Fracture toughness, size of weld defect, fabrication error etc. which control brittle fracture of welded structure are generally thought to be probabilistically distributed factors. It is easy to evaluate deterministically whether fracture occurs or not by using fracture mechanics and the worst values of various factors. However, there exists some ambiguity in determination of the worst values.Reliability analysis of spherical tank for the model case was performed by means of fracture mechanics and Monte Calro simulation under postulation of distribution functions for various factors.The present report describes the postulated probabilistic distributions, model of simulation and formulation of analysis which are used in reliability analysis of the tank. It is recognized that Monte Calro simulation is effective method for reliability analysis of the tank.From the results of simulation, it is shown that 1) probability of failure Pf of the tank is 10-310-4 and 2) average value of δc, distribution parameter αδc, of δc, average size of retained weld defect etc. have significant effect on Pf, but angular distortion and misalignment of butt welded joint have trivial effect on Pf.It is pointed out that the following information and data accumulation are important to reliability analysis of welded structure. That is 1) probabilistic distribution parameters, especially in NDI detection probability and δc distribution function. 2) probabilistic information in what kind of microstructure the tip of weld defects exist. 3) information about difference of δc value between for natural crack and for fatigue crack.
In the process of production of Ti-6Al-4V alloy, which consists of blending of extra-low-chlorine T i and 60mass%Al-40mass%V master alloy, cold compaction, vacuum sintering and hot forging, the influence of compaction stress and sintering conditions on the density of sintered compacts and various forging characteristics of the sintered compacts were investigated.The increasing rate of the compact density during heating up to sintering temperature depends on density of the green compact and heating time, with which it has a linear correlation. The increasing rate of the compact density during holding at sintering temperature is in proportion to the power of the holding time. The empirical equation of the density change during sintering was derived from these findings, and confirmed to coincide well with experimental values.The density after forging is strongly influenced by pressure and plastic deformation during forging. Tile holes in the compact can be rejected completely by suitable forging condition, except the surface area of the compact where the temperature reduces rapidly compared with that of the inner part of the compact.
New premium connections for oil country pipes were developed and the gas tightness, anti-galling capability and joint strength were studied experimentally and theoretically. The sealing mechanism was the two-step-pin-nose radial metal seal and composed of two shoulders and stable gas tightness was ensured by the secondary reserve shoulder even if the primary shoulder was deformed. The stress relief mechanism was provided to alleviate the local pressure on the seal. The sealing area surface was processed by Cu, Zn plating or phosphating depending on the service environment to increase the anti-galling capability. The API Buttress type thread was used because of the easy application and sufficient joint strength. This connections were successfully applied to drill-strings used under severe conditions where stress varied much by thermal expansion and contraction, such as in steam injection wells containing heavy gravity crude oil in California, U.S.A. (13 figs, 3 tabs, 2 photos, 6 refs)
Optimum design analysis in engineering is generally formulated as a problem of nonlinear programming. For casing design problem, there are combinatorial variables, integer variables, which select a casing to be used among alternatives. In spite of including integer variables, nonlinear programming may be able to optimize the casing design problem, since it has an enough optimization potential for continuous variable problem. In this paper, an application study is presented to verify the usefulness of nonlinear programming termed SUMT for casing design problem.
For a design of deviated well trajectory, it is important to estimate torque and drag and to select the most appropriate well trajectory. Some equations of torque and drag have been proposed under an assumption that torque and drag forces are primarily caused by sliding friction. However, since they are formulated as an incremental equation using short element of the pipe, they have a little short versatility, i.e., necessity of computer usage. Under an assumption that azimuth angle and inclination angle do not change together over a bending section, the incremental equation can be transformed to differential equations. Based on conducted differential equations for three bending sections, theoretical formulas for inclination angle change section and approximate formula for azimuth angle change section are both derived as algebraic equation. These formulas can easily calculate torque and drag using only bending condition parameters. It is found that the proposed formulas can be used as a convenient estimating method, since the estimate for hookload including drag agrees well with one obtained by conventional iteration of the incremental equation.
For a horizontal well or an extended reach well, which is promising for well construction in . future, a well casing is subjected to local wear of wall thickness on its inside surface due to rotational contact with tool joint or drill pipe during drilling. Collapse strength of worn casing is analyzed as an unstable problem by the elastic-plastic finite-element method (FEM) and the decrease ratio of collapse strength due to casing wear is formulated by nonlinear programming using parameters of the wear shape. The elastic-plastic FEM is fully applicable as values calculated by the FEM program agree well with those of experiments. It was found that the decrease ratio of collapse strength of a worn casing is linearly proportional to the decrease ratio of wall thickness in both modes of plastic collapse and elastic collapse, and that the apical angle of the wear shape has a slight influence on collapse strength.
Optimum design analysis in engineering is generally formulated as a problem of nonlinear programming or nonlinear mixed integer programming including selection among alternatives. However, all working methods presently available for linear mixed integer programming are confined essentially to linear problems and are short of versatility. To overcome this situation, a new procedure for nonlinear mixed integer programming with constraint functions was developed for solving general optimum design problems. In this paper, an application study on well design is presented as an example to verify the usefulness of the developed optimization procedure.
