly successful and indicate that the device will probably play an important role in future efforts to eliminate connection failures. Fig. 1 illustrates the fluid-eroded pin recovered from a well which blew out, apparently due to one such connection failure.

How Connection Seals Are Formed

An understanding of the seating mechanisms of available types of connections is necessary to obtain satisfactory connection performance. Figs. 2 through 7 illustrate the different types of tubing and casing connections commonly used by the industry. These connections employ several different types of threads and some include special sealing surfaces normally referred to as metal-to-metal seals.