Methodology for developing reliability-based load and resistance factor design (LRFD) guidelines for ship structures

The main objective of ship structural design is to ensure safety and functional performance requirements of a structural system for target reliability levels, for a specified period of time, and for a specified environment. As this must be accomplished under conditions of uncertainty, probabilistic analyses are necessary in the development of such probability-based design criteria of bull structural components for surface ships. A methodology for developing load and resistance factor design (LRFD) guidelines for ship structures is outlined in this paper and demonstrated for surface ship bull girders. Future design guidelines for bull structural components of a marine vessel are currently being developed using reliability methods and are expressed in a special format such as the Load and Resistance Factor Design (LRFD) format. Reliability of these structural elements can be defined as its ability to fulfill their design functions for a specified time period. This ability is commonly measured using probabilities. Reliability is therefore, the occurrence probability of the complementary event to failure. Based on this definition, reliability is one of the components of risk. Safety can be defined as the judgment of risk acceptability for the system making it a component of risk management. The performance of a ship structural component is defined by a set of requirements stated in terms of tests and measurements of bow well the system or element serves various or intended functions over its service life. Risk and reliability measures can e considered as performance measures that can be specified in the form of target reliability levels (or target reliability indices, go's). The selected reliability levels of a particular structural element reflect the probability of failure of that element and the risk associated with it. In this paper, reliability methods for developing LRFD-based partial safety factors (FSFs) for ship bull structural are described. These methods include analytical procedures, such as the First-Order Reliability Method (FORM), for calculating the partial safety factors. These factors can be used in LRFD formats to account for the uncertainties in strength and in the load effects. The FORM procedure can be used to determine these factors based on prescribed probabilistic characteristic of strength and load effects.