Numerical Research On The Interactions Between Marine Structures And Level Ice Based On Cohesive Element Model

The oil,natural gas and other resources are rich in the Arctic region.In the past two decades,the development projects of oil and gas in the Arctic region have present an obviously accelerated growth trend.As the global climate gradually warms and the Arctic ice continues to melt,the Arctic shipping lines get more and more busy,and will become one of the major routes of world trade in the future.There are great difficulties and risks for operating and sailing in polar regions.For example,the shell or device of marine structures may be damaged due to the collision with floating ice,level ice and iceberg,and the structures may even be caused to capsize;Ships may be trapped in area of large floating ice or thick level ice;Machinery and equipment of offshore platforms or ships may malfunction under low temperature,and so on.The development and utilization of the arctic should be based on safe operation and navigation.Therefore,it has great theoretical and practical significance to study the mechanism of interactions between marine structures and sea ice and to predict accurately the ice loads.Under this background,based on the cohesive element model,combined with the homogenized elastoplastic constitutive model of sea ice,this thesis has applied the nonlinear explicit finite element software LS-DYNA to simulate the interactions between different kinds of marine structures and level ice,and to predict the ice loads.Specific research contents and results are as follows:(1)A homogenized elastoplastic constitutive model is used to simulate the microscopic local crushing failure of sea ice on the macroscopic level.In order to simulate the damaged and softening effect of local crushing on sea ice material,the amount of sea ice microcrack development is equivalent to the effective plastic strain of sea ice,and the effective stress of sea ice decreases with the increase of equivalent plastic strain in the plastic zone.Three damaged and softening criteria in the plastic zone are proposed to characterize the different damaged and softening processes of ice materials with the development of micro-cracks.Based on the data of level ice compressive tests,the influence of strain rate is added into the constitutive model to consider the variation of ice compressive strength under different strain rates.The numerical model of an ice cone crushing against a rigid plate is established to validate the ice constitutive model.The mesh convergence in the numerical simulation and the sensitivity of the material parameters in the plastic zone are analyzed.Three damaged and softening criteria for sea ice materials are applied in the numerical simulations,and the simulated impact load curve and the pressure-area curve are compared with model test data for validation.The results show that the impact load curve and the pressure-area curve calculated by the constitutive model with the linear damaged and softening criterion fit best with the model test results,and the pressure distribution during the crushing process of sea ice by the numerical simulation is also consistent with the observed results in the experiment,which proves the validity of the ice constitutive model.(2)The finite element numerical model of the collision between a cone and level ice is established.The local crushing failure and bending fracture failure of level ice are simulated by the combination of the homogenized elastoplastic constitutive model and cohesive element model.The regular triangular prism mesh is applied to ice bulk element to simulate the random fracture path of actual sea ice more accurately.Firstly,the mesh sensitivity of ice bulk element is analyzed.Then the validity of the numerical simulation results is proved by comparing the simulated ice loads with those obtained from the model test.The results show that the calculated ice loads are in good agreement with the model test results in both the time domain and frequency domain when the horizontal size of the ice mesh is close to the radial breaking length of level ice.The numerical model simulates well the local crushing and bending fracture failure of level ice,as well as the subsequent processes of ice rubble accumulation,rotation and sliding of ice fragments during the process of cone-level ice collision.The effectiveness of the numerical model is proved.Then,the influence of the main material parameters of cohesive element on the numerical simulation results is studied.A method is used to extract the load component of ice breaking from the total ice loads of the cone,and the proportion of ice breaking components in total ice loads is calculated.Finally,the effects of impacting velocity,cone angle and cone waterline diameter on ice loads and the proportion of the ice breaking component in total ice loads are studied.The different ice failure patterns and the changes of ice loads between the cone-level ice collision and cylinder-level ice collision are also compared.(3)The ship continuous icebreaking process in level ice is simulated by using the cohesive element model combined with the homogenized elastoplastic constitutive model.Firstly,the results of numerical simulation and model test are compared for validation.Similar to the cone-level ice collision scenario,when the horizontal length of ice bulk element mesh is close to the radial ice breaking length,the numerical simulation results of ice loads are in good agreement with the model tests results in both the time domain and frequency domain.The numerical simulation well reproduces the local crushing and bending fracture failure of level ice,as well as the rotation and sliding process of ice fragments in the continuous icebreaking process.Then,Lindqvist semiempirical formula and Riska semi-empirical formula are introduced,and Lindqvist semi-empirical formula is refined by using the method of hull longitudinal section to consider the influence on icebreaking resistance brought by different ice failure patterns in different bow areas.By comparing the numerical results with the results calculated by the three semi-empirical formulas,the influences of ship speed on ice resistance and ice failure pattern are studied.Then the numerical method and the refined Lindqvist semiempirical formula are used to study the influences of ship draft on icebreaking resistance and ice failure pattern.The variations of ice resistance and ice failure pattern when the ship travels with different heeling angles in level ice are also studied by numerical simulations.The mechanism of variation of ice resistance and ice failure pattern is explained in detail.(4)The cohesive element model and the homogenized elastoplastic constitutive model are used to simulate the propeller milling level ice.The ice block model with finite width and fixed constraints at the two sides are established to simulate the level ice with infinite width.The ice bulk element adopts regular hexahedral mesh with fine division to improve the numerical stability and eliminate the numerical error caused by its shape defect.Firstly,the mesh convergence of the numerical model is analyzed,and the validation of the numerical results is proved by comparing the numerical results with the model test results.The results show that the peak values of resultant force and moment on the propeller blade and their arisen time fit well with the experimental results,and the numerical method simulates well the physical phenomena during propeller blades milling ice block with high speed,including ice local crushing,splashing of ice fragments and the irregular incision shape due to ice fracture failure under high strain rate.Then,the effects of ice compressive strength,shear strength and porosity,as well as propeller advance speed,rotation speed and cutting depth on the ice loads are studied.The results show that the average ice loads decrease linearly with the increase of propeller rotation speed,and accelerate with the increase of advance speed and cutting depth.Therefore,when the propeller is used to break level ice,the rotation speed of the propeller should be appropriately increased,and the advance speed of the propeller and ice cutting depth should be carefully controlled to ensure the safe operation of the propeller.The research results show that the numerical method established in this thesis is suitable for simulating the interaction process between different kinds of marine structures and level ice,meanwhile it can accurately predict the ice loads on the structures.These results can provide the reference for formulating the specification for structure design in polar regions,as well as the effective theoretical guidance and numerical tools for the design and safe use of marine structures in the polar regions.