Page 405 - Marine Structural Design

Chapter 20 Spectral Fatigue Analysis and Design 381

The global motion analysis serves as a basis for dynamic load development. The actual
interface from the global analysis to the structural analysis consists of several loading
components for each analyzed wave period and direction: the real and imaginary applied unit
amplitude, wave diffraction and radiation loads, the associated inertial loads and other cyclic
loading such as tendon dynamic reactions. The successful interface of these load components
is dependent on a consistent geometric and mass model between the motion and the structural
analyses and is also dependent on a consistent generation of the loading components in the
motion analysis. Consistent modeling is obtained by interfacing the model geometry directly
from the motion analysis wherever possible. Consistent mass is obtained by interfacing with
the same weight control database for both the motions and structural analyses, when available.
Load combinations are formed for each wave period and direction. These combinations consist
of the applied wave load, the generated inertial load, and the associated cyclic loadings such as
tendon dynamic reactions for both real and imaginary loadings of the floating structures.
These combinations form the total cyclic load condition for each wave period and direction to
be used in the spectral fatigue analysis.
Analysis and Validation
Hull structural analyses are performed using linear finite element methods. The reaction
forces include total force and moment reactions and the analysis results are verified.
Symmetrical or asymmetrical load conditions are checked to confirm symmetrical or
asymmetrical analysis results.

20.4.2 Local Structural Analysis
Local structural details are included as a part of the analyses for the entire hull structure.
The analysis of the structural details may be performed using the finite element program such
as ABAQUS (HKS, 2002) and other software. The FEM model is three-dimensional and linear
stress analysis is performed. The results from the FEA model are interfaced into the fatigue
model for additional model validation and subsequent spectral fatigue analysis of the local
structural details. The entire model is plotted and revised for accuracy both from the FEA
model and after interface to the fatigue model.
Loading conditions for finite element analysis of local structural details should be based on the
hull's structural analysis since it includes all cyclic loadings of the structure.
The unit loading conditions are frequently applied. The resulting stresses for each unit load
condition are interfaced to the fatigue model for subsequent combination into fatigue design
loads.

20.5 Fatigue Analysis and Design

20.5.1 Overall Design
A spectral fatigue assessment should be carried out for each individual structural detail. It
should be noted that every structural detail, every welded joint and attachment or any other
form of stress concentration is potentially a source of fatigue cracking and should be
considered individually.
The UK DEn procedure or its modified versions are recommended in Europe for the fatigue
analysis and design of floating structures since it is the most widely accepted code. Design