Reliability based inspection planning using fracture mechanics based fatigue evaluations for ship structural details

Survey and inspection of ship structures is essential in order to monitor the integrity of the hull. With increasing ship size, the number of details susceptible to fatigue damage may be significant. Also as the ship nears the end of its design life, more effective inspections are required to ascertain the structural health of the ship. Reliability based inspection techniques are a possible way for ensuring the effectiveness of the surveys. In the present work, the applications of fracture mechanics based fatigue crack evaluations have been demonstrated to prepare reliability based inspection plans for a Very Large Crude Carrier (VLCC). Probabilistic considerations have been applied taking into account the uncertainty in various parameters related to the loads, materials as well as the parameters of fatigue crack growth. Scheme for updating of reliability of a given ship structural detail has been presented which employs Bayesian approach. Effect of utilization of various inspection techniques has been demonstrated. Various cases such as no-detection of crack, detection of crack with & without repair have been considered. Finally, the results have been also compared with the reliabilities in the current practice of fixed periodical inspections. The paper demonstrates that reliability based inspections are a feasible technique for integrity management of ship structural details while maintaining a practicable work schedule.