Investigation On Mechanism Of Marine Structure-Sea Ice Collision Using Finite Element Method

As the climate changes in the Arctic,ice coverage and thickness continues to decrease.Regular transport through the Northeastern and Northwestern Arctic Sea has become possible.Moreover,Arctic region is rich in oil and gas resource.These changes lead to a significant increase in marine and offshore activities in the Arctic in the coming years.The probability of collisions between icebergs or ice sheet and ships or offshore structures increase.Using finite element method,this paper studies the mechanism of marine structure-sea ice collision to give advises to the regulars for the Arctic structures.In order to simulate impact forces,failure process of sea ice and marine structure responses,this paper builds three sea ice models which represent different charactors of sea ice.(1)A temperature-gradient-dependent elastic-plastic material model of ice is proposed.The model is based on the ‘Tsai-Wu'-type yield surface,and an empirical failure criterion is adopted.A series of yield surfaces with different sizes but the same shape are obtained from the linear interpolation of test results to represent the continuous temperature range in an iceberg.Temperature dependence is defined as the change in ice properties due to the temperature gradient as a function of depth of the iceberg.Based on field test data,three types of iceberg temperature profiles are assumed.The ice model is implemented as a user-defined subroutine in the commercial explicit finite element code LS-DYNA.Collisions between a rigid plate and different geometric iceberg shapes are simulated to analyze the influence of iceberg geometry and ice model temperature.The calculated contact area-pressure curves are compared with design laws to further calibrate the proposed ice model.Both a sharp temperature profile and low temperature range can increase the local contact pressure and global contact force as the penetration increases.The simulation results show that the ice model can capture and be used to demonstrate the influence of temperature-gradient on contact force in ship-iceberg collisions;(2)A nonlinear viscoelastic iceberg material model is proposed.A nonlinear Burgers' model in which Kelvin and Maxwell units are strain rate-and stress-dependent is adopted for the iceberg material.The strain rate effect is considered in this model based on the experimental results.The stress of the iceberg model grows linearly(in log form)with increasing strain rate before reaching the transition strain rate,after which the stress remains rather constant.A damage function that reflects microstructure changes and severe fractures in ice is adopted as the failure criterion.The iceberg model is implemented using implicit integration Crank-Nicolson method and is incorporated in the commercial software LS-DYNA by a user-defined material.Laboratory-scale experiments,creep experiments and constant strain rate experiments,and reality-scale experiment,iceberg-rigid steel plate collisions,are simulated to validate the proposed iceberg material model.Simulated time-strain curves are compared with the results of creep experiments.In the constant strain rate experiments,the strain-stress curves for brittle and ductile failure and ultimate triaxial strength of the ice model are analyzed.Area-pressure curves and contact force-displacement relations are investigated for different impact speeds in iceberg-steel plate collisions.Contact force is also studied in view of the kinetic energy of icebergs.The numerical results show that the proposed iceberg material model yields reasonably good results.(3)A simple topology method for insertion of cohesive element in 3D tetrahedron finite element meshes has been developed.Using this method,cohesive elements are inserted in ice sheet model which is meshed with tetrahedron elements.Therefore,crack branching and propagation path can be simulated as a natural outcome of the initial-boundary value problem without added assumptions of fracture path.It should be noticed that this method can also be applied to the numerical study of crack in other field,such as rock fractures.Collision between Norstromsgrund lighthouse and ice sheet is simulated in reality scale.Initiation and propagation of ice sheet fractures are reflected by the deletion of cohesive elements.Contact forces,lighthouse response and failure process of ice sheet are analyzed.A commonly used ice protection equipment,inclined conical steel plate,is added to the lighthouse at the contact interface to reduce the ice load.Ice sheet failure modes and crack patterns calculated from vertical and inclined structures are compared and analyzed,respectively.