RAO Response Plot |
 
|
RAO Response Plot |
|
The RAO Response Plot feature is a useful means of verifying that vessel RAO data has been input correctly into Flexcom. In the most general case, RAO magnitudes and phase angles vary with both incident wave heading and wave frequency, so the complete specification of RAO data for any given vessel can be quite extensive. Furthermore, Flexcom has its own set of conventions regarding the specification of RAO data, and the program supports RAO Conversions from third-party software.
The RAO Response Plot feature graphically presents responses for each of the local Vessel Degrees of Freedom. Heave is shown by default, but the other directions may be examined using the Plot drop-down list. RAO magnitudes are generally of most interest, but it is also possible to examine RAO phase angles. If more than one vessel is included in the input data, the name of the desired vessel may be selected from the Vessel drop-down list.
In 2D mode, vessel response is presented as a function of wave frequency in a 2-dimensional space. If RAO data is specified at several wave headings, separate data series are superimposed on the same plot, each using a different colour, as designated by the plot legend.
In 3D mode, vessel response is presented as a function of both wave frequency and wave heading in a 3-dimensional space. Viewing data in 3D space provides enhanced visualisation and is particularly suited to RAO data.
The following sample plot presents heave RAO data (m/m) as a function of wave heading (degrees) and wave frequency (Hz).
Sample 3D Plot
You can alternate between 2D and 3D views by pressing the 
button.
A vessel may be defined using either one of two keywords, *VESSEL or *VESSEL,INTEGRATED. While the former is perfectly valid, it has effectively been superseded by the latter, which is now the recommended option. *VESSEL,INTEGRATED is a more comprehensive keyword and has several advantages over its predecessor. Most importantly it accepts RAO data also, thereby eliminating the need for a separate *RAO keyword – hence the ‘integrated’ nature of the new keyword. Further details are available in Basic Vessel Concepts.
In general, when performing an analysis where RAOs are specified as a function of wave heading, it is always recommended to examine the time dependent motions of floating vessels. This provides confidence that the specified RAO data does in fact lead to the expected vessel motions. Flexcom also provides a related feature, the Vessel Response Plot option, to readily facilitate such an examination.