This paper reviews the traditional approach to tubular goods design with particular reference to tensional loading. Commonly used design factors are analysed to develop a basis for selecting the values to be used with more rigorous design assumptions.
The loading conditions during running and pulling operations are discussed. After the pipe is landed, it is subject to a variety of induced stresses. The paper focuses on the conditions requiring analysis, such as pressure testing, stimulation, well kill, production and injection. The paper also includes discussion of completion configurations and situations requiring special design considerations.
The increasing use of deep and deviated wells and the unfavourable economic environment, necessitates greater attention to cost effective tubular design.
In complex development situations, the designer is faced with the problem of deciding what additional factors must be considered, and how these should be incorporated into the design techniques normally used. Compounding of "safe" assumptions can easily result in a requirement for such heavy wall, high grade tubulars that not only do costs become prohibitive, but al so it may be virtually impossible to complete the well. Conversely, an underdesigned tubular goods program can result either in the well failing to achieve its objective or target production rate, Or in a potentially unsafe situation.
The objective of this paper is to review the traditional approach to tubular goods design with particular reference to tensional loading. Commonly used design factors are analysed as a basis for selecting values to be used with more rigorous design assumptions. The loading conditions used for tensional design analyses are al so discussed.
Assuming that the tubulars have been adequately sized with respect to the tubing's primary function as a flow conduit, the major design objective is to ensure adequate pressure integrity under allprobable service conditions, at the lowest possible cost.
Although we have the analytical capabilities to assess the theoretical behaviour of the tubulars as ressure vessels, there are considerable uncertainties with respect to the loading conditions, the formation strength, and the in-situ state of the pipe. Moreover, the average drilling or production engineer does not have the time, experience or technical background to attempt such an analysis. The tendency in the oil industry therefore has been not to be overly sophisticated in trying to analyze n extremely complex system, but rather to make designs on the basis of a set of notional loading conditions and design factors that have proved adequate in the past. The concept is illustrated in Figure 1. While this approach will lead to a tendency for overdesign, the relative costs of the engineering convenience is generally fairly small. A typical set of such design criteria is presented in Table 1.
Uniaxial analysis is usually adequate for normal low pressure wells (<3000 m and/or 35 MPa. Biaxial checks should be made for critical wells and conditions, particularly where significant tension can occur during collapse loading of large diameter tubulars.
A much more rigorous triaxial design analysis is often utilized instead of, or in addition to, the above procedures to develop cost effective tubular goods programs for severe loading conditions (>4000 m, and/or 70 MPa, and/or 150 °C).
