| Since the birth of container ships in the 1960 s,their large-scale process has never stopped.By 2020,the world’s largest container ship has reached approximately24,000 TEU.Restricted by the arrangement of containers,ultra large container ships have obvious characteristics of large openings,which will cause more serious combined longitudinal bending and torsion strength problems than conventional ships.Since the21 st century,marine accidents for container ships have occurred from time to time,which has triggered hot discussions on the structural safety of container ships in the international maritime community,and has also triggered the need from International Maritime Organization(IMO)to further improve the codes or standards for the container ship safety.The current structural design approach on container ships is mostly based on the structural rules or guidelines of each classification society and the experience from the designers,and there are differences between them.The Goal-Based Standards(GBS)issued by IMO can minimize the potential structural safety hazards caused by different classification societies or differences in design experience.The research on container ship structural design method oriented to GBS is an important technical guarantee for the structural safety design and optimization of ultra large container ships,and is of great significance to engineering practice.At present,the unified rules by IACS that can meet the requirements of GBS are Common Structural Rules for bulk carriers and oil tankers.This thesis aims to set up the container ship structural design method oriented to GBS,and mainly focuses on the research of container ship load model,direct strength calculation and ultimate strength calculation method.The following work has been completed:(1)With the interpretation of GBS requirement on design loads,the basic principles and calculation procedures of different equivalent design wave(EDW)methods were summarized.The extreme value prediction methods for the hull girder wave loads were discussed,while the effect of non-uniform heading distribution and variant ship speed on extreme values required by GBS.On this basis,the formula of extreme values and distribution factors for horizontal wave bending moment,horizontal wave shear force and wave torsional moment were put forward,which were verified to be more in line with the direct wave loads calculation results.At the same time,the relatively more reasonable approaches to determine the vertical still water bending moment,vertical still water shear force and still water torsional moment were proposed,which considering the characteristics of unequal containers’ loading and the transmission force by container sockets.With the consideration of technical methodology of new CSR and the characteristics of the container ship,a set of load model for strength assessment and prescriptive requirement,which was suitable for container vessels and oriented to GBS,was proposed by direct wave analysis and spectral stress analysis for typical high stress areas in several typical container vessels,on the basis of linear EDW method.(2)With the interpretation of GBS requirement on direct strength calculation,a set of general hull girder load adjustment methods based on local loads applied on finite element model were derived.On this basis,the global direct strength calculation methods were improved,including the nodal force method and the EDW methods combined with hull girder load adjustment approach.With the discussion of hull girder load adjustment,boundary constraints and loading patterns,the cargo hold analysis for container ships based on direct method was improved and extended from amidship to the entire ship,focusing on the influence of transversely asymmetric loading conditions on the stress results.(3)With the interpretation of GBS requirement on ultimate strength calculation,the structural reliability analysis(SRA)method was used to study the statistical characteristics of load component and ultimate capacity,and partial safety factors in the unified requirement on the vertical bending ultimate strength of container ships in UR S11 A,including double bottom bending effect,etc.The degree of reliability of ultimate strength in UR S11 A was evaluated by several typical container ships.On the basis of the research on ultimate shear buckling,a simplified formula for predicting the lower limit of ultimate torsional moment for container ships under restrained torsion was proposed and verified by nonlinear finite element methods and model tests.Finally,the ultimate strength of container ships under combined loads was studied,including the FE calculation method of ultimate capacity of hull girder under combined loads,fitting relationship among various load components(bending-shear,bending-torsion and bending-shear-torsion).The main research results and innovations of this thesis can be summarized in the following aspects:(1)A unified load model for the structural strength calculation of container ships,oriented to GBS requirements,was studied and established.The model gave the extreme values and distribution factors for horizontal wave bending moment,horizontal wave shear force and wave torsional moment,which was verified to be more in line with the direct load prediction results.The hull girder still water loads simulation method or simplified calculation formula was improved by considering the characteristics of unequal containers’ loading and the transmission force by container sockets,which was vertified to be more reasonable than current approaches.(2)An approach for adjusting hull girder load was put forward.This approach improved the direct cargo hold analysis method for container ships based on direct method,which was verified to effectively compensate for the defects that the hull girder bending moment caused by the longitudinal loads was not considered in new CSR.Such approach could fully consider the safety risks caused by the extreme loading conditions.(3)A simplified formula for predicting the lower limit of ultimate torsional moment for container ships under restrained torsion was proposed.By comprehensively considering the combined load conditions,the evaluation scope for the ultimate strength of container ships was broadened. |
