Casing and Tubing Connection Stresses
Authors: Schneider Warren P. 
Published: December 2007

Schneider, Warren P., SPE, Lone Star Steel Co.

By use of a simple theoretical analysis, the transverse stresses in couplings and pipe ends of API eight-round and buttress threaded connections caused by makeup and pressures have been calculated and tabulated. Stresses for 680 specified combinations of casing and tubing size, grade, and thread type are examined.

Introduction

More now than at any other time in the search for petroleum energy reserves, operators are using drilling and production equipment nearer the limits of performance capabilities and, at the same time, facing more costly consequences of failure. The rapid advances in abilities to reach greater depths are challenging designers of equipment that must safely contain the high pressures attendant with today's greater drilling depths. The recent surge in drilling activity has resulted in more frequent encounters with high pressure. Casing and tubing play an important role in both drilling for and extracting oil and gas. Their contribution to the success or failure of the effort and their cost warrant thoughtful design and selection. When unexpectedly high pressures are encountered downhole, these tubulars can be the last defense against loss of equipment, the well, and even lives. It has been established that the use of steel pipe is the most practical means to make a pressure vessel suitable for the drilling and production environment. Many practical considerations require that the pipe be used in separate lengths, each length being joined in sequence as it is lowered into the wellbore. The connection between lengths must sustain high tensile loads and at the same time provide pressure containment from both net internal and external pressures. This connection is made through the joining of threaded pipe ends, most commonly with the aid of a threaded coupling. With the exception of a growing number of proprietary connections, the configuration and specifications of these connections have been standardized through the efforts of the API. To understand the performance of these important connections more thoroughly, the stresses in API round and buttress threaded connections were examined. The results of the application of a previously introduced analysis method to couplings and pipe ends of the full range of sizes, weights, and grades of API casing and tubing are presented.

Connection Description

API casing thread types include short round thread, long round thread, buttress, and extreme-line. Except for extreme-line, the connections all include use of a coupling. Tubing thread types include nonupset (NUE), external-upset (EUE), and integral joint. NUE and EUE connections use a coupling. This discussion is restricted to short, long, and buttress threaded casing connections and NUE and EUE tubing connections because of predominant use of those connection types. predominant use of those connection types. For each of these types, the union is made by screwing a threaded pipe end into a similarly threaded coupling (Fig. 1). Both pipe and coupling threads are cut on a taper such that at a certain point of engagement the threads of the coupling and pipe come into full intimate contact. This is called the hand-tight position. The thread taper for both pipe end and coupling is specified at 3/4 in./ft on the diameter, except for taper on buttress threads on pipe 16-in. OD and larger, which is 1 in./ft. Two thread profiles are used with these connections: rounded "V", more commonly known as round thread, and buttress (Fig. 2). Thread pitch on casing and tubing round threads is 8 threads per inch, or 10 threads per inch in the smaller tubing sizes. Buttress pitch is 5 threads per inch. As the pipe end advances farther into the coupling through rotation, or makeup, past the hand-tight position, an increasing amount of interference between the pipe and the coupling occurs because of the taper. Expansion of the coupling and compression of the pipe end caused by the wedging action is a major source of tangential, or hoop, stress in both members. The amount of makeup to be applied to a given connection is prescribed. prescribed. JPT

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