Research On Fatigue Strength Assessment Method For Oil Tanker Structures

Fatigue damage is one of the main failure modes of ship building and offshore engineering structures,and the fatigue strength has been concerned by ship designers and major classification societies.In this thesis,the fatigue strength evaluation methods and influencing factors of ship structures are studied,including simplified fatigue analysis method,spectrum fatigue analysis method and fatigue screening method,and the effects of design sea conditions,hull girder bending moment and secondary stress on fatigue life are discussed..On this basis,the fatigue strength analysis and comparative analysis of the real ship are carried out.The main contents are as follows:1)The basic principles of ship fatigue analysis are described,including S-N curve,Miner linear damage theory,Weibull distribution and fatigue damage calculation.The simplified fatigue analysis method of CCS,CSR and HCSR rules is studied.The specifications of CCS(2018),CSR and HCSR are discussed in detail from the aspects of hull girder bending moment,cargo and external sea pressure,hull girder stress,ship motions and accelerations,the local bending stress of plate and longitudinal,reference stress and the regulation on mean stress,corrosion allowance,S-N curves and fatigue assessment criteria.The spectral analysis method for fatigue assessment is studied.The basic idea and evaluation process of ship fatigue spectrum analysis method are discussed.2)Based on the simplified analysis methods in CCS(2018),CSR,HCSR and fatigue spectrum analysis method,the fatigue strength at the connection of the longitudinal and web frames of a 27,000-ton tanker was evaluated,and the calculated results were compared and analyzed to study the differences between the methods.In order to improve the efficiency of fatigue life and fatigue strength evaluation,the structure with smaller stress concentration factor in the rules or the structure form which can be exempted from fatigue strength evaluation stipulated in the rules can be preferred in ship design.3)In order to further improve the efficiency of fatigue strength evaluation,the fatigue screening method is studied,the necessity of this method is analyzed,the process of fatigue strength evaluation by using this method is expounded,and a real ship example is given.This paper summarizes and puts forward three measures to improve the efficiency of fatigue life or fatigue strength evaluation as follows:adopting typical anti-fatigue joints with smaller stress concentration coefficient in the rules;adopting the structure of exempt fatigue strength evaluation stipulated in the rules;and giving the fatigue checking joints based on fatigue screening.4)The influence of calculating sea condition,bending moment of hull girder and secondary stress on fatigue life of longitudinal bones is discussed.Whether there is allowance in the assessment results under the North Atlantic sea condition selected in the code is analyzed.The comparative study of bending moment of hull girder is carried out,and the influence degree of secondary stress on fatigue strength is analyzed.