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In recent years, with high pressure, high temperature (HPHT) wellsincreasing, the casing collapsed cases sometimes take place in such wells,which often lead to very serious consequences. The basic principle of API 5C3yield collapse design is that casing failure takes place by initial yield atthe inner wall; in fact, there is no loss of internal pressure integrity andstructural integrity when failure takes place and casing can still bear a lotof collapse pressures. Therefore, an accurate prediction of the through-wallyield collapse strength, which is de?ned as the maximum value of external yieldpressure required to cause the steel casing to fail, is a very importantconsideration in the casing design process. The yield collapse formulas atarbitrary yield radius and the through-wall yield are obtained in this paper.Through comparing calculation values with API 5C3's; it is shown that theinitial yield collapse strength calculated by lamé formula and Mises yieldcriterion is higher than API 5C3's by 15.45%, and the through wall yieldcollapse strength for thick wall pipe (D/t=20) is higher than API 5C3's by28.4%, which provides an important reference to improve casing strength designfor designer and casing quality control for manufacturer.
Key words: equation; casing; through-wall yield; collapse
During development and production, casing can protect borehole, reinforceborehole wall, isolate from oil, gas and water zone in well and lock variousbad ground wreck damage for borehole. Casing generally bears higher collapsepressure. When the external collapse pressure exceeds casing collapseresistance strength, casing will be collapsed; meanwhile, it will result in thewhole wellbore damage and in the worst case, lead to the whole well out ofuse.
Collapse pressure is not only one of the important characteristics of OCTG,but also one of three main elements of casing strength design (anti-internalpressure strength, collapse resistance strength, tensile strength). In current,collapse pressure formula in API 5C3 is used conventionally in the petroleumindustry in the world. Yield collapse strength calculated by API 5C3 yieldcollapse formula is minimum collapse value, and in casing design, it isconsidered that casing failure takes place by initial yield at the inner wall;In fact, there is no loss of internal pressure integrity and structuralintegrity when failure takes place and casing can still bear a lot of collapsepressure. So for thick and heavy thick-walled casing strength design in deepand super-deep well it is important to calculate accurately the through-wallyield collapse value.
Kyogoku, Tetsuo (Sumitomo Metal Industries Ltd.) | Tokimasa, Katsuyuki (Sumitomo Metal Industries Ltd.) | Nakanishi, Hisayuki (Sumitomo Metal Industries Ltd.) | Okazawa, Tohru (Sumitomo Metal Industries Ltd.)
In high-angle and horizontal wells, casing is subjected to severe wear and bending. Effect of wear and bending on the collapse strength was investigated by experiments and was evaluated using analytical approach as well as FEM. Empirical formulas allowing imperfections based on elastic collapse and yield onset pressure are derived.
Highly deviated drilling can cause severe wear on the internal surface of well casing. In these high-angle and horizontal wells, casing is also subjected to large bending also. The casing wear and bending can decrease casing performances. Especially, the reduction of collapse strength in casing due to wear and bending has to be taken into consideration in the casing design process. The effect of wear and bending is not clear, in this paper quantitative evaluation for the effect of wear and bending on the collapse of casing is intensely studied.
Concerning the effect of wear, C. E. Murphey studied the worn casing collapse using one - in. OD steel tubes with flats milled on the exterior. He indicated that the percentage reduction in collapse pressure due to wear is predictable within the upper and the lower bound and is no greater than the percentage reduction in minimum wall thickness using elastic collapse analysis. Although his method gives practical estimation of worn casing collapse pressure for the external wear, confirmation should be carried out in the present paper whether his method can be applicable for the internal wear and in the plastic collapse region.
The purpose of this study is to clarify the mechanism of the collapse strength reduction and to propose formulas which estimate the collapse strength under wear and bending First, collapse tests were performed using steel pipes with internal wear and under bending. Some tests were conducted under axial tension. Second, collapse mechanism of worn casing was discussed with experimental results, analytical evaluation and Finite Element Analysis for bifurcation Problem. Third, empirical formulas for predicting the collapse strength under wear and bending allowing ovality, eccentricity and residual stress are provided.
Specimen and Testing Machine
As the materials of specimens, steel bars and seamless steel pipes were prepared. The steel bars were milled and bored to get ideal pipes with accurate diameter and thickness. The seamless pipes were internally and locally ground to produce worn casing specimens.
Table 1 indicates the dimension and mechanical properties of specimens for worn casing collapse. S55C grade 5-1/2-in. steel bars were milled and bored.
Chen, Qiang (PetroChina Co. Ltd.) | Gao, Shan (PetroChina Co. Ltd.) | Li, Yiliang (PetroChina Co. Ltd.) | Li, Tao (PetroChina Co. Ltd.) | Bi, Xiuling (PetroChina Co. Ltd.) | Han, Weiye (PetroChina Co. Ltd.) | Sun, Qiang (PetroChina Co. Ltd.)
Abstract The paper summarized an experimental and theoretical study of the collapse resistant ability of solid expandable tubular (SET)after expansion, and the formula used to calculate the collapse strength of casing was modified to make it adaptable to the expandable tubular. One key limiting factor of the expandable tubular in its application in the open hole well to replace the casing is its post-expanded mechanical properties. The collapse strength of the expandable tubular after expansion was significantly compromised as the result of the comprehensive influence by the change of factors like the ovality, eccentricity and residual stress. In view of these problems, firstly full-scale expansion experiments were performed to examine all the impact of those factors; secondly the formula was modified based on the experimental data; thirdly the collapse experiments were carried out to test the strength of post-expanded tubulars as well as the accuracy of the modified formula. The results demonstrated that after large-scale plastic deformation, the dimension of the tubular changed dramatically as represented by the increase of eccentricity and ovality, the residual stress also appeared on both inside and outside surface of the tubular, the starting point of the collapse occurred at the position with the least wall thickness. The calculation results which reflected the average collapse strength of the tubular were larger than the actual experimental outcome but the error was kept well within 15%. We expect the research will contribute to the better understanding of the collapse resistant ability of post-expanded tubular and form the necessary technical basis for it future broad use.
ABSTRACT The importance and cost of casing programs justify careful engineering design based on the best available information on strength and loading conditions. A method of design is outlined which considers actual conditions, and which results in a consistent compromise between economy and safety. Factors influencing choice of sizes, weights, grades, etc., are discussed; method of tabulation is shown; and an example of design is included. Collapse and tension data in Table 1 are primarily to illustrate the method, but are believed to be the most reliable available at the moment. By design of casing programs is meant the selection of suitable casing sizes, weights, grades; type of joints; and hole sizes to be used in a particular well to perform most safely and economically the functions of water shut-off and protection at depths predetermined by geological considerations. The term "casing-program design" as herein used is not intended to cover design of casing, but rather covers a method of choosing from available stock the particular casing most suitable for a given well. Published figures on "setting depths" in the past have encouraged selection of casing by methods which failed to recognize actual conditions and, therefore, failed to assure either safety or economy. At best, these methods apply only to "average" field conditions and simple cases, and are far from correct in many instances. A pressure gradient of 1/2 lb. per sq. in. per ft. of depth is assumed, and may result in very large errors, because cases have been reported of formation pressures requiring 172 lb. per cu. ft. of mud, or a gradient of 1.19 lb. per sq. in. per ft, In addition to this, implied strength data on both tension and collapse have been generally unreliable and inconsistent. As a result, casing programs often have been chosen with little recognition of actual loading, and with no reliable knowledge of the strength of the pipe. An attempt is made herein to present a method of design which will recognize actual loading conditions and result in design on a proper engineering basis. Size and Clearance In the design of a casing program the first consideration is the size of pipe to be used, and this determination should start from the size and length of liner to be used. Selection of the liner size controls the size of all other strings, and this has an extremely important bearing on the total casing cost. Large liners have many advantages, and these advantages should be balanced against the comparative costs of alternative programs starting with the different liners. Having selected tentatively the size and length of liner, the necessary hole size is determined from the required clearance. Fig. 1, entitled "normal casing clearance," gives required clearances for various outside diameters and lengths of pipe in open hole. Fig. 1 is based on data from W. C. Main, of the Youngstown Sheet and Tube Company, and is a simplification of the method advocated by Gignoux. The chart gives reasonable results for the majority of cases, but in special cases more or less clearance may be used-depending upon straightness of hole, mud and general hole condition, and other factors.