Modal Analysis |
 
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Modal Analysis |
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This section summarises the theory underpinning the operation of the modal analysis capability in Flexcom. Its aim is not to provide a mathematically exhaustive description of the solution procedure, rather it is intended to provide an adequate introductory background. This section briefly outlines the analytical capabilities of Modes, an ancillary module of Flexcom, which performs modal analysis.
The objective of a modal analysis is to calculate the natural frequencies and mode shapes of a structure. Flexcom possesses an ancillary module, Modes, which computes the natural frequencies and corresponding three-dimensional mode shapes of offshore structures. A large variety of offshore systems can be analysed using Modes, including flexible and rigid risers, mooring lines, TLP tethers and complex multi-articulated towers. Particular features of the software are as follows:
•Modes can find the natural frequencies of any structure that can be analysed using Flexcom, including structures whose geometry definition uses cables.
•Natural frequencies are calculated using the subspace iteration technique, which is fast, accurate and efficient.
•Program output is in both tabular and graphical form. The graphical output consists of full, on-screen, 3D representations and plots of the structure mode shapes.
•Modes is fully integrated into the Flexcom, and the program inputs are specified in a similar fashion.
Further information on this topic is contained in the following sections:
•Mathematical Background describes the theory underpinning the modal analysis procedure.
•Seabed Modelling introduces the topic of riser-seabed interaction in modal analysis.
•Subspace Iteration outlines the subspace iteration technique, used by Modes to converge to the required eigensolution.
•Output File outlines the options for controlling the contents of the modal analysis output file.
•Solution Convergence notes some helpful tips for overcoming convergence issues in a modal analysis.
•SHEAR7 Interface describes the capability of Modes to interface with SHEAR7, a program which estimates fatigue damage caused by vortex induced vibrations. Modes presents information regarding natural frequencies, mode shapes etc. in a format ready for direct input to SHEAR7.
•*ANALYSIS TYPE is used to specify the analysis type.
•*EIGENPAIRS is used to specify the required number of natural frequencies, and also parameters relating to the subspace iteration algorithm.
If you would like to see an example of how these keywords are used in practice, refer to A01 - Deepwater Drilling Riser or B01 - Steel Catenary Riser.
SHEAR7 Interface
•*RISER TYPE ($MODES section) is used to specify the riser type and other parameters relating to the Modes facility for generating output for subsequent input to SHEAR7.
•Numerous keywords are contained in the $SHEAR7 section, a relatively recent addition to Flexcom. This input section provides a streamlined interface with SHEAR7, allowing you to define all of the required SHEAR7 inputs directly within Flexcom itself, resulting in a seamless integration of the modal and VIV fatigue analysis stages.
For full details of how Modes interfaces with SHEAR7, refer to SHEAR7 Interfacing Operation.