Finite Element Analysis
ØDS has extensive experience in Finite Element (FE) Analysis.
The establishment of an FE model from structural plans, size and position of elements is an area where the experience and understanding of marine structures is essential.
Today ØDS is one of the most respected independent consulting companies in the world with state-of-the-art FE tools for marine applications.
- Features
- Local Analysis
- Global Analysis
- Natural Frequency Calculations
- Response Analysis Applications
- Vibration Prediction
- Trouble Shooting
- Foundation Impedance
Introduction
Finite Element (FE) analysis builds on the simple concepts of linearization, discretization, and interpolation. The infinite number of degrees of freedom in a continuous system is reduced to a finite representation well suited for numerical analysis.
General Purpose
The Finite Element analysis is a powerful, state of art tool for:
- Vibration prediction
- Natural frequency calculation
- Corresponding mode shape analysis
- Calculation of vibration level
Marine Applications
A specific use of the FE analysis is made on marine structures in order to:
- Control ship vibrations and keep them at acceptable levels
- Avoid vibration problems on newbuildings
- Perform trouble shooting
The objective of a vibration study is to achieve a combination of excitation sources and structure that will meet the specification requirements. The dynamical behaviour of the structure is to be documented, and possible problems of vibration are to be identified and dealt with in order to ensure that vibration levels will comply with the requirements set out in the specification.
A single model including all the elements and characteristics of the ship will be very complex. Therefore, separate analyses are normally carried out for:
- Global structures
- Local structures
Global Structures Analysis
Natural frequencies and corresponding mode shapes of the entire ship are calculated and checked for resonance with the excitation frequencies of the main sources of vibration (propeller and machinery). If the natural frequencies coincide with the excitation frequencies, there is a risk of resonance problems.
Local Structure Analysis
Local analyses are usually carried out during the design stage in order to minimize vibration problems and future modifications of the structure. Natural frequency calculations should be carried out for carefully selected local structural element. Typical and critical stiffened deck panels should be given priority.
Forced Response Analysis
In order to check that the vibration levels on board the ship comply with the requirements, damping is introduced and excitation forces are applied to the model. The damping values introduced in the calculations is derived from an empirical model and is thus based on experience acquired from measurements on large number of ships.
Shaft Analysis
Torsional and lateral analysis of the drive train is crucial to ensure reliability. Detailed analysis can also be required to prevent vibration problems in the ship structure.
Tools
ØDS uses state-of-the-art-tools for FE analysis, including ANSYS and I-deas. Furthermore, ØDS has developed an inhouse FE code for special applications. These tools enable ØDS to use nonstandard
methods such as:
- Quality mesh generation
- Automated hull meshing
- Modelling of structural-hydrodynamic interaction
- Sub-modelling techniques
- Super elements
- Master degrees of freedom
- Contact elements
- Plastic and non-linear elements
- Topological optimisation
- Stochastic loading
Special assignments requiring tailor made tools can be performed on existing models by importing data from most commercial FE codes.
Verification
ØDS can perform modal excitation measurements on global and local marine structures to verify the calculation results.
Experience
ØDS primarily uses the FE method for design verification and trouble shooting.
It is our experience that many vibration problems can be avoided or resolved with the correct use of FE analysis and correlation with measurements on the physical structures where it is possible.
However, it should be emphasized that the FE method is just one of many tools. ØDS always try to select the best tool for a given task.
FE calculation of natural frequencies of a
thrusher foundation on FPSO vessel.
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