Study On The Ice Load Variations And Strength Assessment Of Hull Structure During Ship-Ice Impact

The use of the Arctic for shipping,research and natural resources in recent years has been marching to higher latitudes accompanied by higher risks for ships navigating in ice covered waters.Although the glancing impact with a giant ice floe has been established as the governing load scenario regarding the hull scantling design of polar class ships,there are still several issues to be addressed,including: 1)Complete observation of the ship-ice impact event is often limited by many factors in the field,while the relevant model test methods are still missing;2)The ship-ice glancing impact load can be characterized by its non-linear spatial and temporal variations,but specific load model has not been proposed yet by the existing studies;3)Although the plastic design philosophy of polar class ships has been accepted by the industry,there is still a lack of consistent and reliable strength assessment methodology under dynamic ice loads;4)Double acting ships bring a new mode of operation,but disagreements still exist in the industry on the design ice load for stern-first scenarios.Regarding the above issues,the present study has carried out a series of model tests and numerical simulations.The main findings are summarized as follows.Based on the understanding of the ship-ice impact process,failure mechanism of ice and technical background of the Unified Requirements for Polar Class ships(UR I),the criteria for the scaled modeling of the ship-ice glancing impact events in ice tank are proposed.Through the step-by-step calibrations of the impact mode,initial impact position and related test parameters,a complete procedure for the scaled modeling of ship-ice impact is developed,accompanied by the load recognition and processing methods based on tactile pressure sensors.The proposed test method and procedure could make up for the potential drawbacks of field observations,where natural environments are uncontrollable and tests under extreme conditions cannot be conducted due to safety concerns,and also provide possible reference for the establish of relevant model testing techniques.Under the guidance of the proposed test method and procedure,several repeated tests are carried out to investigate the spatial and temporal variations of ship-ice impact load.From the perspective of "global" ice load,it can be found that the spatial migration of the ship-ice contact area on the hull surface shapes like a parabola,owing to the relative movement between ship and ice and the downward flexural deformation of ice.On the other hand,the "local" crushing failure of ice results in the evolution of the "high-pressure zone",which can be roughly divided into three types:pure crushing,spalling and non-simultaneous crushing.These processes have been displayed based on the tactile sensor data and also reflect the phenomena observed in published medium-or full-scale tests.Based on the above findings,a method to describe the characteristics of global migration and local non-uniform distribution is proposed by adopting the concept of "sliding load window" and Gauss function.This method is regarded as a regression of the original discrete test data and can be easily applied for the nonlinear plastic response analysis of the hull structure.After introducing the ice loads as input,the dynamic responses of a stiffened panel(regarding as a part of the hull)are analyzed by non-linear finite element simulations.The simulations are based on the application of the "sliding load window" methodology and carried out under three load scenarios,i.e.,uniform load,non-uniform load and migrating non-uniform load.The results reveal that the uniform load model adopted by the current rules may lead to less conservative or safe hull structural strength,when the evolution of high-pressure zone during the crushing failure of ice are neglected.If the non-uniform spatial distribution of the ice load is considered,the load migration will lead to more severe plastic response of the structure.Such result emphasize the importance of the sliding(or moving)-load based method in the structural assessment of polar class ships.In order to investigate the structural strength design of the stern area of double-acting ships,a series of stern first ship-ice glancing impact tests is carried out at different speeds and ship types.The hull area factors(AF)are determined based on the measured ice loads and selected to assess the design strength of the stern area.Based on the evaluation of AF under different probabilities of exceedance,a preliminary knowledge on the stern-first hull area factor is achieved.The present work could be regarded as a first attempt to solve the divergence between the current rules on the AF of the stern area and also provides useful data on this issue.