Fatigue assessment of ship structures in early design stage

Generally ships fatigue assessment is done in detailed design phase with the help of FE analysis. This means that the modeling and calculations are time consuming processes. To improve this process a simplified fatigue assessment approach is generated. This means that the FE analyses are neglected and therefore it could be possible to use this method in earlier design stage. Instead of FE method the beam theory or CB method is used. In this work beam theory is applied. At first the fatigue critical connections are studied together with damage statistics of tankers. This data describes the most fatigue critical connections, their locations and the type of fatigue loading. This significantly constrains the data amount for fatigue assessment. Important part of fatigue assessment is to define local stresses in fatigue critical connections. For that the stress concentration factors are defined based on literature review. This allows transforming nominal stresses into local notch stresses at fatigue critical spots. From notch stresses the fatigue parameter damage ratio is obtained and finally the fatigue life is calculated for the fatigue critical connection. The method is evaluated in case study with FE analysis. The results showed good precision of simplified method and the uncertainties were on the safe side of the deviation. The method weak point is the lack to take into account shear loading as suitable stress concentration factors are complicated to predict