套管磨损后剩余抗挤强度的数值分析

假设套管内壁的磨损缺陷为圆弧,并视为平面问题,采用平面8结点等参元,分别按平面应力和平面应变问题,分析其抗挤强度。按平面应力分析时,得到的抗挤强度数值结果比根据最小壁厚从API标准(API Bul 5C2)中查得的数据还要小。在这两种情况下,抗挤强度都随磨损缺陷深度的增加和半径的减小而降低,而平面应变分析得到的抗挤强度数值结果比平面应力分析得到的结果要高。为了评价平面分析的精度,又假设套管的磨损缺陷为圆柱面加两个球面或椭球面,采用空间20结点等参元进行分析,结果表明:当套管无缺陷部分的长度保持不变时,其抗挤强度随缺陷长度的增加而降低并趋向一个下限值;当套管含缺陷部分的长度保持不变时,其抗挤强度随套管长度的增加而增加并趋向一个上限值;当缺陷较长时,空间数值分析结果与平面数值分析结果相近;当无缺陷部分的长度较长时,若其它条件不变,则缺陷端部为球面所得的结果与缺陷端部为椭球面所得的结果几乎没有差别;端部在轴向施加固定约束所得的抗挤强度比端部自由时要大。

NUMERICAL ANALYSIS ON THE REMAINING COLLAPSING STRENGTH OF WORN CASING

Casing collapse accounts one of the main failures during production, therefore it's of great importance to understand the remaining collapsing strength of worn casing. Suppose the worn part to be a circular arc, and could be looked as a plane model, the paper tries to analyze the collapsing strength from the two aspects of plane stress and plane strain, both applying the 8-node-element. The results show that collapsing strength decreases with the aggravation of casing wear and shortening of radius on both aspects, while the collapsing strength obtained by plane strain is higher than that of plane stress, which is lower than the minimum value of API standard. To further estimate the accuracy of this method, a spatial analysis method is adopted, which takes the worn part as cylinder with two spheres or ellipsoids, and applies the 20-node-element. The results show that the collapsing strength varies with the specific lengths of worn part and unworn part, but with a maximum and minimum value, when there is relative long worn part, approximately results are obtained by the two analysis methods.