Risk Analysis Of Fatigue Failure For Ship Structure During Navigating In Ice-covered Regions

Polar ships are important equipments for arctic waterway transportation,resource extraction,and scientific research.Polar ships must not only meet the structural safety requirements in conventional waters,but also adapt to the working environment of the polar regions.Among them,ice loads pose a serious threat to the structural safety of ship in ice areas.The hull structure needs to have higher ultimate strength to resist the impact of floating ice,and better fatigue performance to carry continuous ice breaking tasks.Even if the dangerous area has been strengthened specifically at the beginning of the ship design,the complex polar environment may still cause serious fatigue damage and structural damage.Therefore,it is necessary to propose an effective method for evaluating the safety of ship structures in ice areas,and to quantitatively judge the risk of structural failure to prevent serious disasters.The fatigue analysis method of ship structure based on S-N curve has a mature theoretical foundation and extensive engineering application.However,due to the lack of complete environmental data and effective ice load calculation methods,there are relatively few fatigue studies on polar ships.In this regard,the sea ice numerical model established by the discrete element method is applied to simulate the ship-ice collision to obtain effective ice loads.During the icebreaking process of a ship,its bow structure is in direct contact with sea ice and is subjected to strong impact and compression.It is necessary to judge the local structural ultimate bearing capacity of the ship ice collision area.In addition,the longitudinal strength of the hull girder is still the focus of the safety of the ship’s structure.The ice load under ice conditions is far greater than the wave load,and the vertical ice force generated during the ice breaking process also has a greater impact on the bending moment of the ship’s midship.influences.Based on the calculation results of ship fatigue strength in ice area,local structural strength of bow,and longitudinal strength of hull girder,this paper adopts reliability theory to obtain structural failure probability,and introduces risk concept to put forward a quantitative assessment method for ship structure safety in ice area.The following research contents have been specifically carried out:(1)Fatigue analysis of ship structure under ice load.Based on the time domain fatigue S-N curve analysis process of polar ship any local structure fatigue calculation,based on actual ship go aerial observation statistics in the process of the polar ice thick-speed working condition of joint probability distribution of fatigue,and combined with discrete element method numerical simulation to get the working condition of ship bow ice force in local area and the hull girder bending moment.(2)Calculation of structural ultimate strength of polar ships.The load-displacement curves of the local bow structure and the curvature-bending moment curves of the hull girder are obtained by the nonlinear finite element method through the static loading step loading,and the ultimate strength of the structure is determined by combining with the ultimate load criterion.The influence of fatigue failure on the ultimate strength of the whole structure is analyzed by selecting dangerous components according to the fatigue calculation results.(3)Based on the concept of risk,a quantitative assessment method of ship structure safety in ice area is proposed.According to the fatigue and strength analysis results of ships in ice area,the fatigue failure probability of bow and midship structural members and the strength failure probability of the whole structure after failure are calculated by using structural reliability theory.The product of fatigue failure probability and strength failure probability is taken as risk index,and the risk matrix is established to judge the safety level of the risk index,It can provide a reference for the structural safety inspection and maintenance plan of polar ships in the whole life cycle.For polar ship,the sudden structural damage accidents are unpredictable,but there are statistical methods for the long-term accumulation of structural fatigue damage.The risk assessment method proposed in this paper effectively considers the correlation between fatigue damage and structural strength,quantificationally calculates the impact of component fatigue failure on the overall structural strength,and determines which dangerous components need to be focused on prevention and maintenance,so as to ensure the structural integrity of polar ships during service.