Thermal-mechanical Sequence Coupling Analysis Of Ultimate Bearing Capacity Of Spudcan Foundation In Polar Marine Permafrost

The arctic contains vast reserves of oil and gas,accounting for about 22 percent of the world's untapped oil and gas resources,most of which are located offshore at depths of 500meters.But there is permafrost on the seafloor in the arctic,where ice age remains and it takes a long time to melt away.When the jack-up offshore drilling platform penetrates its pile foundation into the seabed soil,the heat conduction between the pile foundation and the permafrost will cause the permafrost to melt.The strength of permafrost decreases after thawing,which leads to the reduction of bearing capacity of pile foundation and poses a great threat to the stability of jack-up offshore platform.Therefore,the study of ultimate bearing capacity characteristics of jack-up platform pile foundation in polar offshore permafrost is an important part of Marine engineering science,but there is no international research on this issue.Based on the flat bottom shoe,boots skirt piles,cylindrical pile foundation as the basic research object,based on Abaqus/Standard of pure heat conduction and implicit small deformation finite element method,through the establishment of shoe sequence and permafrost thermal-mechanical coupled finite element model to simulate the pile shoe in the ocean of permafrost thermal conduction process,to explore the change law of vertical ultimate bearing capacity analysis after thawing permafrost strength reduction factor's influence on the ultimate bearing capacity.In the first part of this paper,the heat conduction process of flat footing,skirt footing and circular footing in marine frozen soil is simulated.The temperature field of three kinds of pile footings foundation under different depth ratio?H/D?is calculated,and the influence of three kinds of pile footings on the heat transfer performance of frozen soil and the shape of the pile footings on the temperature field are analyzed.The influence of flat-bottomed pile footings on soil temperature is the greatest,and the thawing boundary of frozen soil is the deepest,while the thawing rate of frozen soil around the skirt of skirt-type pile footings is low,which has the least effect on the frozen soil temperature,and the thermal conductivity of cylindrical pile foundation is almost the same as that of flat-bottomed pile footings.In the second part of this paper,the temperature field data of three kinds of pile footings calculated in the previous part are input into the corresponding ultimate bearing capacity model by means of predefined temperature field through Abaqus/ODB result file,so as to establish the thermo-force sequential coupling finite element model and realize the simulation of thermal-mechanical sequential coupling between pile foots and frozen soil.Based on this model,the variation rule of vertical ultimate bearing capacity of pile footings under the action of temperature field is studied.The vertical ultimate bearing capacity of the three kinds of pile footings is negatively correlated with the time quantity of temperature field and positively correlated with the buried depth rate?H/D?.When the permafrost temperature at the bottom of the pile is in the latent thermal effect temperature zone,the change rate of ultimate bearing capacity reaches the peak.The reduction coefficient of thawing strength of permafrost(s_u,T/s_u0)has a significant influence on the vertical ultimate bearing capacity of flat-bottomed pile footings.