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The most important mechanical properties of casing and tubing are burst strength, collapse resistance and tensile strength. These properties are necessary to determine the strength of the pipe and to design a casing string. If casing is subjected to internal pressure higher than external, it is said that casing is exposed to burst pressure loading. Burst pressure loading conditions occur during well control operations, casing pressure integrity tests, pumping operations, and production operations. The MIYP of the pipe body is determined by the internal yield pressure formula found in API Bull. This equation, commonly known as the Barlow equation, calculates the internal pressure at which the tangential (or hoop) stress at the inner wall of the pipe reaches the yield strength (YS) of the material.
Production casing for deep high pressure corrosive environment wells requires special planning. The criteria by which production casing is planning. The criteria by which production casing is chosen for wells shallower than 20,000' is quite different for deeper wells. If the worst case condition for internal pressure is a tubing leak, then the most accurate method of predicting shut-in tubing pressure must be employed.
The use of the appropriate restricted yield materials is advisable in many high pressure completion systems, even in areas not normally associated with sulfide stress cracking problems.
Uniaxial design analysis is not sufficient to adequately predict load condition in deep high pressure completion systems. A triaxial stress pressure completion systems. A triaxial stress analysis should be performed to insure the yield strength of the material is greater than the worst case stress condition for the most cost effective size/material combination.
A casing connection that has demonstrated performance under extreme combined loads should be performance under extreme combined loads should be chosen to insure the integrity of the production casing system.
From a safety standpoint, the production casing system is one of the most important aspects of well planning. Less than optimum surface and intermediate casing can sometimes be compensated for by good well control procedures. A production casing failure, however, may have disastrous results in that it normally is not tested until a tubing failure, and serves as the back up protection for such an event. protection for such an event. The proper selection of a production casing system insuring both a technically correct and cost effective solution is an iterative precess.
The first step is to predict the load conditions as accurately as possible. The absolute requirement is that the production casing system should maintain integrity during a shut-in tubing leak condition. To this end the use of the most accurate method of predicting shut-in pressures is imperative. An examination of the various methods is offered.
The use of sour service materials is recommended in a high pressure deep completion. Under extreme pressure conditions restricted yield materials may be required, even in areas not normally associated with sulfide stress cracking.
The use of industry standard uniaxial design factors may not result in the most technically correct or cost effective solution. An analysis of the three principal stresses will enhance the probability that the production casing system probability that the production casing system exceeds any load conditions it may be subjected to.
Conventional casing connections may not be sufficient for the extreme tensile and internal pressure loads that the production casing system pressure loads that the production casing system may encounter. In order to preserve the integrity of the production casing system, the connections must be designed for extreme combined loads. What would be expected of a connection and some advanced thread concepts to solve these problems are discussed.
DETERMINE THE REALISTIC LOADS
It is imperative that realistic load condition be estimated in order to choose an adequate wall thickness, material and connection to satisfy the requirements of the severe environment.
In a completion utilizing a conventional closed annulus, or packer completion, the worst case loading can occur during a tubing leak. In this case we assume that will essentially have the shut-in tubing pressure acting on the production casing system at the surface. Accurate prediction of expected shut-in tubing pressure should be used as the starting point in designing production casing.