【作者】 徐永臣；

【导师】 李广雪； 吴建政；

【作者基本信息】 中国海洋大学 ， 海洋地质， 2014， 博士

【摘要】 1976年黄河改走清水沟流路后，黄河废弃的钓口流路水下三角洲遭到强烈蚀退，岸滩冲刷很快，水深加大，海岸线大幅度后退，原有沉积地貌单元遭到破坏和夷平。海底滑坡、沉陷、断裂、冲蚀沟槽、差异冲刷、粉土液化等海底不稳定因素均有发生，这些海底不稳定因素对油田开发带来了巨大挑战。海洋石油钻井平台在施工过程中，发生过桩腿刺穿、滑移甚至平台倾覆，造成了灾难性海洋工程事故。本论文依托多年积累的井场路由调查资料，研究了黄河废弃水下三角洲地基土的强度特征及分布规律。研究区近十年来整体处于冲刷状态，4-10m等深线之间的水下岸坡区平均冲刷大于2m。研究区浅部地基土主要为近期三角洲相的粉土和淤泥、淤泥质粉质粘土和早期的河湖沼泽相的粉质粘土、粉细砂及粉土。海底表层沉积物主要类型有粉土和淤泥、淤泥质粉质粘土。表层粉土，具有较高的力学强度，厚层粉土可以作为插桩式钻井平台的持力层；淤泥、淤泥质粉质粘土属于软弱土，不能满足插桩式钻井平台的要求。河湖相沉积的粉细砂、粉土，厚度一般大于10m，是插桩式钻井平台在该区理想的持力层。根据地基土破坏的一般形式和特征，分析了桩靴式钻井平台插拔桩过程中地基土的破坏形式和特征。插拔桩过程是动态贯入过程，桩端及桩靴周围地基土体受到强烈扰动，地基土破坏主要以冲剪破坏为主，土中并不出现明显的连续滑动面，该过程与土体静力学状态下土体整体破坏存在较大区别。在分析了插桩过程中土体的破坏形式基础上，根据极限平衡理论，假定破坏面的方法，推导出了均匀和双层地基土中基于被动土压力的插桩阻力计算公式。极限承载力的分析计算建立在土体整体剪切破坏模式上，给出了土体极限状态下的承载力。本文推导出的均匀土中和双层地基土中的插桩计算公式，建立在土体冲剪破坏模式上，给出了土体冲剪破坏下的插桩阻力。二者与实际插桩阻力存在一定的差距，分别给出了插桩阻力的上限和下限。插桩阻力分析计算过程中采用假设破坏面更接近于插桩过程土体的破坏模式，实例计算结果也表明该方法计算的插桩阻力较以前的极限承载力更接近于实际插桩阻力，丰富了桩靴式钻井平台插桩深度预测方法。根据插拔桩后井场的调查资料，分析了地基土经历插拔桩过程后的变化特征，探讨了桩坑内土体的强度恢复机理。地基土经历插拔桩过程后，在海底形成一定大小和深度的桩坑，在水动力作用下，桩坑逐渐被夷平。在浅地层声学记录上表现为地层反射面的中断，尚未夷平的桩坑在声纳图像上表现为近视圆形的凹坑。分析结果表明地基土经历插拔桩过程后工程性质发生了较大变化：粉土经历插拔桩过程后，桩穴内的含水量一般小于桩穴外的含水量；桩底下粉土的含水量比周边粉土的含水量小。桩穴内的粉土的压缩模量一般大于桩穴外地基土的压缩模量；软粘土的压缩模量一般小于桩坑周边地基土的压缩模量。研究认为，粉土是研究区桩靴式钻井平台的主要的持力层，具有良好的排水条件，经历插拔桩后的桩坑内的粉土强度一般大于或接近周围土的强度。软粘土由于排水条件很弱，插桩过程中受到扰动破坏，强度降低。受到扰动的地基土强度恢复过程复杂，影响因素主要有再沉积作用、自身的压缩固结、沙土液化和人工预压。需要重复插桩的井场，地基土变化由于经历了插拔桩的过程较原始地层复杂，为了提高地基土的强度，可以采取预压的方式提高地基土的承载力。近年来，随着海洋石油的开发活动加剧，研究区内平台插拔桩作业频繁，地基土受到扰动和破坏，从而使得本来就复杂的井场地层情况变得更为复杂。虽然在海洋水动力作用下，地基土强度会恢复，但由于其强度与周边土体强度存在差别，后续平台插拔桩施工作业过程中存在较大的安全隐患。进一步研究受到扰动破坏土体的工程性质变化及其对平台插桩的影响，有助于减少工程事故的发生。更多还原

【Abstract】 Since rerouting to Qingshuigou flow path in1976, the Yellow River abandonedDiaokou flow path underwater delta is under strong erosion with fast scouring speed,enormous water depth and large stepping back of coastline, which leads to thedestruction and deplanation of original sedimentary geomorphic units. The occurrenceof unstable seabed factors such as submarine slide, sinking，fracture, erosion groove,difference scour and silt liquefaction brings huge challenge for oil field development.The happenings of impaled pile leg, slipping and even overturned platform during theconstruction of offshore drilling platform cause marine engineering catastrophicaccidents.The character distribution law of the abandoned underwater delta foundationsoil strength of Yellow River is analyzed in this dissertation based on wellsite androuting survey data of years. The study area has been under erosion in the past tenyears, whose average erosion depth is greater than2m in subaqueous coastal slopewhere the contour is between4m and10m. The shallow foundation soil in study areais mainly composed of silt, mud and muddy-silty clay in recent delta facies and siltycaly, silt and silty-fine sand which belong to early Lacustrine bog facies. Sea floorsediments include silt, mud and muddy silty clay. Surface silt with high mechanicalstrength can be used as the bearing stratum of pile type drilling platform. Soft soil likemud and muddy siltyclay are disqualified. Silt and silty-fine sand belonging toLacustrine bog facies whose thickness is greater than10m is an ideal bearing stratumof pile type drilling platform in this area.The destruction forms and characteristics of foundation soil in the insertion andextraction of shoe type drilling platform are analyzed based on the general destructionforms and features of foundation soil. Insertion and extraction is a dynamicpenetration process in which the soil on the top and close to pile shoe is violentlydisturbed. The main destruction form is punching shear failure without continuousslipping surface. The process differs largely from the structural failure under staticstate. In accordance with the soil destruction form and limit equilibrium theory, insertpile resistance computational formula in uniform and two-layer foundation soil onpassive earth pressure is deducted through the hypothesis of failure surface.The analysis and calculation of ultimate bearing capacity deduces the bearingcapacity under limit state based on soil general shear failure mode. The insert pileresistance computational formula deducted in this essay calculates the capacity undersoil clash-cut mode according to soil clash-cut model. There is a certain gap betweenthem and the actual insert pile resistance, which calculates the upper limit and lowerlimit of insert pile resistance. The hypothesis of failure surface put forward in this essay is much closer to the actual insert pile failure and calculation examples alsoprove that the insert pile resistance computational result through formula deducted inthis essay is much closer to the actual value compared with the ultimate bearingcapacity, which enriches the prediction methods of insertion depth of shoe typedrilling platform.Based on the survey data of wellsite after insertion and extraction, variablecharacteristics of foundation soil and recovery mechanism of soil strength areanalyzed and explored in this essay. Shoe pits with a certain size and depth are formedon seabed after insertion and pullout process and reduce to deplanation underhydrodynamic action, which is displayed as the interruption of reflection surfaces onshallow acoustic records. Shoe pits which are not yet leveled are represented asapproximate circular pits on sonar image. Analysis results prove that engineeringproperties of foundation soil have changed greatly after insertion and extraction: watercontent inside pile hole is smaller than that outside; silt water content at the bottom ofpile is smaller than that around; silt compression modulus inside pile hole is biggerthan that outside; compression modulus of soft clay inside the pile hole is smaller thanthat outside. Investigation results imply that silt is the major bearing stratum of shoetype platform in study area with well-drained condition, whose strength in the pilehole is closed or greater than that around. Due to poor drainage condition, soft claystrength decreases after disturbance and destruction. The recovery of soil strength is acomplex process, whose influencing factors mainly include redeposition, compressionconsolidation, sand liquefaction and artificial preloading. In the wellsite which needsrepeated pile insertion, soil is more complex than the original. Preloading can improvesoil strength and bearing capacity.With the active development of offshore oil in recent years, the insertion andextraction of platform is conducted more frequently leading to foundation soildisturbance and destruction, which makes wellsite stratum situation even morecomplicated. The strength of foundation soil will be recovered under marinehydrodynamic action, while differences still exist with the strength of surroundingfoundation soil, which causes high security risks to the later platform insertion andextraction. Further investigation and exploration of engineering properties change ofdisturbance and destruction soil and its influence on platform insertion can contributeto the reduction of engineering accidents.更多还原