Reliability-Based Design Guidelines for Fatigue of Ship Structures

Marine and offshore structures are subjected to fatigue loadings primarily due to the action of seawater waves and the sea environment in general. The load cycles in such an environment can be in the order of a million cycles per year. Fatigue failures in these structures can take place at sites of high stress concentration that can be classified by two major categories: baseplate and weldments. The former includes locations of high stress concentration such as openings, sharp re-entry corners, and plate edges. In general, the mechanisms behind these failures are described by the general approaches to fatigue life prediction as discussed in this paper. There are two major approaches for predicting fatigue life: (1) the S-N curve approach and (2) the fracture mechanics (FM) approach. The S-N curve approach is based on experimental measurement of fatigue life in terms of cycles to failure for different loading levels and specimen geometries, while the fracture mechanics (FM) approach is based on the existence of an initial crack and subsequent crack propagation under cyclic load. The objective of this paper is to develop reliability-based methods for determining the fatigue life of structural details associated with conventional displacement type surface monohull ships based on the S-N approach and on the assumption that fatigue damage accumulation is a linear phenomenon (i.e., that follows Miner's rule). The methods are also based on structural reliability theory and can be applied either in direct reliability-based design or in a load and resistance factor design (LRFD) format. The resulting design methods are referred to as the reliability-based design approach for fatigue of marine structures. These design methods were developed according to the following requirements: (1) spectral analysis of wave induced loads, (2) use of conventional fatigue design codes, (3) nominal strength and load values, and (4) achieving target reliability levels. The first-order reliability method (FORM) was used to perform reliability assessments and to develop the partial safety factors (PSFs) for fatigue limit state equations.