Response To Platform Pile Insertion And Pullout Of Yellow River Abandoned Underwater Delta Foundation Soil

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.