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To arrive at the optimal solution, the design engineer must consider casing as a part of a whole drilling system. A brief description of the elements involved in the design process is presented next. The engineer responsible for developing the well plan and casing design is faced with a number of tasks that can be briefly characterized. While the intention is to provide reliable well construction at a minimum cost, at times failures occur. Most documented failures occur because the pipe was exposed to loads for which it was not designed.
Casing and tubing strings are the main parts of the well construction. All wells drilled for the purpose of oil/gas production (or injecting materials into underground formations) must be cased with material with sufficient strength and functionality. Therefore, this chapter provides the basic knowledge for practical casing and tubing strength evaluation and design. Casing is the major structural component of a well. Casing is needed to maintain borehole stability, prevent contamination of water sands, isolate water from producing formations, and control well pressures during drilling, production, and workover operations. Casing provides locations for the installation of blowout preventers, wellhead equipment, production packers, and production tubing. The cost of casing is a major part of the overall well cost, so selection of casing size, grade, connectors, and setting depth is a primary engineering and economic consideration. Tubing is the conduit through which oil and gas are ...
To evaluate a given casing design, a set of loads is necessary. Casing loads result from running the casing, cementing the casing, subsequent drilling operations, production and well workover operations. Internal pressure loads result from fluids within the casing and are modeled with pressure distributions. Pressure distributions are typically used to model the internal pressures. These pressure distributions are discussed next.
Sathuvalli, U. B. (Blade Energy Partners) | Pilko, R. M. (Blade Energy Partners) | Gonzalez, R. A. (Blade Energy Partners) | Pai, R. M. (Blade Energy Partners) | Sachdeva, P.. (Blade Energy Partners) | Suryanarayana, P. V. (Blade Energy Partners)
Abstract Subsea wells use Annular Pressure Build-up (APB) mitigation devices to ensure well integrity. Type I mitigation techniques control APB by reducing radial heat loss from the production tubing to the wellbore. Type II techniques work by controlling the stiffness (psi/°F) of an annulus by modifying its contents and boundaries. Though the physics of APB mitigation is well understood, the reliability of a mitigation strategy or its interaction with other parts of the wellbore is not always quantifiable. This is partly due to lack of a unified approach to analyze mitigation strategies, and partly due to lack of downhole data after well completion. Simply stated, the engineer is hard pressed to find computational-predictive methods to assess alternative scenarios and strategies within the framework of the design basis during the life of the well. In this light, our paper presents a quantitative approach to design the currently used APB mitigation strategies, i.e., rupture disks, syntactic foams, nitrified spacers, and Vacuum Insulated Tubing (VIT). In each case, the design is linked to the notion of “allowable APB” in an annulus, which in turn, is tied to the design of the casing strings, and thus to wellbore integrity. Based on an extensive survey of published literature and patents, we also review APB mitigation techniques that have been used less frequently or awaiting proof of concept/field trial.
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.
1. 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.
2. 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.
3. 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).
4. 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.