Development of a reliability model for crack growth occurrence for a secondary hull component

Abstract

Ship hull vibration is a major contributor to fatigue crack growth and main engine excitation is identified as an important vibration source. A general method to solve any vibration problem arising onboard a ship does not exist, which encourages the use of a reliability-based framework for assessing ship vibration and its consequences. A stochastic model of vibration response is developed for the probabilistic formulation of the failure probability of the occurrence of crack propagation of a secondary structural hull component. The secondary structural component considered is a pipe stack support. The pipe stack support connects a cargo pump pipe stack to the wall inside the cargo tank, and the support is welded directly onto this wall. First, a generic cargo hold model is analysed with engine speed and the relative distance between the engine and the structural component under consideration as stochastic variables. Then, submodels are used to investigate the local vibration of the support and the stress response is evaluated for a combination of different engine speeds and relative distances. A surface is fitted to the vibration response and used for probabilistic analysis by Monte Carlo (MC/DSPS) and FORM/SORM reliability methods. The limit state is formulated as the possibility of fatigue crack growth based on a threshold stress intensity factor. This threshold factor depends on the initial crack size and different initial sizes are investigated. The adequacy of the functional representation for the stochastic model, which is fitted to discrete data points, is also assessed. It is seen that a functional representation using a sum of sine terms give an adequate fit for describing the stress response induced by engine speed, while a polynomial representation was adequate for the relative distance variable. The failure probability estimated by Monte Carlo simulations and SORM indicates that the pipe stack support is not critical for the occurrence of fatigue crack growth. A main observation from the analysis is that the reliability-based design of secondary structural components, also looking at the interaction with the global structure, may help to improve the vibration-induced stresses in local hull details by application of proper design measures.