Fatigue analysis: Results for damage based methods

Fatigue results are presented in a separate window, in both graphical and tabular form.

Results graphs

Range graphs are presented of fatigue damage or life against arc length.

We denote a damage value at arc length $z$ and circumferential position $\theta$ as $D(z, \theta)$. This could be overall total damage or the total exposure damage value for an individual load case. The damage graphs all plot $D(z) = \max\limits_\theta D(z, \theta)$, that is the maximum damage value at a given arc length $z$.

The life graphs plot the corresponding life values $T/D(z)$ where $T$ is the exposure time. For individual load cases, $T$ is the exposure time for the load case; for total life, $T$ is the total exposure time of all the load cases.

Graphs are available for the calculated fatigue damage or fatigue life as follows:

• Total damage/life: the overall total damage/life
• Individual load case damage/life: the total exposure damage/life from the specified load case
• Worst cases: the total exposure damage value from the five most damaging load cases. By most damaging, we mean the load cases with the largest values of $\max\limits_{z, \theta} D(z, \theta)$, the maximum damage value over all $z$ and $\theta$.

The graphs can be customised in a number of ways:

• The arc length axis can be either horizontal or vertical, the latter option being more appropriate for vertical risers.
• The arc length axis can be inverted. When it is inverted increasing values run from right to left (if it is horizontal) or top to bottom (if it is vertical). Again, the latter is appropriate for vertical risers with arc length values that increase from the top end to the bottom end.
• Fatigue damage/life can optionally be plotted on a logarithmic scale.
• Individual arc length intervals can be plotted.

Results tables

Results tables are presented in a spreadsheet consisting of a number of pages: a damage summary, collected damage tables, a page echoing the S-N curve data, and one page for each load case.

Damage summary sheet

The damage summary (not available for the SHEAR7 damage calculation) includes a worst damage table detailing the amount and where on the line it occurred, and an excessive damage table listing all fatigue points at which the overall total damage has exceeded the specified critical damage value.

Damage tables sheet

The damage tables sheet reports the overall damage over total exposure for each arc length, and detailed damage tables for each arc length analysed. These report, for each fatigue point in the cross section at each arc length, the total exposure damage value from each load case and the overall total damage.

In all of these tables, overall total damage values that exceed the specified critical damage value are highlighted in red.

Load case sheets

The load case sheets contain the derived stress results for each fatigue point that has been analysed, together with general information such as the environmental data that applied to that load case.

There is one table of stress results for each arc length covered by the specified arc length intervals. Each such table contains a row of results for each fatigue point in that arc length cross section. These results are the stress ranges (for each of the stress components), the maximum stress range and the resulting load case damage values.

In addition to the detailed tables a damage summary table is presented which tabulates the load case damage at each fatigue point.

For mooring fatigue, tension results rather than stress results are reported. For SHEAR7 fatigue, since the damage calculation has been performed by SHEAR7, only damage values are reported.

Saving and printing

You can save the results by exporting the spreadsheet, as an Excel sheet or plain text. The results can be printed directly from OrcaFlex, but for more control we recommend that you export them in Excel format and then print from Excel.

Customising results output

The tabular results output can be customised using the options on the results page of the main fatigue form.

Output load case tables

You can choose to omit the individual load case sheets from the fatigue results. This can significantly reduce the time and memory required to generate the results tables.

Output detailed load case tables

If you do choose to output load case tables, you can further choose whether or not the detailed tables on the load case sheets are omitted from the fatigue results. Again, this can help to reduce the time and memory required to produce the results.

Load case damage units

Load case damage values may be reported as damage per hour, damage per day or damage per year.