VIVA results

The VIVA results are presented in OrcaFlex as extra worksheets in the summary results, giving the results from the latest VIVA call. There is a separate worksheet for each excited mode, plus one for the multi-mode response. The drag coefficients currently in use are specified by the target mode.

The VIVA results are described briefly below. For more details see the VIVA documentation.

Note: Amplitudes in VIVA results are single amplitudes, i.e. measured from the mean position to the peak.

Single mode results

The worksheet for a given single excited mode gives a table of single mode results as a function of arc length along the line. The table follows the discretisation used by VIVA, so there are NP equally-spaced results points.

These are results predicted by VIVA if single-mode VIV response occurs in that mode. They include

Max amplitude

The amplitude of predicted VIV motion at that point.

Max bend moment, max stress

The dynamic bend moment and stress amplitudes due to VIV. Note that these do not include the bend moment and stress due to the mean position of the line.

Drag coefficient

The predicted drag coefficient, allowing for VIV.

Life

The predicted fatigue life. Note that this value only accounts for the fatigue due to VIV: other sources of fatigue must be accounted for separately.

Multi-mode results

The multi-mode worksheet contains two tables. The second of these is the multi-mode equivalent of the single mode results table; it gives the same information, but this time for the case where multi-mode VIV response occurs.

The first table summarises the bending modes that VIVA predicts as being excited. It has the following columns:

Mode number

The mode number

Natural mode frequency with and without VIV

The natural frequencies of the mode, as calculated by VIVA. Those without VIV are calculated using VIVA's own default added mass coefficients (not those specified in the OrcaFlex line type data); for those with VIV, the VIVA default added mass coefficients have been modified to allow for VIV.

Max amplitude

The largest offset of any node in this mode.