Wave scatter conversion: Data

The wave scatter conversion tool requires the following data, which can be saved to a .sct file.

Regular wave scatter

If calculated from irregular sea state scatter is selected then the regular wave scatter table is derived from a random sea state scatter table using the formula of Longuet-Higgins, 1983. Full details of the calculations involved are given in the theory section.

If user specified is selected then the regular wave scatter table is input directly. This option is useful where the regular wave scatter table is pre-determined but you still want to take advantage of the automatic export of text data files, batch script and fatigue analysis files.

Units

You can select the units of length to be used for the wave scatter data.

Sea state table

These data specify the random sea state scatter table. If the regular wave scatter option is user specified then these data are not required.

Spectral form

All the sea states are assumed to be of the same spectral form, which can be either JONSWAP or ISSC.

Spectral parameters

If you select the JONSWAP spectral form you have three options for specifying the spectral parameters: Automatic, partially specified or fully specified. This setting applies to all sea states.

Sea states

You must define the spectrum for each sea state in your scatter table.

If you have selected the ISSC spectral form then you enter $\Hs$ and one of $\{\Tz,\Tp,\fm\}$.

If you have selected the JONSWAP spectrum the required parameters depend on the selected parameter option:

• automatic parameters mean that you set $\Hs$ and $\Tz$, and OrcaFlex calculates and reports $\Tp\text{ and }\fm$.
• with partially-specified parameters, $\Tz,\Tp,\text{ and }\fm$ are interdependent, so setting any one of them sets the other two accordingly. You must also set the (independent) value of $\Hs$.
• fully-specified parameters require that you set one of the interdependent values $\Tp\text{ and }\fm$, from which OrcaFlex calculates and reports $\Hs\text{ and }\Tz$. You must also set $\gamma$, $\alpha$, $\sigma_1$ and $\sigma_2$.

Number of occurrences

For each sea state in your scatter table you must also define the number of occurrences of that sea state. This value is required to determine the relative likelihood of occurrence of each sea state.

Normally this value comes from a sea state scatter table, where it is usually presented in parts per thousand.

Regular waves (wave scatter calculated from irregular sea state scatter)

These data define the discretisation of the regular wave scatter table into bins.

Wave heights, wave periods

These are the regular wave heights and wave periods that you wish to analyse. OrcaFlex automatically constructs bins around these values. The output from the wave scatter conversion tool includes a table listing these bins.

Total duration

The total duration to be covered by the regular wave scatter table.

Occurrence threshold

Some of the bins in the regular wave scatter table can have very small occurrence values. In order to reduce the amount of computation time required to analyse all the load cases it is customary to ignore such bins. Any bin which has an occurrence value lower than this threshold will be ignored. If you do not wish to ignore any bins then you should set the threshold to 0.

Regular waves (user-specified regular wave scatter)

These data define a complete regular wave scatter table.

Number of bins

The number of bins in the regular wave scatter table.

Wave height, wave period, number of occurrences

These are specified for each bin in the regular wave scatter table.