Page 406 - Marine Structural Design

382 Part III Fatigue and Fracture

standards such as AWS (1997) are used in the USA. However, it should be noted that different
design standards provide different procedures in the fatigue stress determination and S-N
classification, which result in large discrepancies in the predicted fatigue damages. Therefore,
a consistent procedure based on one design standard shall be used.
The safety factors for fatigue design of floating structures are given by the design standards
listed in Section 20.2 based on:
Criticality of the joint
Inspectability and repairability
The criticality of a join is determined based on its structural redundancy. A joint is critical if
its failure will potentially lead to the failure of the structure.
20.5.2 Stress Range Analysis

A stress range analysis is performed using the fatigue software as a precursor to the fatigue
damage calculation. The FEA unit load, model geometry and element stress results are
interfaced into the fatigue calculation model. Loading combinations will then be defined for
each fatigue wave load based on the applied boundary loads.
Geometry and element properties from the space frame model are plotted and revised for
accuracy. Any detected errors are corrected in the FEA input file and the FE analysis repeated.
The finite element model of the specific hotspot region shall be developed based on the
procedures, finite element size requirement defined by the design standards.
In the FEA model, unit load results will be interfaced into the space fiame model database.
These unit loads are then appropriately combined based on the applied boundary loads.
20.5.3 Spectral Fatigue Parameters

Wave Environment
The wave environment consists of wave scatter diagram data and wave directional
probabilities.
The scatter diagram data consists of annual probabilities of occurrence as fimctions of
significant wave heights and peak periods in the structure installation site. For spectral fatigue
analysis, a wave spectrum (e.g. Pierson-Moskowitz) is associated with each cell of the scatter
diagram.
Directional probabilities for fatigue waves are also included in the fatigue assessment. It is
usually unconservative to ignore any non-uniform distribution in directional probabilities.
However, in lieu of such information, the wind directional probability may be used to account
for the non-uniformity in the wave approaching direction and to provide conservatism in the
fatigue damage calculation.
Stress Concentration Factors
The determination of the appropriate SCF in the fatigue analysis is a complex task. It is also
dependent on the S-N classification and stress analysis methods. The general rule of thumb is
that the stress used in the fatigue analysis should resemble the fatigue stress obtained from the
specimen tested when deriving the S-N curves. The fatigue stress does not mean the most
accurate stress determined by the high-resolution fine mesh FEA. It is the pertinent stress, in