| Polar monitoring shows that the global climate change in recent decades has led to the decrease of Arctic ice covering area and ice thickness.As the Arctic Routes has come into being,the shipping and offshore activities are becoming frequent,as well as the exploitation of oil and gas operations,which demonstrating that the Arctic exploitation has great potential economic value.However,the ice floes in summer and ice sheets in winter are still serious threats to the shipping and offshore operations in Arctic regions,which demand safe design and construction of polar ships and offshore structures.In addition,the polar environment is very vulnerable to pollution,so before large amount of Arctic sailing and offshore development activities can be put into reality,safety measurements on preventing serious sea pollution must be guaranteed.As for safe Arctic vessels design,one of the most critical challenges is the prediction of load due to interaction between polar ice and marine structures,which can affect the ship resistance,structure safety and maneuverability.However,polar ice shows complex and variable deformation,and failure modes as well.The ice load comes from different failure mechanisms of sea ice in different interaction scenarios between ice and marine structures.Crushing and fracturing are the main failure modes of polar sea ice.Crushing mainly occurs in ship-iceberg collision and the contact area between ice and ship,while fracturing mainly occur in the scenario that a ship passing through the ice sheet.Numerical method is employed to simulate the failure mode which describes the continuous compressive deformation,crushing and bending failure.They are main failure modes of polar ice,providing reliable estimation of ice load on ship structures due to ship-ice interaction.According to different deformation and failure modes of polar sea ice,ice material model is proposed and numerical program is compiled based on finite element method and extended finite element method,to simulate the ice failure and predict the ice load.The simulation results are compared with those of existing experiments,in order to validate the proposed ice failure mode.The research content and novelty of this thesis can be divided into the following parts:In order to investigate the continuous compression deformation and discrete crushing of isotropic sea ice and predict the ice crushing load,a viscoelastic-plastic material model is proposed for the isotropic ice.The material model is composed of viscoelastic and plastic components.The effects of strain rate,temperature,confining pressure and porosity on ice are considered in the viscoelastic component,and the range of parameters of the material model is given for its application.Based on the viscoelastic model,the Tsai-Wu yield criterion is employed to establish the viscoelastic-plastic material model to simulate the ice crushing.The central difference method and graph return algorithm are used to develop the numerical program of the proposed material model,which is then incorporated into LS-DYNA.The single unit tests are conducted to verify the accuracy of the program to prepare for the subsequent numerical simulations.Finite Element Method and the viscoelastic ice material model are used to simulate the continuous compressive deformation of ice.The constant strain rate experiment and creep experiment of iceberg ice and freshwater ice are simulated.By comparing the simulation results with those of the existing experiments,it is proved that the proposed material model can predict the ice compressive strength and also the creep deformation in a satisfiable accuracy.The material model can describe the influence of the strain rate,temperature,confining pressure and porosity on ice compressive strength.And the application range of the model to accurately predict ice strength is strain rate from 10-5to1.4?10-2,temperature from-30?C to-5?C,and confining pressure from 5MPa to 70MPa.The simulated results of creep experiment show that the model can simulate the primary creep,second creep of ice under different external force levels and the creep recovery after the external force is removed.The analysis shows that the ice compressive strength is very sensitive to temperature and strain rate.Sea ice can reach very high strength under low temperature combined with high strain rate,which is a threat to the safety of ships in ice area.Finite Element Method is adopted to simulate the iceberg collision and predict the ice crushing load.The viscoelastic-plastic material model combining with element deletion are used to simulate the Pond Inlet iceberg indentation experiment.The mesh sensitivity analysis shows that the fluctuation amplitude of the numerically calculated ice load is sensitive to the mesh size,while the average ice load is relatively insensitive to the mesh size.By comparing the numerical ice load and pressure-area curve with the experimental results,the validity and the accuracy of the numerical scheme for the ice crushing load prediction are verified.The stress distribution of ice is simulated and analyzed,and it is found the stress state is related to the distribution of internal damage and macro-cracks in the iceberg.Besides,the collision between a rigid plate and spherical ice is also simulated.The obtained pressure-area curve is found to correspond well with the empirical data.Finally,the effect of iceberg shape and structure shape on ice load is analyzed,and the analysis indicates that a rigid head pressing into the ice interior produces similar ice crushing load with that of a flat structure impacting a spherical iceberg at the same nominal contact area,while the former produces higher plastic deformation at the ice contact area.In order to investigate the crack initiation and extension of level ice and to predict the ice load,Extended Finite Element Method and cohesive model are adopted to simulate the collision between landing craft bow and level ice.Considering that the level ice is mostly columnar grain ice,the transversely isotropic elastic material model is used to describe the constitutive relation of ice bulk element,and a transversely isotropic Tsai-Wu criterion is used as the crack initiation criterion to improve the accuracy of the numerical simulation.The validity of the numerical results is proved by the mesh sensitivity analysis and the comparison with experimental results.The calculation results indicate that the numerical model is able to simulate both crack modes of level ice,bending and splitting.The initiation and extension path of the cracks are investigated.It is found that the bending crack emerges at the midline on the upper surface of the level ice and extends along an arc line towards the free edges.Splitting crack initiates at the middle of the area near the free edge on the bottom surface and extends along the radial direction.It also found that the initial crack is closely related to the tensile hydrostatic stress.Finally,the effects of collision velocity and inclination angle of the landing craft bow on the fracture modes,ice load and bending fracture size are studied.The results show that the increase of the collision speed and the inclination angle makes the level ice tend to bending fail;conversely,the level ice tends to splitting crack.Meanwhile,the increase in collision velocity makes the bending ice load increase and size of the broken ice decrease,while the increase in inclination angle of landing craft bow makes the bending load and broken ice size decrease.This paper conducts a numerical study of two major sea ice failure modes and ice load control mechanisms:crushing and failure.Effective numerical methods and sea ice material models are proposed for different sea ice failure modes,which can simulate the crushing of iceberg and the bending and splitting of level ice,as well as accurately predict the corresponding ice load in ship-ice interaction.The outcomes of this research are expected to provide useful information for the comprehensive and in-depth understanding of ice failure mechanism,ship-ice interaction and ice loading mechanism,as well as theoretical support for the design of polar ships and offshore structures. |
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