Page 32 - Marine Structural Design

Chapter I Introduction 9

of S-N curves and the determination of hot-spot stress. In the IIW code, the S-N curves are
named according to their reference stress range *OR that corresponds to 2 * 106cycles.

Log N

Figure 1.3 S-N Curves for Fatigue Assessment
With the increasing use of finite element analysis, a design approach based on the hot-spot
stress will be more and more popular. The fatigue uncertainties are due to several factors such
as
selection of environmental conditions such as sea-states and their combinations
extrapolation of fatigue stresses in the hot spot points
selection of design codes such as the S-N curves and the stress calculations
combination of wave-induced fatigue with the fatigue damages due to vortex-induced
vibrations and installation
selection of safety factors and inspectionhepair methods
The accumulative fatigue damage for a structural connection over its life-cycle is usually
estimated using Miners rule, summing up the damage due to individual stress range blocks.

where ni and N,denote the number of stress cycles in stress block i, and the number of cycles
until failure at the i -th constant amplitude stress range block. DarrOw is the allowable limit that is
defined in design codes.
A simplified fatigue analysis may be conducted assuming stress ranges follow Weibull
distribution. This kind of analysis has been widely applied in classification rules for fatigue
assessment of ship structures. The Weibull parameters for stress distribution have been
calibrated against in-service fatigue data for ships and more refined fatigue analysis. The value
of the Weibull parameters may be found from classification rules, as a function of ship length
and locations of interests. Alternatively, in offshore design codes API RPZA, a simplified
fatigue analysis is proposed assuming the wave height follows Weibull distributions. The