Uncertainties in the fatigue design of offshore steel structures: Fid-Bau Portal

Uncertainties in the fatigue design of offshore steel structures

Pittaluga, A. / Cazzulo, R. / Romeo, P.

Abstract Fatigue is one of the failure mechanisms of offshore steel structures, which must be more carefully examined during the design stage. It demonstrates many aspects of uncertainty, due to the stochastic nature of the wave motion, considerable dispersion of the physical and mechanical characteristics which govern the phenomenon, and the microscopic way in which it initially develops. The deterministic analyses which are most commonly performed call for safety coefficients, which are able only in a very empirical way to combine the uncertainties relative to the various factors that determine the occurrence of the phenomenon. The most suitable tool to quantify the uncertainties which actually influence the collapse of the structure and to calibrate the respective safety coefficients in a probabilistic way is reliability analysis. The most widely used method for carrying out fatigue reliability analysis of offshore structures is the Wirsching method which represents an extension to the reliability case of the well-known linear accumulation laws of fatigue damage, either Miner's rule, based on S-N curves, or Paris' law, based on fracture mechanics. The purpose of this study is to discuss to what extent present approaches are actually able to distinguish between the different uncertainty contributions and to calibrate the safety factors to be used in reliability based codes, from a practical point of view. The conclusion is that considerable advantages, in terms of an increase in the usage factor, can be obtained by improving the quality of the analyses and inspections.