Frequency Domain Analysis |
 
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Frequency Domain Analysis |
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This section summarises the theory underpinning the operation of the frequency domain analysis capability in Flexcom. The aim of the note is not to provide a mathematically exhaustive description of the solution procedure – it is intended to provide an adequate introductory background, and to explain the different steps in a frequency domain analysis.
Frequency domain analysis is far more efficient than time domain, but its use is restricted to systems whose behaviour is (largely) linear. Where the technique can be applied, the computational effort is only a fraction of that required for a corresponding time domain simulation, resulting in huge savings in terms of run time. A frequency domain dynamic analysis restarts from a converged static solution (e.g. an initial static or current static analysis), in much the same way as a time domain dynamic analysis would. Flexcom performs the frequency domain dynamic analysis (by expressing the applied loading and structural response in terms of sinusoidal components), and then automatically performs a restart static analysis (executed internally by the software so no additional user action is required). This step is necessary because the linearised drag forces due to current, in an analysis with waves and current, are a function of the dynamic response due to wave action (further details are provided later in this section). Appropriate use of the frequency domain is the responsibility of the user, but it should be noted that any non-linearities associated with the applied loading or structural response should be relatively small for the underlying assumptions of frequency domain theory to remain valid.
Further information on this topic is contained in the following sections:
•Evolution of Flexcom Software outlines the rationale for merging the frequency domain analytical capabilities of Freecom (formerly a standalone frequency domain package) into Flexcom.
•Mathematical Background describes the theory underpinning the frequency domain analysis procedure.
•Static Solution discusses the method used to find the mean or static structure response.
•Dynamic Solution explores how the dynamic response about this mean solution is computed.
•Drag Linearisation discusses the techniques used to linearise the drag component of Morison’s equation.
•Summary of Solution Procedures presents a summary of the solution procedures for completeness.
•*ANALYSIS TYPE is used to specify the analysis type.
•*HYDRODYNAMIC SETS is used to assign hydrodynamic coefficients to element sets.
If you would like to see an example of how these keywords are used in practice, refer to B01 - Steel Catenary Riser.