Keyword Based Variations |
 
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Keyword Based Variations |
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While the use of equations may be helpful in certain circumstances, the main benefit of parameter definition probably lies in the area of load case variations. This area of application is illustrated in the following simple example. Supposing you wish to examine a range of near and far vessel offset cases, spanning ±100m, at 25m intervals. In earlier versions of Flexcom, this would require the creation and storage of a separate keyword file for each offset, but the keyword parameterisation facility means that a single keyword file is all that is required to model each load case. So you include all the required data in a single base keyword file, called “Offset.keyx”, restarting from an initial static analysis keyword file called “Static.keyx”, as follows:
$PREPROCESSOR
C Set up a vessel offset parameter
*PARAMETERS
Offset, -100
OffsetFolder, "Near"
C Set up the vessel offset variations
*VARIATION
NAME=NearOffsets
PARA=Offset, GEN=-100, -25, 25
PARA=OffsetFolder, VALUE=Near
NAME=FarOffsets
PARA=Offset, GEN=25, 100, 25
PARA=OffsetFolder, VALUE=Far
C Set up the analysis combinations
*COMBINATIONS
DESCRIPTION=Offset Analyses, SUBDIRECTORY="%OffsetFolder%\%Offset%m Offset", FILENAME="%Offset%m Offset"
VARIATIONS=ALL
$LOAD CASE
*RESTART
TYPE=DYNAMIC/STATIC, LAST="Static"
C Reference the (variable) vessel offset parameter
*OFFSET
VESSEL=FPSO
OFFSET=0.0, =[Offset], 0.0, 0.0, 0.0, 0.0
Flexcom will then automatically create a separate keyword file for you corresponding to each vessel offset, and the *OFFSET keyword will contain the relevant offset value. The names and locations of the files are controlled by the *COMBINATIONS keyword. For the sample data shown above, separate subdirectories are created using a combination of the parameter name (i.e. “%Offset%” in this case) and standard characters (i.e. “m Offset” in this case). Specifically, you will obtain 8 separate subdirectories, entitled “Near\-25 m Offset”, “Far\75m Offset” etc., located in the same directory as the base file. The keyword file names will be entitled “-25m Offset.keyx”, “75m Offset.keyx” etc.
You might typically proceed to defining a range of regular wave load cases, restarting from the vessel offset cases. Supposing you wish examine a range of regular waves, with periods spanning 6-24s, at 2s intervals. When these 10 regular waves are combined with the 8 vessel offsets, this results in a load case matrix totalling 80 separate dynamic analyses. To set up all of these keyword files individually would be quite tedious and time consuming, and this is where the keyword parameterisation facility really comes into its own. It’s possible to set up all the required analyses using a single base keyword file, called “Wave.keyx”, as follows:
$PREPROCESSOR
C Set up wave period and vessel offset parameters
*PARAMETERS
Period, 6
Offset, -100
OffsetFolder, "Near"
*VARIATION
C Set up the wave period variations
NAME=Waves
PARA=Period, GEN=6, 24, 2
C Set up the vessel offset variations (copied from previous file)
NAME=NearOffsets
PARA=Offset, GEN=-100, -25, 25
PARA=OffsetFolder, VALUE=Near
NAME=FarOffsets
PARA=Offset, GEN=25, 100, 25
PARA=OffsetFolder, VALUE=Far
C Set up the analysis combinations
*COMBINATIONS
DESCRIPTION=Wave Analyses for Offsets, SUBDIRECTORY="%OffsetFolder%\%Offset%m Offset", FILENAME="%Period%s Wave"
VARIATIONS=ALL
$LOAD CASE
C Reference the (variable) wave period parameter
*WAVE-REGULAR
1.0, =[Period], 0.0, 0.0
C Reference the (variable) wave period parameter
*TIME
STEP=FIXED
2, =[(Period*5)+2], =[Period*0.05], =[Period]
C Reference the (variable) vessel offset parameter
*RESTART
TYPE=DYNAMIC/STATIC, LAST==[Offset + “m Offset”]
For the above specification, you will obtain 10 separate wave analyses, entitled “6s Wave.keyx”, “8s Wave.keyx” etc., located in each of the 8 subdirectories created previously for the vessel offset analyses. Each dynamic analysis uses an appropriate regular wave period. A unit wave amplitude is used in all cases. Appropriate time variables are computed based on the regular wave period. Each dynamic analysis runs for 5 wave periods, using a time step of 1/20th of the wave period, with the loads ramped on over the first wave period.
The load case matrix is automatically determined by Flexcom based on the specified parameters and combinations. For the sample case described above, 10 different wave periods are combined with 8 different preceding vessel offset analyses, resulting in a load case matrix totalling 80 separate dynamic analyses.
For complete generality, it is also possible to add or remove entire keywords. This facility is intended to cater for variations which are difficult to accommodate by the standard parameter variations, but the option is probably rarely invoked in practice. For example, if you wished to examine different wave spectra or discretisation options, it might be more straightforward to replace the entire *WAVE keyword, rather than attempting to parameterise several different options within it.
While the preceding discussion provides a representative sample of the modelling possibilities using keyword parameterisation, it is by no means exhaustive. You could also use parameterisation to examine different finite element mesh densities, or to automatically compute structural properties required by Flexcom from the engineering inputs available to you. You can make the parameterisation definitions as simple or complex as you wish – the possibilities are endless really – feel free to experiment.
Note that the above example is ideally suited to regular waves, where the wave period tends to vary in fixed increments. Random seastates, which are typically characterised by a mean up-crossing period Tz (this does not vary in fixed increments), are better suited to the alternative Spreadsheet Based Variations modelling approach.
•*PARAMETERS is used to define parameters whose names may be referenced subsequently in the definition of other input variables.
•*VARIATION is used to define parameter or keyword variations.
•*COMBINATIONS is used to control the names and locations of generated keyword files, along with the required analysis combinations.
If you would like to see an example of how these keywords are used in practice, refer to J03 - Summary Postprocessing Collation.