Existing single-string analysis methods may be inadequate for more difficult casing design problems, such as annular fluid heat-up and platform wellhead thermal growth, which require a multistring (or global) analysis of the whole well system. This paper presents a method for such an analysis and describes a finite-element formulation developed to implement it. The formulation is fully general and is applicable to a wide range of casing-/tubing-design problems.
Fluid heat-up pressures in trapped annuli have long been a concern of the petroleum industry and have been the subject of several recent studies. A general analysis method was first developed for BP Exploration, as part of a study into trapped annular stresses in subsea production wells. It had a far wider scope than just solution of heat-up problems and proved to be a significant development for four main reasons.
It is a multistring method and therefore permits analysis of problems like annular fluid heat-up, which was not previously possible with existing single-string techniques.
Multistring analysis also allows quantitative risk analysis (QRA) of the whole well system by use of structural reliability methods. This opens up a whole new area, probabilistic analysis, which is proving to be of key importance to future developments in casing/tubing design.
The solution method uses a finite-element formulation for the axial response; therefore, it does not suffer from the applicability problems of the closed-form approach (discussed later).
It reduces the whole analysis to the two fundamental equations that govern the behavior of the well system. It can be shown that this general approach, using a finite-element implementation, permits analysis of completely general design problems, even for the most complex aspects of the system response. Furthermore, any future theoretical developments can probably be included in this general finite-element treatment.