基于分项系数法的套管实用可靠度设计方法

Practical reliability design method of casing based on partial coefficient method

针对套管服役过程中几何尺寸、力学性能和所承受荷载作用随机性和时变性的特点,结合ISO2394、EN1990、GB50153等国内外结构可靠度设计统一标准,提出了一种基于分项系数方法的套管可靠度设计的实用方法。该方法依据固井水泥环固结前后套管荷载组合形式的差异性和套管强度的折减,将套管整个服役过程分为钻完井阶段和生产阶段,分别建立了上述2个阶段的套管强度设计极限状态方程。考虑荷载和强度的变异系数对套管设计值的影响,采用可靠度理论分别计算套管荷载和强度的分项系数,实现了套管服役期间的可靠度定量表述,可以有效预测套管在整个服役期间的安全可靠性能。计算结果表明:管材强度变异系数对设计结果的影响要远大于荷载变异系数,在设计时应注意选择性质稳定、强度变异系数小的管材。通过与SY/T5724的设计结果相比较,基于分项系数法的分阶段强度可靠度设计方法可以满足工程套管强度的实际需要。

Aiming at the randomness and time-varying characteristics of geometry size,mechanical property and bearing load during casing service process,apractical method is put forward for reliability design of casing based on partial coefficient method in combination with ISO2394,EN1990,GB50153 and other domestic and foreign unified standards for structural reliability design.Based on the differences in casing load combinations and the degradation of casing strength prior and posterior to cement sheath consolidation,the casing service process is divided into two stages using the above method,i.e.,drilling/completion stage and production stage,based on which the limit state equations of casing strength design in the above two stages are established respectively.Taking into account the influences of load and strength variation coefficients on casing design values,the reliability theory was used to calculate the partial coefficients of casing load and strength,and achieve the quantitative expression of reliability in the casing service process,thus effectively predicting the safe reliability of casing in the whole service process.Calculation results show that casing strength variation coefficient has rather higher impacts on design results than load variation coefficient.In the design stage,the casings with property stability and low strength variation coefficient should be selected.Compared with the design results of SY/T5724,the staged strength reliability design method based on partial coefficient method is able to meet the practical strength demand for engineering casing.