Modal Data Specification |
 
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Modal Data Specification |
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Firstly, you must identify your riser as being a certain type. The are two main categories in this context; (i) top-tensioned risers (TTR) and (ii) catenary risers (SCR). Modes bases some important decisions on what modes to include in the SHEAR7 output on this classification. This is discussed in more detail shortly. If your riser does not fall into either of these categories, you may classify it a User defined riser type.
In terms of VIV data, one mandatory input is the number of segments of equal length to be used in generating the SHEAR7 data. The VIV program requires eigensolution details at equally spaced points along the riser, but your finite element model will rarely be based on elements of equal length. However Modes allows you complete flexibility in defining your FE model as you require via the Flexcom main analysis module, and then produces SHEAR7 input for an equal spacing you define in Modes, by interpolation between the values for the elements of your discretisation.
There are several other optional inputs. You may specify the number of modes to consider for output to the SHEAR7 input file(s). This defaults to all modes of the eigensolution, but you might want to exclude higher modes, as these are less accurately determined than the lower modes. If you invoke this option, only the lowest number of eigenpairs are considered for output. You may also specify a particular element set for output, in which case the SHEAR7 output is restricted to the elements of this set only. If you do not specify an element set, the composition of the default set depends on the riser type under consideration. If the riser type is TTR or User, then the default is all elements of the finite element mesh. If on the other hand the riser type is SCR, then the default is all elements that are not wholly on the seabed, starting at that element whose first node is the SCR touchdown point. A further optional input is a damping ratio for use with a subsequent SHEAR7 analysis. Note that this input is only relevant to wake interference analyses, where Flexcom can automatically run a SHEAR7 analysis to compute enhanced drag and lift coefficients.
•*RISER TYPE ($MODES section) is used to specify the riser type and other parameters relating to the Modes facility for generating output for subsequent input to SHEAR7.