OrcaFlex Manual - Orcina

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7 MODAL ANALYSIS
7.1 DATA AND RESULTS
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Modal Analysis, Data and Results
The modal analysis form enables you to calculate and view the undamped natural modes of the whole system, or of
a single line. To open this form, see the Modal Analysis command on the Results menu. Note that the analysis is only
available when the static position of the model has been calculated.
For full details of the calculation, and a discussion of its limitations, see the Modal Analysis theory section.
Performing modal analysis
To perform a modal analysis you need to specify the following:
� What you want to analyse: the whole system or a single line.
� Which modes you want to calculate. You can ask for All modes or a specified range of modes. For large systems
it is much quicker to calculate only a small number of modes – see Modal Analysis Theory for more details.
� Whether you want to calculate the mode shapes or just the natural periods. If you exclude the mode shapes then
the analysis only calculates the natural periods, not the shapes of the natural modes. If you include the mode
shapes then the analysis takes longer.
When you have made your selections click the Calculate button. The modal analysis will then calculate the
undamped natural periods and, if requested, the mode shapes. Each mode is normalised to have largest offset
magnitude equal to 1, i.e. the offsets vectors are scaled so that largest offset vector is a unit vector. The modes are
numbered in order of increasing frequency.
Modes Table
This page displays a spreadsheet giving the mode periods and mode shape results in numerical form. If you do not
calculate the mode shape then the table reports only the periods of the requested natural modes. If you calculate the
mode shapes then the table also gives the shape in the form of the displacements of each degree of freedom. Mode
shapes can be reported with respect to either global axes directions or local axes directions.
If mode shapes are calculated then the table includes the following information:
� Offset distribution displays a measure of how inline, transverse, axial and (if torsion is included) rotational the
mode is. For details see Offset Distribution below.
� 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, mostly axial, rotational and mostly rotational. Mixed means that
none of the components are more than 50% of the total.
Mode View
Note: The Offset Distribution and Mode Type information is only available for single line analyses where
there is relative flow normal to the line. So, if the line is entirely above the water, or there is no
current defined then this information will not be available.
If you requested the mode shapes then the View page displays a 3D view of the system showing one selected mode
shape superimposed on the static position of the system. The current direction is also shown on the view, and you
can control the view angle, zoom etc., as on any 3D view. You may need to zoom out in order to see the system, and
you may need to adjust the view angle to suit the mode that you are viewing. For example an out of plane mode for a
catenary is best viewed by looking along the plane of the catenary.
You can use the mode drop-down list to control which mode is shown on the view. Note that when that drop-down
list has the focus (click it to give it the focus) then you can use the arrow keys to quickly increment or decrement the
mode shape number that is displayed.
The drawing exaggeration value allows you to vary the amplitude of the drawn mode shape. The animate mode
shape and draw node axes options allow further control of the mode shape drawing.
If the mode shape is being animated then there is a further choice to make, the animation period. If you select the
mode period option then the animation has a cycle period equal to the mode period. However, for modes with
either very long or very short periods, this option can make visualisation of the mode shape quite difficult. The
alternative option, fixed, animates the mode with a 5s cycle period.