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Home > User Interface > Results Processing > Fatigue Analysis > Fatigue Analysis Setup > Fatigue based on the spectral approach

Fatigue analysis based on the spectral approach

The pre-processing window associated to spectral fatigue analyses is defined through six different sections:

  • The main section

  • The Load case selection section

  • The Line parts section

  • The Stress calculation diameter section

  • The Stress type section

The fatigue analysis process is summarized in this chart.

Warning : User attention must be paid to the fact that detailed results output files request (through Save Results button) may lead to very consuming CPU time (I/O) and very large files, despite it is binary files. This is true in particular for Bins combinations. Example: A detailed file for 140 elements, 8 section points, 30000 sea-states combinations is about 400 Mo.

Note

The characteristics of each line part are read in the .DSK file from each relevant segment type, in particular the corrosion allowance and the SCF. SCF coefficients must include the equivalent SCF due to thickness effect, according to the thickness effect factor and the reference thickness associated to the fatigue curve.

For each section, the information available or requested is:

Main section

Contains general data definition, options selection and action release.

  • Name: Name of the fatigue analysis, which may be modified.

  • Problem: Select the type of fatigue post-processing, Riser or Mooring fatigue. If Mooring is selected, "Stress calculation diameter", "Stress type" sections, and the number of section points field are unavailable (shadowed). Furthermore, the fatigue curve selection area becomes "T-N curves" instead of S-N curves.

  • Analysis type: The displayed window may be changed at every time depending on the selected method, using check boxes displayed in the main window (Spectral or Rainflow).

  • Line type: you may select either the Generic line type which corresponds to homogeneous pipes or the Trelline type which correspond to bounded flexible pipes and is associated with the hose line type.

  • Save full database: When the Save full database check box is activated, one asks the software to create a detailed results output file (binary file analysis-name.dtf containing the damage values for all elements, all sections points, all line parts, all Bins, all sea-states). The type of results saved in the DTF file is selected through the combo-box:

    • NORMAL means fatigue results based on Miner-Palmgren assumption with Rayleigh correction for narrow banded vibrations, but without Whishing correction.

    • WIRSHING means fatigue results based on Miner-Palmgren assumption with Rayleigh correction for narrow banded vibrations and with Whishing correction.

  • OK: The OK button is used to save the current fatigue analysis. The window is closed.

  • Launch: The Push button Launch is used to save and launch interactively the fatigue post-processing of the current analysis.

Load case selection

This section is used to define the BINs load cases, it means the set of sea- states blocks, their headings, spectrum type, probabilities of occurrence, etc

  • New: To create a new load case BIN

  • Remove: To remove an existing BIN

  • Backward: To display the previous BIN

  • Forward: To display the next BIN

  • Name of BIN: The user must give a name to the new BIN

  • RAOs: Stress/tension RAOs may be recovered from an existing DSO file. Otherwise, the RAOs are built from selected dynamic analyses results database.

    • Type of dynamic analysis: Whatever the RAOs origin is (existing or to be built), the type of dynamic analyses already performed on RAOs building purpose, has to be settled:

    • From regular waves: Regular waves analyses have been performed to build stress or tension RAOs. In this case, the combo-box below provides the list of regular waves Analysis sets. One analysis set must be selected.

    • From irregular waves: Irregular waves analyses have been performed to build stress or tension RAOs. In this case, the combo-box below provides the list of irregular waves Analyses. One analysis must be selected.

    • RAOs recovering: First, one has to select the type of dynamic analysis already carried out. Then Use DSO must be selected. If the DSO file exists and the awaited stress/tension RAOs are included, these former are read and used. On the other hand (no DSO or RAOs not available), RAOs are built as if Use DSO option has not been selected.

    • Advanced parameters : You may find details about the advanced parameters by clicking here.

  • Spectrum: For the current BIN, a wave spectrum type must be selected. This will be used to build the response spectrum for all sea-states included in the scatter diagram associated to the BIN.

  • Wave type: The user must then specify if the damage must be computed for swell alone, local seas alone, swell + local seas not combined or swell and local seas combined. When swell and local seas have to be combined, a global response spectrum is built:

\[ S^{combined}_{stress}(\omega) = S^{swell}_{stress}(\omega) + S^{local sea}_{stress}(\omega) \]

  • Scatter diagram: One scatter diagram must be selected for each BIN. A file browser is used. Once selected, the scatter diagram file may be edited and modified with Edit File. The file format is described here.

  • Probability Distribution: The stress/tension distribution may be built according to different probability distributions that must be selected. The distribution law is applied to the whole scatter diagram of the current BIN.

  • Export: Once the load cases definition has been completed, this can be saved in an external file. The file extension is BIN. The file format is described here.

  • Import: External BIN files may be imported

  • Position: Display the serial number of the current BIN in the list (can not be modified).

  • Number of cases: Display the total BINS number (can not be modified).

Line parts

This section is dedicated to the elements selection. Whole or part of one (or several) line(s) may be selected, using abscissa values.

  • Line: A line object must be selected in the list displayed in the combo-box. Note that the list includes all the lines defined in the DSK.

  • S-N/T-N curves: A fatigue curve file must be selected via the browser. Two files are provided with DeeplinesGUI and are copied in the installation directory during the setup: File SNCurves.csn for S-N fatigue curves, TNCurves.ctn for T-N fatigue curves. The file format is described here. Once the file has been selected, the fatigue curve list must be updated by pressing the Reload button. Fatigue curves may be plotted by clicking the Edit button.

  • Curve: A fatigue curve must be selected via the browser. The fatigue curves available in the previously selected file are listed.

  • Curv. Abscissa: For the selected line object, points at which fatigue damage will be computed are selected via abscissa values. Curvilinear abscissa along the line must be seized, in-between the range of actual abscissa. The convention is: 0 corresponds to the beginning of the line; the last allowable abscissa is the line length (line end). Abscissa may be seized individually or by groups.

    Separator symbol is ;
    Group of abscissa are defined with -
    Example: 10.; 20.; 100.-450.

  • Add Line Part: Once the former parameters have been selected/seized (fatigue curve, line, abscissa), it defines a line part. This can be added in the fatigue analysis using Add Line Part. Then, the parameters may be changed to define a new part. Different fatigue curves may be associated to different parts of lines. Several line parts may be defined and included in the fatigue analysis.

  • Remove Line Part: Used to remove line parts that have been previously defined and included in the fatigue analysis.

  • Abscissa and Fatigue curve: A table summarizes the definition of the selected line parts. The table number of lines is increased, respectively reduced, each time the user adds or removes a line part. The first column gives the current number of the line part. The second column recalls the abscissa. The last column gives the name of the associated fatigue curve.

Stress calculation diameter

Used to specify at which diameter the stress will be calculated. For mooring analysis, this feature is not available.

  • Outer: The stress will be computed at the pipe outer diameter.

  • Inner: The stress will be computed at the pipe inner diameter.

Stress type

Used to specify which stress component will be calculated. For mooring analysis, this feature is not available.

  • Axial: The axial stress only will be used to compute fatigue.

  • Bending: The bending stress only will be used to compute fatigue.

  • Axial+bending: The axial+bending stress component will be used to compute fatigue.

Number of section points: User may choose the number of section points at which stress or tensions will be computed for a riser analysis. The default value is 8. For a mooring analysis, the value is fixed to 1.

Compute envelop of RAOs: the damage will conservatively be calculated using the maximum envelop of stress RAOs among the number of section points.

Real sea-states defined by lines: This option is to specify a specific combination of sea-states. This is useful when sea-states records are used instead of scatter diagrams. When this option is checked, the stress spectra and associated damages will be calculated for the combination of all the waves on the same row for the different BINs. This in particular allow to deal with crossed-seas. The number of lines used to describe the environmentmust be identical in each BIN in this case.