Time history data

Some data items allow you to specify time-varying values in the form of a time history. The time history data can be given either in an external file or as an internal table of data.

This functionality is available for wave elevation, wind speed and direction, vessel motion and constraint imposed motion, and allows you full control over how the variable changes with time.

When using external files to specify time-varying data for more than one object (e.g. for both the wave and wind, or for the wave and a vessel), you can have all the data in a single file (using multiple columns in a single table) or you may use separate time history files for the different objects.

Time history data are specified on a page control like this:

Figure:

Time history data

Source page

The data source determines whether the data are specified in an internal table, or in an external file. In the case of external data, the input file is the name of the time history input file that contains the data. You can give here either the full path or a relative path.

Vessel motion and constraint imposed motion time history data offer you a choice of interpolation method. For most purposes we recommend using cubic spline interpolation, since it gives continuity of velocity and acceleration. Cubic Bessel interpolation typically gives step changes in acceleration at the specified time samples, and linear interpolation gives zero acceleration between the times specified and then an infinite acceleration when the velocity changes at a specified time sample. Such acceleration effects can manifest themselves as steps or spikes in the inertial forces on any connected child objects.

The time origin gives you control of how the times given in the time history data relate to the times in OrcaFlex. This time origin specifies the simulation time that corresponds to zero time in the time history data. You can apply different periods of the time history by adjusting the time origin.

For example, if the time history time origin is 100s and the simulation time origin is 400s, then a simulation consisting of 40s of build-up (i.e. simulation time -40 to 0) followed by 200s of simulation (simulation time 0 to 200) will cover time history time from 260s to 500s. Note that the time history data must cover the whole of the simulation, and, depending on the time history import range data, may need to cover a wider range of times.

For vessel time history data the datum point specifies, with respect to vessel local axes, the point on the vessel at which the motion time history applies.

For wave time history data the minimum number of components determines how many components are used for the Fourier representation of the time history wave.

For constraint time history data the ramping data are applied to smooth the transition between statics and dynamics.

The minimum time sample interval can help to smooth time history data in some situations:

if the time history values have been truncated to a small number of significant figures, or
if the sample interval in the data is very small.

If either of these conditions hold (or worse, if both do) then you may find that the interpolated time history values cause the simulation to contain unrealistic noise.

Using a non-zero minimum time sample interval results in the time history data being sub-sampled at the specified period. For example if the time history contained data sampled at 0.01s intervals and the minimum time sample interval was 0.2s then OrcaFlex would use every 20^th row of time history data.

This sub-sampling technique has proved to be very effective at smoothing noisy interpolated time histories and so improving the quality and stability of simulations.

Note:

If you do not wish to sub-sample the time history input data then you should set this value to 0.

Data page (internally specified)

The time history data are held in a single table, with each row containing a time value and the corresponding data for that time. The columns of the table are as follows:

Wave elevation data contain time and elevation columns.
Wind data contain time and speed columns, and optionally a direction column.
Vessel and constraint imposed motion data are made up of a time column, and six columns for the degrees of freedom.

For a wave time history the time values must be equally spaced, since this is required by the fast Fourier transform (FFT) used. Other time histories do not have this limitation: variable time intervals can be used, since they use interpolation methods rather than FFT.

Data page (externally specified)

You must tell OrcaFlex which columns in the time history file correspond to time, X, Y, Z etc. This is done on the data page using a series of drop down lists.

You can also indicate that degrees of freedom (X, Y, Z etc.) are fixed by selecting none in the drop down list. For a vessel primary motion time history, the degree of freedom will be fixed at the value at the end of the statics calculation. For a vessel superimposed motion or a constraint imposed motion time history, the degree of freedom will be fixed at zero.

Time history file format

Time history files must conform to the following formatting rules:

The file must be a tab-delimited text file; in other words it must be a text file in which the time history data columns are separated by single tab characters. Files of this format can usually be exported from a spreadsheet in tab-delimited text format.
The data must be in standard decimal or scientific form.
One column must contain the time values; these must be given in ascending order.
The data must be given in the same units as are used in the OrcaFlex model. Note that angles must be in degrees.

The data are assumed to start at the first numeric entry in the time column and blank rows are not allowed below this first entry. This means that textual information about the file (titles etc.) can precede the data but not be interspersed within it.

Note:

If you generate your time history text file from a spreadsheet such as Excel, beware that the number of significant figures in the text file will correspond to the number format of the cells in the spreadsheet. Before saving the text file, you should set the number format to display all the significant figures, otherwise the time history text file will lose some precision. In Excel you can do this by first selecting all the cells and then using the format cells command (on the format menu) to set the number format to scientific with 15 decimal places.

Preview page

The preview page presents graphs of the data contained in the time history, to provide you with a quick check on the data. The time axis of the graphs shows the raw data and each graph has superimposed on it two vertical lines indicating the beginning and end of the simulation. As described above, the time history time origin and the simulation time origin can be used to control the relationship between times given in the time history and time in OrcaFlex: these preview graphs alow you to confirm that the time origin data items are set correctly.

Note:

The minimum time sample interval is not taken into account when OrcaFlex displays a time history preview graph; all the data are used.