深水井口导管管土界面摩擦退化试验研究

深水井口导管在黏土中的贯入阻力与导管土体界面的黏性摩擦系数密切相关。在导管安装过程中,管土界面会产生大位移剪切,使得导管-黏土界面剪切力产生退化。本文针对传统剪切试验装置小位移剪切的限制,无法正确描述井口导管贯入时导管-黏土界面大位移剪切时的退化机理,采用GDS多功能界面剪切试验仪在恒应力条件下进行大位移单调剪切试验,研究法向应力对导管管土界面摩擦退化机理的影响规律。试验结果表明,大位移单调剪切时,界面剪应力随剪切位移先增大后减小,最终趋于稳定,界面表现出剪应力退化现象,随着法向应力的增大界面峰值应力与残余应力均增大;土样法向位移随着剪切位移、法向应力的增加而增大最终趋于稳定。最后在对试验数据分析的基础上,基于剪应力随无量纲位移的退化规律,提出了界面剪应力随累积位移退化的计算方法。并结合算例对比分析了井口导管在贯入80 m深度处,由于界面摩擦退化效应导致导管贯入阻力降低了28.48%。

Experimental study on soil-conductor interface friction degradation of wellhead in deepwater

The penetration resistance of a deep-water conductor in clay is closely related to its friction resistance at the conductor-soil interface. During the installation of this conductor, considerable displacement occurs at the soil-pipe interface, causing degradation in the friction at the conductor-clay interface. Conventional shear testing devices cannot accurately describe this degradation mechanism at the conductor-soil interface under large displacement shear during the conductor penetration; therefore, in this study, to address this limitation, the GDS multifunctional interface shear testing apparatus was used to conduct large displacement monotonic shear tests under constant stress conditions. The aim was to investigate the influence of normal stress on the degradation mechanism of friction at the conductor-soil interface. The experimental results indicate that during the monotonic shear, the interfacial shear stress initially increases with shear displacement and then decreases, eventually reaching a stable state. The interface exhibits shear stress degradation. With increasing normal stress, the peak stress and residual stress at the interface also increase. The normal displacement of the soil sample increases with shear displacement and normal stress and eventually reaches a stable value. Based on the analysis of the experimental data, a simple method is proposed to calculate the interfacial shear stress with increasing dimensionless displacement considering the shear stress degradation mechanism. Finally, by combining an engineering case study of a conductor at a depth of 80 m, degradation of the interfacial friction is demonstrated to have reduced penetration resistance by 28.48%.