Research On Direct Analysis Approaches Of Polar Ship Ice Loads

Due to the action of sea ice,the ship operating in the ice area will be subject to ice load.Determining the ice load on ship hull reasonable is the basic requirements for designing ship operating in the ice area.Therefore,many researchers have been paying attention to ice load on ship hull.Similar to the research methods on other engineering problem,theoretical analysis,model test and numerical simulation have been adopted to study the ice load on ship hull by domestic and foreign researchers.Among them,the theoretical formulas based on the ideal assumptions need to be revised by engineer's experience;the data obtained by the tests depends on the test object.At the same time,it also has the characteristics of large investment,high cost,time-consuming and so on.Numerical simulation method can not only be used as an effective supplement to theoretical analysis and model test,but also will be expected to be the main means and effective tool for ship design.In this paper,the general commercial finite element software ABAQUS have been adopted as the secondary development platform,with its user-defined subroutine(User Subroutine)as a development tool.Based on fracture mechanics and damage mechanics,the direct analysis method of ice load have been developed,respectively.Meanwhile,the corresponding user-defined elements have been developed.Then,the direct analysis method of ice load is taken as an effective supplement to the theoretical analysis,and a simplified assessment method for assessing the safety of hull structure under the action of iceberg is studied.The main work of this paper is as follows:Firstly,based on the characteristics of sea ice,the global ice load model based on fracture mechanics and the local ice load model based on damage mechanics are studied.Then,based on the fracture mechanics model and the damage mechanics model,the direct analysis methods of ice load are established,respectively.The fracture mechanics model analyzes the global load and ice resistance of ship advancing in level ice from the global perspective.The damage mechanics model analyzes the interaction between the hull and the sea ice from the local perspective.The local ice load could be used for designing of the hull structure and optimizing the ship profile.At the same time,based on the theory of fracture mechanics and damage mechanics theory,the moving crack fracture element and sea ice damage element are developed.In order to verify the accuracy of the developed elements,the solution obtained by proposed direct analysis methods of ice load is compared with the corresponding analytic solution.Secondly,this thesis has carried out the application research on direct analysis method of global ice load based on fracture mechanics.By using the direct analysis method of global ice load based on fracture mechanics with the Lindqvist formula as reference,the critical strain energy release rate of ice floe is adjusted by numerical test.The numerical relationship between the critical energy release rate of ice floe and the speed of ship(0.5m/s to 3m/s)is established.Based on the numerical relationship between the critical strain release rate of ice floe and the speed of ship,the global ice load of ship advancing and turning in level ice are studied by the direct analysis method.Further,the feasibility of the direct analysis method of global ice load based on fracture mechanics is verified,and the influence of ship's speed and water-plane width on the global ice load is analyzed.At the same time,this thesis has carried out the application research on direct analysis method of local ice load based on damage mechanics.The interaction between hull and level ice is characterized to the interaction of different types of surfaces with level ice.According to the relationship between the normal of level ice and the normal of surface,and the curvature of the surface,the shape of hull structure is divided into three categories: plane,arc surface and sharp surface.Based on the direct analysis method of local ice load based on damage mechanics,the local ice load of plane,arc and sharp structure with different projection length and angle have been studied.Meanwhile,the influence of the parameters such as the projection length and the curvature of the surface on the ice force of the structure are studied by direct analysis method.And the local ice load spectrum of hull structure is constructed based on the calculated results obtained by direct analysis method.Therefore,the variation of the ice load with different shape of the curved surface,and different angle between the surface and the ice sheet can be given qualitatively,which could help the engineer to choose the appropriate hull shape.Finally,this thesis has carried out the research on the simplified assessment method for assessing the safety of hull structure under the action of iceberg.In polar waters,there are not only ice floes,but also floating iceberg.The existence of the iceberg poses a threat to the safety of ship operating in polar waters.In order to improve the safety of the ship operating in polar waters,it is necessary to study the assessment method for assessing the safety of hull structure under the action of iceberg.In this thesis,the simplified assessment method for assessing the safety of hull structure under the action of iceberg has been established with the effective supplement by direct analysis method of ice load.Firstly,the ice force of the hull structure is determined by the direct analysis method which based on damage mechanics.Then,based on the rigid plasticity theory,a simplified analysis method of ultimate load-carrying capacity of hull structure is proposed.Finally,the simplified assessment method for assessing the safety of hull structure under the action of iceberg is established by combining the ice force obtained by direct analysis method and the simplified analysis method.Thus,an available simplified assessment method for assessing the safety of hull structure under the action of iceberg has been provided to the engineers.