Research On Prediction Method And Influence Law Of Ice Resistance Of Ship In Broken Ice Conditions

In the past half-century,due to the effect of global warming,the ice-covered region in the Arctic has declined by 50%.A large area of level ice and icebergs in the Arctic has been melting.The broken ice’s covered area and marginal ice zone,which consist of the small-scale floe and open water,are gradually increasing.In contrast to such extreme conditions of level ice,ice ridges and icebergs,polar ships,particularly the Arctic transport ships,prefer to navigate in the marginal ice zone or the channel full of broken ice.In general,the broken ice condition will not cause considerable damage to the hull’s structure,but it will significantly increase the ship resistance,which has a negative effect on navigation performance and reduces the economy of the Arctic routes.It is of great significance for the safe and economical navigation of polar ships in the Arctic to predict the broken ice resistance accurately,understand the formation of broken ice resistance,and clarify its influence law.At present,the research on the prediction of broken ice resistance is still relatively immature.There is still a dearth of understanding about the formation mechanism and the effects of parameters on broken ice resistance.The complexity and diversity of the interaction between ship and broken ice,the randomness and variation of broken ice distribution and ice field environment have brought great challenges to broken ice resistance research.In this thesis,the investigations of the prediction and characteristics of broken ice resistance are conducted through model test and numerical simulation.The model test with room-temperature artificial ice is applied to predict the ship’s broken ice resistance.The mechanism of formation and the effect of parameters on broken ice resistance are studied.The experimental results provide data support to the numerical simulation.As for the numerical simulation,based on the non-smooth discrete element method,the numerical models for the ship-broken ice collision,ice-water coupled load and broken ice failure mode,are established.The prediction model of the ship’s broken ice resistance is developed as a result.The main works in this thesis are listed:(1)A model test of the prediction of broken ice resistance is performed in the roomtemperature towing tank with artificial ice made from polypropylene material.According to the ice navigation code and the measured statistical data of the ice field in situ,the ship velocity,ice concentration,shape and size distribution of broken ice are designed for the low-to-medium ice concentration in the model test.The random distribution of the ice field is quantified.Based on the established experimental setup,the model test is carried out under the cases of multivelocity and multi-ice conditions.The segmentation principle is applied to evaluate the mean value and uncertainty of ice resistance.In addition,the feasibility of the current model test,the effects of parameters and the physical phenomena such as “ice attachment” and “anti-clockwise rotation” observed during the interaction between broken ice and ship are further investigated.(2)The numerical model for the prediction of broken ice resistance is established based on the non-smooth discrete element method(NDEM).The framework for predicting the ship’s broken ice resistance is developed,in which the non-smooth discrete element method is used to simulate the collision between the ship and broken ice,and the simplified fluid force model is adopted.An ice field generation method is proposed to achieve the numerical reconstruction of the broken ice field,which considers the floe size and shape distribution.The numerical results are directly compared with the results from both model test and empirical formulas.The good agreement in both value and variation of ice resistance demonstrates the accuracy of the present numerical model.Then,the effects of the ice field’s characteristics such as floe geometry,ice thickness and channel width,on the broken ice resistance are further discussed.(3)Based on the framework for predicting the ship’s broken ice resistance,the ice-water coupled load is further considered.The MPS-NDEM coupled model which combines the moving particle semi-implicit method(MPS)with the non-smooth discrete element method(NDEM)is developed for modelling the ice-water coupled load.The pressure oscillation,low accuracy,solid boundary treatment and free surface identification problems existing in the original MPS method are modified to improve the algorithm’s accuracy and stability.The updated MPS method is coupled with NDEM and the corresponding program is parallelized to improve the efficiency.The accuracy of the present coupled model is validated by simulating the dam break,wave motion,ice motion in wave and dam break impacting blocks problem.The model is also applied to simulate the problem of ship moving at high speed through the lowconcentration broken ice field.Meanwhile,the effect of hydrodynamic load on the numerical prediction of broken ice resistance is also pointed out.(4)Based on the framework of predicting the ship’s broken ice resistance,the fracture behavior of the floe is further introduced.The numerical simulation of the ship’s moving through the broken ice field with large broken ice and high ice concentration is conducted.The theoretical model and the non-smooth discrete element method are combined to simulate the crushing,bending,and splitting failure behaviors of broken ice.Then,the feasibility of the present numerical model is validated by simulations of the collision between a single floe and cylinder structure,as well as a cone structure moving through the level ice.Based on the good agreement with statistical data and model test,the prediction of the ship’s broken ice resistance is carried out under the case of large broken ice and high ice concentration.The characteristics of broken ice resistance such as the effects of ship velocity and head angle are further investigated.