Page 403 - Marine Structural Design

Chapter 20 Spectral Fatigue Analysis and Design 379

Determine how these results should be weighted in predicting statistics of the loads
produced in random seas.
The prediction of the time-domain fatigue analysis was compared with frequency-domain
stochastic fatigue analysis that assumes linear model of ship behavior. It was revealed that the
nonlinear effect is significant. The NTF method may also be applied to any offshore structures.

20.4 Structural Analysis
20.4.1 Overall Structural Analysis

Overall structural analyses are usually performed using space frame models and fine FEA
models. The space frame analyses define the boundary loads for local structural models. To
get the stress transfer functions for the fatigue damage assessment, these boundary loads are
used to factor the results of fine, FEA unit load analysis results.
This section presents aspects of modeling, load evaluation, and structural analysis applicable
to the overall structural analysis.
Space Frame Model
The space frame model includes all the important characteristics of the stiffness, mass,
damping, and loading properties of the structure and the foundation for the structural system.
It consists primarily of beam elements. The accuracy of the calculated member end forces is
influenced by the modeling techniques used.
Figure 20.1 shows a space frame model for TLP hull primary structures and deck primary
structures. Although not shown in this figure, tendons are included in the model as supporting
structure to provide the proper vertical stiffness. Tubular beam elements are used to model the
tendons. Applied load cases are, in general, self-balancing and should result in zero net load at
the tops of the tendons. Thus, relatively flexible lateral springs are provided at the tops of all
tendons in order to stabilize the analysis model against small net lateral loads.
The hull's column and pontoon structures are modeled using beam-column elements. Joint
and member definitions are interfaced from the global analysis model because interfaced loads
from this analysis must be consistent with the model. Member properties are determined based
on the member cross-sectional properties and material properties. Yield stresses of plate and
stiffener components are input, along with the maximum bracket spacing for ring stiffener
frames.
Additional joints and members are included to ensure that the tendons and deck structure are
structurally stable and as additional load collectors where appropriate. Deck members are
modeled using the tubular or AISC (American Institute of Steel Constructions) elements.
Deck equipment mass locations are determined for each major deck area and specifically
included in the model so that proper inertial load magnitudes and centers of action are
generated in the analysis.