The Analysis Of Ultimate Strength Calculation Methods And System Reliability Research Of Cracked And Eroded Ship Structures

The ultimate bearing capacity of ship structure will be reduced by the damage of cracks and corrosion during the whole life cycle,which leads to the occurrence of accidents at sea and inestimable loss.Therefore,it is necessary to evaluate the ultimate bearing capacity of the ship structure in the bad sea condition in the initial stage and operation process of ship accurately and quickly.As a large three-dimensional space thin-walled system composed of stiffened plates,the ship structural safety will be affected by many uncertain factors as the working environment is very complex.It is of great practical significance to analyze the reliability of the ship structure according to the probability thought.On the basis of the empirical formula of the ultimate strength of stiffened plate element containing cracks and corrosion,a simplified progressive failure ultimate strength calculation program which can invoke the stress-strain relationship of the element by nonlinear finite element analysis is developed,using the idea of progressive failure and the APDL parameterized design.Taking a Floating Production Storage and Offloading(FPSO)as an example,the ultimate strength of the ship with corrosion and crack in different years was calculated by using the simplified progressive failure procedure and the nonlinear finite element method.The ship structure is simulated using discrete elements.The system reliability analysis of the ship space structure is carried out based on the simplified progressive failure procedure and the probability network estimation method(PNET).The specific research content is included as following:(1)The empirical formulas for calculating ultimate strength of intact stiffened plate,cracked and corroded plate and stiffened plate are analyzed.Based on the empirical formula of the compression ultimate strength of stiffened plate,the empirical formula of the ultimate strength of the stiffened plate with crack and corrosion damage is proposed based on the flexibility of stiffened plate and shell plate.Compared with the results of finite element calculation and empirical formula,the accuracy of the empirical formula for ultimate strength of damaged stiffened plates is verified.(2)Based on the parameterized design of the ultimate strength of the hull element containing cracks and corrosion,the progressive failure procedure is compiled using the element stress strain relationship obtained by finite element method.The fast calculation of the ultimate strength of ship hull are proposed by obtaining the stress-strain relationship of element with the cracked and the corroded quickly.The calculation accuracy of the program is verified by the box girder and a ship hull.(3)The ultimate strength of FPSO hull girder with only corrosion,only cracks,cracks and corrosion are calculated by using nonlinear finite element method and the proposed simplified calculation program respectively.Comparing the calculation results of the two methods,it is found that the simplified progressive failure procedure is feasible and the result is reliable.(4)Based on the reliability of stiffened plate elements of large ship structures,a reliability calculation method of ship girder system based on failure path of progressive failure process is proposed.The reliability of FPSO hull girder containing cracks and corrosion simultaneously is calculated.The results show that the method can quickly evaluate the reliability of ship girder space system.