OrcaFlex Manual - Orcina

VIV Toolbox, Frequency Domain Models
SHEAR7 Mds file
The SHEAR7 Mds file export facility is provided on the Modal Analysis form. To use it:
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� You will need to create a SHEAR7 data file. The simplest way to do this is to use the built-in facility to export
SHEAR7 data files.
� Calculate the static position of the model.
� On the Results menu select the Modal Analysis.
� Do the modal analysis for the line you want to analyse. You must include calculation of the mode shapes and
include all modes that might to be needed by SHEAR7.
� Select the VIV page. For each mode, the VIV page reports the extent to which the mode is transverse (i.e. in the
VIV direction), inline (i.e. in the normal drag direction), axial, or some mixture. This helps you decide which
modes to export to SHEAR7. See Mode Selection Table below for details.
� The final column of the table shows which modes are currently selected to be exported to the .Mds file.
Whenever you calculate the modes OrcaFlex ticks just the Transverse and Mostly Transverse modes, but you
can then change the selection as required.
� Use the Export SHEAR7 Mds File button. A SHEAR7 .Mds file is generated for those modes that are currently
ticked for export. You are asked where to save the file. See Values Exported for details.
Warning: OrcaFlex does not calculate the mode slope; in its place OrcaFlex outputs zero. See Values Exported
for details.
Exporting from Batch Script
The SHEAR7 Mds file can be exported from an OrcaFlex batch script using the SHEAR7MdsFile command.
Mode Selection Table
VIV excitation occurs in the transverse direction. In SHEAR7 mode shapes are 1D since it assumes that all the modes
in the .Mds file are purely in that transverse direction. The OrcaFlex modal analysis is fully 3D, so in principle you
should therefore only export modes that are purely transverse.
In practice the natural modes do not always neatly divide into the transverse, inline and axial directions, so you will
sometimes have to export the modes that are nearest to being transverse. OrcaFlex therefore provides the mode
selection table to help you decide which modes should be exported to SHEAR7.
The table includes the following columns:
� Offset Distribution displays a measure of how inline, transverse and axial the mode is. For details see Offset
Distribution.
� Mode Type classifies each mode according to the offset distribution. Transverse means that the transverse
component is more than 90% of the total, Mostly Transverse means that it is between 50% and 90%, and
similarly for Inline, Mostly Inline, Axial and Mostly Axial. Mixed means that none of the components are more
than 50% of the total.
� Export to SHEAR7 Mds File determines which modes will be exported to SHEAR7. When you calculate (or
recalculate) the modes OrcaFlex ticks just the Transverse and Mostly Transverse modes, but you can then
change the selection as required. See Discussion and Examples.
Values Exported
The first line in the .Mds file contains the number of selected modes and the number of nodes in the line. Then
follows a section giving the angular frequencies of the selected modes, in radians per second. Finally there follows a
section for each selected mode, giving the Mode Offset, Mode Slope and Mode Curvature.
Warning: OrcaFlex does not calculate the mode slope; in its place OrcaFlex outputs zero. The mode slope is
only used in SHEAR7 to correct the predicted RMS acceleration for a vertical riser, in the special
case where one is trying to model the gravitational contamination from tilt that a transversely
mounted accelerometer would measure. See the SHEAR7 documentation for details. The RMS
acceleration is listed in the SHEAR7 .plt plotting files.