抗挤强度新算法对套管柱设计安全系数的影响

Analysis of How New Equations for Calculating Casing Collapse Strength Affects Safety Factor of Casing String Design

新版的API 5C3—2008标准已经颁布实施,其附录中给出了新的套管挤毁强度计算公式。将其与API 5C3—1994标准中套管挤毁强度计算公式(旧公式)进行了对比分析,并按新公式计算了油田常用套管的抗挤强度值。认为新公式建模更合理,计算结果更精确,且计算结果相对于旧公式的值发生了较大的变化。对于140钢级以下的套管,当套管径厚比大于20时,新公式计算值大于旧公式值;当径厚比小于20时,新公式值小于旧公式值。对140钢级以上的套管,这一变化的径厚比的界限值为22。基于对套管抗挤强度值的新认识,给出了套管柱设计时油田常用套管的合理抗挤安全系数建议值。

API 5C3-2008 standard has been promulgated and implemented.The new formula to calculate the casing collapse strength has been given in its appendix.In this paper,it is conducted that comparative analysis of formula to calculate the casing collapse strength in API 5C3-1994.It suggests that the new formula is more accurate,the model is more reasonable,because the effect of manufacturing factors and material properties on casing collapse strength is considered.The collapse strength of common oilfield casing is calculated according to the new formula.The value of new formula is very different from the results given in API 5C2-1999.The results show that：For casing steel grade below 140,the value of the casing collapse strength is greater than that of API 5C2 when the casing of diameter thickness ratio is greater than 20;It is less than that of API 5C2 when the diameter thickness ratio is less than 20.For casing steel grade over 140,the limit value of diameter thickness ratio is 22.So this paper gives reasonable collapse safety factor of common casing.