Integrated procedure for fatigue life evaluation of critical structural details in oil tankers

This dissertation addresses the problem of fatigue cracks in oil tankers. In order to effectively control fatigue damage it is necessary to combine efforts in the areas of inspection, monitoring, load assessment, design and repair.; A tanker information database system is developed containing general vessel information in combination with the results of crack and corrosion surveys. About 7,000 crack records for 16 vessels are entered to test the system and to develop the target failure probabilities necessary to verify the fatigue life evaluation process.; For the development of an automated system for the calculation of the fatigue damage of ship critical structural details it is essential to use hot-spot stresses instead of nominal stresses. This requires a modified representation of fatigue strength.; A calibrated S-N curve is developed based on original S-N test specimen that can be used in combination with hot-spot stresses obtained from linear-elastic finite element analyses. The calibration process excludes the stress concentration factor that is inherent in the original S-N curves and thus makes it possible to replace the three curves that have been used in the calibration with a single curve. This curve has to be used in conjunction with the defined finite element mesh size and the defined procedure for the assessment of the fatigue design stress.; The fatigue evaluation procedure is based on the Miner summation rule and uses a probabilistic approach originally developed for offshore platforms. A fracture mechanics approach has been developed that allows it to determine S-N curves for ship details with initial cracks.; For three different ship details the developed software system has been verified based on target failure probabilities obtained from the ship database. The load estimation is based on the vessel response characteristics, the travel route, the maneuvering philosophy and hot-spot stresses from finite element analyses. This loading in conjunction with the calibrated S-N curve is used to calculate the failure probabilities. The calculated probability values indicate low failure probability.; In order to facilitate the selection of S-N curves a database structure has been developed that includes the necessary curve information in conjunction with usage rules.; This study shows that an integrated software system for the evaluation of fatigue damage in combination with a ship database makes it possible to effectively control fatigue damage in ship and will lead to improved fatigue design of critical structural details.