Page 115 - Marine Structural Design

Chapter 5 Ship Hull Scantling Design by Analysis 91

Short-term response is then obtained by multiplying the frequency response functions by the
wave spectra. The long-term response is calculated by using the short-term response and wave
statistics, which consist of wave scatter diagrams.

5.3 Strength Analysis using Finite Element Methods
5.3.1 Modeling
In principle, strength analysis by means of finite element methods should be performed with
the following model levels:
Global Analysis
A global analysis models the whole structure with a relatively coarse mesh. For a huge
structure like ships, the global model mesh must be quite rough; otherwise too many degrees
of fieedom may consume unnecessary man-hours and cause computational difficulty. The
overall stiffness and global stresses of primary members of the hull should be reflected in the
main features of the structure. Stiffeners may be lumped, as the mesh size is normally greater
than the stiffener spacing. It is important to have a good representation of the overall
membrane panel stiffness in the longitudinal and transverse directions. This model should be
used to study the global response of the structure under the effects of functional and
environmental loads, to calculate global stresses due to hull girder bending, and to provide
boundary conditions for local FE models. Design loads should reflect extreme sagging and
hogging conditions imposed by relevant operation modes such as transit, operating, storm
survival, installation, etc.

Local Structural Models
For instance, cargo hold and ballast tank models for ship shaped structures may be analyzed
based on the requirements of classification rules.
Cargo Hold and Ballast Tank Model
The local response of the primary hull's structural members in the cargo and ballast area is
analyzed, for relevant internal and external load combinations. The extent of the structural
model shall be decided on, by considering structural arrangements and load conditions.
Normally, the extent covered is the tank itself, and one half the tank outside each end of the
considered structure (Figure 5.2).
The mesh fineness shall be determined based on the method of load application. The model
normally includes plating, stiffeners, girders, stringers, web-frames, and major brackets.
Additional stiffness may be employed in the structure for units with topsides, and should be
considered in the tank modeling.
From the results of the global analysis, the boundary conditions for the cargo hold and ballast
model may be defined. The analysis results of the cargo hold/ballast model may be used as the
boundary conditions for the frame and girder models.
The following basic loads are to be considered in the model:
Static and dynamic loading from cargo and ballast,
Static and dynamic external sea pressure,
Dead weight, topside loading, and inertia loads