Stofat

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Combining results from different loading conditions in Stofat

As of today, there is no possibility to combine results from different runs inside Stofat (for instance 80% operation 20% transit). This issue has been adressed previously and is listed as a development task.

There are two general workarounds for the issue, and neither of them is particularly simple. The first way is to combine the loadfiles (L#.FEM) in a small utility program called Waloco before running Sestra. However, the wave data in the S#.FEM files also has to been considered, which may not be so straightforward. This may also be problematic since Stofat has some model size limitations, and you may very well end up splitting up the runs either way since the model becomes too big.

The second way would be to write scripts (Excel VBA/Phyton or similar) to manually combine text output vtf files with fatigue results from the different runs. This is perhaps not so straightforward and most likely very time consuming and prone to errors.

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How is Stofat calculating stresses for fatigue calculation?

Stofat is based on the documentation in DNV Class Note 30.7 and DNV RP-C203. We recommend looking up these documents, in addition to the Stofat user manual.

In Stofat, the stresses are extrapolated from the user defined interpolation points to the hotspot, as given in the RP-C203 section 4.3.1, fig. 4-3.

Stofat will first interpolate the component stresses from the element stress points to the two user defined interpolation points. Next, the component stresses are extrapolated from the interpolation points to the hotspot, where the principal stresses are calculated.

The principal stresses are used together with the wave spectra to calculate the respons spectra. Spectral moments are computed from the respons spectra. The stress ranges used in the fatigue calculation are computed based on the spectral moments.

Stofat is not using equation 4.3.1 (in RP-C203) directly, but will use the principal stresses together with the 'Weld normal line' method, as shown in fig. 2-3 and 2-4 in the RP, to sort out the principal stresses parallel to a weld. When the main principal stress is within the sector defined by the weld normal line, it will be used. If the main principal stress is outside this sector, the damage is set to 0.

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How to enable stochastic fatigue for a submodel with no local wave loads?

Including local loads like wave loads is normally recommended in a submodel analysis. However, if the wave loads are not important for the submodel analysis, the local wave load analysis may be skipped. In that case the file S#.FEM, required for stochastic fatigue, needs to be created for the submodel.

The S#.FEM file has to be present when Sestra is run for the submodel, together with the T#.FEM file(s) - and L#.FEM file(s) for possible local loads. If not, no stochastic fatigue can be computed later.

As the wave load cases for the submodel have to match the wave load cases for the global model, the global S#.FEM file (e.g. S1.FEM) can be copied and renamed to the submodel superelement number (e.g. S4.FEM). Renaming the file is okay because the contents are Sestra commands. (Renaming a T#.FEM model file is not okay.)

Please notice that the contents of the S#.FEM file are described in the Sestra user manual.

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How to export pictures from Framework, Profast, Stofat, Postresp?

Update November 2017:As Office has been more restrictive to Postscript due to a security vulnerability in EPS please try this workaround:select "Microsoft print to PDF" or a similar PDF printer driver as default printer on your computerinside Postresp set the Plot to Format Windows-printersWhen you Plot the image, this will be saved in a pdf file

Please note that the file will not be properly saved until you close Postresp.

It was possible to import CGM files directly into MS Office in the past. However, a Windows update rendered MS Office uncapable of recognizing this file format.

In order to export pictures from the software it is recommended to export as Postscript format and change the file extension from .ps to .eps.

Then it is possible to import into MS Office.

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How to generate a PDF file for a plot from Framework, Profast, Stofat, Postresp

User can follow the below steps to create a PDF file for a Framework plot.

  • After a desired plot displayed on the screen, on **Set **tab select **Plot **and then choose Postscript. Click Apply or OK to save the selection.
  • Click Plot, the file with the defined name, such as FrameworkActivity1.PS, is created in the analysis folder. Now exit Framework.
  • In the analysis folder, double click the generated postscript file FrameworkActivity1.PS to open Acrobat, click YES to generate the PDF file.

See also the attached document for details.

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Is there a thickness correction in solid elements?

No. The thickness correction is only done for 2D shell elements, not for 3D solid elements.

To do this for solid elements is too complex. The need of a thickness correction is not that important when using solid elements anyway. These elements may be used as a number of elements through the plate thickness, capable of describing the stress variation a lot better than the shell elements.

The thickness correction is used to adjust for a different stress variation through the plate thickness for thin plates as opposed to thick plates. This may influence the calculated damage. If solid elements are used with just one element through the plate thickness, a correction may still be needed. This can be included as a stress concentration factor (SCF).

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Read jnl file (Postresp/Stofat/Framework)

A JNL command file cannot be read from within Postresp (or Stofat, Framework) when it is located in a folder using blanks (spaces) in the name or path. Workaround: copy the file to another folder, e.g. C:\DNV\workspaces

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What is the exceedence probability levels?

The commands DEFINE SHELL-FATIGUE-CONSTANTS EXCEEDENCE-PROBABILITY-LEVELS or DEFINE FATIGUE-RESULTS-DUMP EXCEEDENCE-PROBABILITY in Stofat are more or less doing the same thing, which is to define the number of stress exceedence levels that are to be included in the dump file from a certain fatigue run. The default amount of levels are set to 11, but this may be changed if neccesary. Below is an excerpt of a dumpfile (.pex) for a certain hotspot or element (294) with 15 exceedence levels (becomes 16 in reality since level 0 is included):

Stress range  
Exceedence probability
294 8 All 0 4.25488950E+06 1.94639291E-08
294 8 All 1 4.09540550E+06 6.57393002E-08
294 8 All 2 3.80287650E+06 5.52219262E-07
294 8 All 3 3.51034725E+06 3.98817792E-06
294 8 All 4 3.21781850E+06 2.47715889E-05
294 8 All 5 2.92528950E+06 1.32427696E-04
294 8 All 6 2.63276050E+06 6.09929557E-04
294 8 All 7 2.34023150E+06 2.42231088E-03
294 8 All 8 2.04770262E+06 8.29821639E-03
294 8 All 9 1.75517362E+06 2.45170034E-02
294 8 All 10 1.46264475E+06 6.24469407E-02
294 8 All 11 1.17011588E+06 1.37006059E-01
294 8 All 12 8.77586875E+05 2.57967234E-01
294 8 All 13 5.85057938E+05 4.12830949E-01
294 8 All 14 2.92528969E+05 5.52937984E-01
294 8 All 15 0.00000000E+00 6.10746741E-01

Notice the significant change in exceedence stress range probability between the levels. If both these commands are used, the command with the highest exccedence level will be the active one. Please also have a look at chapter 2.7 in the user manual for more info about the dump file, and also try the commands in Stofat to look at the actual results.

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What is the number in the FILE TRANSFER command?

The number used in the FILE TRANSFER command is the superelement identification KEY number, not the *superelement *number. This KEY number is defined by the sequence of assembling superelements in Presel.

When running Stofat interactively, a table showing the connection between the superelement type number (T#.FEM) and the key number is displayed.

From the Prepost user manual:

A superelement identification key is simply an integer number in the range 1 to number of 1st level superelements, numbered from left to right in the superelement tree.

PREPOST will present a list showing the available superelements and their corresponding keys.

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