With the development of the national economy, pressure pipes have been more and more widely used. In order to use pressure pipes safely and efficiently, a reliability analysis should be conducted on them, and their safety conditions should be measured with reliability indicators.
2 Reliability analysis SAPV-95, that is, "Safety Assessment Regulations for Defective Pressure Vessels in Service" adopts the failure assessment chart for the safety assessment of structures with plane defects under static load. The failure assessment chart is based on the British Central Electricity Authority (CEGB) Based on the "dual criterion" evaluation method (usually called the R6 evaluation method), the failure evaluation chart developed based on the deformation plastic theory solution obtained by the General Electric Company (GE) of the United States of America is based on the stress intensity factor Ki near the crack tip and the material. The ratio of fracture toughness Kic, that is, the dimensionless number Kr is the ordinate, and the ratio of the primary load P that the structure bears to the material plastic yield limit load P0, that is, the dimensionless number Lr is the abscissa, and the international setting Kr is subject to the average value, The coefficient of variation is the normal distribution of VKr; Li follows the normal distribution with the mean value of -L and the coefficient of variation is VLr, and Ki and Li have no standardized transformation, so that the LlKr coordinate system is transformed into the ZlZ2 coordinate system.
Then the interval where Zi is located is if the interval where Zi is located is x-kh, x + h otherwise, calculate u (input + 3h), U (X + 5h) until the interval where Zi is located is the second step, use Fibonacci method to determine the solution The approximate value of Zi. Set X among them! The iteration accuracy given in advance makes this repeated n times, and the approximate solution Zi is obtained. 3.2 Model test operation Using the existing NMH1 horizontal well and E117 lateral well history data, the comprehensive model is analyzed based on the strength of the drill string as a confirmatory test Run and program to perform strength calculation and analysis. The results are basically consistent with the results of the whole well drill string strength finite element calculation, indicating that the accuracy is credible, but it is simpler and more convenient than that. It has good practicability. 13.3 The accident analysis and analysis of 3242m accounts for 53 segments. Accurately investigate the stress distribution and take The calculation result of the friction coefficient between the drill string and the borehole wall is 0.22. See figure ab. The symbols â–¡ A, X, and + represent the bending stress, torsional stress, axial tensile and compressive stress, third equivalent stress, shaft axis and other curves. The maximum bending stress of the drill string is 643.3 MPa, where the well depth is 2087.06 m; the maximum torsional stress is 895.7 MPa, the maximum equivalent stress is 1893 MPa, where the well depth is 2089.05 m; the maximum tensile stress is 137.7 MPa, here at the wellhead.
The stress distribution of the whole drill string is shown in Figure a. It can be seen that the drill string above the window is dominated by axial tension and compression, and the stress decreases with the increase of the well depth, which is much smaller than the working stress of the bending section; the bending section drill string is bent and twisted Mainly, and the stress value is very large, especially near the bottom of the well is particularly prominent, mainly reflected in the curvature of the shaft.
The stress distribution of the drill string in the bending section is shown in Figure b. The bending stress, torsional stress and the third equivalent stress all reach the entire drill 506 ~ 1893MPa. Even if the steel grade S135 with a higher yield strength of 930.79MPa is used, the equivalent working stress far exceeds The strength requirements and even the working stress with such a high yield strength can cause damage at any time in this interval. In this case, the actual (first) damage is often those with size or geometric defects or mutations (these places are prone to stress Concentrated and more easily destroyed) The drill string failure of this well happened in this interval (at 2 thread buckles and the equivalent working stress above 2085.08m is mostly less than the yield strength of 087.06 ~ 2089.05m, the axial tensile and compressive stress is relatively It is negligible, although the bending curvature is as high as 3. However, the bending stress is much smaller than the yield strength, which is within the range of strength requirements. The equivalent working stress reaches such a high value, mainly due to the large change in orientation (about 2 / m), resulting in a large It can be seen that due to the torsional stress, the larger azimuth change has a greater threat to the strength of the drill string. In that the curvature of the wellbore at a particular head orientation changes in performance) is too large, a rather large operating stresses (mainly reflected in torsional shear stress), followed by both the presence of stress concentration, resulting in the drill string shear fracture.
2) In the construction of horizontal wells with large curvatures, the strength of the drill string is more sensitive to changes in azimuth, so pay special attention to controlling the change in azimuth not to be too large, so as to avoid damage to the strength of the drill string.
Exam 1 Liu Yanqiang. Simplified method for strength analysis of large deflection drill string. See: Du Qinghua, Mechanics and Applications 3 Liu Yanqiang, Lu Yingmin. Calculation and analysis of drag resistance of drill string. Acta Petroleum Sinica, 1996, 17 (3): 110-115 (Continued from page 68) With the increase of vl, v., The U value decreases, that is, as the dispersion of L and Kr increases, the security state becomes worse This is in summary with the reliability theory, and the following conclusions can be drawn: 1) The reliability index can quantitatively represent the safety state of the pressure pipeline with plane defects under static load 2) The two-step method can quickly solve the problem with plane defects The reliability index of the pressure vessel under static load.
3) The reliability index of pressure pipes with plane defects under static load decreases with the increase of the average value and coefficient of variation of Lr and Kr.
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