Unraveling the Mysteries of Proprietary Connections
Authors: Klementich E.F. 
Published: December 2007

This paper is SPE 35247. Technology Today Series articles provide useful summary information on both classic and emerging concepts in petroleum engineering. Purpose: To provide the general reader with a basic understanding of a significant concept, technique, or development within a specific area of technology. Journal of Petroleum Technology, December 1995.

Introduction
When Alexander the Great was confronted with the task of unraveling the Gordian knot, he simply drew his sword and sliced the jumbled mess in two. Unfortunately, such a direct approach is not feasible for unraveling the mysteries of the numerous interwoven proprietary connections available today. These mechanical devices, often erroneously called premium connections, can whipsaw a petroleum engineer into frenzied indecision because of the number of competing designs available and the myriad conflicting cross claims made of alleged benefits by their manufacturers. The purpose of this article is to provide some insight to the potential users of proprietary connections and to facilitate a rational selection process and procurement procedure.

Types of Proprietary Connections
First, it must be noted that the standard and commonly used API casing connections [short, round thread casing and coupling (STC); long, round thread casing and coupling (LTC); buttress thread casing and coupling (BTC)] and API tubing connection EUE-8R (external upset ends, eight round thread tubing and coupling) can provide reliable service for the vast majority of wells drilled and completed. Proprietary connections can and have been misapplied at considerable expense; for example, 95/8 drilling (intermediate/protective) casing. API BTC works more than adequately in drilling-mud environments. There is no need to use a proprietary connection for such service. In fact, pin nose metal-to-metal seal proprietary connections in this type of service can be less reliable than the API BTC connection since the metal-to-metal seal is much more damage prone than the thread seal of API BTC. Also, very reliable leak resistance, even to gas, is obtained with tin plated BTC couplings.
Table 1 provides reasonable application limits for the common API casing and tubing connections. For specific circumstances, such as very high pressures, large diameters, sealing reliability, and small clearances, the standard API connections are not applicable or do not provide the degree of reliability required for the application. Thus, the need for proprietary connections.
Proprietary connections can be classified into six generic groups, which are described in Table 2. Large-diameter (20.000-in.-OD or larger) weld-on connectors are an additional type of connection, but because no threads are machined on a pipe body, they are considered to be connectors for pipe, not pipe connections.

Principal Loading Modes
The principal loading modes on pipe strings in wells are somewhat more complicated than commonly perceived. Five loading modes exist to which a pipe string and their connections can be subjected.Axial (tension or compression).Pressure (internal and/or external).Bending (directional well and/or helical buckling).Torsion (makeup and rotation).Nonaxisymmetric side loads (point, line, or area).
Note that the commonly considered burst, collapse, and tension loads constitute only two of the five principal loading modes.
The first three primary loading modes are generally the most significant and dominant service loads for casing and tubing. Torsion is important for liners that may need to be rotated in or into the hole. Side loading can be important in extended-reach or horizontal wells and for plastic salt (or shale) zones.

The Ideal Pipe Connection
The ideal casing and tubing connection is 100% transparent to the pipe body. That is, a made-up pipe string behaves as if the connections were not present from both a performance (rating) and a geometric (dimensional) standpoint. Other than welding, no methods currently exist by which complete connection transparency can be achieved, although one recent connection design will essentially achieve that goal.
Historically, performance transparency has been achieved with the metal-to-metal seal integral-joint (MIJ) connectors, but at a great expense of geometric transparency (i.e. massive hot-forged upsets). On the other hand, near-geometric transparency has been achieved with integral-flush-joint (IFJ) connections, but at a substantial loss of performance transparency. Unfortunately, too often proprietary connections have been optimized for a noncritical loading mode at the expense of the dominant load. For example, in IFJ connections, axial-compression resistance (which is most needed by such a connector for deep well applications) is sacrificed, or not even considered, to achieve a little extra tension efficiency. Fortunately, recent advances in pipe-connector technology have provided substantial increases in performance transparency, without an associated decrease in geometric transparency: in essence, MTC performance at slim-line high-performance (SLH) geometries and SLH performance at IFJ geometries.

Proprietary-Connection Ratings
Invariably, proprietary connections are incompletely rated by their manufacturers. Too often, critical loading modes are neglected or, worse, overrated for a strictly monotonic load, completely disregarding the cyclic nature of actual loads in a well. Table 3 provides a complete listing of proprietary connection characteristics that might be needed by a user to evaluate the suitability of a proprietary connection for a specific application. Fig. 1 shows the service-life loads 1, 2, 3, etc. (envelope), of a long, deep-set 9 5/8-in. drilling/production liner. Note that many connections in the string see both tension with internal pressure and compression with external pressure. Furthermore, while cycling between these service loads, compression with burst and tension with collapse loads can also be expected. Consequently, a proprietary connection catalog in which only a monotonic tension rating is provided and pressure (burst and collapse) limits are merely implied does not provide adequate data (documentation) to make a sound engineering decision concerning the suitability of the connection for the intended application.
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