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Home > User Interface > Results Processing > General Post-Processing > Extracting Results RAOs

Extracting Response Amplitude Operators of Results

The RAO tab allows you to compute Response Amplitude Operators for any variable, such as floater position but also tension or stress for instance, with respect to the wave elevation or any other variable you want.

Warning

With time-domain dynamic analyses, results RAO are only available for Analysis Sets that are based on Wave Set components. Extraction of results RAO will perform correctly only in case all individual waves included in the Wave Set are given similar headings.

Results RAO cannot be extracted from Analysis Sets that are based on Environment Sets.

Results RAO may also be processed from a single frequency-domain dynamic analysis provided that the number of periods, upper and lower bound periods are properly setup in the Calculation parameters.

Type of RAO: Two types of requests are available :

  • One DOF: RAO will be computed for the selected variable.

  • 6 DOFs: RAO will be computed for the selected group of variables (for the 6 degrees of freedom).

    Note

    When 6 DOFs is selected, the following variables only are available:

  • Position

  • Displacement
  • Velocity
  • Acceleration

According to: The RAO can be computed according to the wave elevation (at a fixed point or at the point where RAO are computed) or according to another variable. By clicking the Edit button in the According to window, you can specify the location of the primary variable of the RAO.

The secondary variable of the RAO is chosen as in the static or dynamic curves windows.

Time steps: to specify the time range to be used for computing the RAOs. It is generally recommended to remove the ramp time at the beginning of the simulation. You may either specify both minimum and maximum time values, or select the number of wave periods

Advanced parameters: The Advanced window allows to select advanced options for computing the RAO:

  • For the RAOs computed from a regular wave: FFT method or standard deviation.

  • For the RAOs computed from a irregular wave: Selection of standard spectrum ratio formula or cross spectrum analysis, and related advanced parameters.

Once you have input all the data required, click the Add curves button. Three different graphics will then be computed:

  • RAO amplitude: this is the amplitude of the dynamic part of the result that would arise from a unit wave height.

  • RAO phase: phases are phase lead with respect to the wave crest.

  • RAO consistency: the consistency is a measure of the RAO quality and accuracy. Consistency may vary between 0 and 1, where 1 indicates perfect consistency. Values less than 0.5 indicate that processing could not perform well for certain wave periods. Consistency is always equal to 1 for analyses based on regular waves. In case the consistency is found to be 0.5 or smaller with irregular waves, it is recommended either to extend the duration of the irregular wave simulation, or increase the number of wave frequencies used to model the wave spectrum, or even change the wave spectrum to get a more uniform wave energy distribution over the frequencies of interest.

Warning:

RAOs requests (either when defined as a predefined result or not)
may be generated from any analysis in the active analysis set. However RAOs
processing is correct  only in case all the .DSS files from the analysis set
have been created at the same time. Otherwise, the following message would
appear:

DeeplinesGUI
Number of nodes set is <=0

Erroneous RAOs would then be built and displayed. Therefore, in case where one
or several  analyses has to be modified (further to a convergence problem for
instance), user must regenerate all .LOG and .DSS files for the whole analysis
set (Launch computation (analysis set object popup menu) -> Generate all)
before opening any .DSS file for RAOs post-processing purpose.