Discrete Element Analysis Of Sea Ice Failure Mode And Ice Load During The Interaction With Marine Structure

With the increase of human activities on resource exploitation in cold regions,the interaction between sea ice and marine structures poses a serious threat to structural stability and personnel safety.Therefore,a lot of researches have been carried out on the mechanical properties of sea ice and structural ice load.The main research methods include field monitoring,model test and theoretical analysis.In recent years,the numerical simulation method has also been developed in the calculation of ice loads during the interaction between sea ice and marine structure.Traditionally,the finite element method is difficult to accurately describe the failure mode of sea ice since it has the mechanical properties of brittle materials.The discrete element method(DEM)is widely adapted to analyzs the breakage process of brittle materials,such as rock,ceramic and glass.The dynamic process of sea ice fracture,rubbling and pile-up can be effectively simulated by DEM with the bond and failure function.In addition,the GPU-based parallel computing technology can improve the computational efficiency and scale of DEM simulations,so that the method can be applied to large-scale numerical simulation for sea ice engineering.At present,the important studies of DEM simulation for sea ice engineering includes developing reasonable bond and fracture criterion in DEM to describe the mechanical behaviors of sea ice interacting with marine structures,revealing the relationship between failure mode of sea ice and ice loads on structure.In this thesis,the DEM approach for sea ice engineering is developed to simulate the interaction between sea ice and marine structure.The bond and failure criterion,the relationship between macro-micro parameters for sea ice,and the GPU-based parallel algorithm are studied.The research mainly focuses on the characteristics of ice loads on structure and the failure mode of sea ice under different types of marine structures and sea ice parameters.The main works in this thesis are listed:(1)The sea ice model is constructed by spherical elements with bond function.The parallel bonding model,tension-shear fracture criterion,hybrid fracture criterion,linear contact model and motion equation of spherical elements are introduced in details.An optimizing and improving method for engineering sea ice application based on GPU parallel algorithm of DEM is proposed.(2)The relationship between sea ice strength and micro-discrete element parameters is established by DEM simulations of the uniaxial compression test and three-point bending test of sea ice.The bonding strength and internal friction coefficient between particles are determined by the compressive strength and flexural strength of sea ice.According to the principle of orthogonal experiment,the sensitivity of the DEM parameters to the simulation results is analyzed,and thus the reasonable range of the important parameters is determined.(3)The interaction between sea ice and conical structure is simulated by the DEM.The ice load and sea ice failure modes are validated with the field data in the Bohai Sea and the model test results in the HSVA to verify the rationality of parameters of DEM.It is found that the bending failure mainly occurs when the sea ice interacts with conical structure.The failure process is affected by the inclination angle of cone,the diameter of cone,ice velocity,ice thickness and the position of ice-cone interaction,which can lead to the change of ice load.From the aspects of the generation and propagation of macro-cracks and the bonding failure mode of bonded particles,the evolution process of the failure mode of sea ice is analyzed.A formula for calculating static ice force of conical structure is proposed.(4)The breaking process of sea ice interacting with the vertical structure is simulated by employing the tension-shear fracture model and the hybrid fracture model considering damage respectively.The comparison results show that the hybrid fracture model is more suitable for simulating the crushing failure of sea ice.The influence of ice velocity on the crushing failure is studied with this model.The results illustrate that the brittle crushing failure of sea ice at higher ice velocity caused the random vibration of structure,while the ductile crushing failure occurs at lower ice velocity caused the steady vibration of structure.Combining with the variation characteristics of ice load,structural vibration,relative velocity of sea ice and structure and the number of failure times of particle bonding,the physical process of brittle-ductile transition of sea ice at lower ice velocity is revealed.The results show that the crushing failure process of sea ice is not only related to its mechanical properties,but also affected by structural vibration,which verifies that the steady-state vibration of structure has self-excited vibration property at low ice velocity.In addition,the ice pressure distribution characteristics on vertical structures are analyzed by simulating the crushing process of sea ice acting with the vertical structure.The mechanism of high pressure zones is studied.(5)The ice pile-up at intake of nuclear power plant and the interaction between sea ice and nuclear floating platform are simulated by the DEM.The numerical model of ice floe in the DEM is established according to the floe shape to simulate the pile-up at intake with the effect of wind and current.The influence of the mean size,density and velocity of sea ice on the ice pile-up is analyzed.The height of ice pile-up is reasonably predicted to reduce the risk of ice jamming caused by ice pile-up at the intake of nuclear power plant.Nuclear floating platform should have good anti-icing performance when it is operated in ice regions.The interaction process between sea ice and nuclear floating platform is simulated by DEM to analyze the ice load on platform,which can provide reasonable reference for structure design.At last,the research of DEM analysis for the interaction between sea ice and marine structure is summarized,while the research directions of the future work are pointed out.