Study Of Analysis Methods For Force And Structural Response In Ship Collisions

Collision events including ship-ship/platform and ship-ice collisions are of particular importance in marine accidents.The consequences of collisions can be severe in terms of the structural integrity,the safety of personnel and the pollution of environment.If collision mechanics can be well understood,then structural damage or economic losses caused by the collision can be effectively avoided or reduced.The main purpose of the work herein is to study analysis methods for force and structural response in ship collisions,which can be references for ship structural crashworthiness design.Due to the complexity of ship collision problem,in a collision scenario,the analysis procedure is commonly decoupled into two independent parts:the external mechanics and the internal mechanics.The external dynamics addresses the energy released for dissipation and the impact impulse of the collision.The internal mechanics is concerned with contact force,structural deformation and how the strain energy is dissipated.This decoupled approach is simplified,and can be used as a fast and valid approach to address the ship collision problems under certain situations.In contrast to the decoupled approach,the fluid-structure interaction(FSI)approach can provide solutions to fully coupled ship collision problems.External mechanics,internal mechanics and FSI approaches are used to investigate force and structural response in ship collisions.For the problem of interaction between ice and ship,the prediction of ice forces is most important for the ship design.Based on the external mechanics,a simplified analytical method is proposed to estimate the force in oblique ship bow-iceberg collisions.The eccentricities of both iceberg and ship and the friction effect are included.This method includes three main steps:first,the total energy dissipation(energy dissipation on normal and tangential directions)during the collision is computed;second,the energy dissipation-indentation and the force-indentation relationships are established;Third,the impact force is calculated.This method is compared to Popov model and Matskevitch model.In addition,the effects of the global shape of the iceberg,the normal frame angle of the ship,the ice pressure-area relationship,the friction and the local shape of the iceberg on the impact force are studied.The results indicate that all these parameters have a significant effect on the impact force.The forces obtained by the present method are compared with those specified in relevant regulations(IACS)and numerical studies.The reasons of differences are discussed,which can be references for structural crashworthiness design of the ship sailing in ice-area.In order to quickly assess the ship side structural response during ice floe impact under Accidental limit state,a new simplified uniform pressure method is proposed to assess the maximum indentation and energy dissipation of the ship side structure by analyzing the characteristic of ice pressure-area relationships during the collisions.The integrated NLFEA method is used as a reference to verify the proposed method,in which the ice material model is calibrated by empirical fomula of ice force and validated by pressure distribution over the contact area and crushing specific energy of ice.The results show that the proposed method can be used as a first-level prediction to analyse ship structural strength during the initial stages of design.To get a more accurate dynamic response of collision,the Arbitrary Lagrangian Eulerian(ALE)FSI method is used to simulate the ice-structure collision,in which the ice material model and the fluid model of LS-DYNA are verified.The differences in assessment of collision response between the FSI method and the constant added mass(CAM)method are investigated.To validate and benchmark the two methods,the test data are used.The results indicate that the FSI method can provide a more realistic simulation of the collision,and predicts a more accurate collision results.However,the CAM method overestimates the contact force peak and the energy dissipation of the ice.To perfect the research of ship collision problems,the FSI numerical simulation of full-scale ship-ship collision is carried out,in which the interaction between the two ships and the surrounding fluild(water and air)is considered.The effects of forward velocity of the struck ship,the mass of the struck ship and the collision angle on collision results are investigated.The results show the importance of forward velocity of the struck ship.Furthermore,the differences among the analytical method based on momentum conservation,the CAM method and the FSI method are investigated for the two cases where the struck ship is static and the struck ship has a forward velocity,and the reasons of differences are analyzed.The comparisons indicate that the CAM method can be used for the simple collision scenarios due to its simplicity and its fastness.However,for more complex collision scenarios,the FSI method should be used.These numerical results will be used for the study of nonlinear analysis for ship collisions.